International Seismological Summary Research Articles

Digitization of BCIS bulletins and the ISC quest to verify pre-digital earthquakes

We report on the effort of the International Seismological Centre (ISC, www.isc.ac.uk) to digitise station measurements for earthquakes as well as other types of seismic events that are currently listed in the ISC database without station data. The association of instrumental measurements to a seismic event is fundamental to verify its occurrence and it facilitates, when possible, its hypocentre relocation. We search for relevant station data in the Bureau Central de l’Association Internationale de Sismologie (BCIS). The BCIS bulletins were produced at the same time of the International Seismological Summary (ISS) but they appear to list more events with data than the ISS from the 1950s. Unfortunately, BCIS bulletins have not been digitised in a systematic way. We show that during the period going from the mid-1950s up to 1963 the BCIS is a unique source of station data that allowed us to verify thousands of earthquakes worldwide and significantly expand the global record of seismicity in those years. The data digitised in this work will play a key role in future works regarding earthquakes in the pre-digital era.

The 1927 earthquakes and aftershocks in the Crimea: relocation based on instrumental data

We used the most complete instrumental data supplied by the global network, a modern velocity model (ak135), and a modified method of generalized beamforming to relocate the Crimean earthquakes of June 26 and September 11, 1927, and for the larger (magnitudes above 4.5) aftershocks that occurred during the year 1927. For the relocation, we collected all available seismic bulletins from the global network using data from the ISC-GEM project (International Seismological Centre-Global Earthquake Model), EuroSeismos project, the ISS (International Seismological Summary) Bulletin, the archive of the Geophysical Survey of the Russian Academy of Sciences bulletins, and the archive of the Russian State Library. There are many onsets of P and S phases as recorded by numerous seismic stations for the two Crimean earthquakes and six aftershocks; the network geometry is favorable for reliable determination of hypocenter parameters. We believe that our results will be a contribution to the study of aftershocks following the 1927 Crimean earthquakes, and this will make it possible to learn new things about the space-time evolution of the seismic process during the active period from 1927 to 1931.

Relocalizing a historical earthquake using recent methods: The 10 November 1935 Earthquake near Montserrat, Lesser Antilles

This study investigates the hypothesis of Feuillet et al. (2011) that the hypocenter of the seismic event on November 10, 1935 near Montserrat, Lesser Antilles (MS 6 1/4) (Gutenberg and Richter, 1954) was mislocated by other authors and is actually located in the Montserrat-Havers fault zone. While this proposal was based both on a Ground Motion Prediction Equation and on the assumption that earthquakes in this region are bound to prominent fault systems, our study relies on earthquake localization methods using arrival times of the International Seismological Summary (ISS). Results of our methodology suggest that the hypocenter was really located at 16.90° N, 62.53° W. This solution is about 25 km north-west of the location proposed by Feuillet et al. (2011) within the Redonda fault system, northward of the Montserrat-Havers fault zone. As depth phases that contribute valuable insights to the focal depth are not included in the ISS data set and the reassociation of these phases is difficult, the error in depth is high. Taking into account tectonic constraints and the vertical extend of NonLinLoc's uncertainty area of the preferred solution we assume that the focus is most probably in the lower crust between 20 km and the Moho. Our approach shows that the information of the ISS can lead to a reliable solution even without an exhaustive search for seismograms and station bulletins. This is encouraging for a better assessment of seismic and tsunami hazard in the Caribbean, Mexico, South and Central America, where many moderate to large earthquakes occurred in the first half of the 20th century. The limitations during this early phase of seismology which complicate such relocations are described in detail in this study.

Can We Consider the 1951 Caviaga (Northern Italy) Earthquakes as Noninduced Events?

Online Material: Table of Mercalli–Cancani–Sieberg (MCS) intensities, onset times and associated phases, complete parameter sets for each 1951 location and computation of the isostatic stress variation due to unloading in the upper crust. On the night of 15–16 May 1951, two moderate earthquakes with estimated magnitudes of M w 5.4 and 4.5 occurred in northern Italy, about 40 km southeast of Milan, close to the small town of Caviaga. They were recorded by several observatories worldwide, as reported by the International Seismological Summary (ISS) On‐Line Bulletin (ISS, 1951; International Seismological Centre [ISC], 2011). Despite the moderate magnitudes, these two events caught the attention of seismologists and have been studied in detail, in particular by Caloi et al. (1956), because they were close to Caviaga in an area that was assumed to be aseismic. Moreover, their shallow hypocenters (∼5 km in Caloi et al. , 1956) indicated a possible anthropogenic source, related to wells for gas withdrawal (Fig. 1; see Data and Resources). Figure 1. (top) Maps of the area of interest, showing the tectonic setting and seismicity between 1981 and 2012 (Italian Seismological Instrumental and Parametric Data‐Base Working Group, see Data and Resources; Catalogue of Italian Seismicity, Castello et al. , 2006). Symbols in color indicate earthquakes (circles for M w>2.0 and squares for M w>4.5). Symbols are scaled with magnitude and colors accord to depth scale. Focal mechanisms are for seismicity with M w>4.5 from 1977 to present (Pondrelli et al. , 2006). Top left inset: The black box is the study area and the red dashed line marks the boundary of the Adria plate. The (red) tectonic structures and (light green lines) geological sections are extracted from Pieri and Groppi (1981) and Cassano et al. (1986). Top right map: Blue stars indicate the original locations of the events …

The Curious Case of the 1919 Swaziland Earthquake

The locations of seismic events during the early twentieth century were sometimes plagued by mislocations, especially in Africa where there were few seismograph stations. In this regard, it is common to find false earthquakes erroneously listed among true ones. Without critical assessment, such events, especially in historical catalogs, have remained in subsequent earthquake catalogs including modern ones. A number of these false earthquakes have been disclosed and their nature revealed and discussed in many places around the world (e.g., Giardini and Balassanaian, 1997; Albini et al. , 2010; Grunthal and Walhstrom, 2011). The most difficult catalog records to identify and expose as false events are mislocated true earthquakes (e.g., Guidoboni and Ferrari, 1986; Bellettati et al. , 1993; Stucchi and Camassi, 1997; Glavcheva, 2004; Grunthal, 2004; Albini, 2011). These mistakes can only be identified and corrected by reviewing earthquake catalogs starting from the most recent tracing back to earlier studies for reinterpretation (Camassi, 2004; Albini, 2011). The combination of felt effects and instrumental data has helped in the relocation of some earthquakes and in some instances, in the discarding of mislocated earthquakes. The case of the 31 October 1919 Swaziland earthquake ( M s 6.5) is one example of a false earthquake found today in the catalog for Africa. The event was reported in the International Seismological Summary (ISS) bulletin with phases from 22 stations around the globe but was not reported in the popular media at the time, with no records of it having been felt even in the epicentral area. With the presence of missionaries, British agents, and many reporters in the region at that time, the lack of felt reports for this event raises the question “Did the event really occur in Swaziland or was this epicenter a mislocation?” Another event of …

Focal mechanism and depth of the 1956 Amorgos twin earthquakes from waveform matching of analogue seismograms

Abstract. Historic analogue seismograms of the large 1956 Amorgos twin earthquakes which occurred in the volcanic arc of the Hellenic subduction zone (HSZ) were collected, digitized and reanalyzed to obtain refined estimates of their depth and focal mechanism. In total, 80 records of the events from 29 European stations were collected and, if possible, digitized. In addition, bulletins were searched for instrument parameters required to calculate transfer functions for instrument correction. A grid search based on matching the digitized historic waveforms to complete synthetic seismograms was then carried out to infer optimal estimates for depth and focal mechanism. Owing to incomplete or unreliable information on instrument parameters and frequently occurring technical problems during recording, such as writing needles jumping off mechanical recording systems, much less seismograms than collected proved suitable for waveform matching. For the first earthquake, only seven seismograms from three different stations at Stuttgart (STU), Göttingen (GTT) and Copenhagen (COP) could be used. Nevertheless, the waveform matching grid search yields two stable misfit minima for source depths of 25 and 50 km. Compatible fault plane solutions are either of normal faulting or thrusting type. A separate analysis of 42 impulsive first-motion polarities taken from the International Seismological Summary (ISS bulletin) excludes the thrusting mechanism and clearly favors a normal faulting solution with at least one of the potential fault planes striking in SW–NE direction. This finding is consistent with the local structure and microseismic activity of the Santorini–Amorgos graben. Since crustal thickness in the Amorgos area is generally less than 30 km, a source depth of 25 km appears to be more realistic. The second earthquake exhibits a conspicuously high ratio of body wave to surface wave amplitudes suggesting an intermediate-depth event located in the Hellenic Wadati–Benioff zone. This hypothesis is supported by a focal mechanism analysis based on first-motion polarities, which indicates a mechanism very different from that of the first event. A waveform matching grid search done to support the intermediate-depth hypothesis proved not to be fruitful because the body wave phases are overlain by strong surface wave coda of the first event inhibiting a waveform match. However, body to surface wave amplitude ratios of a modern intermediate-depth event with an epicenter close to the island of Milos observed at stations of the German Regional Seismic Network (GRSN) exhibit a pattern similar to the one observed for the second event with high values in a frequency band between 0.05 Hz and 0.3 Hz. In contrast, a shallow event with an epicenter in western Crete and nearly identical source mechanism and magnitude, shows very low ratios of body and surface wave amplitude up to 0.17 Hz and higher ratios only beyond that frequency. Based on this comparison with a modern event, we estimate the source depth of the second event to be greater than 100 km. The proximity in time and space of the two events suggests a triggering of the second, potentially deep event by the shallow first one.

ISC-GEM: Global Instrumental Earthquake Catalogue (1900–2009), I. Data collection from early instrumental seismological bulletins

In order to produce a new global reference earthquake catalogue based on instrumental data covering the last 100+years of global earthquakes, we collected, digitized and processed an unprecedented amount of printed early instrumental seismological bulletins with fundamental parametric data for relocating and reassessing the magnitude of earthquakes that occurred in the period between 1904 and 1970. This effort was necessary in order to produce an earthquake catalogue with locations and magnitudes as homogeneous as possible. The parametric data obtained and processed during this work fills a large gap in electronic bulletin data availability. This new dataset complements the data publicly available in the International Seismological Centre (ISC) Bulletin starting in 1964. With respect to the amplitude-period data necessary to re-compute magnitude, we searched through the global collection of printed bulletins stored at the ISC and entered relevant station parametric data into the database. As a result, over 110,000 surface and body-wave amplitude-period pairs for re-computing standard magnitudes MS and mb were added to the ISC database. To facilitate earthquake relocation, different sources have been used to retrieve body-wave arrival times. These were entered into the database using optical character recognition methods (International Seismological Summary, 1918–1959) or manually (e.g., British Association for the Advancement of Science, 1913–1917). In total, ∼1,000,000 phase arrival times were added to the ISC database for large earthquakes that occurred in the time interval 1904–1970. The selection of earthquakes for which data was added depends on time period and magnitude: for the early years of last century (until 1917) only very large earthquakes were selected for processing (M⩾7.5), whereas in the periods 1918–1959 and 1960–2009 the magnitude thresholds are 6.25 and 5.5, respectively. Such a selection was mainly dictated by limitations in time and funding. Although the newly available parametric data is only a subset of the station data available in the printed bulletins, its electronic availability will be important for any future study of earthquakes that occurred during the early instrumental period.

ISC-GEM: Global Instrumental Earthquake Catalogue (1900–2009), II. Location and seismicity patterns

We present the final results of a two-year project sponsored by the Global Earthquake Model (GEM) Foundation. The ISC-GEM global catalogue consists of some 19 thousand instrumentally recorded, moderate to large earthquakes, spanning 110years of seismicity. We relocated all events in the catalogue using a two-tier approach. The EHB location methodology (Engdahl et al., 1998) was applied first to obtain improved hypocentres with special focus on the depth determination. The locations were further refined in the next step by fixing the depths to those from the EHB analysis and applying the new International Seismological Centre (ISC) location algorithm (Bondár and Storchak, 2011) that reduces location bias by accounting for correlated travel-time prediction error structure. To facilitate the relocation effort, some one million seismic P and S wave arrival-time data were added to the ISC database for the period between 1904 and 1970, either from original station bulletins in the ISC archive or by digitizing the scanned images of the International Seismological Summary (ISS) bulletin (Villaseñor and Engdahl, 2005, 2007). Although no substantial amount of new phase data were acquired for the modern period (1964–2009), the number of phases used in the location has still increased by three millions, owing to fact that both the EHB and ISC locators use most well-recorded ak135 (Kennett et al., 1995) phases in the location.We show that the relocation effort yielded substantially improved locations, especially in the first half of the 20th century; we demonstrate significant improvements in focal depth estimates in subduction zones and other seismically active regions; and we show that the ISC-GEM catalogue provides an improved view of 110years of global seismicity of the Earth. The ISC-GEM Global Instrumental Earthquake Catalogue represents the final product of one of the ten global components in the GEM program, and is available to researchers at the ISC (www.isc.ac.uk) website.

A history of British seismology

The work of John Milne, the centenary of whose death is marked in 2013, has had a large impact in the development in global seismology. On his return from Japan to England in 1895, he established for the first time a global earthquake recording network, centred on his observatory at Shide, Isle of Wight. His composite bulletins, the “Shide Circulars” developed, in the twentieth century, into the world earthquake bulletins of the International Seismological Summary and eventually the International Seismological Centre, which continues to publish the definitive earthquake parameters of world earthquakes on a monthly basis. In fact, seismology has a long tradition in Britain, stretching back to early investigations by members of the Royal Society after 1660. Investigations in Scotland in the early 1840s led to a number of firsts, including the first network of instruments, the first seismic bulletin, and indeed, the first use of the word “seismometer”, from which words like “seismology” are a back-formation. This paper will present a chronological survey of the development of seismology in the British Isles, from the first written observations of local earthquakes in the seventh century, and the first theoretical writing on earthquakes in the twelfth century, up to the monitoring of earthquakes in Britain in the present day.

Systematic Relocation of Early Instrumental Seismicity: Earthquakes in the International Seismological Summary for 1960 1963

Abstract We have relocated 2556 events that were reported in the bulletins of the International Seismological Summary (ISS) during the period 1960–1963 using the phase arrival time data listed in the bulletins. Parts of the data were already available in digital form, and the rest we obtained by scanning the printed bulletins and applying an optical character recognition procedure. Earthquakes were then relocated with a teleseismic location method that uses an improved 1D global travel-time Earth model, depth phases, and corrections for ellipticity, bounce-point bathymetry/topography, and station elevation. The aspherical velocity structure below stations was partially taken into account through the use of teleseismic station (patch) corrections. The quality of the newly obtained locations and the improvements in data fit are similar to relocations of earthquakes in the early bulletins of the International Seismological Centre (ISC). The most significant improvement of the new locations is in focal depth, and a significant number of blatant mislocations (such as earthquakes deeper than 100-km below midocean ridges) have also been corrected. The relocations presented here extend by 10% the time period for which homogeneous and reliable hypocenters are available on a global basis. This is the first of a series of studies addressed towards relocating, using the same methodology, all the instrumentally recorded earthquakes listed in the bulletins of the ISS (1918–1963).

A Digital Hypocenter Catalog for the International Seismological Summary

Research Article| September 01, 2005 A Digital Hypocenter Catalog for the International Seismological Summary Antonio Villaseñor; Antonio Villaseñor Institute of Earth Sciences “Jaume Almera” CSIC Lluís Solé i Sabarís s/n 08028 Barcelona, Spain antonio@ija.csic.es (A.V.) Search for other works by this author on: GSW Google Scholar E. Robert Engdahl E. Robert Engdahl University of Colorado Department of Physics Campus Box 390 UCB Boulder, CO 80309-0390, USA engdahl@colorado.edu (E.R.E.) Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2005) 76 (5): 554–559. https://doi.org/10.1785/gssrl.76.5.554 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Antonio Villaseñor, E. Robert Engdahl; A Digital Hypocenter Catalog for the International Seismological Summary. Seismological Research Letters 2005;; 76 (5): 554–559. doi: https://doi.org/10.1785/gssrl.76.5.554 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search A reliable catalog of earthquake locations and magnitudes spanning long periods of time is a fundamental requirement for a wide variety of seismological studies, including seismotectonics, Earth structure, and seismic hazard. On a global scale, the most comprehensive source of earthquake locations, phase data, and magnitudes is the Bulletin of the International Seismological Centre (ISC). Locations, magnitudes, the data used to compute them (i.e., phase arrival times and amplitude/period readings), and other data are collected and processed at ISC and made readily accessible in digital form (Willemann and Storchak, 2001). ISC, which published its first bulletin... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Revised world seismicity catalog (1950-1997) for strong (Ms≧ 6) shallow (h≦ 70 km) earthquakes

AbstractUsing the standard earthquake catalogs provided by the International Seismological Centre (ISC), its predecessor the International Seismological Summary (ISS), and the National Earthquake Information Center (NEIC), I analyze the worldwide consistency of teleseismic reporting, completeness of the seismicity record, and homogeneity of magnitude determination, for strong shallow earthquakes (surface-wave magnitude, Ms ≥ 6; depth, h ≤ 70 km) for the period 1950 to 1997. Under the postulates that the rate of earthquake occurrence for the entire world is constant on a time scale of decades and that well after the installation of the World-Wide Standardized Seismograph Network in the middle 1960s the earthquake catalog for strong (Ms ≥ 6) shocks is complete, and the seismicity rates are typical of all periods in the century, we find that, due to the use of different formulations and criteria to calculate the parameter magnitude, the Ms of moderate (6 ≤ Ms < 7) events during the period 1950 to 1963 was systematically overestimated by as much as 0.5 magnitude unit, relative to the Ms assigned to shocks occurring after 1963. When this correction is taken into account, the new catalog of events with Ms (corrected) ≥ 6 in the period 1950 to 1997 becomes largely homogeneous in Ms. Under the foregoing postulates, this new catalog is shown to list nearly all and only the strong shocks [Ms (corrected) ≥ 6; h ≤ 70 km] that occurred in the Earth during the period, a notable exception being the time span from 1964 to 1968. The revised catalog, including the scalar moment and moment magnitude for each event, is listed on the worldwide web page http://www.ldc.usb.ve/~ojperez/catalog.

The Rhodes earthquake of 26 June 1926

We use macroseismic and instrumental data to re-examine the large earthquake of 26 June 1926 in the Hellenic Arc and other associated events. The earthquake was felt over a large area in the Eastern Mediterranean region and caused sporadic damage, and in places destruction, over a large area in Rhodes, southwestern Anatolia, eastern Crete and in the Nile Delta. Despite its size, there has been uncertainty as to its position, depth and magnitude. The earthquake was well recorded instrumentally and we have relocated it using the readings listed in the International Seismological Summary and the present location procedure of the International Seismological Centre. We have also inspected seismograms from 14 stations. We find the intermediate depth of the event to be well established at about 115 km, with a formal error of about 10 km and supported by depth phases. There is evidence of complexity in the source, with a secondary release of energy from the same focus after about six seconds. From original records and bulletins we establish the surface-wave magnitude (Ms as 7.4 ± 0.3, corresponding to a moment release of about 2.2 × 1027 dyn.cm. The felt information is consistent with these findings, and although the earthquake was felt very widely, nowhere did it produce excessively high intensities. Our intensity assessments, usually from original sources, tend to be lower than those assigned in previous studies, the highest being IX MSK on Rhodes.

The seismicity, seismotectonics and earthquake hazards of Libya, with detailed analysis of the 1935 April 19, M = 7.1 earthquake sequence

SUMMARY We have investigated the seismicity of Libya in order to understand better the earthquake hazards, the tectonics of the region, and the present-day evolution of the Sirt basin. We first relocated all events occurring between 1931 and 1988 using a bootstrap location technique. We then determined focal mechanisms from firstmotion data. Finally, we used waveform modelling techniques to determine source parameters for the 1935 main shock and its two largest aftershocks. Relocation of the 1930s’ events are in some cases significantly different from those given by the International Seismological Summary (ISS). The first-motion results suggest a combination of strike-slip and normal faulting for the 1935 and 1939 sequences. The waveform modelling results of the 1935 mainshock suggest a strike-slip mechanism with one fault plane similar to the strike of the eastern edge of the Hun Graben. About 5 hr after the mainshock, an aftershock (mb = 6.0) occurred about 40 km to the south, which was followed about 9 hr later by another aftershock (mb = 6.4) in the same area. Waveform modelling results for this earthquake sequence suggest focal depths ranged from 15-21km, with rupture lengths of 14-16 km. An average displacement of 1.8 m is estimated for the mainshock, but no surface rupture has been noted for the event. In the vicinity of the Hun Graben area, the number of large earthquakes that have been reported through history is an indication that the area is seismically active. Most of the activity appears to be concentrated along the eastern margin of the NW-SE trending Hun Graben. The focal mechanisms suggest that the maximum compressive stress in northwestern Libya is oriented WNW. In the north-eastern part of the country, the stress direction changes to a more NE-SW orientation, as evidenced by the 1963 February 21 Al-Marj and the 1967 Al-Jabal Al-Akhdar mainshocks.

Seismic cycles along the Aleutian arc: Analysis of seismicity from 1957 through 1991

We catalog and relocate Aleutian arc seismicity. Between 1957 and 1991, two great earthquakes ruptured the same 250‐km‐long portion of the central Aleutian arc: the 1957 Aleutian Islands earthquake and the 1986 Andreanof Islands earthquake. Because accurate estimates of the moment distribution of the 1957 earthquake are not available, the spatial distribution of aftershocks for each of these events is compared and tested against models describing the modes of occurrence of great subduction zone earthquakes. Earthquake relocations are based on P wave arrival times published in the International Seismological Summary, the Bureau Central International Seismologique, and the International Seismological Centre bulletins and include corrections for the near‐source velocity structure associated with the down‐going slab. Magnitude estimates are extracted from bulletins and prior to 1964 are estimated by us from microfilmed records. Our catalog is complete above magnitude 5.5. Aftershocks associated with the 1957 and 1986 earthquakes appear to occur in different areas. East of the main shock epicenters, aftershock locations are anticorrelated. West of the main shock epicenter, aftershocks of the 1986 earthquake tended to concentrate along the updip edge of aftershock clusters associated with the 1957 earthquake. If we assume aftershocks rim the distribution of seismic moment release associated with each event, these observations imply that the moment distribution of the 1986 earthquake was different from that of the 1957 earthquake. This suggests that we should use caution in identifying mechanically strong portions of a fault, asperities, by simply mapping the moment distribution of a single great earthquake. A fundamental tenet of the asperity model, that rupture always occurs on the strongest portions of the fault with weaker portions rupturing either aseismically or dynamically as a result of rupture on a strong fault patch, may in the case of the central Aleutian arc not be correct. Thus observing the moment distribution from a single great earthquake may tell us little about what the distribution of moment release will look like during the next earthquake.

The Makran earthquake of 1983 April 18: A possible analogue to the Puget Sound earthquake of 1965?

Summary On 1983 April 18, a large earthquake (mb∼6.5, M0 = 1.2X1019 Nm) occurred in the Makran region near the Pakistan-Iran border. This earthquake is the only event of mb > 6.2 in the entire Makran region since the occurrence of the great Makran earthquake (M= 8.0–8.3) in 1945. We determined the source parameters of the 1983 event by formally inverting its P- and SH-waves recorded by the World-Wide Standard Seismograph Network (WWSSN) and the Canadian Seismograph Network (CSN) at teleseismic distances. The best-fitting centroidal solution shows predominantly normal faulting at a depth of about 65 ± 5 km. These results provide an additional constraint on the configuration of the down-going slab previously inferred from the occurrence of three small- to moderate-sized events located 100–300 km to the west of the event reported here. We also discuss the possibility of drawing an analogy between the occurrence of the 1983 Makran event and that of the Puget Sound earthquake of 1965 (M0–1.4X1019Nm, mb ± 6.5), and between the seismotectonic settings of the two regions. If the very large (M= 8.0–8.3) earthquake off the Makran coast in 1945 is a low-angle thrust due to slip along the plate interface, its occurrence calls for the possibility of similar seismic risk in the Puget Sound region. However, in contrast to inferences made from geological observations in the field, about a dozen P-wave first motions read from historical seismograms and those reported by the International Seismological Summary suggest that the 1945 Makran earthquake has a large component of normal faulting which can be attributed to the relatively old age of the subducted lithosphere.

The depth of the deepest historical earthquakes

We useP andS times listed in the International Seismological Summary to relocate 23 historical earthquakes (1927–1963) reported as occurring at or below 670 km. In all cases, our relocated hypocenters are shallower than the starting depths; furthermore, all events converge to 691 km or less, with a precision estimated at ±10 km. This study upholds the results of Stark and Frohlich, who had usedpP−P times of post-WWSSN earthquakes to constrain reliable hypocentral depths to no greater than 684 km. In particular, we reject Rothe's claim that a 1963 event in the vicinity of New Guinea occurred at a depth of more than 780 km.

A catalogue of seismicity in Greece and adjacent areas

A new earthquake catalogue for Greece has been formed to cover the instrumental period 1901–78, in particular 605 earthquakes for the period 1917–63 inclusive are relocated using first arrival data from the International Seismological Summary. These relocations incorporate macro-seismically and other well-controlled master events into an ensuing joint epicentre determination technique. The largest annual average shift is 165 km for 78 earthquakes during the decade after 1917, decreasing to 17 km for the later years 1957–63. Magnitudes are redetermined mainly using readings from the Swedish network and Uppsala since as early as 1908. Catalogue completeness exists for magnitudes around 5.5 for at least the last 60 years, but magnitudes of about 4.7 are completely reported only during the most recent 15 years. The new epicentral positions lead to a better delineation of the seismic zones than has previously been achieved. The majority of shallow earthquakes are contained in a belt parallel to the Hellenic Arc which extends north into Albania, in the south-east the epicentral locations give a more diffuse extension of this zone into the west coast of Turkey. Depths of intermediate earthquakes along the Hellenic Arc tend to relocate to shallower depths, in the south-west part of Crete no earthquake focus deeper than 100 km is found, although in the south-eastern section of the arc a tendency to increased depths is observed. A second zone starts at Leukas Island in the west and extends through central Greece to near Volos on the east coast, where it divides into two branches, which are less well defined, but which eventually join the seismicity of western Turkey. A third zone follows the Saronikos and Corinth gulfs.

The Seismicity of the Marmara Sea Region of Turkey

Summary The seismicity of the Marmara Sea area of Turkey is investigated by epicentral data from three sources: computer revisions of epicentres reported by the International Seismological Summary in the period 1923 to 1963, epicentres reported by the International Seismological Centre in 1964 to 1970, and micro-earthquake locations determined by a radiolinked seismometer network operating for two months in the summer of 1971 as part of a co-operative project between Kandilli Observatory and the Institute of Geological Sciences. The Marmara area is at a triple junction between the Turkish, Aegean, and Eurasian Plates in a complicated area of small block structures, and an understanding of the behaviour of the Marmara area is important in the interpretation of the tectonics of the whole region. The present paper delineates lines of seismicity, which suggest that the Anatolian Fault splits into three branches in the Marmara area diverting some of the east-west orientated stresses associated with the right-lateral Anatolian Fault southwards into Western Turkey and the Eastern Aegean.

The History of the International Seismological Summary

Without belittling the important contributions to the early growth of earthquake science by a number of countries of the world, it is fair to say that the International Seismological Summary (hereafter referred to as the I.S.S.) has been essentially a British undertaking, both in its origin, its history and its format. For its early history much credit must be given to the energetic labours of John Milne, who may well be called ‘ The Father of British Seismology ’. It will be remembered that Milne went to Japan in 1876 at the invitation of the Emperor of Japan. With the co-operation of a distinguished band of British scientists, W. E. Ayrton, J. A. Ewing, C. G. Knott and J. Perry, who were resident at that time in Japan, Milne in the course of 20 years helped to bring the infant science of seismology to a high level. Japan, a highly seismic country, was ideal for such early researchers, and Milne, with interests in engineering and geology, did much to develop seismology, both instrumentally and theoretically. It is impracticable to set out this growth in strictly chronological order; in this article an attempt is made to record those happenings that bear particularly on the origin and development of the I.S.S., noting in detail some matters that are not found in standard treatises and not seeking to present a properly balanced exposition. At this stage it is of interest to recall that Milne, on his return to England in 1895, settled in Shide, near Newport, Isle of Wight, with his Japanese wife, Tone Milne, where he set up a seismological observatory. The Milne horizontal pendulum seismograph (later developed by J. J. Shaw into a much more sensitive instrument, the Milne-Shaw seismograph) came to be installed at a number of stations and institutions, especially in the British Empire; these stations sent their ‘ station registers ’ to Milne, and the registers were the basis of much of Milne’s researches. It should be mentioned here that although the British Association had published seismological reports at intervals from 1841 onwards, including a series from 1881 to 1895 by Milne with the title ‘ Reports of the B.A. Committee for Earthquake Phenomena in Japan ’, the present series began with the 1894 ‘ Report on Seismological Investigation ’ (sic). These Reports give much information on the development of seismology. The registers were published annually in full in the ‘B.A. Circulars ’ until their discontinuance, partly on the grounds of economy, as from the end of 1912. Annual lists of large earthquakes were published, and in the Report for 191 1 we see the beginnings of a project destined to develop into the I.S.S.; this Report lists to the nearest minute the ‘ time of origin ’ of instrumentally observed shocks and the calculated positions, to the nearest degree, of the corresponding epicentres. The number of stations available to Milne, 22, at the time of the first circular (1899) had increased by the time of Milne’s death in 1913 to 35.