Source mechanism of Astor Valley Earthquake of November 20, 2002 inferred from teleseismic body waves
Abstract
Teleseismic body waves recorded at the IRIS Global Seismographic network are analyzed to investigate the source mechanism of the Astor valley earthquakes of Nov. 20, 2002 occurred in northern area of Pakistan about 60 km southeast of Gilgit near Bunji. The inversion method of Kikuchi and Kanamori is applied to the records of 11 seismic stations that included vertical P and horizontal SH components for this purpose. The azimuthal coverage of seismic stations was good enough to resolve some details of heterogeneous moment tensor. The fault area is estimated from the first four days’ aftershock distribution as 25 x 20 km; and their distribution corresponds to the local tectonic trends The estimated seismic moment of the November 20, 2002 main shock is 9.4x 1017 Nm which gives Mw=5.9. The solution of body waves indicates normal faulting with a small component of strike slip on source fault whose strike is parallel to the local trend of MMT, with the tension axis perpendicular to it. The location of the main shocks along with aftershock area suggests that the nodal plane oriented south west-northeast dipping to the north west is preferred for the fault plane. Fault Plane solutions of November 01, 2002 and November 20, 2002 earthquakes derived from first motion polarity data recorded by MSSP local seismic network and some global seismic stations also show normal faulting. The results indicate that the Astor valley earthquakes may be considered to have occurred in an uncoupled region, in response to the gravitational pull due to internal gravity sliding.
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