Abstract: The purpose of this work is to determine the quality of the information obtained from the seismological stations that constitute the Broadband Network of the Mexican Servicio Sismológico Nacional (National Seismological Service, SSN). We estimate the orientation of the seismometer installed at each site of the Broadband Network for different epochs; each epoch is defined by changes in digitizer or the seismometer. The correct orientation of the sensors is relevant to many seismological studies that involve rotation or require exact information regarding the orientation of the components. However, orienting a triaxial sensor is not an easy task, even for an experienced field engineer. Several factors can affect it during the installation of a seismometer. Between 2015 and 2017, all the seismometers were reoriented using a gyroscope, 62% of them were misoriented by more than 5°; however, the majority of those were less than 15°. This misorientation corresponds to the last instrumental epoch. To estimate the seismometer orientation during previous periods, we use two methods. The first one is based on the polarization of Rayleigh surface waves (Chael, 1997; Selby, 2001). The second uses a principal component analysis, a linear technique widely used in different branches of physics (Walck and Chael, 1991). The estimations obtained with both methods for the last epoch show high correlation with the measurements with the electronic gyroscope, validating the orientation estimation for the earlier instrumental epochs. Furthermore, we show the effect of sensor misorientation in earthquake location and estimation of receiver functions.
Media has not been submitted for this Presentation
Abstract: We investigate the Lg wave attenuation characteristics for the Western Tibet using a tomographic regionalization method. A total of 378 regional events of magnitudes (Mb) greater than 4.5, and focal depth less than 50 Km, recorded at 52 stations are considered. The Lg wave attenuation coefficient at 1 Hz (LgQ0) is calculated in the group-velocity window 3.6-2.8 Km/s by using the two-station method (TSM). This method considers the spectra from two stations which are (1) aligned approximately with the source and (2) separated enough to permit the use of the standard TSM for Lg Q measurement and also has the advantage to remove the source effect during the analysis. Out of initial 11,767 TSM pairs, 3,371 high-quality inter-station paths have been selected as input for the tomographic inversion. We have observed the lowest LgQ0 (< 50) structure in the western Tibetan Plateau, lower LgQ0 (50-100) values in the Himalayas and higher LgQ0 (100-300) zone in the Lhasa Terrain. We interpret these observations in terms of both intrinsic and scattering effects caused by the presence of partial melting, crustal thickness as well as heterogeneities present below the area.
Media has not been submitted for this Presentation
Abstract: The characterization of high-frequency (>10 Hz) ground motions for hard rock sites in Eastern North America is a critical seismic response issue for major infrastructure, particularly nuclear power plants. The diminution of amplitudes with increasing frequency is modeled using kappa (Anderson and Hough, 1984 BSSA), which is a measure of the slope of amplitude decay at high frequencies in the spectral domain. This study examines kappa and its variability for 7 hard rock sites near Charlevoix, Quebec (Canada), using hundreds of recording from earthquakes of M>3 within 150 km. Kappa values are compared to bedrock velocities measured at the recording stations to gain insight into the relationship between kappa and physical rock properties. We also examine whether there is evidence of source or path effects on kappa.
Media has not been submitted for this Presentation
Abstract: It is a well-known fact that critical structures are required to be designed for the vertical effect of ground motions as well as the horizontal effects. We present a much-needed new model for the spectral ratio of vertical to horizontal component of earthquakes (V/H ratio) for Central and Eastern North America (CENA). The V/H ratio model has the advantage of considering the earthquake magnitude, source to site distance, and shear-wave velocity of soil deposits in the upper 30m of the site for PGA and a wide range of periods (0.001 to 10.0 seconds). The model evaluation will be based on a comprehensive set of regression analysis of the newly compiled Next Generation Attenuation (NGA-East) database of available CENA recordings with M ≥ 3.0 and RRUP < 1000 km. The median value of the geometric mean of the orthogonal horizontal motions rotated through all possible nonredundant rotation angles, known as the GMRotD50 (Boore et al., 2006), will be used along with the vertical component to perform regression using the maximum likelihood method. The earthquakes and recording stations in the Gulf Coast region are excluded due to their different ground-motion attenuation (Dreiling et al., 2014). However, the excluded data are evaluated in a parallel study to see if they can be represented by the same source parameters but different anelastic attenuation. The predicted ratios from the proposed model will be compared with recently published V/H ratio models and can be used to develop the vertical response spectra for CENA sites.
Media has not been submitted for this Presentation
Abstract: The seismic and acoustic activity of the Colima Volcano is studied and monitored by the Seismic Network of the State of Colima (RESCO), which belongs to the University of Colima. Nine broadband stations around the volcano record seismic activity in real time. Additionally, an acoustic sensor installed by the UNAM near the seismic station of Montegrande recorded the most important explosions in January and February 2017. The seismic-acoustic monitoring includes the location of volcano-tectonic events (VTs), explosions, rockfalls, count of events automatically with Hidden Markov Models, evaluation of the seismic and acoustic energy of the explosions, RSEM, etc. In recent years (2013-present), the activity of the volcano has been intense, lava domes growths, lava flows, lava dome collapses, as well as moderate to large explosions. Particularly the explosions in 2017, in their acoustic signals had a characteristic waveform of ‘N’. The seismic energy observed in the explosions had values above 1e + 8 Joules, while that the acoustic energy only presented 5 explosions that exceeded this value. With these values the ratio between acoustic and seismic energy remained mostly less than 1. This indicates that the seismic-acoustic source is probably generated in a long and narrow conduit, likewise, the volcanic columns were charged with ash according to the models of Johnson and Aster (2005). Similarly, the time between the arrival of the seismic wave and the acoustic wave is observed between 15.6 to 19.6 sec, which tells us that there are a small variation in the depth of the source. These explosions are lower in the seismic energy released compared to the 2005 activity.
Media has not been submitted for this Presentation
Abstract: The method widely used for the simulation of seismic wave propagation in three dimensional sedimentary basin is the finite element method combining with viscous-elastic artificial boundary. However, in the finite element method, the boundary surfaces are often limited in regular cases, such as a horizontal free surface, to determine the input wave motion on the artificial boundary. A degenerate model method is proposed in this study for determining the input wave motion on artificial boundary in three dimensional finite element model, in which the solution of the free fields outside and on the lateral artificial boundaries in three dimensional model is given based on four two-dimensional models, and the solution of the free fields outside and on the lateral artificial boundaries for each two-dimensional model is given based on two one-dimensional models. Furthermore, an explicit finite element method with viscous-elastic artificial boundary is established for the simulation of seismic wave propagation in three dimensional site. The numerical results obtained from a free half space model indicates that the proposed input wave motion method is reasonable. In the Mw 7.9 Wenchuan earthquake on 12 May 2008, the Wudu township which is located in a sedimentary basin suffered serious damage even if it is far away from the fault rupture (the distance >100km). In order to interpret the abnormality of damage phenomenon, the elastic wave field simulation in the Wudu Basin is conducted by the proposed method based on the ABAQUS software. The simulation results reveals the basin edge and the basin focusing effect. The topography scatters the body waves and the surface wave generated is trapped at the shallow part of the basin, and most of the energy is reflected from the interfaces of soil strata and focused back into the basin. It results in extraordinary strong shaking patterns in Wudu basin area.
Media has not been submitted for this Presentation
