SPATIAL AFTERSHOCK DISTRIBUTION - EFFECT OF NORMAL STRESS

We study the spatial clustering of shallow aftershock hypocenters with respect to focal mechanisms of mainshocks. We use the Harvard centroi d moment tensor (CMT) global catalog, the Preliminary Determination of Epicenters (PDE) earthquake list, the California Institute of Technol ogy/U.S. Geological Survey catalog of earthquakes in southern Californ ia, and a catalog of focal mechanisms for all earthquakes since 1850 i n southern California with magnitude larger than 6. We need to account for possible systematic bias in hypocenter distribution due to the ge ometry of seismogenic zones, especially that of subduction zones. We a lso select only strike-slip earthquakes from the catalogs to investiga te aftershock clustering in circumstances more favorable for direct ob servation. We compare the spatial distribution of hypocenters before e ach strong earthquake with the distribution during the first 250 days after the earthquake and for the time interval extending beyond 250 da ys. If the friction coefficient in the Coulomb criterion is positive o ne expects that after a strong earthquake, aftershocks and other earth quakes would concentrate in the direction of the P axis (dilatational quadrant) rather than in the direction of the T axis (compression quad rant). Such correlations have been pointed out previously for selected earthquakes sequences, but is such correlation a general feature of e arthquake occurrence? We study spatial earthquake distributions before and after each event for several choices of focal sphere partition, c utoff magnitude, focal mechanisms of large events, time periods, dista nce from a mainshock, etc. Although some earthquake distributions agre e with a nonzero friction coefficient, others produce the opposite pat tern, suggesting that the concentration of events along the P and T ax es is due to random effects. This result implies that the friction coe fficient in the Coulomb law is close to zero.