Abstract

We observe enhanced high‐frequency radiation of P waves from aftershocks of the 4 April 2010 (Mw 7.2), El Mayor–Cucapah, Baja California, earthquake that may reflect isotropic radiation generated by rock damage in the source regions. To eliminate path, radiation pattern, and site effects we use spectral ratios of four pairs of collocated events with similar size and focal mechanism that occurred within short time intervals (less than 24 hrs) and were recorded at the same stations. The P/P and the S/S spectral ratios calculated for the selected pairs of events show that events with similar size may have differences of high‐frequency radiation up to a factor of 4 at 10 Hz and up to a factor of 10 at 4 Hz for P and S waves, respectively. To evaluate the differences between P‐ and S‐wave energy radiated at high frequencies, we calculate the (P/P)/(S/S) ratio of the ratios at high frequencies (f>1.5  Hz) in a band for which the signal‐to‐noise ratio is adequate. Since the pairs of used events have approximately the same size, the ratio of ratios is expected to be near unity. We observed high (P/P)/(S/S) spectral ratios at high frequencies (f>6  Hz), up to a factor of 9, which may reflect isotropic radiation associated with rock damage. To find additional possible signatures of rock damage, we evaluate changes of coda Q using the same data. The results show small change of Q for a doublet located on the Laguna Salada fault. However, for the doublets located south of the Sierras El Mayor–Cucapah, where there was no surface rupture before the sequence, changes of Q at 5 Hz by a factor of about 3 indicate significant temporal variations of rock properties along the source–receiver path.

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