Shear-wave attenuation (Qβ) in Northwestern Baja California from spectral ratios and inversion analysis
Atenuación de ondas transversales (Qβ) en el noroeste de Baja California a partir de tasas espectrales y análisis de inversión
Luis Munguía1,*, Rogelio Arce1
1 División de Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE). Carretera Ensenada-Tijuana 3918, Zona Playitas, 22860. Ensenada, Baja California, México.
* Corresponding author: (L. Munguía) This email address is being protected from spambots. You need JavaScript enabled to view it.
How to cite this article:
Munguía, L., & Arce, R., (2026). Shear-wave attenuation (Qβ) in Northwestern Baja California from spectral ratios and inversion analysis: Boletín de la Sociedad Geológica Mexicana, 78(2), A121125. https://doi.org/10.18268/BSGM2026v78n2A121125
Manuscript received: July 29, 2025. Corrected manuscript received: October 18, 2025. Manuscript accepted: October 28, 2025.
ABSTRACT
This study presents new estimates of the frequency-dependent shear-wave quality factor (Qβ) for the Peninsular Ranges of Baja California (PRBC), a key segment of the Pacific–North America plate boundary characterized by complex crustal structures and active deformation. Accurate characterization of seismic attenuation is essential, as its proper correction is required to obtain unbiased earthquake source parameters, tomographic images, and ground-motion predictions. The analysis is based on broadband velocity recordings from 16 earthquakes (Mw 4.0–5.2, focal depths 5–15 km) at source-to-site distances of 40–191 km, providing high-quality waveforms with good azimuthal coverage. Two independent methods were applied: (i) joint inversion of multiple-event velocity spectra and (ii) a single-station spectral ratio method using a reference frequency, both based on the geometric mean of the horizontal displacement spectra. The two methods yielded consistent results, with the joint inversion demonstrating greater robustness due to the simultaneous incorporation of multiple events and propagation paths. The joint inversion produced a power-law relationship Qβ = 41.2 (± 0.20) f 0.84(± 0.08) over 2–31 Hz, while the reference-frequency method yielded a nearly identical model, Qβ = 41.3 (± 0.20) f 0.84 (± 0.08), within the 8–20 Hz range where a clear linear trend was observed. The resulting attenuation parameters are consistent with previous models for the region, confirming their reliability and internal consistency. These findings establish robust and regionally representative attenuation constraints for the PRBC, improving understanding of crustal heterogeneity and energy dissipation mechanisms. The results provide quantitative evidence of moderate to high crustal attenuation (Qβ ≈ 250–350 at 10 Hz) and extend the frequency coverage and stability of Qβ estimates. Overall, the proposed model offers reliable attenuation parameters for use in regional seismological and earthquake-engineering applications.
Keywords: Frequency-dependent Qβ, seismic attenuation, spectral ratio method, inversion analysis, northwestern Baja California.