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Broadband Spectral Characteristics of Moderate-sized Earthquakes Using Nearby Recordings

Description: 

We propose characterizing earthquake source spectra using three measures: seismic moment M₀, apparent stress σ_a, and stress parameter Δσ_B, corresponding to frequencies less than the corner frequency f_c, bracketing f_c, and much higher than f_c. To minimize path effects, σ_a and Δσ_B are estimated using S wave records within 50 km. Depth dependent variations in velocity and density at the source are accounted for. The results are calibrated using observations at rock sites. We model near-surface impedance as a function of frequency using a generic model created using the corresponding Vs30 measure. We conduct a grid search with F-tests to constrain a frequency-dependent crustal Q model (Q(f)=q₀ f^α) and a site attenuation constant κ₀ for each station, assuming Aki’s ω^-2 model.

We use this approach to study the Ridgecrest earthquake sequence. Taking advantage of high signal-noise-ratios (SNR) in lower (<0.5 Hz) and higher frequencies (~10-40 Hz), we first use 42 relatively larger Ridgecrest events (4.0≤M_w≤5.4) to construct the attenuation models: q₀=60, α=0.675, and κ₀ ranging from 0.01s—0.05 s among different stations. We subsequently use them to retrieve the source spectra of 122 3.5≤M_w≤5.4 Ridgecrest earthquakes. We validate our approach by inspecting the high frequency falloff rate γ, estimated using the stacking spectra for frequency ranging from 15 Hz to 36 Hz. For Mw≥4.0 earthquakes, γ is 2.0±0.20 as expected. For 3.5≤Mw≤3.9 events, γ decreases to 1.8±0.30, correlated with the decrease in SNR ratios. The scaling of σ_a and Δσ_B with magnitude follow the relationship 10^(c×Mw) with c ~ 0.24 for σ_a and 0.30 for Δσ_B. Geometric mean σ_a and Δσ_B for Mw 4.6 events with centroid depths ≥ 6 km are 1.0 MPa and 2.9 MPa, respectively, approximately 4 times larger than the geometric means σ_a and Δσ_B for events of the same magnitude but with centroid depths < 6 km. The ratios of Δσ_B/σ_a increase with magnitude slightly and have a geometric mean of 2.9, suggesting the source spectrum in this magnitude range is close to a Boatwright single-corner spectral model.

Session: Advances in Reliable Earthquake Source Parameter Estimation - III

Type: Oral

Date: 4/16/2025

Presentation Time: 02:45 PM (local time)

Presenting Author: Ralph

Student Presenter: No

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