CORRELATION BETWEEN INCREASED EARTHQUAKE FREQUENCY AND STATIC STRESS CHANGES INDUCED BY THE 2004 SUMATRA-ANDAMAN AND 2005 SUMATRA EARTHQUAKES

The effects of the static stress change induced by the M_w = 9.3 2004 Sumatra-Andaman earthquake and the M_w = 8.7 2005 Sumatra earthquake are investigated using the 3-D boundary element program Poly3D. The geometry of the Sumatra subduction zone and the strike-slip Sumatra Fault was approximated using triangular discontinuity elements on 40-km and 20-km centers, respectively, generating a high-resolution model of the complex subduction zone. Maximum reverse offset along the ruptured portion of the subduction zone ranged from 7 m to 23 m and is modeled independently between fault segments. The resultant geometric model closely approximates the actual geometry and rupture distribution of the subduction zone, reducing errors associated with oversimplification and model inaccuracies. A study of >1000 shallow (0-20 km) upper plate earthquakes occurring between 26 December 2004 and 17 July 2005 indicates that areas of elevated static Coulomb stress change on the strike-slip Sumatra fault are associated with increased earthquake frequency, while areas with little to no stress change are associated with the lowest earthquake frequencies. Larger subduction zone earthquakes occur in areas of elevated stress on segments adjacent to the rupture zones. A model of the statistical correlation between static stress change and earthquake frequency through time is being constructed in order to observe the timing of stress relaxation following a major earthquake.