ISC-GEM: Global Instrumental Earthquake Catalogue (1900–2009), I. Data collection from early instrumental seismological bulletins
Introduction
The instrumental monitoring of earthquakes on a global scale began more than 100 years ago. Since then seismologists around the world store and exchange the results of standard observational seismological practice (e.g., picking of arrival times, amplitude-period measurements, etc.) or more complex waveform analyses (e.g., moment tensor inversion) in seismological bulletins/catalogues. Such products contain fundamental parametric data characterizing the seismicity of a region or the entire globe. However, instrumental parametric data (earthquake source related such as hypocentres, magnitudes, moment tensor solutions, or seismic station related such as phase arrival times, amplitudes and periods of different seismic waves) were available in computer-readable format, with a few exceptions, for only the last 50 years. A large volume of earthquake data starting with 1960 is publicly available from the International Seismological Centre (ISC, www.isc.ac.uk), where different types of parametric data are collected, processed and reviewed in order to produce the definitive summary of the Earth’s seismicity. In this respect, the ISC database can be considered the most comprehensive repository of instrumental earthquake data on a global scale thanks to the collaborations of the ISC members (www.isc.ac.uk/members/) and reporting agencies (www.isc.ac.uk/iscbulletin/agencies/). However, for the time period before 1964, only the basic earthquake parameters such as location and magnitude retrieved from different catalogues (e.g., Gutenberg and Richter, 1954, Rothé, 1969, Båth and Duda, 1979, Abe, 1981, Abe and Noguchi, 1983a, Abe and Noguchi, 1983b) were available in the electronic format. A bulletin differs from a catalogue by containing the station data used in the computation of the earthquake location and, whenever possible, the magnitude. Therefore the lack of digitally available station parametric data (e.g., phase picks and amplitude-period measurements) represented a huge limitation for the task of re-computing hypocentres and magnitudes using modern techniques and standards for the vast majority of global earthquakes before 1960.
Before the era of modern computer-based bulletin production, individual observatories and research institutions around the world stored seismic station reports in printed or hand-written seismological bulletins. Many such bulletins have been collected by the ISS and ISC over the years and represent a fundamental source of seismological data for earthquakes that have occurred from the beginning of 20th century until the 1960s and to a lesser extent into the 1970s. This allowed us to digitize and process station parametric data from a multitude of printed bulletins covering approximately 70 years of instrumental seismology.
The availability of data in the ISC database at the start of the project is summarized in Table 1, where we distinguish between phase arrival times and amplitude-period data essential for relocation and magnitude re-computation, respectively. Although the ISC Bulletin starts in 1964, phase data covering the period 1960–1963 was already available in the ISC database based on the work of Villaseñor and Engdahl (2007). Both phase and amplitude-period data were completely missing in the ISC database for earthquakes before 1960, whereas only amplitude-period data needed to be retrieved from printed seismological bulletins for events in 1960 and partially also in the1970s.
Due to time and resource limitations however, we searched for station data relevant to selected earthquakes according to the following cut-off magnitudes for different time periods:
- 1.
1900–1917: MS ⩾ 7.5 worldwide plus a selection of smaller shallow events in stable continental areas with MS between 6.5 and 7.5;
- 2.
1918–1959: MS ⩾ 6¼;
- 3.
1960–2009: MS ⩾ 5.5.
For the initial selection of earthquakes before 1976 we used all types of magnitude estimates (many from individual stations), compiled from all available sources of location and magnitude, in an expanded version of the Centennial Catalogue (Engdahl and Villaseñor, 2002). MS was our reference magnitude type before the introduction of the Global Centroid Moment Tensor database (GCMT, www.globalcmt.org), and when that was not available we used other magnitude estimates (including magnitudes of unknown type). From 1976 onwards we used an event selection cut-off moment magnitude of MW 5.6 (MS 5.5) from GCMT when available, or MW proxy values from available MS or mb. In the latter case we used the Bormann et al. (2009) relationships, and lowered the cut-off magnitude down to MW 5.5 during 1964–1977 in light of the larger uncertainty estimation in that period. Thus we avoided overlooking earthquakes with MW ⩾ 5.6 in the modern period.
In Storchak et al. (2013) a brief overview of the data added in the early instrumental period was given, concerning both arrival time and amplitude-period measurements. Here we describe in detail the printed sources considered for the production of the ISC-GEM catalogue and the procedures used to digitize the data stored in the printed bulletins. This description will be useful to the users of the ISC-GEM catalogue in obtaining a better overview of the underlying data used for relocation (see Bondár et al., 2015) and magnitude re-computation (see Di Giacomo et al., 2015) in the early instrumental period. We also point out some limitations of the data collection, which, in some case, may have affected the quality of some earthquake parameter determinations.
The newly digitized data fills a significant time gap where no parametric data were previously available in the ISC database for relocating earthquakes (before 1960) and for re-computing standard magnitudes such as MS and mb (before 1971, except the WWSSN short-period body-wave amplitude data available from 1964). For the period 1960–2009, station data can be found in the ISC Bulletin via the ISC website (www.isc.ac.uk). We also give a summary of the newly available digital parametric data that effectively doubles the overall time length of the ISC Bulletin. Finally, we briefly discuss possible areas of improvement.
Section snippets
Phase data collection (1900–1959)
To re-compute locations of the ISC-GEM earthquakes that occurred before 1960, we collected arrival times of seismic phases from different sources that were only available in either printed or manuscript form. These have been converted to a digital form using one of two methods. For good quality printed bulletins with formats consistent over a long period of time we have used optical character recognition (OCR) techniques (see Engdahl and Villaseñor, 2002). For all other sources we entered the
Amplitude-period data collection (1900–1970)
A major drawback with the ISS and other data sources is the lack of the basic measurement data (e.g., amplitude, period and component information) for seismic phases useful for re-computing the standard magnitude scales such as MS and mb. In order to re-calculate magnitudes for the relocated hypocentres, we needed to retrieve all necessary information from the early instrumental seismological bulletins (either individual stations or network bulletins). This was the core of the work for this
Conclusions
In order to deliver an updated catalogue based on instrumental data covering the last 100+ years of global earthquakes, we processed and digitized an unprecedented amount of phase and amplitude data from various collections of printed bulletins over about 1.5 years. This task was necessary for retrieving the basic data to run current techniques for event relocation and magnitude re-computation. Therefore, we processed individual and network station seismological bulletins to retrieve the
Data and resources
Most of the figures were drawn using Generic Mapping Tools (GMT, Wessel and Smith 1991) software.
Acknowledgements
Members of the Data Entry Team: Rebecca Verney, Natalia Safronova, Rosemary Wylie, Agne Baranauskaite, Jessica Wilson and Hepsi Simpson.
This work was supported by the GEM Foundation as one of the five GEM Global Hazard Components.
We wish to recognize the initiative of the late Edouard Arnold and Pat Willmore, former Directors of the ISC, for their efforts in organizing first the transfer of a large fraction of the ISS paper bulletins onto punch-cards and preparing the Shannon tape. We also wish
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2015, Physics of the Earth and Planetary InteriorsCitation Excerpt :Unfortunately the authors never published the stations that contributed arrival times, amplitudes and periods of seismic waves for each event in the catalogue. Thanks to the later efforts of their colleagues, the so-called Gutenberg Notepads have been preserved by microfilming (Goodstein et al., 1980) and digital scanning (Di Giacomo et al., 2015a). The International Seismological Centre (ISC) was established as the successor to the International Seismological Summary (ISS) based in the United Kingdom and to the Bureau Central International de Seismologique (BCIS) based in France (Berry, 1989).