GPS-MEASURED SLIP RATES VS. SEISMIC MOMENT RELEASED BY PAST SEISMICITY ALONG ALBORZ MOUNTAIN RANGE IN IRAN

摘要

The area under study in this paper covers the northern boundary of Central Iranian Block which coincides with the Alborz Mountains. This belt has been responsible for several known catastrophic earthquakes in the past.The Manjil Earthquake of June 20, 1990 with over 50,000 human casualties and more than $7biliion of economic damage is an example of such events in Alborz. The occurrence of moderate to large earthquakes in the Alborz indicates that significant portion of the shortening between Iran and Eurasia takes place in the Alborz. In this study, the records of historical earthquakes along Alborz are used to calculate and plot the geographical distribution of seismic moment released in time. A two dimensional distribution function is proposed and used here to spread the seismic moment along causative tectonic features. Using accumulated seismic moment, the average slip rates across seismogenic faults are estimated for 32 sub-zones along the Alborz Mountains and western Kopet Dag. The estimated horizontal slip rate of 4-5 mm/year is in good agreement with the geodetic vectors recently estimated from a regional GPS survey in this region. The study also reveals geographical variation of slip rate along some 900 km length of this zone which may suggest appearance of seismic gaps or areas with aseismic nature. The completeness of the historical earthquake catalogue and its reliability with regard to the estimates of earthquake magnitudes, locations and rupturing systems are among many plausible factors controlling the credibility of such results. Therefore, any conclusions derived from these results remain as reliable as the data used for the analyses.