Spectral decay parameter (κ) and seismic quality factor (Qβ) in northern Baja California, Mexico

Parámetro de decaimiento espectral (κ) y factor de calidad sísmica (Qβ) en el norte de Baja California, México

Luis Munguía1,*

1 División de Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada, B. C., Carretera Ensenada-Tijuana No. 3918, Zona Playitas, C. P. 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. (2026). Spectral decay parameter (κ) and seismic quality factor (Q β) in northern Baja California, Mexico: Boletín de la Sociedad Geológica Mexicana, 78(2), A111025. https://doi.org/10.18268/BSGM2026v78n2A111025 

Manuscript received: June 27, 2025.  Corrected manuscript received: September 23, 2025.  Manuscript accepted: October 6, 2025. 

ABSTRACT

Understanding the spectral attenuation parameter κ and the seismic quality factor Q β is essential for seismology and earthquake engineering. This study estimates both parameters using seismic data from northern Baja California, including earthquakes of magnitudes 4.0 to 5.8 recorded within 140 km of the epicenters at sites with diverse geological conditions. Kappa was independently estimated from horizontal acceleration spectra recorded at eight sedimentary and seven rock sites. It was derived from the slope of the high-frequency amplitude decay in smoothed spectra, using frequency bands between 25 and 42 Hz.

A linearly increasing trend of κ with distance was observed at both rock and alluvium sites, with average slopes of 0.00025 s/km and 0.00016 s/km, respectively. The zero-distance intercepts of κ (κ₀) were 0.0200 s for rock and 0.0367 s for sediment. A frequency-dependent Q β model for the Mexicali Valley was also developed using S-wave spectra from eight earthquakes. Shear-wave attenuation was modeled with the factor exp (−πf[R/(Q β(f) β) + κ₀]), which incorporates the site-specific κ₀ = 0.0367 s, estimated in this study to account for high-frequency decay near the surface. Q β was initially estimated through individual event inversions and subsequently through a joint inversion of the spectra from all eight earthquakes. In both stages, Q β was estimated separately from the spectra of each horizontal component, as well as from their averaged spectra. 

All inversions utilized 20 discrete frequencies spanning the 2–40 Hz range. The first stage yielded an average Qβ = 16.4 f 1.0, while the second produced Qβ= 24.1 f 0.98. Although both results are consistent, greater confidence is placed in the model obtained from the joint inversion of all events. The κ and Q β( f) estimates obtained in this study are essential for improving strong ground motion predictions and refining regional seismic hazard assessments.

Keywords: attenuation, spectral decay parameter κ, quality factor Q β, Baja California.