Eric Robert Engdahl

A data set of earthquake hypocenters and associated traveltime residuals for seismic phases recorded by seismograph stations globally is an essential starting point for most studies of global seismicity and Earth structure. Such data sets have been produced in various forms by national and international agencies since the beginning of instrumental seismology at the turn of the twentieth century. We have reprocessed the comprehensive data used to produce the routinely distributed bulletins of the International Seismological Centre (ISC) since 1964 to construct a new refined data set of hypocenters with improved focal depths and phase residuals. This data set, called ISC‐EHB, is used to reveal features of the seismotectonic zones in downgoing slabs in greater detail than previously routinely available.

We outline the work done to extend and improve the ISC-GEM Global Instrumental Earthquake Catalogue, a dataset which was first released in 2013 (Storchak et al., 2013, 2015). 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 https://doi.org/10.31905/D808B825.

The Engdahl-van der Hilst-Buland (EHB) Bulletin of hypocentres and associated traveltime residuals was originally developed with procedures described by (Engdahl et al. 1998) and ended in 2008. It is a widely used seismological data set, which is now expanded and reconstructed, partly by exploiting updated procedures at the International Seismological Centre (ISC), to produce the ISC-EHB. The reconstruction begins in the modern period (2000–2013) to which new and more rigorous procedures for event selection, data preparation, processing and relocation are applied. The selection criteria minimize the location bias produced by unmodelled 3-D Earth structure, resulting in events that are relatively well-located in any given region. Depths of the selected events are significantly improved by a more comprehensive review of near station and secondary phase traveltime residuals based on ISC data, especially for the depth phases pP, pwP and sP, as well as by a rigorous review of the event depths in subduction zone cross-sections. The resulting cross-sections and associated maps are shown to provide details of seismicity in subduction zones in much greater detail than previously achievable. The new ISC-EHB data set will be especially useful for global seismicity studies and high-frequency regional and global tomographic inversions.

A highlight of the Decade of North American Geology (DNAG) program is the publication of new continent-scale maps of North America. The map set includes a Geological Map, Gravity Anomaly Map, Magnetic Anomaly Map, Seismicity Map, Stress Map, Neotectonic Map, and Thermal Aspects Map. The maps are wall size (four 42-in x 55-in sheets per map), in color, and made on a common base map at a scale of 1:5,000,000. A GSA-sponsored workshop was held at the 1984 Seismological Society of America meeting in Anchorage, Alaska, to discuss construction of the Seismicity Map. Consensus was reached that the goal of the Seismicity Map would be to provide a useful resource on North American seismicity for seismologists, earth scientists at the postgraduate level, and industry, and to accurately portray North American seismotectonic features using the entire earthquake history from the early 1500s (pre-instrumental) through the modern period. It was agreed that construction of the map database would require the rationalization of hundreds of thousands of earthquake hypocenters from global, national, regional, and local catalogs, and from other source materials, and that because modern data are more useful than historic data for resolving seismotectonic features, a scheme of data selection and representation would have to be devised that could reveal details of the seismotectonic fabric of North America yet preserve a perspective of historical earthquake occurrence. The problem was further complicated by the highly variable monitoring capability of seismic networks and by the different types and rates of occurrence of earthquake activity

The International Seismological Centre Global Earthquake Model (ISC–GEM) Global Instrumental Earthquake Catalogue (1900–2009) is the result of a special effort to substantially extend and improve currently existing global catalogs to serve the requirements of specific user groups who assess and model seismic hazard and risk. The data from the ISC–GEM Catalogue would be used worldwide yet will prove absolutely essential in those regions where a high seismicity level strongly correlates with a high population density (Fig. 1 ). The Catalogue is also expected to have a multidisciplinary use in a wide range of studies such as global seismicity assessment, tectonics, as well as the rapid determination of seismic hazard. Because of the large volume of digitized seismic traveltime and amplitude data generated during this project, we expect the Catalogue to be useful in studies of the inner structure of the Earth as well as in nuclear monitoring research. This global catalog was also designed to serve as a reference to be used for calibration purposes by those compiling regional seismicity catalogs that contain events of much smaller magnitudes. This will guarantee that the catalogs prepared by other teams for different regions will contain comparable earthquake locations and magnitude parameters, especially in border areas. The work on the Catalogue was funded by the GEM Foundation as part of the five Global Hazard Components and is a result of a 27-month-long project. This project was led by the ISC and performed by the team of international experts in accordance with the requirements of the Scientific Board of GEM and following recommendations of the International Association of Seismology and Physics of the Earth’s Interior (IASPEI ) observers. Further, eight IT, administrative, and technical staff at the ISC worked on this project. Details can be found in the Acknowledgments.Peer Reviewe

The geometry, kinematics, and mode of back‐arc extension along the Andaman Sea plate boundary are refined using a new set of significantly improved hypocenters, global centroid moment tensor (CMT) solutions, and high‐resolution bathymetry. By applying cross‐correlation and double‐difference (DD) algorithms to regional and teleseismic waveforms and arrival times from International Seismological Centre and National Earthquake Information Center bulletins (1964–2009), we resolve the fine‐scale structure and spatiotemporal behavior of active faults in the Andaman Sea. The new data reveal that back‐arc extension is primarily accommodated at the Andaman Back‐Arc Spreading Center (ABSC) at ~10°, which hosted three major earthquake swarms in 1984, 2006, and 2009. Short‐term spreading rates estimated from extensional moment tensors account for less than 10% of the long‐term 3.0–3.8 cm/yr spreading rate, indicating that spreading by intrusion and the formation of new crust make up for the difference. A spatiotemporal analysis of the swarms and Coulomb‐stress modeling show that dike intrusions are the primary driver for brittle failure in the ABSC. While spreading direction is close to ridge normal, it is oblique to the adjacent transforms. The resulting component of E‐W extension across the transforms is expressed by deep basins on either side of the rift and a change to extensional faulting along the West Andaman fault system after the Mw = 9.2 Sumatra‐Andaman earthquake of 2004. A possible skew in slip vectors of earthquakes in the eastern part of the ABSC indicates an en‐echelon arrangement of extensional structures, suggesting that the present segment geometry is not in equilibrium with current plate‐motion demands, and thus the ridge experiences ongoing re‐adjustment.