The seismicity, seismotectonics and earthquake hazards of Libya, with detailed analysis of the 1935 April 19, M = 7.1 earthquake sequence

Abstract

SUMMARY We have investigated the seismicity of Libya in order to understand better the earthquake hazards, the tectonics of the region, and the present-day evolution of the Sirt basin. We first relocated all events occurring between 1931 and 1988 using a bootstrap location technique. We then determined focal mechanisms from firstmotion data. Finally, we used waveform modelling techniques to determine source parameters for the 1935 main shock and its two largest aftershocks. Relocation of the 1930s’ events are in some cases significantly different from those given by the International Seismological Summary (ISS). The first-motion results suggest a combination of strike-slip and normal faulting for the 1935 and 1939 sequences. The waveform modelling results of the 1935 mainshock suggest a strike-slip mechanism with one fault plane similar to the strike of the eastern edge of the Hun Graben. About 5 hr after the mainshock, an aftershock (mb = 6.0) occurred about 40 km to the south, which was followed about 9 hr later by another aftershock (mb = 6.4) in the same area. Waveform modelling results for this earthquake sequence suggest focal depths ranged from 15-21km, with rupture lengths of 14-16 km. An average displacement of 1.8 m is estimated for the mainshock, but no surface rupture has been noted for the event. In the vicinity of the Hun Graben area, the number of large earthquakes that have been reported through history is an indication that the area is seismically active. Most of the activity appears to be concentrated along the eastern margin of the NW-SE trending Hun Graben. The focal mechanisms suggest that the maximum compressive stress in northwestern Libya is oriented WNW. In the north-eastern part of the country, the stress direction changes to a more NE-SW orientation, as evidenced by the 1963 February 21 Al-Marj and the 1967 Al-Jabal Al-Akhdar mainshocks.