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.