Characterizing Broadband Source Spectra of Moderate Earthquakes: Revising Site Response Uncertainty and Its Impact on Source Parameters

Chen Ji, & Ralph J. Archuleta

Submitted September 7, 2025, SCEC Contribution #14880, 2025 SCEC Annual Meeting Poster #TBD

We characterize the broadband source spectra of moderate earthquakes with three measurements: seismic moment M_0, apparent stress σ_a, and stress parameter 〖Δσ〗_B, corresponding to frequencies below the corner frequency, bracketing the corner frequency, and well above the corner frequency, respectively. Both σ_a and 〖Δσ〗_B are related with spectral amplitudes, avoiding the challenges in precisely determining the corner frequency of earthquake spectra using spectral-fitting techniques; both relate tightly to strong ground motion. Because both are sensitive to site effects, we update our previous approach (Ji, Archuleta, and Peyton, 2024) by further considering frequency dependent uncertainties of synthetic site responses at individual stations constructed using Vs30 and κ_0. The remaining uncertainty in these updated site responses is the average site response of selected stations. It is noteworthy that in the low frequency range this average site response results in excellent agreements in seismic moment, and in the high frequency range it is related with the mean high-frequency falloff rate of 2, which is obtained by averaging all corrected source spectra. For the Mw 4-5.4 Ridgecrest earthquakes, we find that 〖Δσ〗_B, which is estimated using the geometric mean source spectrum (Ω ̅_0), is reduced by only 3% though the mean within-event variation is reduced significantly from 0.16 to 0.057 in log 10 units. In contrast, σ_a is decreased by 40%, much more significantly, because it was defined as the geometric mean of the estimates using spectra of individual stations (hereafter σ_a^1). If instead we estimate σ_a also using Ω ̅_0 (hereafter σ_a^2), it would decrease only by 8%. Note the analytic inequality σ_a^1≥σ_a^2 (Ji, Archuleta, and Peyton, 2024) still holds but the mean of the σ_a^1/σ_a^2 ratios decreases from 1.8 to 1.4, reflecting the improved similarity between within-event spectra. We evaluate the solutions of path and site responses using 80 Mw 3.5-4.0 Ridgecrest earthquakes with satisfactory results. The mean ratio of 〖Δσ〗_B/σ_a^2 is 5.7 with a factor of 1.43 uncertainty. We subsequently apply the same methods to about 200 Mw 4-5.5 earthquakes in Southern California, using direct S wave records at close distances.

Key Words
Site response, apparent stress, stress parameter

Citation
Ji, C., & Archuleta, R. J. (2025, 09). Characterizing Broadband Source Spectra of Moderate Earthquakes: Revising Site Response Uncertainty and Its Impact on Source Parameters. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology