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

Abstract

We studied the local seismicity of the Dead Sea basin for the period 1984–1997. Sixty percent of well-constrained microearthquakes ( M L≤3.2) nucleated at depths of 20–32 km and more than 40% occurred below the depth of peak seismicity situated at 20 km. With the Moho at 32 km, 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 380°C at a depth of 31 km. For the upper mantle, the brittle to ductile transition occurs in the model at 490°C and at 44 km depth. The absence of micro-seismicity in the upper mantle remains difficult to explain.