THE EMERSON LAKE BODY: A DENSE, MAGNETIC KNOT WITHIN THE EASTERN CALIFORNIA SHEAR ZONE

Gravity and magnetic data indicate a dense, magnetic body that lies between the Hector Mine October 16, 1999 (Mw=7.1) and Landers June 28, 1992 (Mw=7.3) epicenters. The aftershocks and ruptures of these two events avoided the interior of the body. Two- and three-dimensional modeling of the potential-field anomalies shows that the source, here named the Emerson Lake body (ELB), extends to a depth of approximately 15 km. Two-dimensional gravity models indicate that the ELB has a density of about 2760 kg/m3. The ELB most likely is composed of Jurassic hornblende diorite because exposures of these rocks coincide with the apex of both gravity and magnetic anomalies west of Emerson Lake. Seismic tomography also shows higher velocities within the region of the ELB.

Seismicity before the Landers earthquake also tended to avoid the ELB, suggesting that the ELB affects the distribution of stress in this part of the Mojave Desert. Even though the 1975 Galway Lake earthquakes (M5.4) overlap the subsurface extent of the body in plan view, the earthquakes were shallow (< 5 km) and tended to occur above the ELB. The 1979 Homestead Valley earthquakes (M5.3) were also shallow, occurring along a set of conjugate faults. The Homestead Valley aftershocks occurred within an embayment in the western margin of the ELB.

Because the ELB is older than the Miocene initiation of movement on the eastern California shear zone, we predict less cumulative offset on faults within the body than on faults outside the body. Analysis of high-resolution aeromagnetic data does not indicate measurable apparent right-lateral offset (> 1 km) on faults within the ELB, but does find higher cumulative offset (3-20 km) on faults outside the ELB. Detailed gravity indicates an east-striking stream channel that crosses the Hidalgo fault without any right-lateral deflection. Thus, the ELB influences the way strain is accommodated throughout this part of the Mojave Desert.