Methods for Measuring Seismicity Rate Changes: A Review and a Study of How the Mw 7.3 Landers Earthquake Affected the Aftershock Sequence of the Mw 6.1 Joshua Tree Earthquake

The development of fault interaction models has triggered the need for an accurate estimation of seismicity rate changes following the occurrence of an earthquake. Several statistical methods have been developed in the past to serve this purpose, each relying on different assumptions (e.g., stationarity, gaussianity) pertaining to the seismicity process. In this paper we review these various approaches, discuss their limitations, and propose further improvements. The feasibility of mapping robust seismicity rate changes, and more particularly rate decreases (i.e., seismicity shadows), in the first few days of an aftershock sequence, is examined. To this aim, the hypothesis of large numbers of earthquakes, hence the use of Gaussian statistics, as is usually assumed, must be dropped. Finally, we analyse the modulation in seismicity rates following the 1992, June 28 Mw 7.3 Landers earthquake in the region of the 1992, April 22Mw 6.1 JoshuaTree earthquake. Clear instances of early triggering (i.e., in the first few days) followed by a seismicity quiescence, are observed. This could indicate the existence of two distinct interaction regimes, a first one caused by the destabilisation of active faults by the travelling seismic waves, and a second one due to the remaining static stress perturbation.