Dead Sea Earthquake-Source Scaling Using Masada Deep Borehole Data

Abstract

AbstractThe low-noise environment in the Masada deep borehole (MDBI), an abandoned oil well on the western shore of the Dead Sea rift, allows the recordings of many small-magnitude earthquakes. MDBI has a 2 Hz triaxial seismometer installed at a depth of 1256 m below ground and 1516 m below sea level. We used MDBI P- and S-wave data and the general source relationship u(f)=u(0)e−(πft/Q)/[1+(f/f0)2y]1y, in which f0 is corner frequency and 1<y<2, to estimate source parameters for 1031 earthquakes along the Dead Sea fault. The seismic moments, corner frequencies, and source radii as determined by this model show a complete breakdown in earthquake scaling. The spectra of many small earthquakes are characterized by drop-offs of roughly f−2. Q0(P) and Q0(S) increase with event distances up to ∼25 km, beyond which they are ∼1510 and 2285 for p=1 and ∼2040 and 2985 for p=2. For these two end members, the ratio Q0(S)/Q0(P) for distances of 20–150 km has values ranging between 1.3 and 1.4. At shorter distances, this ratio is somewhat higher, indicating that P waves attenuate faster than S waves. The energy versus seismic moment results shows no azimuthal dependence, implying that it is a valid measure of source strength. For the end-member models, this relationship follows E∝M01.35 and E∝M01.49 for the P and S waves, respectively.