R. D. Adams

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 Algerian earthquake of 24 June 1910: A case history

Abstract In Algeria, as in many other places, the lack of reliable macroseismic and instrumental information makes the study of long-term seismicity difficult. The earthquake of 24 June 1910 provides an example of these difficulties, and demonstrates the uncritical way in which such information is often used in the preparation of earthquake catalogues. This earthquake, with a magnitude ( M s ) of 6.6. is equal in size to the Orleansville (El Asnam) earthquake of 9 September 1954, and is the second largest shock in Algeria this century.

Discrepancies in earthquake location between ISC and other agencies

The different operating requirements of the International Seismological Centre (ISC) from those of the National Earthquake Information Service of the US Geological Survey(NEIS), and of the prototype International Data Center to monitor the Comprehensive Test Ban Treaty (pIDC), result in some discrepancies between earthquake locations computed by the three agencies. For larger events recorded by many stations the differences are small, but for some smaller events differences in location of up to 20° may occur. The largest discrepancies are found for small events in areas where later analysis by ISC has the benefit of additional readings from regional seismograph networks and where ISC has made a different interpretation of the station readingsavailable to pIDC and NEIS. We identify regions where such discrepancies occur most frequently, and give some examples for which the augmented data set of ISC has resulted in significant improvement for specific earthquakes. NEIS and, particularly, pIDC produce their results more speedily than ISC, and these form a valuable starting point for the later, more complete ISC analysis, which is commonly considered the most definitive compilation of global earthquake information.

Local earthquake quantification

Abstract The diversity of noise conditions, and, consequently, the diversity of types of seismographs that are found in different environments, make it unlikely that a single method of determining magnitudes for local earthquakes will be developed. In each area local studies must be carried out to determine the appropriate attenuation-distance function, taking into account known structural features. Each locally determined magnitude scale should be related to either a standard teleseismic scale, or preferably, to Richters original local magnitude scale, ML.