Man-made microseismic disturbances

Abstract

A recently installed seismograph network in Israel enables, for the first time, a systematic examination of the microearthquake activity and its relation to the regional tectonics. The observed microearthquake activity is widespread and not limited to the Dead Sea rift plate boundary; in the rift itself, the more seismically active parts are north of the Sea of Galilee and in the Dead Sea basin. A possible seismic quiescence appears in the Jordan Valley to the north of the Dead Sea basin and may be associated with that segment of the plate boundary which broke recently (1927), thus indicating a low strain level. Inside Israel the active nature of the Carmel-Farah fault systems was determined; its present level of seismicity appears to be comparable to that of the rift itself, indicating the significance of this fault system in the active tectonics of the area. The high microseismic activity on these faults, and their location in densely populated areas, mark them as possibly important sources of seismic hazards. Diffused seismicity is observed throughout much of Israel, although the nature of these earthquakes is not yet understood. The small-magnitude earthquakes near the rift are, most probably, part of a seismic halo around the plate boundary. Pronounced seismicity is also observed in the Mediterranean Sea off the coast of southern Lebanon and northern Israel. This microearthquake activity decreases from north to south and, to some extent, from east to west. Whereas near the coast of northern Israel and southern Lebanon the microearthquake activity is relatively high, in the south near the coast of the Sinai Peninsula it is very low. This difference is attributed to a possible combination of a higher strain level in the African plate as it approaches the Africa-Eurasia plate boundary and hinge faulting near the coast.