An Attenuation Study of Body Waves in the South-Central Region of the Gulf of California, Mexico

Abstract

We studied the seismic attenuation of body waves in the south-central region of the Gulf of California (GoC) with records from the Network of Autonomously Recording Seismographs of Baja California (NARS-Baja), from the Centro de Investigación Científica y de Educación Superior de Ensenada's Broadband Seismological Network of the GoC (RESBAN), and from the ocean-bottom seismographs (OBS) deployed as part of the Sea of Cortez Ocean Bottom Array experiment (SCOOBA). We examine 27 well-located earthquakes reported in Sumy et al. (2013) that occurred from October 2005 to October 2006 with magnitudes (M w ) between 3.5 and 4.8. We estimated S-wave site effects by calculating horizontal-to-vertical spectral ratios and determined attenuation functions with a nonparametric model by inverting the observed spectral amplitudes of 21 frequencies between 0.13 and 12.59 Hz for the SCOOBA (OBS) stations and 19 frequencies between 0.16 and 7.94 Hz for NARS-Baja and RESBAN stations. We calculated the geometrical spreading and the attenuation (1=Q) factors for two distance intervals (10-120 and 120-220 km, respectively) for each frequency considered. The estimates of Q obtained with the SCOOBA (OBS) records for the interval 10-120 km indicate that the P waves attenuate more than S waves (Q P 34 1:2f 0:820:10 , Q S 59 1:1f 0:900:03 ) for frequencies between 0.6 and 12.6 Hz; whereas for the 120-220 km interval, where ray paths travel deeper, S waves attenuate more than P waves (Q P 117 1:3f 0:440:19 , Q S 51 1:2f 1:120:11 ). The estimates of Q obtained using NARS-Baja and RESBAN records, within 10-120 km, indicate that P waves attenuate more than S waves (Q P 69 1:2f 0:870:16 , Q S 176 1:4f 0:610:26 ) at frequencies between 0.3 and 6.3 Hz; whereas at the 120-220 km distance interval S waves attenuate slightly more than P waves (Q P 39 1:1f 0:640:06 , Q S 48 1:1f 0:370:07 ) at high frequencies (f > 3 Hz). These results, based on a unique OBS dataset, provide an indirect mean to constrain future models of the thermal structure beneath the GoC.

Site Response of the NARS-Baja and RESBAN Broadband Networks of the Gulf of California, México

“…In general, the softer the soil, the higher the amplification is. Attenuation studies in the Gulf of California (Vidales-Basurto et al, 2014) indicate that P waves attenuate considerably more than S waves. Thus, P waves from small magnitude earthquakes disturb the geotechnical characteristics of the soils less than moderate and large magnitude events.…”

Rayleigh wave dispersion measurements reveal low‐velocity zones beneath the new crust in the Gulf of California

“…More specifically, where Q ~ 100, a −6% shear wave velocity anomaly reflects a −117 ° C temperature anomaly (assuming ∂ln Vs /∂ T = −1.54 × 10 −4 K −1 [ Karato , ]). The associated temperature anomaly is expected to be even smaller in the Gulf near the plate boundary, where Q S = 59 at 1 Hz [ Vidales‐Basurto et al ., ]. However, higher Q farther away from the plate boundary, where the LVZs are more frequently found rule out an interpretation of the LVZs based on temperature alone.…”

Three-dimensional shear-wave quality factor, Qs(f), model for south-central Gulf of California, Mexico obtained from inversion of broadband data

“…The Gulf of California (GoC) is relatively young, oblique rift system that formed with the cessation of the Farallon plate subduction under the North American plate about 12 million year ago (Lonsdale, 1989). The evolution of the GoC extensional province occurred in two stages; stage-I: extension and separation of the continents that resulting in the formation of marine basins; stage-II: is being continued till today is the expansion of seafloor and structure of transform faults (Stocks and Hodges 1989;Bennett et al, 2007;Vidales-Basurto et al, 2014). The GoC is comprised a group of stepping faults, seafloor spreading, ridges and the right lateral strike-slip transform faults which collectively form a continental rift system.…”

“…The site response functions determined by Vidales-Basurto et al, (2014) and Avila-Barrientos and Castro (2016) for both the NARS-Baja and the RESBAN broadband stations were used to correct the spectral amplitudes for site effects.…”

“…Later, Ortega and Quintanar (2011) used the offshore earthquake data and compared the propagation characteristics of S waves reported by Ortega and González (2007) with the P wave propagation characteristics following the same method. Vidales-Basurto et al, (2014) has conducted the attenuation study in the south central GoC region to understand the tectonic structure of gulf region. The results obtained based on the different source receiver distance depicts the higher body wave attenuation along the oceanic spreading centres.…”