A body wave inversion of the Koyna, India, earthquake of December 10, 1967, and some implications for body wave focal mechanisms

Abstract

With a generalized inverse technique, WWSSN (World-Wide Standard Seismograph Network) longperiod P and SH wave forms are analyzed from the Koyna earthquake. The effects of local plane-layered earth structure near an imbedded point dislocation source are put in by using a modified plane-wave ray theory which includes the standard reflection and transmission coefficients plus source corrections for radiation pattern and geometrical spreading. The generalized inverse compares synthetic seismograms to the observed ones in the time dom!lin through the use of a correlation function. By using published crustal models of the Koyna region and primarily by modelling the crustal phases P, pP, and sP, the first 25 s of the Jong-period wave forms is synthesized for 17 stations, and a focal mechanism is obtained for the. Koyna earthquake which is significantly different from previous mechanisms. The fault orientation is 67° dip to the east, -29° rake plunging to the northeast, and Nl6°E strike, all angles being ±6°. This is an eastward dipping, left lateral oblique slip fault which agrees favorably with the trend of fissures in the meizoseismal area. the source time duration is estimated to be 6.5 ± 1.5 s from a triangular time pulse which has a rise time of 2.5 s, a tail-off of 3.9 s, source depth of 4.5 ± 1.5 km, and seismic moment of 3.2 ± 1.4 x 102• dyn cm. Some short-period complexity in the time function is indicated by modelling shortperiod WWSSN records but 1s complicated by crustal phases. The long-period P wave forms exhibit complicated behavior due to intense crustal phase interference caused by the shallow source depth and radiation pattern effects. These structure effects can explain much of the apparent multiplicity of the Koyna source. An interpretation of the Koyna dam accelerograms has yielded an S-P time which can be used along with the IM D (Indian Meteorological Department) epicenter and present depth determination to place the epicenter directly on the meizoseismal area.

Seismological aspects of the Guatemala Earthquake of February 4, 1976

“…In the synthesis the surface reflections pP and sP were included (Figure 6). This type of modeling has been successfully applied to the determination of source parameters of relatively simple events [Langston, 1976]. The seismic moment of this first event was estimated to be 1.6 X 11Y 6 dyn cm.…”

Seismicity, faulting, and structure of the Koyna‐Warna seismic region, Western India from local earthquake tomography and hypocenter locations

“…Focal mechanisms and moment tensor inversions indicate complexly faulted basement underlying the Deccan Traps in the Koyna-Warna area, with general agreement of strike-slip movement on one or more NE-SW trending faults and normal faulting on NW-SE oriented faults [Rastogi and Talwani, 1980;Gupta et al, 1980;Langston, 1976Langston, , 1981Langston and Franco-Spera, 1985;Dziewonski et al, 1988;Chadha et al, 1997;Mandal et al, 1998;Gahalaut et al, 2004;Dura-Gomez and Talwani, 2010]. One of the NNE-SSW trending faults is believed to be the strike-slip causative fault for the 10 December 1967 M 6.3 Koyna earthquake, but to the south, the NW-SE oriented faults are predominantly normal faults that coincide with the general trend of the Warna lineament [Singh et al, 1975;Langston, 1976Langston, , 1981Gupta et al, 1980;Rastogi and Talwani, 1980;Langston and Franco-Spera, 1985;Talwani, 1997b;Rastogi et al, 1997;Gahalaut et al, 2004;Shashidhar et al, 2011]. Regional structures inferred from geomorphology and supported by geophysical data also suggest structures ranging from NW-SE to NE-SW [Langston, 1976[Langston, , 1981Talwani, 1997b], and drilling in a NNE-SSW fault zone showed a 60°WNW dipping fault at shallow (tens of meters) depths [ Figure 1; Gupta et al, 1999].…”

Anatomy of surface rupture zones of two stable continental region earthquakes, 1967 Koyna and 1993 Latur, India

“…Fault plane solutions have shown the strike-slip nature of the seismic fault; surface observations have pointed to lef-lateral movement (Talwani, 1997). Whether the dip of the fault plane is towards WNW or ESE has been uncertain (Langston, 1976 Reconstruction of the shatter zone between the two boreholes and its extrapolation to the surface fissure (Fig. 4) indicates that the fault dips towards WNW at -60 ø, thus resolving a longstanding debate about the direction of dip of this fault.…”