ANALYSES OF EARTHQUAKE RELATED UPPER CRUSTAL STRESSES AND GROUND DISPLACEMENTS IN THE HIMALAYA

Abstract

The present work is motivated by the need to' understand the occurrence of earthquakes in the Himalaya. Towards this end, firstly, the stresses are analytically simulated in several homogeneous and elastic wedge shaped models of the Outer and Lesser Himalaya. Secondly, some observed ground displacement data are analysed using the Finite Element Method to estimate slip rates on the detachment under the Himalaya. In the first part of the study, we have identified certain linear and hyperbolic stress distributions which provide plausible explanations to the distribution and mode of faulting during moderate and small earthquakes in the Himalaya. The identified stress distributions are those in which stresses are on the verge of failure on the whole or a major portion of the base of the wedge. A set of stress distributions, for models with angles of internal friction of 30° and 25 for wedge and base respectively, satisfy failure criterion in the 50 km long rear portion of the wedge interior and conform to the location of the small and moderate magnitude intrawedge Himalayan earthquakes. The stress distributions in models with angles of internal frictions of 26° and 20° for wedge and base respectively, in general, satisfy failure condition in the deeper reaches (depth * 10 km) at the rear 50 km long portion of the wedge interior, thus may provide plausible explanations to the occurrence of moderate magnitude earthquakes in deeper reaches of the rear of the Himalaya. In another set, stresses are below critical value required for failure, everywhere inside the wedge. This feature is observed for models with angles of internal friction of 26° and about 18° for ii i wedge and base respectively. These stress distributions can simulate the occurrence of moderate earthquakes along the base of the Himalayan wedge. For different models mentioned here, the pore pressure has a largest value of about 300 MPa and.cohesion of wedge and base have values of 25 MPa and 0.01 MPa respectively. The magnitude of the simulated shear stresses on optimally oriented intrawedge faults is less than about 117 MPa for the last of the three sets mentioned above, compared to about 140 and about 144 MPa for the other two sets. We prefer the stress distributions of this last set to those given in the other two sets. In the second part, we were able to interpret the available interseismic geodetic levelling data in terms of thrust fault type slip rate (distributions) on the detachment only. Thrust fault type slip, rather than normal fault type (down dip) motion, is more likely to occur during interseismic period. Thus, inferring that the overriding and the subducting media may not be fully coupled, as suggested by the stick slip motion models. From the estimated interseismic slip rates on the detachment, we have identified a section of high coupling (low thrust fault type slip) bounded in front and rear by sections of low coupling (large thrust fault type slip). This section, which primarily exist below the Lesser Himalaya, coincides with the inferred rupture zones of great Himalayan earthquakes as documented in the literature. The portion of detachment, on which moderate magnitude earthquakes have been reported in the literature is a part of this section. We propose that the recoverable earthquake generating strains for occurrence of thrust type earthquake will be accumulating even when thrust fault type slip is taking place on the detachment during interseismic period.