ALBERT

Description: 〈span〉〈div〉ABSTRACT〈/div〉To see any change in seismic velocities that may be associated with an abrupt change in the regional geology (granitic rock in contact with sediments), we conducted a refraction seismic study in the Peninsular Ranges of Baja California, which is in the Mexico–southwestern Laguna Salada (LS) region. We installed 30 three‐component portable seismic stations, supplemented with two permanent six‐component stations of the Northwest Mexico Seismic Network (RESNOM). The stations, spaced ∼6  km along a refraction profile, recorded two blasts; these were the direct shot located to the south of the city of Ensenada and the reverse shot in the southwestern LS (southwest–northeast direction). Record sections show seismograms with impulsive 〈span〉P〈/span〉 arrivals at nearby stations. Rays from the two blasts were modeled (using asymptotic ray theory) to obtain a 〈span〉P〈/span〉‐wave velocity model from 0 to ∼15  km depth along the refraction profile. Our modeling results are as follows: in the southwestern part of the profile (0–25 km distance), a low‐velocity zone of ∼2  km/s exists between the depths of 0 and 3.5 km; in Sierra Juárez, the mean 〈span〉P〈/span〉‐wave velocity is ∼5.6  km/s between the depths of 0 and 5 km; and in southwestern LS, a low‐velocity layer of ∼2.5  km/s exists between the depths of 0 and ∼3  km. We also modeled a layer of ∼6.5  km/s between 4 and 12 km in the Ensenada–Ojos Negros region, and between the depths of 4 and 8 km below the southwestern LS. From a profile distance of 0 to 50 km, a velocity zone of ∼6.7  km/s appears between the depths of 12 and 15 km.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉To see any change in seismic velocities that may be associated with an abrupt change in the regional geology (granitic rock in contact with sediments), we conducted a refraction seismic study in the Peninsular Ranges of Baja California, which is in the Mexico–southwestern Laguna Salada (LS) region. We installed 30 three‐component portable seismic stations, supplemented with two permanent six‐component stations of the Northwest Mexico Seismic Network (RESNOM). The stations, spaced ∼6  km along a refraction profile, recorded two blasts; these were the direct shot located to the south of the city of Ensenada and the reverse shot in the southwestern LS (southwest–northeast direction). Record sections show seismograms with impulsive 〈span〉P〈/span〉 arrivals at nearby stations. Rays from the two blasts were modeled (using asymptotic ray theory) to obtain a 〈span〉P〈/span〉‐wave velocity model from 0 to ∼15  km depth along the refraction profile. Our modeling results are as follows: in the southwestern part of the profile (0–25 km distance), a low‐velocity zone of ∼2  km/s exists between the depths of 0 and 3.5 km; in Sierra Juárez, the mean 〈span〉P〈/span〉‐wave velocity is ∼5.6  km/s between the depths of 0 and 5 km; and in southwestern LS, a low‐velocity layer of ∼2.5  km/s exists between the depths of 0 and ∼3  km. We also modeled a layer of ∼6.5  km/s between 4 and 12 km in the Ensenada–Ojos Negros region, and between the depths of 4 and 8 km below the southwestern LS. From a profile distance of 0 to 50 km, a velocity zone of ∼6.7  km/s appears between the depths of 12 and 15 km.〈/span〉

Description: In this article, we implement a web questionnaire (available in Spanish at the www.fct.uanl.mx , last accessed March 2014, as Encuesta sismo ) to gather citizens’ observations on felt earthquakes in an area of northeastern Mexico. We generated a seismic intensity map due to the fact that some events from the October 2013 to March 2014 seismic sequence (1.9≤ M c ≤4.5) were recorded and felt in the region. This sequence occurred in the central area of the state of Nuevo León. The data obtained from this questionnaire and by interviewing people living near the epicentral area are useful for a rapid postearthquake evaluation. This is important because the maximum intensity values were of V–VI near the epicentral area. Finally, the attenuation curves obtained from our study proved to be similar to those for central and eastern United States.

Description: An extensive data set of teleseismic and strong-motion waveforms and geodetic offsets is used to study the rupture history of the 2008 Wenchuan, China, earthquake. A linear multiple-time-window approach is used to parameterize the rupture. Because of the complexity of the Wenchuan faulting, three separate planes are used to represent the rupturing surfaces. This earthquake clearly demonstrates the strengths and limitations of geodetic, teleseismic, and strong-motion data sets. Geodetic data (static offsets) are valuable for determining the distribution of shallower slip but are insensitive to deeper faulting and reveal nothing about the timing of slip. Teleseismic data in the distance range 30°–90° generally involve no modeling difficulties because of simple ray paths and can distinguish shallow from deep slip. Teleseismic data, however, cannot distinguish between different slip scenarios when multiple fault planes are involved because steep takeoff angles lead to ambiguity in timing. Local strong-motion data, on the other hand, are ideal for determining the direction of rupture from directivity but can easily be over modeled with inaccurate Green’s functions, leading to misinterpretation of the slip distribution. We show that all three data sets are required to give an accurate description of the Wenchuan rupture. The moment is estimated to be approximately with the slip characterized by multiple large patches with slips up to 10 m. Rupture initiates on the southern end of the Pengguan fault and proceeds unilaterally to the northeast. Upon reaching the cross-cutting Xiaoyudong fault, rupture of the adjacent Beichuan fault starts at this juncture and proceeds bilaterally to the northeast and southwest.

Description: Within the framework of performance-based earthquake engineering, the intensity-measure approach (IMA) has become the standard option for the characterization of engineering-demand parameters (EDPs) for systems undergoing significant inelastic behavior. Within this approach, the rates of exceedance of the EDPs are computed from a hazard curve corresponding to an intensity measure (IM; usually the spectral acceleration at the first mode period S A ( T 1 )) and the conditional probability density function (CPDF) of the EDP given the IM. In view of the lack of strong ground motion records associated with large values of currently used IMs, the parameters required to establish a CPDF are obtained from an incremental dynamic analysis that considers the S A ( T 1 ) linear scaling of motions recorded during seismic events of moderate intensity. However, from a seismological perspective, the linearly scaling method is too simple and may lead to unrealistic ground-motion records that may affect the accuracy of the IMA. The dynamic response of single-degree-of-freedom systems subjected to simulated ground motions is analyzed in order to assess the limitations of the S A ( T 1 ) linearly scaling method. These simulated ground motions were obtained via a stochastic simulation technique that has a solid seismological basis. The results presented herein are useful to numerically understand the limitations of the S A ( T 1 ) linearly scaling method and to identify situations in which a more sophisticated analysis is warranted.

Description: Seven slip models currently available from kinematic inversions, derived from near-source strong-motion and teleseismic body waves in the 0–1.25 Hz frequency range from Mexico’s subduction zone, are used to estimate source-scaling relationships applicable to the region. Our results are compared with existing scaling relations for subduction environments. The relationships for the rupture area of our results are closer to those of Somerville et al. (2002) than to any other, but, like the others, they have smaller areas than predicted by Somerville et al. (2002) . Concerning the combined area of asperities, Murotani et al. (2008) and our results predict smaller areas than those obtained by Somerville et al. (2002) . Concerning the area of largest asperity, the relationships obtained in this study are slightly smaller than those described by Somerville et al. (2002) ; this is a consistent result with the relationships of total rupture area and combined area of asperities. In general, the error estimates for the constrained equations derived in this study in all cases are smaller than those relationships compared here. This might suggest that the expressions obtained in this study could be appropriate for the simulations of strong ground motion for a specific scenario of earthquake slip in the region. Also, these results could be an indication that the relationships vary depending on a specific subduction tectonic region. On the other hand, Aguirre and Irikura (2007) estimated the source area for 31 Mexican earthquakes using corner frequencies; these areas show close resemblance with those predicted by the relationships derived in this study. Based on these findings, an important implication is that two different methodologies to determine the total area of asperities based on either low- or high-frequency data generate similar results. Online Material: Figures of fault models.

Description: The 2006 Kiholo Bay, Hawaii, earthquake triggered high concentrations of rock falls and slides in the steep canyons of the Kohala Mountains along the north coast of Hawaii. Within these mountains and canyons a complex distribution of landslides was triggered by the earthquake shaking. In parts of the area, landslides were preferentially located on east-facing slopes, whereas in other parts of the canyons no systematic pattern prevailed with respect to slope aspect or vertical position on the slopes. The geology within the canyons is homogeneous, so we hypothesize that the variable landslide distribution is the result of localized variation in ground shaking; therefore, we used a state-of-the-art, high-resolution ground-motion simulation model to see if it could reproduce the landslide-distribution patterns. We used a 3D finite-element analysis to model earthquake shaking using a 10 m digital elevation model and slip on a finite-fault model constructed from teleseismic records of the mainshock. Ground velocity time histories were calculated up to a frequency of 5 Hz. Dynamic shear strain also was calculated and compared with the landslide distribution. Results were mixed for the velocity simulations, with some areas showing correlation of landslide locations with peak modeled ground motions but many other areas showing no such correlation. Results were much improved for the comparison with dynamic shear strain. This suggests that (1) rock falls and slides are possibly triggered by higher frequency ground motions (velocities) than those in our simulations, (2) the ground-motion velocity model needs more refinement, or (3) dynamic shear strain may be a more fundamental measurement of the decoupling process of slope materials during seismic shaking.

Description: 〈span〉〈div〉Abstract〈/div〉We performed a seismic noise‐level analysis from records of seismic stations located in northern Baja California, Mexico. We used data from stations of the Northwest Mexico Seismic Network, with the goal to characterize the noise spectrum for each station as a function of time. The seismic stations are located in the sedimentary environment of the Mexicali Valley (MV) and the granitic Peninsular Ranges of Baja California (PRBC). The ambient seismic noise was characterized using the power spectral density (PSD) technique (〈a href="https://pubs.geoscienceworld.org/bssa#rf12"〉Peterson, 1993〈/a〉). We found that, at periods between 0.1 and 2 s, the median noise levels (MNLs) of stations in the MV are up to 25 dB higher, and near the new high‐noise model (NHNM), than MNLs of PRBC stations. For periods 〉2  s, the MNLs are similar for both regions and are between the NHNM and new low‐noise models. We found differences in the noise levels when seismic sensors (different brands but same bandwidth) were interchanged. For periods 〈1  s, the MNLs computed from records of Güralp sensors are ∼20  dB higher than MNLs from Nanometrics sensors; for periods 〉10  s the MNLs of Nanometrics sensors are ∼20  dB higher than the MNLs from Güralp sensors. We observed daily variations in short‐period noise, related to human activity, such as higher noise levels for periods 〈1  s at daylight in a station in the city of Mexicali. No influence of variations of the sea level of the Pacific Ocean on the PSD of stations of north Baja California was observed. At least for two sites, in and south of MV, there is a direct relationship among variations of pressure and temperature with seismic noise: high pressure and low temperature are related with high‐noise levels in the 4.0–8.5 s period band and vice versa.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We performed a seismic noise‐level analysis from records of seismic stations located in northern Baja California, Mexico. We used data from stations of the Northwest Mexico Seismic Network, with the goal to characterize the noise spectrum for each station as a function of time. The seismic stations are located in the sedimentary environment of the Mexicali Valley (MV) and the granitic Peninsular Ranges of Baja California (PRBC). The ambient seismic noise was characterized using the power spectral density (PSD) technique (〈a href="https://pubs.geoscienceworld.org/bssa#rf12"〉Peterson, 1993〈/a〉). We found that, at periods between 0.1 and 2 s, the median noise levels (MNLs) of stations in the MV are up to 25 dB higher, and near the new high‐noise model (NHNM), than MNLs of PRBC stations. For periods 〉2  s, the MNLs are similar for both regions and are between the NHNM and new low‐noise models. We found differences in the noise levels when seismic sensors (different brands but same bandwidth) were interchanged. For periods 〈1  s, the MNLs computed from records of Güralp sensors are ∼20  dB higher than MNLs from Nanometrics sensors; for periods 〉10  s the MNLs of Nanometrics sensors are ∼20  dB higher than the MNLs from Güralp sensors. We observed daily variations in short‐period noise, related to human activity, such as higher noise levels for periods 〈1  s at daylight in a station in the city of Mexicali. No influence of variations of the sea level of the Pacific Ocean on the PSD of stations of north Baja California was observed. At least for two sites, in and south of MV, there is a direct relationship among variations of pressure and temperature with seismic noise: high pressure and low temperature are related with high‐noise levels in the 4.0–8.5 s period band and vice versa.〈/span〉