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A Breakdown of Source Self-Similarity at M5.3 Inferred from Strong Ground Motion Parameters

The strong ground motion parameters, such as peak ground acceleration (PGA) and peak ground velocity (PGV), contain important high frequency source information that is difficult to be extracted with conventional finite fault studies. We note that the magnitude dependence of mean log(PGA) and mean log(PGV) estimated using the NGA West-2 database features a sharp change in slope at about M 5.3, which cannot be reproduced by the stochastic ground-motion modeling using a self-similar double-corner-frequency (DCF) source spectrum model. Here we present a DCF model JA19_2S that has two magnitude ranges; this model can well match this abnormal feature. Its lower corner frequency f_c1 satisfies for log(f_c1)=1.474-0.415M for 3.3<M<5.3; log(f_c1)=2.375-0.585M for 5.3<M<7.3. Its high corner frequency f_c2 follows log(f_c2)=3.25-0.5M, for 3.3<M<7.3. f_c1 is related with the average source duration T d =1/\pi f_c1 and f_c2 is related with the average rise time T_R ~1/ f_c2 .T_d then increases with seismic moment as M₀^1/3+a , with a=0.614 for 3.3<M<5.3 but a= -0.436 for 5.3<M<7.3. This can be the result of either rupture velocity increasing 47% or static stress drop increasing 3.2 times when earthquake magnitude increases from 3.3 to 5.3 and subsequently decreasing by the same amount when magnitude further increases from 5.3 to 7.3. The scaled energy (ratio of seismic radiated energy and seismic moment) of this DCF model scales with two corner frequencies as M₀f_c1² f_c2. This model then also implies that the scaled energy varies from 2.2x10^-5 to 4.8x10^-5 when magnitude increases from 3.3 to 5.3 and subsequently decreases to 2.2x10^-5 when magnitude further increases to 7.3. While these predictions can be evaluated with independent seismic observations, for 5.3<M<7.3 the predicted acceleration spectrum using JA19_2S remarkably agrees with the additive DCF model proposed by Atkinson and Silva (2000) for California earthquakes, though they were constructed with entirely different approaches.

Presenting Author: Chen Ji

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