## Apparent stress and stress parameter of moderate earthquakes during the 2019 Ridgecrest, California, earthquake sequence: uncertainty in depth dependent stress variation

Chen Ji, Ralph J. Archuleta, & Aaron J. PeytonSubmitted September 10, 2023, SCEC Contribution #13237, 2023 SCEC Annual Meeting Poster #183

As part of the community stress-drop validation study, we use S wave strong motion records at ∆<50 km to constrain the apparent stress \sigma_a and stress parameter ∆σ (Boore, 1983) for selected moderate earthquakes during the 2019 Ridgecrest sequence. The centroid depths and moment tensors of these events were previously constrained using 3D Green’s functions (Wang and Zhan, 2020). To reduce the contamination of other seismic phases (e.g., Lg), we use magnitude-dependent S wave time windows that range from 3 to 6 s. We first adopt two methods to estimate seismic radiated energy E_R: a time domain modified Gutenberg-Richter method (MGR, Kanamori et al, 2020) and a spectral technique similar to Boatwright et al. (2002) (FSSE). The MGR method models the path attenuation of an E_R measurement as exp(-k∆), where the constant k is constrained by inspecting all E_R measurements (Kanamori et al., 2020). FSSE adopts a frequency-dependent crustal Q model, q_0f^\alpha, and near-surface \kappa_0 for every station. q_0=60, \alpha=0.675 and \kappa_0 are constrained using all E_R\ measurements and the assumption of flat high-frequency acceleration source spectral level. We find that at a given station, the standard deviation of the \kappa_0 estimates during different earthquakes is about 0.005 s. The mean \kappa_0 estimates at these selected stations vary from 0.01s to 0.05s. These two methods produce very consistent \sigma_a measures, though the discrepancy between them becomes slightly larger for earthquakes with relatively smaller magnitudes, presumably due to their relatively shorter durations. The geometric mean \sigma_a is about 1.1 MPa for MGR and 1.2 MPa for FSSE. Second, we use the constrained q_0, \alpha, and \kappa_0 values to retrieve the apparent S wave source spectra at individual stations. The corrected S wave source spectra for a given earthquake are then stacked together and used to constrain the corner frequency and stress parameter ∆σ. The geometric mean of ∆σ is 3.8 MPa. FSSE \sigma_a and ∆σ scale with Mw as {10}^{\ (0.18-0.22)Mw}, consistent with the prediction of the JA19_2S model (Ji and Archuleta, 2020) and the analysis of Bindi et al. (2023). However, a statistically significant depth dependence (e.g., log(\sigma_a)\propto0.1Z) is also captured. We will further verify this interesting result using a depth-dependent crustal attenuation model and synthetic seismograms.

**Key Words**

apparent stress, site response

**Citation**

Ji, C., Archuleta, R. J., & Peyton, A. J. (2023, 09). Apparent stress and stress parameter of moderate earthquakes during the 2019 Ridgecrest, California, earthquake sequence: uncertainty in depth dependent stress variation. Poster Presentation at 2023 SCEC Annual Meeting.

**Related Projects & Working Groups**

Ground Motions