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Estimates of seismic wave attenuation in northern Baja California

CENTRO DE INVESTIGACION CIENTIFICA Y DE EDUCACION SUPERIOR DE ENSENADA, BAJA CALIFORNIA, APARTADO POSTAL 2732, ENSENADA, BAJA CALIFORNIA, 22830, Mexico

Abstract

By applying the spectral ratio method and the coda wave theory of Aki and Chouet (1975) to digital records of microearthquakes in Baja California, we have determined the seismic quality factors Q, Q_ß independent of frequency, and Q_c(f) and Q_ß(f) as a function of frequency. We found that Q and Q_ß, independent of frequency and in the range from 3 to 25 Hz, increase with the difference in epicentral distance between two stations. At short distances ( 16 km), Q_ß is approximately 65, and at longer distances (83 km), Q_ß increases to 700. We obtained only three values of Q_ß/Q (1.11, 1.47, and 0.87).

Q_c(f) and Q_ß(f) were calculated from events recorded around the San Miguel fault at distances not greater than 30 km and source depths between 5 and 15 km. Q_c(f) = (37 ± 13)f^0.87±0.04 was found in the range of frequencies from 3 to 24 Hz. We found that the minimum mean free path was almost constant (15 km) for the range of frequencies analyzed. Determinations of Q_ß(f) were performed using two lapse times. At short lapse time (10 sec), Q_ß(f) was almost linear with frequency, increasing from 43 at 3 Hz to 392 at 24 Hz [Q_ß(f) = 13f^1.03]. These values correspond to near-surface attenuation. On the other hand, at relative long lapse time (16 sec), Q_ß(f) increases from 147 at 3 Hz to 625 at 24 Hz [Q_ß(f) = 70f^0.74]. These last values of Q_ß(f) are within the range of variation of Q_c(f).

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