ALBERT

Description: Displacement time histories from double-integrated accelerograms typically cannot be used to recover near-field terms because of noise in the acceleration traces. To minimize this problem, empirical mode decomposition (EMD) is used to derive a baseline correction scheme. The scheme is tested against several models composed of a single frequency or two frequencies and offsets in acceleration. It is verified against real displacement time history using Global Positioning Systems (GPS) measurement. For single- and double-frequency-content models, obvious discontinuities are found at drift times for lower intrinsic mode function (IMF) components. A drift model, however, can be clearly found from summing higher IMFs and the residuals of simple frequency-content waveform models. On the other hand, results show that the lesser frequency-content signal has the greater decomposed result. Therefore, a suitable corner frequency for low-pass filtering is first implemented to reduce frequency content. In this case, the trend in the given drift model is easily found by summing higher IMFs for a complex frequency content model (model 3 of this study). A suitable corner frequency for low-pass filtering is attained using a grid-search method. A new semiautomatic EMD-derived baseline correction scheme is tested. The corrected coseismic deformation value (CDV, denoted hereafter as from preevent displacement to the final offset during strong motion), peak ground displacement, and displacement time history from this method have good agreement with 1-Hz continuous GPS measurement for the 2011 Tohoku earthquake.

Description: The seismic potential of southern China is associated with the collision between the Indian and the Eurasian plates. This is manifested in the western Sichuan Plateau by several seismically active systems of faults, such as the Longmen Shan. The seismicity observed on the Longmen Shan fault includes recent events with magnitudes of up to 6.5, and the one of 12 May 2008 M (sub w) 7.9 Wenchuan earthquake. Herewith, as part of an ongoing research program, a recently optimized three-dimensional (3D) seismic wave propagation parallel finite-difference code was used to obtain low-frequency (〈 or =0.3 Hz) 3D synthetic seismograms for the Wenchuan earthquake. The code was run on KanBalam (Universidad Nacional Autonoma de Mexico, Mexico) and HECToR (UK National Supercomputing Service) supercomputers. The modeling included the U.S. Geological Survey 40X315 km (super 2) kinematic description of the earthquake's rupture, embedded in a 2400X1600X300 km (super 3) physical domain, spatially discretized at 1 km in the three directions and a temporal discretization of 0.03 s. The compression and shear wave velocities and densities of the geologic structure used were obtained from recently published geophysical studies performed in the Sichuan region. The synthetic seismograms favorably compare with the observed ones for several station sites of the Seismological and Accelerographic Networks of China, such as MZQ, GYA, and TIY, located at about 90, 500, and 1200 km, respectively, from the epicenter of the Wenchuan event. Moreover, the comparisons of synthetic displacements with differential radar interferometry (DinSAR) ground deformation imagery, as well as of maximum velocity synthetic patterns with Mercalli modified intensity isoseist of the 2008 Wenchuan earthquake, are acceptable. 3D visualizations of the propagation of the event were also obtained; they show the source rupture directivity effects of the M (sub w) 7.9 Wenchuan event. Our results partially explain the extensive damage observed on the infrastructure and towns located in the neighborhood of the Wenchuan earthquake rupture zone.

Description: The great 2008 Wenchuan earthquake (12 May 2008) with a moment magnitude of 7.9 and a surface-wave magnitude of 8.0 in Shichuan, China, caused unprecedented loss of human life and widespread severe damage to many types of structures. Thirty-two strong-motion records were obtained within a source distance of 300 km, and three near-source records were obtained within a source distance of 20 km. We present the preliminary results on the characteristics of the near-source records and the strong-motion aspects of this great earthquake. This earthquake may be divided into four subevents, according to the rupture time history and the final slip distribution. Three of the four subevents have large surface fault displacement, and we consider the subevents that generated the three near-source records as surface-rupture earthquakes, supported by the comparisons made to records from other surface-rupture or buried-fault earthquakes. One station recorded strong ground motions from two subevents in two well-separated time windows, and this allows us to examine the effect of earthquake parameters for each of the subevents. We find that, in the spectral period range of 0.5-2 s, the response spectra of the near-source records from the Wenchuan earthquake are significantly less than those of buried-fault earthquakes, such as the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake that have a much smaller moment magnitude than the Wenchuan earthquake. In the fault-normal direction the displacement spectra at long period for the closest station are similar to those of the Lucerne record from the 1992 Landers earthquake but significantly smaller than those of the TCU052 and TCU068 records from the 1999 Chi-Chi, Taiwan, earthquake. At short and intermediate period, the near-source spectra are much larger than the design spectra in the previous version of the Chinese design code for the heavily damaged area, but they are comparable at long spectral periods.

Description: Seismic instruments record horizontal ground motion into two orthogonal components, and the orientations of the components are dependent on the installed sensors. Ground-motion measures directly derived from the two components vary with the installation orientation, which will influence the result of the seismic design of structures and the variance of ground-motion prediction equations. Various orientation-independent measures have been proposed recently. We provide a thorough analysis of these measures and propose a new fast approach to compute most of them. The approach is based on the projected envelope algorithm that generates the omni-spectrum of horizontal ground motion, and its computation speed is faster than previous methods for this purpose. Visualizing the omni-spectrum helps to identify the maximum demand orientation and to capture effects of directionality. Moreover, most previous orientation-independent measures can be represented concisely in the functional form of the omni-spectrum.

Description: High-precision modeling of seismic-wave propagation in heterogeneous media is very important to seismological investigation. However, such modeling is one of the difficult problems in the seismological research fields. For developing methods of seismic inversion and high-resolution seismic-wave imaging, the modeling problem must be solved as perfectly as possible. Moreover, for long-term computations of seismic waves (e.g., Earth's free-oscillations modeling and seismic noise-propagation modeling), the capability of seismic modeling methods for long-time simulations is in great demand. In this paper, an alternative method for accurately and efficiently modeling seismic wave fields is presented; it is based on a multisymplectic discrete singular convolution differentiator scheme (MDSCD). This approach uses optimization and truncation to form a localized operator, which preserves the fine structure of the wave field in complex media and avoids noncausal interaction when parameter discontinuities are present in the medium. The approach presented has a structure-preserving property, which is suitable for treating questions of high-precision or long-time numerical simulations. Our numerical results indicate that this method can suppress numerical dispersion and allow for research into long-time numerical simulations of wave fields. These numerical results also show that the MDSCD method can effectively capture the inner interface without any special treatment at the discontinuity.

Description: We compared response spectra from the M (sub w) 7.9 2008 Wenchuan earthquake with five modern ground-motion prediction equations (GMPEs). Ninety-three strong-motion records within 300 km of the fault plane were selected for comparison with the GMPE models of Zhao, Zhang et al. (2006), Abrahamson and Silva (2008), Boore and Atkinson (2008), Campbell and Bozorgnia (2008) and Chiou and Youngs (2008) for spectral periods up to 5.0 s. The site class of the recording stations used for the Zhao, Zhang et al. (2006) model was inferred from response spectral ratios of the horizontal and vertical components (H/V) computed from the strong-motion records in moving and overlapping time windows. The average shear-wave velocity of the top 30 m (V (sub S30) ) was only available for two stations. V (sub S30) was extrapolated from the average of the top 20 m (V (sub S20) ) when possible and inferred from the H/V response spectral ratios when necessary. The average predictions of all models were acceptable. The Zhao, Zhang et al. (2006) model gave the best predictions for peak ground acceleration and short spectral periods, especially up to 100 km of the source distance. All Next Generation Attenuation (NGA) models predicted the recorded spectra very well for periods of 0.5-1.0 s and at 5.0 s. The Chiou and Youngs (2008) model gave the best overall predictions. The standard deviations of all attenuation models were similar at a 5% significance level. However, differences between spectra estimated by various NGA models were statistically and practically significant, with the largest difference between the average predictions being nearly a factor of 1.4 at the 0.1-s period and 2.3 at the 5.0-s period for data within a source distance of 100 km. Although one earthquake did not produce median ground motions that the GMPEs are designed to predict, such a large difference represents a challenge for empirical models when estimating spectra from very large crustal earthquakes.

Description: A large number of strong-motion records were obtained by the National Strong-Motion Observation Network System (NSMONS) of China for the Wenchuan earthquake. There are 1350 components of strong-motion records from the mainshock at 455 stations. In these stations, 164 stations are located in the range of rupture distance within 400 km and 37 stations are within 100 km. The largest peak ground acceleration is 957.8 Gal from the Wolong station in the hanging wall area with rupture distance of 23 km. The records from 164 stations in the range of rupture distance within 400 km are used in this study to investigate the hanging wall/footwall effects, directivity effects, and attenuation characteristics of ground motions from the Wenchuan earthquake. The characteristics of ground motion attenuations are compared in four subareas: hanging wall area, footwall area, forward directivity area, and backward directivity area. The study results show a clear hanging wall/footwall effect in near-fault ground motions within a rupture distance of 40 km, but it appears only in the peak ground acceleration and components at short periods below 1.0 s, and a strong forward directivity effect appears in ground motions over the whole range of rupture distance and period. Furthermore, the fitting attenuation curves of the peak ground acceleration and spectral accelerations of ground motions from the Wenchuan earthquake are compared with the predicted curves by an empirical attenuation model in the hanging wall and footwall areas.

Description: Broadband seismic data from the regional seismic network operated by the China Earthquake Administration and 32 temporary seismic stations are used to image the crustal velocity structure in the northeast Tibetan plateau. Empirical Rayleigh- and Love-wave Green"s functions are obtained from interstation cross correlation of continuous seismic records. Group velocity dispersion curves for Rayleigh and Love waves between 10 and 50 s are obtained using the multiple-filter analysis method with phase-matched processing. The group velocity variations of Rayleigh and Love waves overall correlate well with the major geologic structures and tectonic units in the study region. Shear-wave velocity structures were then inverted from Rayleigh- and Love-wave dispersion maps. The results show that the Songpan-Ganzi terrane is associated with a low velocity at depth greater than 20 km. The northern Qilian orogen, with higher elevation and thicker crust compared to the southern Qilian orogen, is also dominated by low velocity at depth greater than approximately 25 km. However, there is no clear evidence of the low-velocity mid-to-lower crust beneath the southern Qilian orogen as the crustal flow model predicts. The low-velocity zone (LVZ) beneath the northern Qilian orogen may suggest that the crustal thickening and surface uplift of the northern Qilian orogen are related to the LVZ, and the LVZ may be considered as an intracrustal response to bear the ongoing deformation in the northern Qilian orogen.

Description: We construct a coseismic deformation interferogram for the April 2010 Yushu earthquakes using ALOS/PALSAR data from the ascending track (path 487). We then infer the trace of the Yushu fault using the coherence image, and we build five fault models for the Yushu fault. To determine the fault geometry parameters that give the best fit to the coseismic interferogram, we apply an elastic dislocation algorithm. Our preferred fault model consists of two faults dipping to the northeast. One strikes approximately N60 degrees W with a dip of 82 degrees ; the other strikes approximately N67 degrees W with a dip of 86 degrees . Lastly, we infer the coseismic slip distributions of the Yushu earthquakes by the inversion of the displacement in the line of sight (LOS). The results show that three high-slip concentrations are located at a depth of 5 approximately 8 km, with a peak slip of 1.32 m at (96.93 E, 33.03 N). The Yushu fault is a left-lateral strike-slip faulting with small north-side-up, dip-slip components.

Description: The collision of the Indian and Asian plates since about 55 Ma has created the largest plateau with the thickest crust on Earth. There is, however, no general agreement on the modes of the crustal thickening. The crustal thickness of the Tibetan plateau still remains poorly determined. It is generally accepted that the Tibetan crust has roughly double normal thickness and that it thins somewhat toward the north, but individual observations of Moho depth vary spatially and for different techniques at the same place by greater than 20 km. In this work we compare P and S receiver functions at station LSA, located in the southern Lhasa terrane, to determine the crustal thickness beneath the station. A doublet Moho structure with two significant interfaces at about 60 and 80 km depth in the lower crust is seen clearly in the P receiver functions. The deeper phase (Moho) is, however, absent in the S receiver function data. Here we model the observed P and S receiver functions by a strong Moho topography that dips to the east at an angle of 30 degrees . This result may indicate that the Moho structure beneath Tibet can be very complicated and has strong lateral variations and suggests that a detailed map of Moho depth is only possible with 2D dense seismic experiments. The Moho dips locally perpendicular to the direction of the Indian plate motion, suggesting that the lower crustal deformation is decoupled from the underlying Indian mantle lithosphere. The extremely strong Moho variation beneath and east of station LSA may also imply a crust-mantle interaction, specifically delamination or foundering of lower crust down to the upper mantle.