Characteristics
The earthquake had a magnitude of 7.0. A maximum intensity of 9 on the MSK scale was observed. The calculated focal mechanism showed that the earthquake was a result of low-angle reverse faulting on a fault plane dipping at about 35° to the north-northeast. This was confirmed from the distribution of aftershocks, which defined a clear plane of this orientation. Analysis of the detailed velocity structure around the rupture zone suggested that it coincided with a marked change in seismic velocity, consistent with it representing the interface between Mesozoic sediments and the underlying crystalline basement. The 1500 m high Racha Ridge is thought to have been uplifted by repeated earthquakes of this type.
The mainshock was followed by a complex series of aftershocks extending over several months, which caused further damage and casualties The largest of the aftershocks, which consisted of two events about 2 seconds apart, had a magnitude of Ms = 6.5 and occurred on 15 June with an epicenter near Java. On 23 October 1992, a magnitude 6.7 earthquake occurred about 100 km east of the aftershock zone. It was also due to reverse faulting on a northnortheast-dipping plane, although with a significant dextral (right lateral) strike-slip component.
Read more about this topic: 1991 Racha Earthquake