We outline the work done to extend and
improve the ISC-GEM Global Instrumental Earthquake Catalogue, a dataset which
was first released in 2013 . In its first
version (V1) the catalogue included global earthquakes selected according to
time-dependent cut-off magnitudes: 7.5 and above between 1900 and 1918 (plus
significant continental earthquakes 6.5 and above); 6.25 between 1918 and
1959; 5.5 between 1960 and 2009. Such selection criteria were dictated by
time and resource limitations. With the Extension Project we added both
pre-1960 events below the original cut-off magnitudes (if enough station data
were available to perform relocation and magnitude recomputation) and added
events with magnitude 5.5 and above from 2010 to 2014. The project ran over a
4-year period during which a new version of the ISC-GEM Catalogue was released each
year via the ISC website
(http://http://www.isc.ac.uk/iscgem/, last access: 10 October 2018). For each year, not only have we added new events to the
catalogue for a given time range but also revised events already in V1 if
additional data became available or location and/or magnitude reassessments
were required. Here we recall the general background behind the production of
the ISC-GEM Catalogue and describe the features of the different periods
in which the catalogue has been extended. Compared to the 2013 release, we
eliminated earthquakes during the first 4 years (1900–1903) of the catalogue
(due to lack of reliable station data), added approximately 12 000 and 2500
earthquakes before 1960 and between 2010 and 2014, respectively, and improved
the solution for approximately 2000 earthquakes already listed in previous
versions. We expect the ISC-GEM Catalogue to continue to be one of the most
useful datasets for studies of the Earth's global seismicity and an important
benchmark for seismic hazard analyses, and, ultimately, an asset for the
seismological community as well as other geoscience fields, education and
outreach activities. The ISC-GEM Catalogue is freely available at
10.31905/D808B825.
Introduction
Earthquake catalogues are used in many activities by the
seismological community. Usually these list basic focal parameters of seismic
events (e.g. location, origin time, depth) along with the magnitude, and,
eventually, other parameters (e.g. moment tensor or fault plane solutions).
Studies concerning seismic hazard and the Earth's global seismicity often
require as input an earthquake catalogue that (ideally) has been obtained
using the same procedures over a long period of time. For such and other
purposes, global instrumental earthquake catalogues have been produced by
many authors since the beginning of the last century. Among others,
catalogues from , ,
, and
have been extensively used over the past decades until
and released the Centennial Catalogue and PAGER-CAT,
respectively, both covering the period 1900–2007. Although such catalogues
proved to be important resources for many years, they cover different time
periods and, more importantly, are often characterised by either large
heterogeneities in their parameters and/or produced with undocumented or
mixed procedures and/or underlying data e.g..
For example, the Centennial Catalogue lists both locations from various
catalogues (including the ones mentioned above) and recomputed ones (from
1964 onwards and only for selected large earthquakes between 1918 and 1964)
using the methodology (normally referred to as EHB),
whereas magnitudes are not recomputed but compiled from several different
sources/authors see. Very similar considerations
also apply to PAGER-CAT, which is based on the Centennial Catalogue up to
1973 . In addition, most of these catalogues terminate at
different times and are no longer maintained. In this context, in 2010 the
International Seismological Centre (ISC,
http://www.isc.ac.uk/, last access: 10 October 2018), as
requested by the GEM Foundation
(https://www.globalquakemodel.org/, last access: 10 October 2018), undertook a major effort to reprocess 100+ years of instrumental
seismological data to reassess both locations and magnitudes of global (i.e.
having magnitude 5.5 and above in our framework) earthquakes and,
consequently, to produce a new earthquake catalogue using homogeneous and
documented methodologies over the longest possible period of instrumental
seismology (i.e. since the early 20th century). In January 2013, after a
27-month project, the ISC and a team of international experts
(http://www.isc.ac.uk/iscgem/people.php, last access: 10 October 2018) released on the ISC website
(http://www.isc.ac.uk/iscgem/, last access: 10 October 2018) the first version V1, for a general description
see of the ISC-GEM Global Instrumental
Earthquake Catalogue (1900–2009).
Since then the ISC-GEM Catalogue has been used by many researchers
investigating seismicity rates, patterns of seismicity and earthquake
forecast e.g. as well as by groups working on
earthquake catalogues for seismic hazard purposes e.g. and other seismological studies
e.g..
In recognition of the value of such a homogeneous (to the largest extent
possible) instrumental catalogue, funding from public and commercial
organisations (http://www.isc.ac.uk/iscgem/sponsors.php, last access:
10 October 2018) has been given to the ISC since November 2013 to work on the
extension of the ISC-GEM Catalogue over a 4-year project which aimed, in a
nutshell, at adding as many earthquakes as possible before 1960 and
prolonging the catalogue beyond 2009. The Extension Project was also
motivated by the fact that damaging pre-1960 earthquakes were below the
cut-off magnitude of 6.25 (e.g. the 30 October 1930 central Italy event,
which caused collapse and severe damage in various towns) and many pre-1960
events had no initial magnitude and therefore could not be selected for V1,
yet they could be large enough to be part of the ISC-GEM Catalogue.
Below we detail the work done during the 4 years of the Extension Project
(which ended in December 2017) and discuss features of the different time
periods extended. Then we outline the overall state of the ISC-GEM Catalogue
in its latest version (V5) and, finally, present the outlook for its further
advancement.
The 4-year plan of the Extension Project
The Extension Project of the ISC-GEM Catalogue has been designed to add
earthquakes smaller than magnitude 6.25 before 1960 and extend it beyond 2009
with events of magnitude 5.5 and above. In addition, many earthquakes
pre-1960 with no magnitude information needed to be processed to reassess
location and magnitude, if enough station data were available.
Figure summarises the annual number of events before 1960
included in V1 of the ISC-GEM Catalogue along with the pre-1960 events
available in the International Seismological Summary (; see also
), and
the Centennial Catalogue plus additional hypocentres (hereafter we refer to
it as the augmented Centennial Catalogue) that were not processed for the V1
release (see also Fig. 8 in ). Note that ISS and BAAS
earthquakes are also listed in the Centennial Catalogue but throughout the
paper we try to refer to the original sources as much as possible. For
simplicity, in the following we refer to earthquakes in grey in
Fig. as extension events (i.e. not listed in V1), meaning that
those are the events we digitised station data for but not necessarily all
will be selected for processing and then included in the ISC-GEM Catalogue.
The station data collection and the selection process will be discussed in
the following sections.
Annual number of pre-1960 earthquakes in V1 of the ISC-GEM Catalogue
(black, total =2439 events) and the events that are available in the ISS
between 1918 and 1959 and the augmented Centennial Catalogue/BAAS between
1904 and 1917 (grey, total of 20 865 events) that were not processed for V1
(in the text referred to as extension events). The hachure patterns on top
outline the period extended in each year of the Extension Project:
1950–1959, 1935–1949, 1920–1934 and 1904–1919 in Year I, II, III and IV,
respectively. The period 2010–2014, not shown here, has also been
progressively added during the Extension Project.
The annual number of events in V1 oscillates between 4 and 12 for 1904–1917,
31 and 92 for 1918–1959 and 235 and 489 for 1960–2009. Such variations
reflect the cut-off magnitudes adopted for selecting earthquakes in different
time periods: 7.5 and above before 1918 (plus significant continental
earthquakes 6.5 and above); 6.25 between 1918 and 1959; 5.5 from 1960 onwards
seefor more details on the V1 earthquake selection
criteria. It is worth remembering here that the cut-off
magnitudes are simply thresholds set for selection purposes (not all pre-1960
events have known or reliable magnitudes) and should not be interpreted as
completeness levels (variations of the completeness over different time
periods for V1 were briefly outlined by , and
investigated in more detail by ).
Considering the number of pre-1960 earthquakes available (nearly 21 000,
i.e. about 2000 more than the V1 release covering 1900–2009) in the ISS
(1918–1959), BAAS (1913–1917) and augmented Centennial Catalogue for which
we had to look for station data (and, consequentially, digitise), we planned
to extend the catalogue following a 4-year schedule as outlined in
Fig. . Such a time frame was necessary to allow us to be as
comprehensive as possible in the station data collection task and also to
assess the ∼60 % of extension events that had no initial magnitude
information (in our database), and, therefore, could not have been selected
just using any cut-off magnitude criteria (details in the next section). In
addition, the extension of the catalogue beyond 2009 would benefit from the
data concurrently released in the ISC Bulletin and would follow the original
selection criteria (i.e. earthquakes with magnitude 5.5 and above).
At the end of each project year an upgraded version of the catalogue was made
available for download at http://www.isc.ac.uk/iscgem/ (last access:
10 October 2018). The catalogue is distributed in CSV format and is composed
of two parts (the Main catalogue, also available as a KMZ file for use with
Google Earth, and the Supplementary catalogue, the latter including events
with either poor location and/or magnitude quality; see
). Location parameters and magnitudes (either direct or
proxy moment magnitude Mw; ) come with
formal uncertainties and quality flags (from A to D, denoting well and poorly
constrained parameters, respectively), followed, if available, by the
solution of the Global Centroid Moment Tensor (GCMT;
http://www.globalcmt.org, last access: 10 October 2018,
). The criteria to assign the quality
flags for location, depth and magnitude are summarised in
Table . For the location quality flag we
consider the secondary azimuthal gap largest azimuthal gap filled by a
single station;hereafter referred to as SGAP, the eccentricity
of the error ellipses and the event location accuracy if
it is of high confidence to become a candidate for the IASPEI Reference Event
List GTCAND in Table ; seeand
http://www.isc.ac.uk/gtevents, last access:
10 October 2018. For the depth quality flag we consider the
availability of very close stations (within 10 km, NSTA10) and in the local
distance range (within 150 km, NSTAlocal), the depth constrained by
depth-phases (if available, depdp in Table ) and
the location accuracy (GTCAND). For the magnitude quality flag we consider
the author GCMT or literature; for direct Mw
values, whereas for the Mw proxy based on our recomputed
MS or mb, the quality flag
depends on combinations of the magnitude value, type (MS or
mb), uncertainty, number of stations used and the uncertainty of
Mw proxy.
Criteria to assign the location quality flags for location, depth
and magnitude. SGAP is the secondary azimuthal gap, GTCAND denotes a high
confidence location accuracy that makes the event a candidate for the IASPEI
Reference Event List (; see also
http://www.isc.ac.uk/gtevents, last access: 10 October 2018), depdp is the depth constrained by depth phases (if
available), NSTA10 is the number of stations within 10 km and NSTA (local)
is the number of stations within 150 km . MS
is considered well constrained when it is obtained from more than four
stations, within 5.5–7.5, and has uncertainty ≤0.2. See text for
details.
Quality flagLocationDepthMagnitude (Mw, direct or proxy)ASGAP < 120 && eccentricity < 0.75or GTCANDdepdp orGTCAND orNSTA10GCMTBSGAP < 160NSTAlocal > 2Literatureor Proxy based on well-constrained MSCOther casesOther casesLiterature or Proxy based on poorly constrained MSor Proxy based on mbDManually assignedManually assignedNo magnitudeor Proxy uncertainty > 0.7or Mw proxy based on MS< 5
One of the key features of the ISC-GEM Catalogue is that all events since
1904 have been reprocessed using instrumental station parametric data and the
ak135 model . To extend the catalogue, we followed the
same steps and methodologies used to create V1, as described in the
following.
and references therein was followed for digitising from
printed bulletins body-wave arrival times and
amplitudes/periods (of surface waves in particular) for the pre-1960 events to allow
relocations and magnitude recomputation, respectively.
For the extension events, the most important source of body-wave arrival
times was the ISS, whereas amplitudes and periods were retrieved from individual
station or network printed bulletins.
describe the two-tier relocation approach, which benefits
both from the EHB location algorithm
and the new ISC locator used to constrain the depth and the
epicentre, respectively. As the EHB and ISC location
algorithms are also used to cross-check each other, the location consistency is checked
twice.
describe the magnitude recomputation, particularly
for the surface wave magnitude MS, which,
in turn, is used as the basis for Mw conversion for most of the events
pre-1960.
are referred to for the literature search of reliable and direct
computations of seismic moment M0 and,
therefore, of Mw, for events before the GCMT solutions start in 1976.
The data collection has been the most time-consuming task and indispensable
part, not only to extend the catalogue but also to revise and better
constrain solutions of events already in V1 (details in the next sections).
Indeed, compared to the data collected for the V1 release, we made a
significant improvement in the number of amplitude and period data digitised,
particularly for MS recomputation, thanks both to additional
bulletins donated (or lent) to the ISC from various institutions and
individuals (including the personal collection of Nicholas Ambraseys; more details
are available at
http://www.isc.ac.uk/iscgem/acknowledge.php) and station bulletins that were not processed for V1 due
to time and resource limitations. Later we also show how the additional data
gathered during the last 4 years helped us revise and better constrain the
MS of pre-1960 earthquakes already listed in V1. With the end of
the Extension Project in December 2017, in the following we outline the
improvements and features of different time periods during which the ISC-GEM
Catalogue has been extended.
Extension for the period 1920–1959
In this section we describe the work done in the first 3 years of the
Extension Project to add earthquakes in the predigital period between 1920
and 1959. Note that throughout this work we consider as predigital
earthquakes those that occurred before 1964 (i.e. before the beginning of the
ISC Bulletin).
Distribution of stations listed in the ISS in each decade
contributing body-wave arrival times to the extension events and colour-coded
by number of body-wave arrival times. The annual number of
stations (b) and body-wave arrival times (c) are also
summarised.
Station data collection and earthquake selection
The variations in the annual number of the extension events shown in
Fig. are the result of various factors. For example, a
significant increase in the annual number of events can be seen in 1918
coinciding with the beginning of the ISS, whereas a dip in the late 1930s to
mid-1940s is associated with the disruption caused by World War II (more
details later) and another dip in the mid-1950s is due to the censoring
introduced by ISS procedures (more details on page 3 of the ISS, 1953) to
reduce the workload. The annual variations in the number of the extension
events also introduce an issue in selecting earthquakes for the ISC-GEM
Catalogue. For example, between 1950 and 1952 the annual number of extension
events in the ISS is between 782 and 1384, and such numbers are above the
annual number of earthquakes of magnitude 5.5 and above in the ISC-GEM
Catalogue in recent years (e.g. the largest annual number of earthquakes is
654 for 2011). This means that a subset of the extension events in 1950–1952
should not be part of the ISC-GEM Catalogue as it falls below the cut-off
magnitude of 5.5. However, since, as mentioned earlier, about 60 % of such
events have no magnitude information in our database, we could not use the
original magnitude criteria of 5.5. Thus, for the extension events we decided to base our selection
criteria both on the distribution of stations in the ISS and the number of stations contributing amplitudes/periods for
magnitude recomputation. This required a major effort to digitise both all
ISS pages (not available in any electronic format) and amplitude and period
pairs (of surface waves, in particular) from the station/network printed
bulletins for all extension events. Here we
briefly summarise the station data collected for the extension events and
highlight some features that are relevant to the ISC-GEM Catalogue.
Figure shows the distribution of stations listed in the ISS for
each decade (1920s, 1930s, 1940s, 1950s) colour-coded by their body-wave
arrivals contribution to the extension events along with the annual number of
stations and body-wave arrivals digitised from the ISS. The number of
stations listed in the ISS generally increased from the 1920s to the late
1930s before World War II affected various seismic stations, and it is only
around 1953 that the station contribution improved significantly. The
box-and-whisker plot of Fig. summarises the median number of
stations per event in each year. It shows that only a limited number of
stations (median number ranging from 9 to 26) are usually associated with the
extension events until 1952, whereas from 1953 onwards there is a general
improvement in this respect (median number of stations ranging from 66 to
99). Another relevant feature to point out is the uneven station
distribution, with Europe showing the highest density particularly before the
1950s, and the lack of stations in Africa and vast parts of the Southern
Hemisphere.
Box-and-whisker plot for the extension events of the median number
of ISS stations per event in each year. The box represents the 25 %–75 %
quantile, the band inside the box represents the median and the ends of the
whiskers represent the minimum and maximum of all data.
Distribution of the stations in each decade contributing amplitudes
to the extension events and colour-coded by number of amplitudes. The annual
number of stations (b) and amplitudes (c) are also
summarised.
Box-and-whisker plot for the extension events of the median number
of stations supplying amplitudes per event in each year. The box represents
the 25 %–75 % quantile, the band inside the box represents the median
and the ends of the whiskers represent the minimum and maximum of all data.
Figures and , similarly to Figs. and
, show the distribution of stations contributing amplitudes for
each decade and the median number of stations supplying amplitudes in each
year, respectively. The number of stations reporting amplitudes increased
until World War II, dropped in the 1940s and improved significantly from
1953. European and many Russian stations are the most important contributors
to amplitude readings compared to stations in other continents, except for La
Paz (LPZ, Observatorio San Calixto, Bolivia) and Riverview College (RIV,
Sydney, Australia) from the Jesuit seismic network . The
number of stations per event contributing amplitudes ranges from 0 to above
40, with the median per year oscillating from 0 to 6 (Fig. ).
We based the selection of the extension events on combinations of the number
of body-wave arrival times and the number of stations supplying amplitude
data. Considering that our relocation approach relies
largely on teleseismic observations (i.e. above 18∘ distance) and the
magnitude reassessment on the availability of three
(or two in some case) station magnitudes, we first excluded events with no
teleseismic phases and fewer than two stations contributing amplitudes. After
this first cut, we further excluded earthquakes with a limited number of
body-wave arrival times and fewer than two to three stations with amplitudes.
These are earthquakes for which we could not obtain a reliable solution (due
to poor station coverage and/or arrival times) after preliminary relocation
attempts. It is worth pointing out we have tried to be as comprehensive and
conservative as possible by not rejecting all poorly constrained relocations
(see next section). Also, we included all extension events between 1953 and
1956 available in the ISS (due to their small number; see Fig. 1) and
well-recorded earthquakes but without amplitudes. As a result, out of the
19 341 extension events between 1920 and 1959 we relocated 11 572. The
annual numbers are shown in Fig. , where the variations are
linked to the state of the global network during those years and the
operational practice changes at the ISS, as mentioned earlier.
Annual number of the relocated extension events between 1920 and
1959. See text for details.
Relocations
The location reassessment of previous hypocentres (from ISS or other authors
adopted by it) of the selected extension events is one of the fundamental
tasks of this work. The relocations are obtained by closely following the
approach described by . In Fig. the
box-and-whisker plots of the defining stations (i.e. stations with at least
one arrival time that constrains the location, hereafter referred to as
NDEFSTA) and SGAP for each year are shown. The NDEFSTA gradually increases
from the 1920s to the 1950s (except for the slight dip in the 1940s, for
reasons explained earlier), whereas the SGAP gradually improves over time.
This in general leads to improved confidence in locations.
Figure shows the location and depth differences between the
previous (ISS or authors adopted by ISS) and the ISC-GEM hypocentres. With a
few large exceptions, median location differences range from about 100 km in
the 1920s to about 20 km in the late 1950s. With depth differences, one must
consider that for 9418 relocated extension events the original depth was
unknown and nominally set to zero. Also, it is important to point out that
about half of the relocated extension events have no depth phases; therefore
for those the depth was assigned to a default depth resulting from the
tectonic setting or nearby earthquakes. However, as already pointed out by
, we remove the artefact of having most shallow earthquakes
set at zero km depth.
Box-and-whisker plots of the number of defining stations (NDEFSTA,
a) and the secondary azimuthal gap (SGAP, b) in each year.
Box-and-whisker plots of the epicentre (a) and depth
(b) differences between previous hypocentres (before) from ISS (or
authors adopted by ISS) and ISC-GEM (after) locations in each year. For 9417
of the 11 572 extension events relocated between 1920 and 1959 the depth for
the previous hypocentres (before) was unknown and nominally set to zero.
We checked the reliability of the ISC-GEM relocations in terms of network
coverage and deviation from the available hypocentres grouped for an event,
performed a cross-check between the EHB and ISCloc algorithms and considered
the nearby seismicity. At times we also used available comments in the
individual station bulletins as a guide in solving uncertain cases.
Obviously, relocations for events with large SGAP (>270∘) and/or
small NDEFSTA are not well constrained and we decided case by case whether to
manually assign location flag D (i.e. the event will be listed in the
Supplementary Catalogue). A typical case in this respect (although
time-dependent) is represented by earthquakes in the North Atlantic ridge
where most of the phase data would come from European stations and SGAP could
be even larger than 300∘ simply because North American stations (see
Fig. ) would not systematically report data for such earthquakes
(except for large ones).
Summary of the location and depth quality flags for the extension events between 1920 and 1959.
Table summarises the location and depth quality flags
for the relocated extension events between 1920 and 1959. The most frequent
quality flag both for location and depth is C. However, despite the
limitations of the global seismic network, particularly before the 1950s, it
is possible to recognize the improvements of the ISC-GEM locations with
respect to the original ones even on a global scale, as shown in
Fig. . Although we do not claim that the ISC-GEM locations are
the best possible solutions in this period for every single event, we
recommend that any regional or focused study of predigital earthquakes
instrumentally recorded should start from the ISC-GEM locations as they are
obtained from (currently) the most comprehensive set of instrumental data.
(a) ISS (or authors adopted by ISS) (before) and
(b) ISC-GEM locations (after) for the extension events relocated
between 1920 and 1959. plate tectonic boundaries are also
shown. It is possible to observe how the ISC-GEM locations better depict the
seismicity of the Earth even on a global scale.
Magnitude reassessment
We used the approach described in to reassess the
magnitude of the extension events consistently with their ISC-GEM
relocations. Due to the lack of short-period body-wave amplitudes before the
1960s, here we focus on recomputed MS as the basis for the
calculation of the proxy Mw. The MS recomputation is
based on the amplitudes and periods of surface waves digitised during this
work (Figs. and ). Before accepting an
MS value, we checked the station distribution and, when possible,
cross-checked our magnitudes with other magnitude information to investigate
cases of large differences with previous results. Figure shows
the timeline of the recomputed MS and their annual counts.
Besides the recurrent features discussed earlier (i.e. general increase in
the annual counts from the early 1920s and the dip in the 1940s), there are
2304 events with MS below 5.5. This occurs because our selection
criteria for this period, as explained earlier, had to be based on station
data availability rather on magnitude. Although events with magnitude below
5.5 would not normally be part of the ISC-GEM Catalogue, we did not exclude
them because of the importance of reassessing the magnitude of predigital
earthquakes. Most of these events with MS<5.5 are mostly
located in an area covering the mid-oceanic ridge of the North Atlantic to
the European Mediterranean region. This
is not surprising considering the distribution of stations contributing
amplitudes (Fig. ). Also, there are 80 earthquakes with
MS≥6.5 that should have already been in V1. These events
were not originally selected because the available magnitude information was
considered not reliable or it was below the cut-off value of 6.25. This
further highlights the necessity of a comprehensive and systematic magnitude
reassessment with homogeneous procedures.
Timeline of the 6575 recomputed MS (bottom) for the
relocated extension events during 1920–1959 and their annual counts (top).
2304 earthquakes have MS<5.5 and 80 have MS≥6.5.
In total, we recomputed MS for 6575 (∼57 %) of the
relocated extension events and obtained a magnitude (MS or any
other type) for the first time (at least to the best of our knowledge) for
3011 of them. A lack of stations reporting amplitudes is normally the cause
for not having a recomputed MS as we normally require a minimum
of three stations. The only exception occurs when we have two station
magnitudes from a subset of specially selected stations that do not differ
more than 0.3 magnitude units (m.u.). In such circumstances we allowed
MS recomputation for 276 earthquakes and assigned MS
uncertainty of 0.5 m.u.
If no direct Mw value is available for an event, the recomputed
MS values are then used as the basis for proxy calculations of
Mw and magnitude quality flags .
Table summarises the counts for the magnitude quality
flags for the relocated extension events between 1920 and 1959. Only five of
the extension events have direct values of Mw from the literature
search . The high number of magnitude quality flags D is
largely due to events for which no recomputed magnitude (MS,
mb
or Mw from the literature) is available and for which
MS, as the basis for Mw conversion, is below 5.
Figure shows the timeline of the earthquakes without recomputed
magnitudes along with their annual counts and depth frequency. Although
MS is not estimated for deep earthquakes according to
, the clear majority (nearly 70 %) of events without
magnitude are shallow (depth ≤50 km). For such shallow earthquakes we
continue to look for additional amplitudes (more details in a later section)
so that we can calculate MS and eventually move some of those
events from the Supplementary to the Main catalogue.
Timeline (b) of the relocated extension events without
magnitude between 1920 and 1959 along with their depth frequency (c)
and annual counts (a).
Summary of the magnitude quality flags for the relocated extension
events between 1920 and 1959. Included in the D flag are 4984 events for
which no magnitude was recomputed.
QualityCount forflagmagnitudeA0B3030C2824D5719
Timelines of the extension earthquakes already in our record (black
circles) and added ones (grey diamonds) split by original location
author/source. See text for the augmented Centennial Catalogue authors
(black) and a brief descriptions of the additional location sources (grey).
The total counts for each location source are shown on the right-hand side.
The annual counts (a) of the extension earthquakes already known and
added ones (black and grey histograms, respectively) are summarised. The
station data sources (b) are also outlined and shown in different
grey colours for the time ranges they have been used for (see text for
details). For 1908 we have added station data from the “Kleinere Beben”
part of ISA (black dots) and during 1913–1918 we also looked into the GUTE
notepads for earthquakes not listed in the BAAS and ISS (dark grey dots).
Individual/network station bulletins have been used to add both surface wave
amplitudes and body-wave arrival times between 1904 and 1919.
Station data contributions from ISA (a),
SHIDE (b), GUTE (c), RUS (d), BAAS and
ISS (e) and BULLETINS (f) identifying the
individual/network station bulletins. The stations are colour-coded by number
of phases digitised (see text for details). For each station data source, the
annual station counts are shown below the corresponding map (note the
different scales for each contributor).
Extension for the period 1904–1919
During the last year of the Extension Project we focused on the first part of
20th century and made special efforts to gather not only body-wave arrival
times and amplitude of surface waves, but also known earthquakes not
available in the ISC database. We did not add any station data before 1904
basically only stations belonging to the Milne network are available;
see, e.g. and, consequently, we decided to drop the 10 pre-1904
events listed before V5 from the ISC-GEM Catalogue and have the catalogue
starting in 1904.
Data collection
Before the ISS was put in production starting with earthquakes that occurred
in 1918, other seismic bulletins were compiled by different authors/agencies
e.g.and references therein. For this
work we gathered station data from the following sources.
International Seismological Associations bulletins are the most comprehensive both in terms of earthquakes and stations
listed for those years.
They are composed of two parts, one for the large/significant earthquakes
(in German “Hauptbeben”) and one for the small ones (“Kleinere Beben”).
Unfortunately, the 1908 “Hauptbeben” part was not printed (at least to the best of our knowledge).
These bulletins are referred to as ISA in the following.
Shide Circulars were used for 1908–1912, referred to as
SHIDE.
Gutenberg's notepads were utilized for 1908–1916 (mostly up to 1912), referred to as
GUTE.
Russian network bulletins were consulted for 1908 and 1911–1912, referred to as
RUS.
bulletins (predecessor of the ISS) listed both locations and station
data.
The ISS was used for 1918–1919.
Individual station bulletins (1904–1919, referred to as BULLETINS) were consulted, not only for what concerns the surface wave amplitudes
but also for body-wave arrival times (in support of relocation rather than
only magnitude reassessment) as some stations
(e.g. Uppsala, Nordlingen, Munich) were partially or completely missing in the BAAS or ISS.
Furthermore, the body-wave arrival times from individual station bulletins are fundamental
for the newly added earthquakes that we describe later in this section.
The ISA, SHIDE and RUS bulletins are available from the supplementary
material of , whereas scanned images of GUTE notepads
were kindly provided by Katsuyuki Abe. The
ISA, BAAS and ISS bulletins list arrival times from most of the stations
operating at that time, whereas SHIDE mostly includes data from Milne
stations and the GUTE notepads only a subset of global stations. Except for
ISS 1918–1919 (already electronically available), the various sources of
body-wave arrival times (ISA, SHIDE, GUTE, RUS and BAAS) for the 1904–1917
extension events were all manually typed in text files and then parsed into
the ISC database.
As shown in Fig. , the annual number of recorded earthquakes, at
least up to 1917, is smaller than an approximate average rate of ∼100 yr-1 for events of magnitude 6
and above. Therefore, for this period we also tried to add as many known
earthquakes as possible that are not listed in the augmented Centennial
Catalogue, BAAS or even the ISS 1918–1919. To do that we considered the
following sources:
Catalog of Damaging Earthquakes in the World
http://iisee.kenken.go.jp/utsu/index_eng.html, last access: 10 October 2018,, referred to as UTSU in
the following;
ISA (only for 1904–1907);
SHARE European Earthquake Catalogue (SHEEC) 1900–2006
, referred to as SHEEC in the following;
catalogue of the European area (referred to as KAR)
and
catalogue for Greece and surrounding areas (available at
http://geophysics.geo.auth.gr/ss/CATALOGS/seiscat.dat (last access: 10 October 2018), referred to as GRE)
for earthquakes before 1908 with station data in ISA (either not available in
SHARE or for which the KAR/GRE solution would be a better starting
point considering the ISA station data);
Significant Earthquake Database of the
(https://www.ngdc.noaa.gov/nndc/struts/form?t=101650&s=1&d=1, last access: 10 October 2018), referred to as NGDC.
As we have a rather mixed set of starting points for hypocentre relocations,
in Fig. we show the timelines of the extension earthquakes
1904–1919 split by original location author, along with their counts and the
time coverage of the station data sources we digitised. The augmented
Centennial Catalogue location sources G&R, B&D, ABE, CENT and BJI are from
, , and
, Centennial itself and Chinese catalogue,
respectively. In total we have found 405 additional earthquakes (mostly
before 1917) on top of the 1530 earthquakes already listed between 1904 and
1919 in the augmented Centennial Catalogue, BAAS and ISS. Notably, between
1904 and 1907 the annual number of earthquakes we added (mostly from ISA and
UTSU) is larger than the annual number of extension earthquakes previously
available in our record. Between 1908 and 1912 the annual number of
earthquakes added is comparable or smaller than the ones already available,
whereas from the beginning of the BAAS and then ISS the annual number of
newly added earthquakes drops significantly during the BAAS and then it is
zero with the beginning of the ISS.
For all earthquakes outlined in Fig. we tried to associate as
many body-wave arrival times and surface wave amplitudes as possible from the
station data sources mentioned earlier. The contribution of each station data
source is presented in Fig. . For the early years of the past
century, ISA was comprehensive in compiling data from stations around the
world, whereas the other sources only included subsets of the stations
operating at that time. Unfortunately, between 1908 and 1912 (coinciding with
the end of ISA, “Hauptbeben” part, in 1907 and before the beginning of BAAS
in 1913) we do not have a comprehensive bulletin such as ISA in preceding
years or BAAS in the following ones. Therefore, we gathered station data from
SHIDE, GUTE, RUS and individual/network station bulletins. From 1913 onwards,
the overall station data collection improves significantly thanks to BAAS and
then ISS.
Considering all sources depicted in Fig. , Fig.
shows the overall annual counts for the number of stations, phases and,
finally, the box-and-whisker plot of the annual number of stations per event.
A significant dip is present in the station data between 1908 and 1912 since
the station (and location) sources available to us for these years are not as
comprehensive as ISA or BAAS/ISS. The box-and-whisker plot of
Fig. also shows that several earthquakes have none to three
associated stations (59 from the augmented Centennial Catalogue, BAAS and ISS
and 116 from the newly added ones). Obviously, the limitations in the
collection of station data influenced the earthquakes that we finally
selected for processing and the quality of the relocations/magnitude
reassessment. The results are discussed in the next two subsections.
Annual number of station (a), phases (b) and
box-and-whisker plot (c) for the overall station data collected
between 1904 and 1919.
Relocations
Not all extension earthquakes have sufficient station data to perform a
relocation using our approach. First, we have discarded 175 earthquakes with
fewer than four stations, as pointed out earlier. We then progressively
discarded another 650 as either the relocation failed or was considered
unreliable. We may go back to the discarded earthquakes if additional
station data become available to us. In the end, we accepted the relocation
for 1110 out of the 1935 extension earthquakes. Figure shows
the annual counts of the relocated extension earthquakes 1904–1919. Note the
dip in the annual number of the relocated extension earthquakes for
1908–1912, reflecting the absence (to the best of our knowledge) of a
comprehensive global bulletin between ISA and BAAS.
As in Fig. , Fig. shows the box-and-whisker plots
of NDEFSTA and SGAP. For this period the relocations are usually based on a
small number of stations (median between 6 and 16) resulting in a large SGAP
(median between 201 and 310∘), even during the years covered by BAAS
and ISS. Figure shows the median location, depth and origin
time differences between previous (see Fig. ) and ISC-GEM
locations. The median location differences oscillate between 70 and 205 km,
with large differences above 1000 km for 46 earthquakes (16 above 2000 and 4
above 3000 km). Such large location differences can occur for various
reasons (from typos in the latitude/longitude of previous locations to poorly
recorded earthquakes having low confidence locations). One extreme example is
the epicentre change from Bristol Bay, offshore Alaska (G&R location), to
offshore Jamaica (ISC-GEM location) for an event that occurred on
22 August 1907 (∼22 h 23 m). The reason for such a large difference
originates from the fact that G&R ignored the report that the event was felt
in Kingston (see, e.g. 1907, part B, p. 73) and preferred
to fit the phase data to an intermediate-depth event offshore Alaska. As for
1920–1959, most of the earthquakes have no depth resolution and the previous
depths were largely unknown or set to zero, and this occasionally results in
large depth changes (±100 and ±300 km for 51 and 10 earthquakes,
respectively). Figure also shows the box-and-whisker plot of
the origin time (OT) differences in each year. We show the OT differences
because in this period (particularly before BAAS) the OT listed in the
previous location sources was at times truncated to the minute or with some
minute error that we were able to address thanks to the stations data we
digitised. Although ∼90 % of the OT differences are within 1 min,
some large OT changes of ±5 min or more occur for 16 earthquakes (8
originally from ABE).
Annual number of relocated extension events between 1904 and 1919. See text for details.
As for Fig. but for the period 1904–1919.
Box-and-whisker plots of the epicentre (a),
depth (b) and origin time (OT) differences between previous (before,
see Fig. ) and ISC-GEM hypocentres (after) in each year. For 880
of the 1110 extension events relocated between 1904 and 1919 the depth of the
previous locations was unknown and nominally set to zero.
Similar to the 1920–1959 period, we assigned location quality flag D if the
location was not constrained well enough. This time this task was done not
only by considering the usual criteria (see Sect. ) but
also consulting available information on the earthquake's effects (e.g.
tsunami, damage). In this respect we made systematic use of the earthquake
effect information available in UTSU and NGCDC. Table
summarises the location and depth quality flags for the relocated extension
events between 1904 and 1919. The limitations of the global network in this
period are generally more prominent than for 1920–1959 and this translates
in most of the earthquakes having location and depth quality C and about 246
of them have location quality D. As for the discarded earthquakes, if
additional station data become available we will try to improve the location
quality and eventually move some of the location flag D earthquakes from the
Supplementary to the Main catalogue. As for Fig. ,
Fig. compares the previous (before) and ISC-GEM locations
(after) on global maps for which, again, a general improvement in the
earthquakes' distribution along plate boundaries is delineated. This is
particularly the case for several global earthquakes along the subduction
zone of the Pacific and Indian oceans whose previous locations were hundreds
of kilometres away from plate boundaries.
Summary of the location and depth quality flags for the relocated
extension events between 1904 and 1919.
Even for this period the magnitude reassessment is mostly based on our
recomputed MS. Following the same procedures described earlier,
we obtained 927 MS for the relocated extension earthquakes, as
shown in Fig. . For 500 of them we have computed a magnitude for
the first time (in our record). Notably, for 137 earthquakes MS<5.5, whereas MS≥6.5 for 306 of them and >7.5 for 12 of
them. The latter includes six earthquakes originally from GUTE, four from ABE
and two from BAAS that were not selected for V1 because the magnitudes
available were not considered reliable or were below 7.5 (the original
cut-off magnitude for the V1 selection before ISS started in 1918). Nearly
all earthquakes with MS<5.5 occurred in the
European Mediterranean area (because in
this period the stations contributing surface wave amplitudes are strongly
concentrated in Europe; see Fig. ). In 1904 the collection of
surface wave amplitudes is limited to two stations, GTT (Göttingen) and
POT (Potsdam), until December, when we could also add data from station LEI
(Leipzig). Consequently, in 1904 we were able to recompute MS for
three earthquakes only, all occurring in December. For 18 earthquakes between
1905 and 1919 we accepted MS based on two station magnitudes.
Except for four earthquakes for which we have direct Mw values
from the literature search, all recomputed MS
values are used as the basis for Mw proxy calculations
. Table summarises the counts
for the magnitude quality flags for the relocated extension events between
1904 and 1919. About 50 % of the 183 relocated extension earthquakes for
which we do not have a magnitude (no direct Mw or recomputed
MS) are deep (MS not allowed in our procedures).
As for Fig. but for 1904–1919. The location authors
of the map in (a) are outlined in Fig. .
As for Fig. but for 1904–1919.
As for Table but for 1904–1919. Included in the D flag
are 183 events for which no magnitude was recomputed.
QualityCount forflagmagnitudeA0B420C427D262Summary of the Extension for 2010–2014
The extension of the ISC-GEM Catalogue beyond 2009 (last year in V1) benefits
from the data already available in the ISC Bulletin and the review of global
earthquakes by ISC analysts. The earthquake selection for recent years is
based on magnitude (5.5 and above). Table
summarises the number of earthquakes added per year during 2010–2014. The
relatively high number of earthquakes in 2011 is due to the
11 March Mw=9.1 Tohoku earthquake that was followed by about
120 aftershocks with magnitude 5.5 and above just in the first 24 h. In
contrast to the predigital period, global earthquakes in recent years are
recorded by a dense global network that usually allows us to constrain the
location with hundreds of stations and a relatively small SGAP. This is shown
in Fig. (note the difference in scale for the plot of the
number of stations compared to Figs. and ). The
ISC-GEM epicentres do not move significantly from the previous ones (ISC
locations), although occasional significant changes in depth occur, as shown
in Fig. .
Box-and-whisker plots of (a) the number of stations (NSTA,
black) and defining stations (NDEFSTA, grey) and (b) of the
secondary azimuthal gap (SGAP) in each year for the earthquakes added for the
period 2010–2014.
As for Fig. but for 2010–2014. Note the different
scales compared to Figs. and .
(a) Histogram distributions of the number of stations
(NSTA) contributing to MS for the pre-1960 earthquakes already in
previous versions of the ISC-GEM Catalogue (grey) and after revision (black)
using the newly added amplitude data. (b) Comparison between
original and revised MS colour-coded by the difference of NSTA
used to obtain MS. The black dashed and the dotted-dashed lines
are for the 1:1 and the ±0.3 values, respectively. Note that during
revision we dropped the MS for four earthquakes.
Number of earthquakes added between 2010 and 2014.
YearCount20105042011672201241420134842014521
As to magnitude, we largely list direct Mw from GCMT
(2347 earthquakes). Proxy Mw values from recomputed
MS or mb are given for 248 earthquakes. The location and
magnitudes of these earthquakes will be included in the figures of the
section outlining the state of V5.
Review of events that have already been part of the catalogue
The ISC-GEM Catalogue comes with a version number that keeps track of the
catalogues's updates and/or additions. Even when an earthquake is listed in
the catalogue, we continue to look for additional station data and
information that could help us to improve, whenever necessary, the
earthquakes' parameters we list in the catalogue. At the same time, we
cooperate with users of the catalogue who inquire about earthquakes of their
interest in different parts of the world, at times resulting in an updated
location, depth and/or magnitude for one or more earthquake. Examples of
updates we made thanks to users' help are available on the ISC-GEM Catalogue
update log web page
(http://www.isc.ac.uk/iscgem/update_log, last access: 10 October 2018). We also run internal checks as progress is made with the
Rebuild of the ISC Bulletin and/or the ISC-EHB dataset
. We try to keep the number of releases to a minimum and
recommend users quote the version number when using the ISC-GEM Catalogue for
their studies.
As mentioned before, during the Extension Project we gathered station data
(particularly for amplitudes of surface waves) from printed station bulletins
that were not available to us. Therefore, during the data collection task of
the Extension Project we did not limit the search for amplitude data to
extension earthquakes but also to earthquakes that were already listed in
previous versions (before V5) of the catalogue. This way we revised the
MS of earthquakes already listed in the catalogue even if we
added just one or two station readings. Figure shows the number
of stations contributing to MS as well as the comparison between
original and revised MS for pre-1960 earthquakes already listed
in previous versions of the catalogue. The increase in the number of stations
contributing to the recomputation of MS is significant:
∼30% and ∼74 % of the original MS were constrained
using fewer than 6 and 11 stations, respectively, whereas with the revised
MS these percentages drop to ∼8.5% and 31 %. Also, the
station data added allowed us to gain about 50 earthquakes with
MS. About 97 % of the revised MS are within ±0.3 m.u. of the original ones, with only five earthquakes with
MS differences above ±0.6 m.u. (often due to originally
mis-associated readings, also resulting in the loss of four original
MS values).
Overall status of the new (V5) ISC-GEM Catalogue
The new version (V5) of the ISC-GEM Catalogue covers the period 1904–2014
and was released on the ISC website
(http://www.isc.ac.uk/iscgem/, last access: 10 October 2018) on 27 February 2018. It is composed of 35 225 earthquakes in total,
with 7126 listed in the Supplementary catalogue (about 93 % of them having
occurred before 1960). The annual number of events in the Main and
Supplementary files is shown in Fig. . The magnitude sources are
the same four described in and the updated
composition is as follows: 45.72 % for Mw from GCMT, 42.85 %
for Mw proxy from recomputed MS, 8.1 % for
Mw proxy from recomputed mb and, finally, 3.33 % for
Mw from the literature search. As outlined in
, the Mw proxy values based on
MS are mostly for pre-GCMT earthquakes (i.e. with few exceptions,
before 1976).
Annual number of earthquakes in the Main (black) and Supplementary
(grey) ISC-GEM Catalogue; from the count of the Supplementary catalogue we
have excluded a subset of earthquakes with MS below 5.
Map showing the earthquakes listed in V5 of the ISC-GEM Catalogue
(more than 28 000 earthquakes, see Fig. ). The symbols are
plotted according to and colour-coded according to the
ISC-GEM depth. The earthquakes shown are from the Main catalogue plus those
earthquakes in the Supplementary catalogue that have reliable magnitude but
poor location (i.e. magnitude quality flag not equal to D and location/depth
quality flag equal to D).
The primary use of the ISC-GEM Catalogue is seismic hazard (including
calibration of regional seismic catalogues) and Earth's seismicity pattern
studies as is it the longest and most homogeneous record of natural global
seismicity recorded during the instrumental period. For this reason, in
Fig. we plot V5 of the ISC-GEM Catalogue using
symbols to emphasise the magnitude of the earthquakes in
the catalogue. To produce the figure, we included earthquakes in the Main
catalogue plus those earthquakes in the Supplementary catalogue that have
reliable magnitude but poor location (i.e. magnitude quality flag not equal
to D and location/depth quality flag equal to D). The subduction zones and
the mid-oceanic ridges are well depicted as are areas where global
earthquakes occur more frequently.
(c) Time–magnitude distribution colour-coded for cells of
0.1 m.u. in each year for the earthquakes used to produce
Fig. . (b) Cumulative annual number of earthquakes with
Mw≥5.5 (red), ≥6.5 (blue) and ≥7.5 (yellow),
along with the annual counts of intermediate-depth
(60 km < depth < 300 km, solid black line) and deep (≥300 km, solid white line) earthquakes; the blue and yellow lines are
obtained considering all earthquakes in V5 (i.e. both Main and Supplementary
Catalogue). (a) Annual completeness magnitude (Mc, black
circles ±1 standard deviation) estimated with the maximum curvature
method of implemented in the R-code of .
Note that we skipped 1904 for the Mc assessment due to the small
number of earthquakes.
The current magnitude content as well as a basic magnitude completeness
(Mc) assessment is shown in Fig. update on
Fig. 20 of. It is not our aim to do a detailed
completeness study as ; here we use the magnitude content
and Mc to highlight the following features of the catalogue.
The predigital period is not as complete (average annual Mc
varying between 5.7 and 6.8) as more recent decades (average annual
Mc between 5.5 and 5.7 since 1964). Important fluctuations in the
annual number of earthquakes/Mc are present in specific periods
or years. For example, because of World War II there is a significant
decrease in the number of recorded earthquakes (particularly below magnitude
6) consistent with the disruption of the global network during the 1940s;
other minor fluctuations are present in almost every decade (e.g. slight rise
in Mc in the early 1960s and late 1970s). The fluctuations over
time of the number of earthquakes (i.e. variations of Mc) in the
full catalogue (especially at the lower magnitudes, below ∼6.5) should
be checked before using it in its current status for studies concerning
temporal and seismicity patterns.
The number of intermediate-depth (between 60 and 300 km) and deep (≥300 km) earthquakes
per year before the 1950s–1960s is significantly smaller compared to more recent decades.
The reason is not fully clear and will be a matter for further investigation (see Sect. ).
Most likely, it is the result of a combination of factors, which include the
detection capability for moderate deep-focus earthquakes of analog seismographs
see, e.g. deployed around the world before the 1950s,
the lack of stations close to subduction zones
for many decades (Figs. and ) and the earthquake
selection criteria.
For global earthquakes, instruments such as the Wiechert, Bosh-Omori, Maika and
Galitzin were able to record surface wave signals
(medium period range, centred around 20 s) better than body-waves (higher frequency
signals, particularly P-waves, from around 10 s and below).
The effect could have been that many stations would not report station data for
moderate deep-focus earthquakes
and, therefore, the ISS would not compile data for such earthquakes (i.e. the
earthquake would not be recorded).
The selection criteria could also play a role, although the earthquakes not
selected for processing either lack station data (and depth resolution)
or, more importantly, are usually too small to account for the small number
of deep-focus earthquakes depicted in Fig. .
In addition, users should be aware that the magnitude uncertainty for
predigital earthquakes is inevitably larger than for earthquakes in the GCMT
era (from 1976 onwards). The timeline of the Mw uncertainty in
the ISC-GEM Main Catalogue is shown in Fig. . This is to further
remind users of the full catalogue that, for patterns of seismicity studies,
they should be aware of the larger magnitude uncertainty in the first part of
last century.
Timeline of the magnitude uncertainty in the Main file of the ISC-GEM Catalogue.
On the right the percentage distribution is shown.
Outlook
We plan to continue maintaining the ISC-GEM Catalogue for years to come and
work on its advancement by
adding recent years (2015 onwards);
regularising the magnitude for earthquakes between 1960 and 1990
to remove as many fluctuations as possible in the Mc over those decades;
adding earthquakes between magnitude 5 and 5.5 that have
occurred in continental areas from 1960 onwards;
improving the content for the predigital period (before 1964) by
filling gaps in the station reports
(particularly for what concerns surface wave amplitudes) and possibly bringing
additional earthquakes and station data from the
Bureau Central International de Seismologie ; we will also consider
any other source (if available) not considered so far
that will bring useful data (station data and/or earthquake information)
that will allow us to improve the catalogue;
with this task we aim at moving as many earthquakes as possible from
the Supplementary to the Main catalogue (see Fig. );
integrating the results from the ISC Bulletin Rebuild project
1964–2010;
see
and the ISC-EHB reconstruction 1964 onwards,;
continuing and extending our literature search for new or updates of direct
estimation of Mw for pre-GCMT earthquakes
as well as general focal parameters; we also aim at including fault plane
solutions from the literature for predigital earthquakes.
A more detailed description of the Advancement Project of the ISC-GEM
Catalogue is available at
http://www.isc.ac.uk/iscgem/advancement.php (last access: 10 October 2018). We will continue releasing a new version after the end of
each year of the Advancement Project. In this way we will be able to provide
the seismological, as well as the broader geoscience community, with the most
comprehensive and homogeneous account of earthquake global seismicity
recorded instrumentally at any point in time.
Data availability
Since 27 February 2018, V5 of the ISC-GEM Catalogue has
been available for download at http://doi.org/10.31905/D808B825. All data used in this paper are maintained at the ISC
(http://www.isc.ac.uk/, last access: 10 October 2018). The ISC-GEM
Catalogue is released without the associated seismic wave arrival times and
amplitudes used for this work. These underlying parametric data are either
already available or will be before the end of 2018 as part of corresponding
events in the ISC Bulletin (http://www.isc.ac.uk/iscbulletin/, last
access: 10 October 2018).
Conclusions
We presented the procedures and results of a 4-year project which extended
and improved the ISC-GEM Catalogue first released in 2013
. We have added about 12 000 more events between 1904
and 1960 and the new version (V5) ends in 2014 instead of 2009. To extend the
catalogue before the 1960s we have digitised ∼650 000 phase arrival
times from various sources (ISS, BAAS, ISA, Shide Circulars, Gutenberg
notepads, etc.) in different periods and added ∼140 000 amplitudes from
printed station bulletins. The features and limitations of the global network
before 1960 have been outlined and the results show that the relocations,
based on our two-tier approach , provide solutions
distributed along main tectonic boundaries, even though they are usually
based on a small number of stations compared to relocations of earthquakes in
recent years. We have recomputed over 6000 MS values for pre-1960
earthquakes and obtained (to the best of our knowledge) a magnitude for the first time for
more than 3000 of them. For the period 2010–2014 we have greatly benefited
from both the station data available in the ISC Bulletin and the reviews done
by ISC analysts which provide us with robust starting points for the
relocations and the Mw from the GCMT.
At the same time as the digitisation from printed sources of stations
supplying amplitude data (of surface waves in particular), we also looked for
additional data for predigital earthquakes (pre-1960) already listed in
previous versions of the ISC-GEM Catalogue. The newly added amplitude data
made us revise a significant number of pre-1960 earthquakes listed in V1 and
improve the magnitude solutions as the revised magnitudes are now based on a
much higher number of stations.
The current state of V5 of the ISC-GEM Catalogue has been summarised and its
features outlined. With the Advancement project we aim to further improve and
extend the catalogue in coming years and address some of the limitations that
have been pointed out here during different periods of time.
Author contributions
DDG was the leading author of the paper and responsible for the station
data collection, earthquake selection, second step of the relocation task,
magnitude reassessment and final checks. ERE provided scientific input and
determined the depths and starting locations which were used in the second
step of the relocation process. DAS obtained the funding for the project,
oversaw its progress and gathered additional station bulletins. All authors
contributed to the paper and approved the final
version.
Competing interests
The authors declare no competing interests in the
production of the ISC-GEM Catalogue.
Acknowledgements
This project was supported by the NSF (Award 1417970), USGS (Award
G15AC00202), FM Global, OYO Corporation, the Lighthill Risk Network, the
Aspen Re, Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) and 65
members of the ISC (http://www.isc.ac.uk/members/, last access:
10 October 2018). Year I and II of the Extension Project were also supported
by the GEM Foundation. We thank two anonymous reviewers for their comments
that helped us to improve the manuscript. Daniela Olaru and Elizabeth Ayres
were instrumental in the data collection from printed station bulletins and
the ISS. We used computer codes by Antonio Villase to digitise the ISS phase
data. Lynn Elms checked and streamlined the text. We are deeply indebted to
various institutions and individuals that provided additional station
bulletins to the ISC (more details at
http://www.isc.ac.uk/iscgem/acknowledge.php, last access:
10 October 2018). We are grateful to Josep Batlló for sharing additional
data on the 1919 Torremendo (Spain) series and related
discussions which allowed us to correct the corresponding events originally
listed in the ISS. For the Mw literature search we thank Paolo
Harabaglia for pointing out the papers from for
two significant earthquakes in Italy (12 December 1908 Messina and 23 July
1930 Irpinia) as well as the centroid solution for the 29 November 1975 Hilo
(Hawaii) earthquake from . Further acknowledgements
to users of the ISC-GEM Catalogue are available at
http://www.isc.ac.uk/iscgem/update_log/ (last access:
10 October 2018). Figures were drawn using the Generic Mapping Tools
.Edited by: David Carlson
Reviewed by: two anonymous referees
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