![]() Subsurface geological structure is the most likely cause of the amplification. One accelerograph, located on a ridge, recorded an anomalously high amplitude ringing at 2 Hz on the transverse component only. This value is between 300 and 500 for the assumed spectral shapes. In the process of matching these spectra, an average value of Q can be inferred to be consistent with the spectra at high frequencies. This leads to a smaller estimated source radius and a higher stress drop. If the displacement spectrum is assumed to be approximated by two asymptotes, f 0 at frequencies less than the corner frequency and f −2 at frequencies greater than the corner frequency, the level of the f −2 asymptote can be constrained by rms acceleration (Hanks, 1979). S-wave spectra, from three accelerograms which recorded the complete S wave, imply a seismic moment of 1.5 × 10 24 dyne-cm and, by the method of Brune (1970), a source radius of 2 km and a stress drop of 90 bars. Magnitude 5.7 earthquake hits off northern California: USGS. ![]() This suggests that the correction slightly overcompensates in this magnitude and distance range, in which Luco lacked data, at least for this earthquake. A magnitude 5.7 earthquake struck on Friday off the coast of Northern California. ![]() When M L is estimated from accelerograph data using the distance correction to the M L scale proposed by Luco (1982), an average of 5.3 is obtained. From the accelerograms, the average estimate for M L is 6.0, compared to an average of M L = 5.5 from more distant recordings. Seven timed strong motion accelerograms were recorded between the distances of 45 and 80 km from the Santa Barbara Island earthquake.
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