Estimates of seismic wave attenuation in northern Baja California

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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 Qα, Qβ independent of frequency, and Qc(f) and Qβ(f) as a function of frequency. We found that Qα and Qβ, 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), Qβ is approximately 65, and at longer distances (∼83 km), Qβ increases to 700. We obtained only three values of Qβ/Qα (1.11, 1.47, and 0.87). Qc(f) and Qβ(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. Qc(f) = (37 ± 13)f0.87±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 Qβ(f) were performed using two lapse times. At short lapse time (10 sec), Qβ(f) was almost linear with frequency, increasing from 43 at 3 Hz to 392 at 24 Hz [Qβ(f) = 13f1.03]. These values correspond to near-surface attenuation. On the other hand, at relative long lapse time (16 sec), Qβ(f) increases from 147 at 3 Hz to 625 at 24 Hz [Qβ(f) = 70f0.74]. These last values of Qβ(f) are within the range of variation of Qc(f).