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

Soil-helium surveys in the surface rupture zones of the 1993 Latur (M-w 6.2 ) and the 1967 Koyna (M-w 6.3) stable continental region (SCR) earthquake s ites in Peninsular India showed anomalies defining surface traces of the ca usative faults. Propagating from the Archaean crystalline basement through the Deccan basalt cover, the seismic fault produced a scarp by uplift along a thrust in the Killari area of the Latur earthquake, whereas the Koyna ea rthquake was associated with a strike-slip fault which expressed itself as an en echelon fissure zone. Core drilling has confirmed that the fault at K illari extends downward from the surface rupture zone with an approximate d ip of 50 degrees towards SSW. The level differences of flow contacts obtain ed by drilling in the hanging wall and foot wall sides of the fault, do not unequivocally establish the amount of displacement, but suggest that it mi ght be anything from 1 m to 6 m. If the higher figure of 6 m is accepted, i t would indicate reactivation of an old fault. Drilling established a WNW d ip of the Koyna fault, resolving a long-standing debate.