Earthquake Risk in Western Saudi Arabia and the Red Sea from Seismic Moment
Ian C.F. Stewart
The seismicity within the onshore areas of Saudi Arabia is generally quite low, but areas of exceptional earthquake activity are associated with the Dead Sea transform fault in the Gulf of Aqaba and also along the spreading Center in the Red Sea. Previous studies indicate that significant levels of risk occur near the Gulf of Aqaba and also in the extreme south of the shield near Yemen. Most studies subdivide large areas into zones of uniform activity, and the resulting risk maps necessarily reflect these zone boundaries. There can be considerable difficulty in defining the zones to allow for known and anticipated events, based on instrumental and historical records, as well as the geology. In addition the recurrence and wave attenuation parameters may not be well defined, and hence the risk maps are often only very approximate at best. A major problem arises from the short time span and hence the incomplete nature of the instrumental earthquake record. The active zones of seismicity are therefore imperfectly defined and the risk contours are subject to some modification with additional data. In the present study the active zones are defined only by the instrumentally-determined earthquake data, which was obtained from the International Seismological Center. The seismic moment release in small unit cells is used to give the recurrence rate for each cell, and the risk or expected ground acceleration at any given site is then found by summing the effects of all active cells, using suitable equations for seismic wave attenuation. Apart from possible temporal variations in the seismicity and the relatively short time span of the data set used (1970 through 2005), there are also considerable uncertainties in the attenuation equations used in the risk estimates. Results are presented for a number of different equations, and the preferred one here is that given by Sigbjörnsson and Ambraseys (2003). Most of the risk maps shown here are for a 950 year return period, which is equivalent to a 10 percent probability of exceedance in 100 years. The preferred risk estimates indicate that near the Gulf of Aqaba there is significant risk, with acceleration levels likely to be greater than 500 cm s-2, or 50 percent of g, for a 950-year return period. Elsewhere over most of the shield the acceleration levels tend to be low, less than 10 cm s-2, and even along most of the Red Sea coast the risk does not appear to be appreciable. The present study does not allow for the effects of site amplification, over thick alluvium for example, which may increase the risk appreciably at some locations.
Stewart, I.C.F., 2007, Earthquake risk in western Saudi Arabia and the Red Sea from seismic moment: Saudi Geological Survey Technical Report SGS-TR-2007-4, 41 p., 32 figs.