Raúl R. Castro

Crustal thickness of the Peninsular Ranges and Gulf Extensional Province in the Californias

We estimate crustal thickness along an east-west transect of the Baja California peninsula and Gulf of California, Mexico, and investigate its relationship to surface elevation and crustal extension. We derive Moho depth estimates from P-to-S converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at ;318N latitude. Depth to the Moho is ;33 (63) km near the Pacific coast of Baja California and increases gradually toward the east, reaching a maximum depth of ;40 (64) km beneath the western part of the Peninsular Ranges batholith. The crust then thins rapidly under the topographically high eastern Peninsular Ranges and across the Main Gulf Escarpment. Crustal thickness is ;15-18 (62) km within and on the margins of the Gulf of California. The Moho shallowing beneath the eastern Peninsular Ranges represents an average apparent westward dip of ;258. This range of Moho depths within the Peninsula Ranges, as well as the sharp ;east-west gradient in depth in the eastern part of the range, is in agreement with earlier observations from north of the international border. The Moho depth variations do not correlate with topography of the eastern batholith. These findings suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges and that a local Airy crustal root does not support the highest elevations. We suggest that Moho shallowing under the eastern Peninsular Ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the Gulf Extensional Province that commenced in the late Cenozoic. Support of the eastern Peninsular Ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and/or upper mantle.

S wave attenuation and site effects in the region of Friuli, Italy

We used strong motion records from the 1976 Friuli earthquake (M 6.4) and 10 of the biggest aftershocks recorded by the National Accelerograph Network of the Electrical Power Company of Italy to estimate the quality factor Q of S waves in this region. The wide distance range of the recordings (10 < r < 190 km) permits us to analyze the spectral amplitude decay of the records using a nonparametric approach [e.g., Anderson and Quaas, 1988; Castro et al., 1990; Anderson, 1991]. We obtained attenuation functions for a set of 18 frequencies ranging between 0.4 and 25.0 Hz. The values of Q retrieved from the attenuation functions obtained follow the frequency-dependent relation Q = 20.4f. A test of the method was made using a second data set consisting of digital seismograms from the Friuli-Venezia Giulia Seismograph Network. In spite of the different size of the volume sampled by these data (10 < r < 131 km), the frequency dependence of Q obtained (Q = 16.1f0.92) is similar to that obtained with the strong motion data set. The near-surface attenuation was also estimated using the model proposed by Anderson and Hough [1984] and Anderson [1991]. We found that κ0 is smaller for the strong motion stations located on rock compared to stations located on either shallow or soft sediments. To estimate the site response of the strong motion stations, we corrected the spectral records for the attenuation effect and then inverted the corrected records to separate source and site effects using the inversion scheme proposed by Andrews [1986]. To verify the site amplification estimates obtained, we also calculated the transfer function of each site using Nakamuras [1989] method for S wave [e.g., Lermo and Chavez-Garcia, 1993]. In general, the shapes of the site functions obtained with the inversion are consistent with the transfer functions obtained calculating the horizontal to vertical component ratio.

Site Response of Strong Motion Stations in the Umbria, Central Italy, Region

We used near-field and regional records from 16 earthquakes ( M L 4.6-5.9) located in the epicentral area of the 1997 Umbria-Marche sequence to estimate the site response of 40 strong motion stations. The distribution of hypocenters covers a distance range from 5 to 100 km. We estimated site amplification factors using two techniques, namely a generalized spectral inversion method and horizontal-to-vertical-component spectral ratios (HVSRs) of ground acceleration. For the first approach, we inverted observed spectral amplitudes for site, Q , and source using a reference site. Since the site response estimated using HVSRs assumes that the vertical component of motion is amplification free, we also separated source and site effects by inverting the vertical component of the acceleration spectra. We found that although most of the stations do not show important amplifications on the vertical component, a few of them have a significant vertical amplification at low frequencies ( f Q , representative of the S -wave train, shows a frequency dependence that can be approximated by the relation Q S = 31.2 f 1.2 , between 0.3 and 9.5 Hz. However, at high frequencies ( f > 10 Hz), this dependence weakens, and Q S takes an approximately constant value of 438.

Stochastic Simulation of Strong-Motion Records from the 26 September 1997 (Mw 6), Umbria-Marche (Central Italy) Earthquake

We simulated the strong-motion time histories recorded during the main shock ( M w 6.0) of the Umbria-Marche seismic sequence (Central Italy) of September–October 1997. Ground-motion waveforms were computed using the stochastic modeling technique proposed by Beresnev and Atkinson (1997, 1998) for finite faults. In this approach, the high-frequency amplitudes are simulated as a summation of stochastic point sources. We used the FINSIM code (Beresnev and Atkinson, 1998), which incorporates regional attenuation and frequency-dependent site-amplification factors. We divided the fault plane into 60 elements whose length and width are 1.2 km and 1.5 km, respectively. The resulting subfault corner frequency and rise time are 0.91 Hz and 0.5 sec. We found that the site-amplification functions play an important role in the simulation process, improving the fit to the observed time histories and spectra. The strong-motion waveforms recorded at the Nocera (NOC) station, located at the northern end of the causative fault, show an important directivity effect. Thus, to fit the observed ground motions, we used an inhomogeneous slip distribution, weighting the slip on the fault heavier toward the north. We tested two models: one that simulates a fault rupture with two main slip patches and a second model that ruptures northward with a less heterogeneous slip distribution where slip is mostly concentrated near the rupture nucleation at the southern edge of the fault plane. The simulated low-frequency amplitudes at NOC, however, require an additional frequency-dependent directivity correction (e.g., Bernard et al., 1996). In conclusion, we found that stochastic finite-fault simulations calculated using adequate site amplification functions and crustal attenuation reproduce reasonably well the ground motions from the M w = 6.0 Umbria-Marche earthquake.

New array monitors seismic activity near the Gulf of California in Mexico

The Gulf of California rift forms a geologically young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. Although this is a classical example of a transform-rift plate boundary, the tectonic evolution of the Gulf of California and surrounding regions is complex and poorly understood due to a lack of geological and geophysical data. In 2002, the Network of Autonomously Recording Seismographs(NARS)-Baja network was installed.lt consists of 19 broadband seismic stations deployed in the Baja-California and Sonora provinces of Mexico (Figure 1). Since NARS-Baja surrounds the Gulf of California rift system, it is ideal for constraining earthquake faulting processes and the crust-mantle structure of the region. Moreover, NARS-Baja, in combination with permanent Mexican and U.S. arrays, forms a unique linear array in excess of 4000 km that should lend itself ideally to seismological studies of the North American-Pacific plate boundary on a larger scale. NARS-Baja is planned to operate for at least 5 years. To promote involvement from the entire research community the data collected from the stations will be made available immediately following routine data quality checks.

The spectral decay parameter κ in the region of Umbria-Marche, Italy

We analyze 44 earthquakes (M2.1–4.1) recorded by the Marchesan Seismograph Network during the 1997 Umbria-Marche sequence. We calculated the spectral decay parameter κ [Anderson and Hough, 1984] using 33 events (M2.2–4.1) and ƒc < 10 Hz located in the middle of the rupture area. Although the source-station paths have approximately the same distance (±2.5 km) and azimuth for a given site, the estimates of κ show great variability at all the stations. Even events with small standard deviation on the estimate of κ can have a value 2–3 times different from that estimated for other events located within the 2.5 km. This observation suggests that the attenuation near the source can change significantly in a small distance range. We separated source and path effects by performing a generalized spectral inversion. We dismissed events with source functions having corner frequencies between 10 and 20 Hz, the frequency band where κ was estimated. With the best estimates of κ we calculated the mean for each site, and we observed an apparent azimuthal variation of the attenuation. We used the observed changes of κ to estimate the near-source attenuation, finding that in a distance of 2 km, Q may vary between 16 and 52, depending on the source-station direction. The results of the analysis suggest that the most likely cause of the observed variability of κ is the spatial variation of Q near the seismogenic zone.

Source Characteristics of a 5.5 Magnitude Earthquake that Occurred in the Transform Fault System of the Delfin Basin in the Gulf of California

Portable and permanent broadband seismic stations in the neighborhood of the Gulf of California recorded a moment magnitude Mw 5.5 event on 26 Novem- ber 1997. This is the first time that a moderate event located in the Gulf of California extensional province was well recorded by local broadband seismic stations. The event was located at 29.754 N and 113.708 W and at a focal depth of 5.0 km in the southeastern end of the transform fault that connects the lower and upper Delfin basins. The hypocentral location and the results of the wave modeling indicate that this is a complex event that originated in the pull-apart Delfin basin. The focal mech- anism estimated from first motions ( 310, d 83, k 97) and body-wave modeling of P waves in the frequency band 0.05-0.5 Hz suggests that the rupture started with dip-slip (reverse faulting) motion and ended releasing the bulk of energy through strike-slip motion. Synthetics of surface waves in the frequency band 0.05- 0.1 Hz were also calculated using a triangular source-time function of 3 sec. The best match between the synthetics and observed surface waves recorded at 90 km from the epicenter was obtained using a fault geometry defined by a strike of 330 15, dip 85 5, and slip of 165 15. The spectral analysis of the Lg phase recorded at stations in the Peninsular Ranges gives a seismic moment of 1.28 10 17 N m (1.28 10 24 dyne cm), a source radius of 6.3 km and a stress drop of 0.22 MPa (2.2 bar). The source parameters inferred with S-wave spectra and the same model (Brune, 1970) give similar values.

Attenuation and Site Effects in the Region of Guadeloupe, Lesser Antilles

The accelerograph network of Pointe-a-Pitre operated by the Bureau de Recherches Geologiques et Minieres recorded strong motions from earthquakes located in the northern Lesser Antilles. Twenty-three small to moderate earthquakes ( M 2.9–5.5) were used to estimate S -wave attenuation and site effects. We characterized the spectral amplitude decay with distance at discrete frequencies between 0.6 and 25.0 Hz using a nonparametric approach. The resulting attenuation functions were used to correct the strong-motion records, and then we separated source and site effects using a spectral inversion technique. We compared the site functions obtained with previous estimates reported by Le Brun et al. (2001) using a standard spectral ratio technique and the Nakamura method (1989). We found that the three approaches give similar site-response functions. The site amplification, inferred from the inversion results, is significant at several sites. For instance, the soft soil sites at station GS show a maximum amplification factor of 18 at 1.3 Hz, station SF a factor of 9.5 at 5 Hz, and station T2 a factor of 8.5 at 1.0 Hz. To estimate the quality factor Q of the S waves, we model the empirical attenuation functions using a parametric attenuation function of the form \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[A(f,r)=G(r){\cdot}\mathrm{e}^{-{\pi}fr{/}Q{\beta}},\] \end{document} where f, r, β , and G ( r ) are frequency, hypocentral distance, mean shear-wave velocity, and the geometric spreading function, respectively. We constrain G ( r ) = 10/ r b, finding that the coefficients b and Q are both frequency dependent. In the frequency band analyzed (0.3 < f < 25 Hz) Q takes values between 20 and 500, and b between 0.6 and 2.3.

Regional variations of seismic attenuation of Lg waves in southern Mexico

Attenuation of Lg waves is estimated using 61 events located in the subduction zone of the Middle American trench and recorded by the seismic network of Laguna Verde, Veracruz, Mexico. We estimate the anelastic attenuation coefficient γ of Lg waves or, equivalently, QLg by calculating the rate of acceleration spectra decay with distance. We consider paths from two regions: Guerrero-Veracruz (NE-SW direction) and Oaxaca-Veracruz (North-South direction). Assuming a frequency dependence of γ of the form γ(ƒ) = γ0ƒη, we find that η=0.175±0.05 for both regions and that γ0 was lower for paths from Guerrero to Veracruz (γ0=0.0071±0.002) than for paths from Oaxaca to Veracruz (γ0=0.0161±0.003) in the frequency range from 2 to 7 Hz. In terms of the quality factor QLg, values of QLg= 134±30ƒ0.83±0.2 and QLg=59±10ƒ0.81±0.2 were found for the Guerrero-Veracruz and the Oaxaca-Veracruz paths, respectively. This difference in attenuation may be due to the state of stresses that prevails in both regions including the density and fluid content of fractures, which are attenuation mechanisms also suggested for other regions [Mitchell, 1995].

Site Response of the Dam El Infiernillo, Guerrero-Michoacan, Mexico

We estimated site effects of the earth dam El Infiernillo, located between the states of Guerrero and Michoacan, Mexico. We analyzed 125 strong-motion records from stations located at three different levels of the dam (80, 120, and 180 m) and in the free field. The selected earthquakes are located in the Cocos–North America subduction zone, with magnitudes between 3.8 and 7.9 and epicentral distances that range between 18 and 433 km. We used three methods to estimate the characteristic frequency of vibration ( f n) of the recording sites. First, we calculated spectral ratios between the horizontal and the vertical components of motion (H/V ratios). Second, we estimated f n calculating standard spectral ratios using a free-field station as a reference site. Finally, we made a third estimate using cross-spectral ratios. We found that although the three methods give consistent estimates of f n, on average the H/V ratio gives more stable estimates. The H/V ratios, however, subestimate the site amplification at low frequencies ( f < 8 Hz) at the top and middle sites. We suggest that this result is due to the lateral heterogeneity of the structure that favors wave-field propagation in the vertical direction.