N. N. Ambraseys

Some characteristic features of the Anatolian fault zone

Abstract The study of the Anatolian fault zone shows that major earthquake sequences associated with faulting have been occurring in the zone since historical times with periods of quiescence of 150 years. The fault zone is a broad belt of crushed rocks a few kilometres wide rather than a single continuous rupture. Recent surface breaks within the zone consist of large en echelon ruptures with individual uninterrupted linear features that do not exceed a few kilometres. The average displacement of the two sides of the zone since 1939 is about 90 cm. There is some evidence to show that creep is taking place in some parts of the zone, of the order of a few centimetres per year. Preliminary calculations show that the angle of residual shear resistance mobilised on the fault at failure should be very small.

The Seismic Activity of the Marmara Sea Region over the Last 2000 Years

We examined the seismicity of the region of the Sea of Marmara in northwest Turkey over the last 2000 yr using the historical record of the region. We found no evidence for truly large earthquakes of a size comparable to those in the North Anatolian Fault zone in the Marmara Basin. Events are smaller in keeping with the known fault segmentation of the Basin. Over the last 2000 yr seismic mo- ment release accounted for the known right-lateral shear velocity across the Marmara region observed by Global Positioning System measurements. Its average rate is relatively constant and varies between 1.6 and 2.4 cm/yr. The long-term seismicity in the Marmara Sea region shows that large earthquakes are less frequent than pre- dicted from the 100-yr-long instrumental period.

NEAR-FIELD HORIZONTAL AND VERTICAL EARTHQUAKE GROUND MOTIONS

Strong-motion attenuation relationships are presented for peak ground acceleration, spectral acceleration, energy density, maximum absolute input energy for horizontal and vertical directions and for the ratio of vertical to horizontal of these ground motion parameters. These equations were derived using a worldwide dataset of 186 strong-motion records recorded with 15 km of the surface projection of earthquakes between Ms = 5:8 and 7.8. The effect of local site conditions and focal mechanism is included in some of these equations.

PREDICTION OF VERTICAL RESPONSE SPECTRA IN EUROPE

Equations for the prediction of vertical peak and absolute acceleration spectral ordinates in terms of magnitude, source-distance and site geology are presented. Comparison to similarly derived horizontal equations shows vertical spectral values to be 1/2-1/4 of the horizontal. The influence of site geology on vertical ground motion is reduced with respect to the horizontal. Ratios of peak vertical to peak horizontal ground acceleration in the near-field of thrust faults are magnitude and distance dependent, reaching values in excess of one very near the fault of large magnitude events. For strike-slip faults the ratio exceeds one for moderate events, decreasing for larger events, and is distance independent. Spectral acceleration ratios exceed one at short periods but are less than one at intermediate and long periods, irrespective of the source mechanism.

SEISMICITY AND STRAIN IN THE GULF OF CORINTH (GREECE) SINCE 1694

We use the record of earthquakes in central Greece since 1694 to estimate the seismically-released strain in that region over the last 300 years. The highest strain rate is seen in the Gulf of Corinth, an extensional graben which is the most prominent active geological structure. Over the 300-year period, the earthquakes account for an average extension rate of about 11 ram/yr between the Peloponnese and the island of Evvia, of which about 10 mm/yr occurs in the Gulf of Corinth. The earthquakes occurring in shorter time periods, perhaps as little as 100 years, account for extensional velocities similar to the 300 year average, though the 20th Century has been relatively quiescent. We are not confident that the historical record can be used to assess seismic strain release in areas smaller than the Gulf of Corinth as a whole. The similarity between the average rate of seismic strain release over 300 years and the short-term strain rates measured geodetically suggests that the bulk of the tectonic strain in...

Earthquake induced displacements of slopes

Abstract 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.

Earthquakes and Associated Deformation in Northern Baluchistan 1892-2001

We examine a century of seismicity and earthquake-related deformation on the western margin of the Indian plate in the Pakistan region of northern Baluchistan. Several catalogs of earthquakes for this region currently exist, but early catalogs in particular suffer from errors, incompleteness, inhomogeneity, and location bias. We form a new catalog of more than 1000 earthquakes using original sources to confirm macroseismic locations and assign M S magnitudes to earthquakes since 1892. In doing so we reveal a systematic east-northeast bias in locations caused largely by the uneven global distribution of seismic stations used in their determination. An appendix provides narrative accounts and historical references to 34 significant earthquakes in the region. The pattern of seismicity in the past century shows activity over a 700-km-long, 200-km-wide segment of the plate boundary with predominantly strike-slip faulting to the west and thrust faulting to the east. At its narrowest near 29° N, transpression of the plate boundary is partitioned into reverse and strike-slip components separated by approximately 100 km. The M 7.3 1931 Mach earthquake (slip 1.1 m on a 40° east-southeast-dipping reverse fault) released fault-normal stresses that may have “unclamped” the subsequent M 7.7 left-lateral Quetta earthquake 4 years later. The northern Chaman fault system in the past century has been largely inactive, suggesting that this time period is not representative of long-term activity in the region and that up to 4 m of potential slip is currently available to drive one or more future M > 7 earthquakes. Despite triangulation installed in the late nineteenth and early twentieth centuries, no recent geodetic data are available to permit plate boundary velocities to be measured directly.

SURFACE-WAVE MAGNITUDE CALIBRATION FOR EUROPEAN REGION EARTHQUAKES

Abstract This paper is concerned with the calibration of the surface-wave magnitude scale for the European region using the Prague formula. Two issues in particular have been addressed: ˙Does the Prague formula require a correction of the distance dependency term? ˙Is a source depth correction required? The issue of distance dependency has been investigated by examining the distance dependency of residuals from individual station Msmagnitudes and “representative” Ms magnitude values. We find that Ma magnitudes determined for the European region, using the Prague formula show slight dependence on recording station distance. The subsequent distance dependency of the mean Ma magnitude determination is therefore a function of the distance distribution of the recording stations. The question of depth correction has been investigated by examining the regression of Ms magnitudes versus seismic moment as a function of depth. We find that the Ms magnitudes determined by the Prague formula are only valid for crusta...

Tsunami Catalogs for the Eastern Mediterranean, Revisited

We critically examine tsunami catalogs for the eastern Mediterranean published in the last decade, by reference to the original sources. Such catalogs have been widely used in the aftermath of the 2004 Boxing Day tsunami for probabilistic hazard analysis, even for projections in a ten-year timeframe. We correct classification and other spurious errors and posit a new list. We conclude that, for some events, any assignment of magnitude (even on a six-point intensity scale) is inappropriate due to lack of information. Furthermore, we assert that any tsunami catalog, including ours, can only be used in conjunction with sedimentologic evidence to quantitatively infer return periods of large events. Statistical analyses correlating the numbers of tsunami events only from catalogs with inferred intensities are meaningless, at least when focusing on specific locales where only a handful of tsunamis have been historically reported. Quantitative hazard assessments based on scenario events of historic tsunamis for which—at best—only the size and approximate location of the parent earthquake is believed known should be undertaken with extreme caution and only with the benefit of geologic studies to enhance the understanding of the local tectonics.

Middle East—A Reappraisal of the Seismicity

The Middle East covers a land area of 13.5 million square kilometers, approximately 2.8 times the size of Europe. Its population amounts to more than 185 million as compared to 450 million in Europe, distributed unequally with most living in a mere 6 percent of the total area which is regarded as cultivable and also in the cities. Also, its gross national product per capita is distributed unevenly ranging from about £38 in Ethiopia to £725 in Israel and £2200 in Kuwait with an overall average of only £120 as compared to £760 for Europe. Until recently the rate of investment in large urban and industrial developments in the Middle East was small and in places insignificant. Consequently there have been essentially no significant economic losses from the large number of damaging earthquakes that occur in this part of the world. Although during the first 70 years of this century earthquakes have taken a toll of 130 000 lives in the Middle East, of which 73 000 perished in 22 earthquakes in Iran and 57 000 in 26 earthquakes in Turkey, damage in economic terms has remained relatively small. The average replacement cost of the 300 000 adobe houses destroyed in Turkey during the period 1900–1966 hardly amounts to a few tens of millions of pounds (Ambraseys 1972). The replacement cost of similar types of construction destroyed in other, less developed parts of the area, is even smaller. For instance, the Silakhor earthquake of 1909 in Iran, in which more