Piezornagnetic field for Parkfield fault model

Abstract

A theoretical piezomagnetic field is calculated as a function of time and ground position for an earthquake instability model applied to moderate (M ∼ 6) earthquakes on the San Andreas fault near Parkfield, California. The instability model simulates fault slip and stress for all parts of a 32‐year earthquake cycle, including unstable (mainshock) slip. Mainshock slip occurs when a nearly locked patch of the fault becomes sufficiently loaded by surrounding aseismic slip that it reaches a condition of rapid failure. The piezomagnetic field is calculated directly from fault slip using analytic solutions based on piezomagnetic nuclei that are analogous to strain nuclei. During the interseismic interval, increasing stress concentration on the locked patch is accompanied by a gradual evolution of the magnetic anomaly field, leading to maximum cumulative changes of about ±2 nT in the vicinity of the pending epicenter. Mainshock slip cancels about half of the interseismically accumulated changes, and rapid post seismic relaxation cancels most of the rest. At certain sites near the locked patch the rate of change of the field progressively increases during the interseismic interval, while at others it progressively decreases. This effect is most pronounced during the last 3–4 years before the mainshock and, at suitably chosen pairs of sites, results in reversal of the temporal trend of the differential field. Such reversals, if separable from geomagnetic variation due to other sources, would constitute precursory indicators of approaching failure.