Lower-crustal strength under the Dead Sea basin from local earthquake data and rheological modeling

We studied the local seismicity of the Dead Sea basin for the period 1984-1997. Sixty percent of well-constrained microearthquakes (M-L less than or equal to 3.2) nucleated at depths of 20-32 km and more than 40% occurred below the depth of peak seismicity situated at 20 kin. With the Moho at 32 kin, the upper mantle appeared to be aseismic during the 14-year data period. A relocation procedure involving the simultaneous use of three regional velocity models reveals that the distribution of focal depths in the Dead Sea basin is stable. Lower-crustal seismicity is not an artifact created by strong lateral velocity variations or data-related problems. An upper bound depth uncertainty of +/-5 km is estimated below 20 km, but for most earthquakes depth mislocations should not exceed +/-2 km. A lithospheric strength profile has been calculated. Based on a surface heat flow of 40 mW m(-2) and a quartz-depleted lower crust, a narrow brittle to ductile transition might occur in the crust around 380degreesC at a depth of 31 km. For the upper mantle, the brittle to ductile transition occurs in the model at 490degreesC and at 44 km depth. The absence of micro-seismicity in the upper mantle remains difficult to explain. (C) 2003 Elsevier B.V. All rights reserved.