Dead Sea Earthquake‐Source Scaling Using Masada Deep Borehole Data

The 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 $∼25km$⁠, 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.