Moment Tensor Solutions and Triggering Environment for Earthquakes in Koyna-Warna Water Reservoirs Region,India |
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| Authors: | S N Bhattacharya |
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| Institution: | (1) Department of Geology and Geophysics, Indian Institute of Technology, Kharagapur, West Bengal, 721302, India |
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| Abstract: | We consider nine earthquakes in the Koyna-Warna reservoir region on the western side of the Peninsular India. The deviatoric
moment tensors of these earthquakes have been evaluated by minimizing the least-squares misfit between observed and synthetic
seismograms. We use broadband seismograms of observatories at KARD and PUNE which are at distances of nearly 50 and 150 km,
respectively, from the epicenters. Both surface wave inversion and the difference between the arrival times of SH and SV show
the presence of an anisotropic crust. However, we have obtained an equivalent isotropic structure by improving the published
crustal structures of this area through inversion of surface wave group velocity data. The deviatoric moment tensors of the
earthquakes are decomposed into two components: double-couple and compensated linear vector dipoles (CLVD). The double-couple
components of all the nine earthquakes show normal faulting with minor strike slip; the T axis is consistently subhorizontal
with an average azimuth of 260.6° and the P axis is nearly vertical. The fault planes of six events give average strike direction
and dip, respectively as 194.0° and 51.8° and are associated with the main fault of the area. The other three events lie in
the southern part of this area and have strike direction between SSE and SE which is parallel to the tectonic features in
this part. The CLVD component is generally within 20 percent of the total moment tensor. Recent studies show that anisotropy
can produce source mechanism with CLVD up to 30 percent and can also cause high pore fluid pressure leading to fault instability
more rapidly compared to conventional mechanism in an isotropic medium. It appears that the anisotropic crust, noted in the
present work, is generating the CLVD component and also gives the proper environment to trigger earthquakes by reservoirs
through pore fluid pressure. |
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| Keywords: | Earthquake-source-mechanism seismicity inversion |
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