The ISC Electronic Archive of Printed Station and Network Bulletins

We present an archive of scanned instrumental seismic bulletins pertaining to either a single station or a set of stations (network). This new service by the International Seismological Centre (ISC), called the ISC Electronic Archive of Station and Network Bulletins, is openly available at the ISC website. The archive is likely to be the most comprehensive to date, thanks to the collection from various sources done by the ISC in the past several decades. The search for scans of a seismic bulletin is based on the location of the town of the institution producing a bulletin. As such, the electronic archive is easy to use and is likely to facilitate the work of a wide community interested in studying past earthquakes and involved in preservation and digitization of analog recordings.

Spatiotemporal Evolution of Earthquakes in Longmenshan Fault and Adjacent Area, before and after the 2008 Wenchuan Earthquake

Seismicity sequence following a main earthquake usually contains much meaningful information for unveiling the focal mechanism and predicting the reoccurrence interval of large earthquakes. The spatiotemporal evolution of earthquakes before and after the 2008 Wenchuan earthquake (Ms 8.0) is analysed comprehensively in this study. The frequency-magnitude relation of the 3493 earthquake events retrieved from the database of the International Seismological Centre indicates that the adopted catalogue is complete for magnitudes ≥Ms 3.4. The seismicity during the 10 years before the Wenchuan earthquake remained stable, including the magnitudes and focal depths. However, seismicity attenuated sharply in the year following the Wenchuan earthquake, and the magnitude of earthquakes before the Wenchuan earthquake decreased gradually. The area of the seismogenic zone of the 2008 Wenchuan earthquake was smaller than the earthquake stricken area. The earthquakes that occurred in the Longmenshan fault area and adjacent area in the study period were mainly shallow earthquakes. The focal depths of earthquakes in the study area became stable gradually after the Wenchuan earthquake, mainly within the range from 10 to 16 km. The earthquakes in the study area were mainly distributed with an along-dip distance of 0–20 km, and the seismicity was distributed uniformly along the fault strike.

A homogeneous earthquake catalogue for Turkey

A new homogenized earthquake catalogue for Turkey is compiled for the period 1900–2018. The earthquake parameters are obtained from the Bulletin of International Seismological Centre that was fully updated in 2020. New conversion equations between moment magnitude and the other scales (md, ML, mb, Ms, and M) are determined using the general orthogonal regression method to build up a homogeneous catalogue, which is the essential database for seismic hazard studies. The 95 % confidence intervals are estimated using the bootstrap method with 1000 samples. The equivalent moment magnitudes (Mw*) for the entire catalogue are calculated using the magnitude relations to homogenize the catalogue. The magnitude of completeness is 2.7 Mw*. The final catalogue is not declustered or truncated using a threshold magnitude in order to be a widely usable catalogue. It contains not only Mw* but also the average and median of the observed magnitudes for each event. Contrary to the limited earthquake parameters in the previous catalogues for Turkey, the 45 parameters of ∼378 000 events are presented in this study.

Complementing regional moment magnitudes to GCMT: a perspective from the rebuilt International Seismological Centre Bulletin

Seismologists and geoscientists often need earthquake catalogues for various types of research. This input usually contains basic earthquake parameters such as location (longitude, latitude, depth, and origin time), as well as magnitude information. For the latter, the moment magnitude Mw has become the most sought after magnitude scale in the seismological community to characterize the size of an earthquake. In this contribution we provide an informative account of the Mw content for the newly rebuilt Bulletin of the International Seismological Centre (ISC, http://www.isc.ac.uk, last access: May 2021), which is regarded as the most comprehensive record of the Earth's seismicity. From this data, we extracted a list of hypocentres with Mw from a multitude of agencies reporting data to the ISC. We first summarize the main temporal and spatial features of the Mw provided by global (i.e. providing results for moderate to great earthquakes worldwide) and regional agencies (i.e. also providing results for small earthquakes in a specific area). Following this, we discuss their comparisons, by considering not only Mw but also the surface wave magnitude MS and short-period body wave magnitude mb. By using the Global Centroid Moment Tensor solutions as an authoritative global agency, we identify regional agencies that best complement it and show examples of frequency–magnitude distributions in different areas obtained both from the Global Centroid Moment Tensor alone and complemented by Mw from regional agencies. The work done by the regional agencies in terms of Mw is fundamental to improve our understanding of the seismicity of an area, and we call for the implementation of procedures to compute Mw in a systematic way in areas currently not well covered in this respect, such as vast parts of continental Asia and Africa. In addition, more studies are needed to clarify the causes of the apparent overestimation of global Mw estimations compared to regional Mw. Such difference is also observed in the comparisons of Mw with MS and mb. The results presented here are obtained from the dataset (Di Giacomo and Harris, 2020, https://doi.org/10.31905/J2W2M64S) stored at the ISC Dataset Repository (http://www.isc.ac.uk/dataset_repository/, last access: May 2021).

A 24-Yr-Long Seismic Bulletin for the European Arctic

To understand the seismic activity in the European Arctic, the seismic bulletins for the region should be as complete as possible. We present the collection results of a new seismic event bulletin of the European Arctic (70° to 90° N, −15° to 75° E) for the 24-yr-long period 1990–2013. The contribution describes in detail the merging of the different sources taken in account for the compilation, the homogenization of the data, and the relocation of the seismic events. With respect to the International Seismological Centre bulletin, the new bulletin contains 5957 new seismic events and 58,242 new seismic onset readings from stations mostly located at regional distances. The gains are distributed over the entire study region, with the most significant contributions across the Svalbard archipelago, along the Knipovich and northern Mohns ridges, as well as northern Fennoscandia.

A Homogeneous Earthquake Catalogue for Turkey and Surrounding Region

A new earthquake catalogue for Turkey and surrounding region (32°–47° N, 20°–52° E) is compiled for the period 1900–2017. The earthquake parameters are obtained from the Bulletin of International Seismological Centre that is fully updated in 2020. New conversion equations between moment magnitude and the other scales (md, ML, mb, Ms and M) are determined using in the General Orthogonal Regression method to build up a homogeneous catalogue, which is the essential data for seismic hazard studies. The 95 % confidence intervals are estimated using the bootstrap method with 1000 samples. The equivalent moment magnitudes (Mw*) for the entire catalogue are calculated using the magnitude relations to homogenise the catalogue. The magnitude of completeness is 2.9 Mw* and 3.0–3.2 Mw* for Turkey and Greece generally. The final dataset is not declustered or truncated using a threshold magnitude because of motivation for generating a widely usable catalogue. It contains not only Mw*, but also the average and median of the observed magnitudes for each event. Contrary to the limited earthquake parameters in the previous catalogues, the 45 parameters of approximately 700 k events occurred in a wide area from the Balkans to the Caucasus are presented.

Rebuild of the Bulletin of the International Seismological Centre (ISC)—part 2: 1980–2010

The Bulletin of the International Seismological Centre (ISC) remains in demand for a wide range of geophysical studies. Users of the ISC Bulletin benefit from its long-term coverage, the most comprehensive set of measurements from the majority of permanent seismic stations and networks, and homogeneity of included parameters of earthquakes and other seismic events worldwide. Despite a reasonably conservative approach in preserving the operational procedures, changes are inevitable in order to achieve the best possible accuracy of published earthquake parameters. The last fundamental change in procedures has fully affected the data for years 2011 and beyond (Bondár and Storchak in Geophys J Int 186: 1220-1244, 2011). This is why it was decided to rebuild the earlier part (1964–2010) of the ISC Bulletin to achieve compatibility in earthquake hypocentres and magnitudes through its entire period. The first article on this project (Storchak et al. in Geosci Lett 4: 32, 2017) covered the period from 1964 to 1979, as it was publicly released. This article covers the remaining period from 1980 to 2010 with all seismic events re-processed and re-located in line with the modern ISC procedures, with ~ 489,000 new events, ~ 2600 new stations, ~15.5 million new seismic arrivals, and more robust and reliable mb and MS magnitude evaluations. We give statistics of departures in epicentre location and depth between the original and rebuilt ISC hypocentres. We also provide a summary for the entire rebuilt period (1964–2010). We give a brief account of events deleted from and added to the Bulletin, based on additional reports from permanent and temporary seismic deployments. We explain that newly re-computed (rebuilt) mb is on average larger that the original mb by 0.05 unit of magnitude. The rebuilt MS is only 0.01 unit larger that its predecessor. We announce that the entire reviewed part of the ISC Bulletin (1964–2017) is now based on the same velocity model, ak135, and the same earthquake location and magnitude computation procedures. Finally, we provide advice on how to obtain the original ISC Bulletin data, where necessary.

The August 17, 1914 Bilimbaev earthquake: location based on instrumental data

The parameters of the hypocenter of the Bilimbaev earthquake that occurred on August 17, 1914, are located. We performed earthquake location based on our compilation of all available seismological bulletins of the time that included data from the ISC-GEM (International Seismological Centre–Global Earthquake Model) project, the EuroSeismos project, and the Geophysical Survey of the Russian Academy of Sciences. The location was performed using a modified generalized beamforming method based on the ak135 travel time model and regional model by the Ural region. The new epicenter is removed from the epicenters, previous-ly determined from macroseismic and partly instrumental data, at a distance not exceeding 28 km. The previously calculated earthquake epicenters are in the region of the error ellipse of the new epicenter. The depth values indicated earlier also lie in the range of possible depths of the focus of the specified hypocenter. Thus, all the epicenters, including the new one, are relevant and with equal probability can be considered as the true epicenter of the earthquake

Reassessment of source parameters of ‘major’ southern African earthquakes

Moderate to large earthquakes within an earthquake catalogue contribute significantly to the seismic hazard and risk assessment results of any region. Thus it is prudent to ensure these events have reliable source parameters (epicentres and magnitude). The dataset of events compiled in this study contains a total of 117 instrumentally recorded events of magnitude M ≥5.0, whose parameters were obtained from the Council for Geoscience (CGS) and International Seismological Centre (ISC) databases. The events are mostly located in South Africa with a few in neighbouring countries. Parametric data made up of all available phase data and amplitudes associated with each of the earthquakes were compiled. The availability of these data enabled the earthquake epicentres and magnitude values to be recalculated using the velocity model and the local magnitude relation that are currently being used by the CGS in its analysis of national seismic data. The accuracy of the relocations was determined by producing and analysing three parameters, the azimuthal distribution of seismograph stations (GAP), root-mean-square of travel time residuals (RMS) and epicenter location error data. The analysis of these parameters showed that there was an improvement in the accuracy of the relocated events. Using the ISC location algorithm, iLOC, eight preselected events were further analysed. From this analysis, two earthquakes were found to satisfy the conditions for Ground Truth (GT595%) candidacy whilst four events satisfied the criteria for GT2090% candidacy.