Time‐space variation of seismic S‐wave coda attenuation (Qc−1) and magnitude distribution (b‐values) for the Petatlan Earthquake

Abstract

Average S‐wave coda‐Q (Qc) at 6 Hz and to about 10 sec after the S‐wave arrival shows: (1) temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 Petatlan, Mexico earthquake; (2) spatial variation through the aftershock area, positively correlated with b, high Qc ‐b values at the two activity centers, intermediate values in between and landward, and low values toward the Middle America Trench, which remain low for two weeks after the main shock; and (3) five‐foreshock average Qc = 135±1.6% and eleven‐aftershock average Qc= 175±1.3% for the eastern asperity. The b values—and possibly Qc values—seem to correlate with the level of aftershock activity.