Seismic observations of microearthquakes from the masada deep borehole

A better understanding of the Dead Sea basin (DSB) tectonics and hazards can be achieved through high-sensitivity observations of its seismic activity. Accordingly, we have taken advantage of the Masada deep borehole (MDBI), an abandoned oil well on the western shore of the Dead Sea rift, to install a seismometer at 1516 m below sea level-a 2 Hz three-component seismometer. The MDBI station is near the east Masada fault, part of the western boundary fault of the DSB. Seismological measurements conducted at these depths can provide more information than possible with surface observations- in this case, recording 1491 high-quality seismograms as compared to 103 by the local national network. The MDBI seismograms also have significantly higher signalto- noise ratio than the on-surface observations of nearby Israel Seismic Network (ISN) stations. This lowers the detection threshold down to well below M= 0 on the ISN scale. Many of these earthquakes, sometimes occurring in clusters, take place almost under MDBI, at depths of 10-25 km. Using cross-correlation techniques, we found several sequences of earthquake clusters underneath both the station and along the Dead Sea fault (DSF). The b-value obtained by the frequency-magnitude relationship of these events is somewhat higher than the value for the DSF area taken as a whole.