Seismicity of the Cyprus region

We have used a combination of macroseismic and instrumental techniques to re-evaluate the seismicity of Cyprus and the surrounding region for the period 1890–1900. We identified 56 events which we believe to have a surface-wave magnitude (Ms) of 4.5 or greater. Early events are located by felt effects alone, and macroseismic information is used to improve the location of many later events. We find the broad pattern of seismicity follows that given by standard listings: an area of weak seismicity to the north-west of Cyprus in the Gulf of Antalia includes intermediate-depth events as are found in the Hellenic Arc to the west; a gap of reduced activity separates this activity from that of the main region of Cyprus, where we could find no evidence for sub-crustal foci; there appears to be no present seismological connection along the presumed plate boundary to the north-east to the junction of the East Anatolian fault and the extension of the Dead Sea rift system. We find a much greater level of activity in the first part of this century than is given in the standard listings, such as that of the ISC. Up to 1963 we identified 47 events of Ms > 4.5, compared with only 10 given in the ISC files. Recent improved station coverage locates more events in the region, but these have not been of large magnitude, and the moment release in recent decades has been appreciably less than earlier in the century. For the events in our list the total moment release is 2.7x 1019 Nm with 75% of this originating in the main Cyprus region. We reaffirm the danger of uncritical use of standard earthquake listings for tectonic interpretation and in particular for hazard evaluation in areas such as this, where neither the historical nor the instrumental seismicity record is complete.     

The relocation of epicentres in Iran

Summary. It is demonstrated that routine computer relocations of early epicentres in Iran seldom result in new positions which are much more reliable than the original ISS locations. This is because the early arrival data are generally of such poor accuracy and azimuthal distribution that the epicentre can be resolved only to the average limits given by the input data; the resolution of focal depth is even worse. It appears that the only reliable method for improving or confirming early instrumental epicentres in areas such as Iran is to attempt to find correlations with macroseismic data.     

Reappraisal of major African earthquakes,south of 20°N, 1900–1930

We have re-examined those earthquakes in Africa south of 20°N, in the period 1900–1930, that appear from instrumental or macroseismic evidence to have a magnitude of 5 3/4 or greater. We identify more than 50 such events, about twice as many as listed by Gutenberg and Richter (1954). We find that the combined use of early instrumental readings and macroseismic information gleaned from previously untapped sources gives the best control of location. Instrumental relocation is difficult because of the lack of stations in Africa and the very uneven global distribution. For the low-gain, medium-period instruments then in use, the best control often comes from using the maximum Airy phase of surface waves. Similarly, there is a lack of sources of macroseismic information, and the simple building practice makes it difficult to assess intensity. We have recalculated magnitude Ms uniformly using the Prague formula. We discuss these problems and show that it is likely that our list is complete only down to magnitude about 6, and that the seismic record for Africa before this century will probably remain incomplete for events of all magnitudes.Of the 54 events in our list 20 are between magnitude 6 and 7, and the largest is the Rukwa earthquake of 1910 in Tanzania (Ms 7.4). The only other African event known to rival it in size is that in southern Sudan on 20 May 1990 (Ms 7.2).     

Seismic displacements on shear surfaces in cohesive soils

The paper is concerned with the earthquake-induced displacements on pre-existing shear surfaces in cohesive soils. Results from ring shear tests have shown that during fast shearing the strength of such surfaces depends on displacement and rate of shearing. The test results have been used in a numerical analysis to assess the displacement of a rigid block sliding on a plane surface. The results from the analysis show that the earthquake-induced displacements on pre-existing shear surfaces are influenced significantly by the soil behaviour under earthquake loading conditions. The results are consistent with the field performance of pre-existing slides in cohesive soils during earthquakes.     

Large-magnitude Central American earthquakes, 1898–1994

We have collected and re-examined macroseismic information for large Central American earthquakes since the beginning of the period of instrumental recording about one hundred years ago, and combined this with a reassessment of early instrumental information to produce a catalogue of 51 events that, we believe includes ail those with magnitudes ( Ms ) greater than 7.0. We have reassessed surface-wave magnitudes by consulting station bulletins and we have derived a correction that gives an equivalent Ms for events of intermediate depth. We have also developed a regional relationship between Ms and seismic moment, which enables us to estimate the seismic slip rate across the Middle American Trench. Our best estimates give an average slip rate several times smaller than suggested convergence rates, but with the seismic slip in the central segment of the trench almost an order of magnitude smaller than that in the segments on either side. The low seismic slip rate may indicate aseismic crustal deformation     

Earthquake-induced ground displacements

The paper brings up to date and amplifies earlier work on earthquake-induced ground displacements using near-field strong-motion records, improved processing procedures and a homogenizing treatment of the seismological parameters. A review of upper bound limits to seismic displacements is given and a predictive procedure is examined that allows the probabilistic assessment of the likelihood of exceedance of predicted displacements to be made in the near field of earthquakes in the magnitude range 6.6 to 7.3. Using a considerable number of unscaled ground motions obtained at source distances of less than half of the source dimensions, graphs and formulae are derived that allow the assessment of permanent displacements of foundations and slopes as a function of the critical acceleration ratio.     

Earthquake induced displacements of slopes

The estimation of permanent slope displacements induced by earthquakes is examined. Predictive formulae and graphs for co-seismic and post-seismic permanent displacements for translational movements are presented which allow the assessment of the vulnerability of natural and man-made slopes subjected to earthquakes.     

Comparison of frequency of occurrence of earthquakes with slip rates from long-term seismicity data: the cases of Gulf of Corinth, Sea of Marmara and Dead Sea Fault Zone

In this article, through the comparison of knowledge relating to historical earthquakes with the understanding of present-day earthquake mechanics and overall GPS slip rates in the eastern Mediterranean region, it has been possible to obtain an idea of how frequently large earthquakes may be expected in some parts of the region. It has also been possible to make an assessment from these early events of slip rates over a long period of time for the Gulf of Corinth in Greece, the Marmara Sea in Turkey and the Dead Sea Fault System, as well as deriving long-term magnitude–frequency relations for these same regions.
It has been demonstrated that slip rates calculated from historical data are in general comparable to those calculated from GPS measurements and field observations, while the size of historical earthquakes and their uncertainty can be quantified. This permits a more reliable estimation of the long-term hazard, the calculation of which is the concern of the engineering seismologist. It has also been shown that in most cases large earthquakes are less frequent when they are estimated from long-term data sets rather than from the instrumental period making the notion of recurrence time and of hazard assessment, questionable.
This study focuses on some of the few areas in the world for which long-term macroseism information exists and which facilitate this kind of analysis.