Ground Motions In Sedimentary Basins Research Articles

Comparative analysis of Rayleigh and Love waves detected propagating in the Nobi and Kanto basins during the 2004-, 2007- Chuetsu and 2011 Tohoku earthquakes

Propagation of long-period ground motion in sedimentary basins has been a subject of great interest among seismologists and engineers. Intense long-period ground motions consist primarily of surface waves that get trapped or generated locally as seismic energy travels through sedimentary deposits. In the present work, we investigate the propagation of surface waves in the basins of Kanto and Nobi in Japan, during three relatively recent events: The Mw 6.6 2004 Niigata Chuetsu, the Mw 6.6 2007 Chuetsu-Oki and the Mw 9.0 2011 Tohoku earthquakes. We identify the surface waves using a signal processing technique that detects their polarization characteristics, in the time-frequency space, using orthogonality relations among phase vectors. Then, by applying the “Normalized Inner Product” (NIP), regions of a particular type of polarization are delineated and filters are applied to isolate the associated surface waves, along with their direction of propagation. With our investigation, we attempt to follow the ‘flow’ of seismic energy as it approaches just outside the basins, and then how it evolves once inside the basin. Our analysis shows that the long period (<0.1 Hz) surface wave energy approaching the Kanto basin during the 2011 Tohoku earthquake consists of Rayleigh waves, and that part of the seismic energy is converted to Love waves. In a higher frequency range (0.1–0.5 Hz), prograde Rayleigh and Love waves were detected in selected areas such as the Chiba sub-basin and the Tokyo lowlands. Regarding the Nobi basin, we find that whereas the Rayleigh waves in the frequency range (0.1–0.5 Hz) radiated during the 2011 Tohoku earthquake strongly interact with the basin, the Rayleigh waves radiated by the Chuetsu events appear to propagate through the basin unaffected. This difference in basin response is attributed to the different azimuthal direction of incidence of the surface wave energy.

3-D numerical simulations of earthquake ground motion in sedimentary basins: testing accuracy through stringent models

SUMMARY Differences between 3-D numerical predictions of earthquake ground motion in the Mygdonian basin near Thessaloniki, Greece, led us to define four canonical stringent models derived from the complex realistic 3-D model of the Mygdonian basin. Sediments atop an elastic bedrock are modelled in the 1D-sharp and 1D-smooth models using three homogeneous layers and smooth velocity distribution, respectively. The 2D-sharp and 2D-smooth models are extensions of the 1-D models to an asymmetric sedimentary valley. In all cases, 3-D wavefields include strongly dispersive surface waves in the sediments. We compared simulations by the Fourier pseudo-spectral method (FPSM), the Legendre spectral-element method (SEM) and two formulations of the finite-difference method (FDM-S and FDM-C) up to 4Hz. The accuracy of individual solutions and level of agreement between solutions vary with type of seismic waves and depend on the smoothness of the velocity model. The level of accuracy is high for the body waves in all solutions. However, it strongly depends on the discrete representation of the material interfaces (at which material parameters change discontinuously) for the surface waves in the sharp models. An improper discrete representation of the interfaces can cause inaccurate numerical modelling of surface waves. For all the numerical methods considered , except SEM with mesh of elements following the interfaces, a proper implementation of interfaces requires definition of an effective medium consistent with the interface boundary conditions. An orthorhombic effective medium is shown to significantly improve accuracy and preserve the computational efficiency of modelling. The conclusions drawn from the analysis of the results of the canonical cases greatly help to explain differences between numerical predictions of ground motion in realistic models of the Mygdonian basin. We recommend that any numerical method and code that is intended for numerical prediction of earthquake ground motion should be verified through stringent models that would make it possible to test the most important aspects of accuracy.

Radiation and Generation of Short- and Long-Period Ground Motions from the 2011 Off Tohoku, Japan,Mw9.0 Earthquake

Ground motion from theMw9.0 March 11, 2011, Off-Tohoku earthquake recorded by dense seismic networks in Japan, K-NET and KiK-net, clearly demonstrated the high-frequency seismic wavefield radiating from the earthquake source and developing longperiod ground motion in sedimentary basins. The photographic sequence of the visualized wavefield demonstrated the process in which the high-frequency seismic waves radiated from large slips at the top of the subducting Pacific Plate at relatively deeper depth of 25-50 km, which caused multiple large shocks of large (&gt;1000-2000 cm/s2) ground acceleration and several minutes lasting ground motions over a wide area along the Pacific Ocean side of northern Japan. An efficient seismic wave propagation along the subducting Pacific slab and ground motion amplification in a superficial thin low-velocity layer overlying rigid bedrock also enhanced high-frequency (&gt;5 Hz) ground motions very drastically. However, the dominant frequency of the strong ground motion recorded in nearfield station was too high such as to cause serious damage to wooden-frame residences having relatively longer-period resonance period (T= 1-2 s); The velocity response in this frequency band was only about one third to one half of those observed in severely damaged area during the destructiveMw6.9 1995 Kobe earthquake. The 2011 Off-Tohoku earthquake also produced long-period ground motion in sedimentary basins such those at Tokyo’s population center but observation of the long-period ground motion withinT=6-8 s was rather weak and of a level comparable to that of anM7.9 Tonankai earthquake occurring along the Nankai Trough in 1944. This was because the surface wave in this period band was not generated efficiently by the relatively deeper slip over the source fault of the Off-Tohoku earthquake.

Geometrical focusing as a mechanism for significant amplification of ground motion in sedimentary basins: analytical and numerical study

We study the geometrical and material conditions which lead to focusing of seismic waves traveling across a concave velocity interface representing the boundary of a sedimentary basin within a denser rock. We approximate, using geometrical analysis for plane-waves, the combination of interface eccentricities and velocity ratios for which the seismic rays converge to a near surface region of the basin. 2-D finite difference modeling is used to compute Peak Ground Velocity (PGV) and spectral amplification across the basin. We show that effective geometrical focusing occurs for a narrow set of eccentricities and velocity ratios, where seismic energy is converged to a region of \(\pm \)0.5 km from surface. This mechanism leads to significant amplification of PGV at the center of the basin, up to a factor of 3; frequencies of the modeled spectrum are amplified up to the corner frequency of the source. Finally, we suggest a practical method for evaluating the potential for effective geometrical focusing in sedimentary basins.

Control Factors of Spatial Variation of Long-Period Strong Ground Motions in the Yufutsu Sedimentary Basin, Hokkaido, during the Mw 8.0 2003 Tokachi-oki, Japan, Earthquake

We successfully reproduced important features of long-period (7 to 8 sec), large-amplitude ground motions observed in the Yufutsu basin, Hokkaido, during the M w 8.0 2003 Tokachi-oki, Japan, earthquake through numerical simulation of wave propagation. Recordings from the dense nationwide strong ground motion networks in Japan demonstrated significant spatial variation of long-period strong ground motions within the Yufutsu basin and revealed that the long-period shaking in the Tomakomai western port was the strongest of all the sites within the basin. In interpreting or predicting long-period strong ground motions in sedimentary basins, the importance of deep basin structures, such as the depth of bedrock with an S -wave velocity over 3 km/sec has been emphasized. However, the characteristics of the underground structure model of the Yufutsu basin that explain the observations indicate that thickness of near-surface (

An analysis of the infrasound signal from the Miyagi-Oki earthquake in Japan on 16 August 2005

AbstractFollowing the 16 August 2005 Miyagi-Oki earthquake in Japan, coherent atmospheric infrasonic waves were observed at regional distances (1200–1500 km) using three seismo-acoustic arrays on the Korean Peninsula. A source-location procedure was applied to the distinct long-duration infrasonic signals to construct earthquakegenerated infrasound source regions on the Japanese island arc. The results showed that the long-duration infrasonic signal was attributable to extensive seismic ground motions on land areas from the southwestern through to the northeastern part of the island arc as well as regions close to the earthquake epicenter. In many coherent infrasonic signals, an effect of seismic ground motions in sedimentary basins could be identified as a source of infrasound radiation from the large earthquake. These observations and interpretations were confirmed using predictions of possible infrasound arrival azimuth variation by converting real seismological data from the dense Japanese seismic network.

Comment on “A hybrid method for the estimation of ground motion in sedimentary basins: Quantitative modeling for Mexico City” by D. Fäh, P. Suhadolc, St. Mueller, and G. F. Panza

Abstract Two possible extensions of the commented article are suggested, with the aim to more efficiently couple the source and path simulation on one side, and the site-effect simulation on the other: (1) New excitation lines, surrounding the sedimentary basin, reduce the necessary size of the 2D finite-difference model of the site. They also better represent the 3D nature of the excitation, including the focal-mechanism effect. (2) New (2.5D approximate) equations of motion improve the way the 3D excitation is propagated within the 2D site model. The two suggested improvements can help in solving problems when a purely 2D treatment is oversimplified, but a true 3D treatment is too expensive.

A hybrid method for the estimation of ground motion in sedimentary basins: Quantitative modeling for Mexico city

To estimate the ground motion in two-dimensional (2D), laterally heterogeneous, anelastic media, a hybrid technique has been developed that combines modal summation and the finite-difference method. In the calculation of the local wave field owing to a seismic event, both for small and large epicentral distances, it is possible to take into account the source, path, and local soil effects. As a practical application, we have simulated the ground motion in Mexico City caused by the Michoacan earthquake of September 19, 1985. By studying the one-dimensional (1D) response of the two sedimentary layers present in Mexico City, it is possible to explain the difference in amplitudes observed between records for receivers inside and outside the lake-bed zone. These simple models show that the sedimentary cover produces the concentration of high-frequency waves (0.2 to 0.5 Hz) on the horizontal components of motion. The large amplitude coda of ground motion observed inside the lake-bed zone and the spectral ratios between signals observed inside and outside the lake-bed zone can only be explained by 2D models of the sedimentary basin. In such models, the ground motion is mainly controlled by the response of the uppermost clay layer. The synthetic signals explain the major characteristics (relative amplitudes, spectral ratios, and frequency content) of the observed ground motion. The large amplitude coda of the ground motion observed in the lake-bed zone can be explained as resonance effects and the excitation of local surface waves in the laterally heterogeneous clay layer. Also, for the 1985 Michoacan event, the energy contributions of the three subevents are important to explain the observed durations.

Variability of seismic ground motion in complex media: the case of a sedimentary basin in the Friuli (Italy) area

A computational hybrid technique is presented to estimate ground motion in complex two-dimensional, anelastic media. The technique combines modal summation and finite-difference methods, and it can take into account the source, path and local soil effects to calculate the local wavefield due to a seismic event. The hybrid technique is applied to study wave propagation in a sedimentary basin in the Friuli region during the September 11, 1976, Friuli aftershock (16h 35m 04s). Special emphasis is given to the understanding of the different features of ground motion in sedimentary basins. The most important effects that can be observed are the excitation of local surface waves at lateral heterogeneities, and local resonances. Within the sedimentary basin studied, the coda of the transverse component is mainly composed of the local, fundamental-mode Love wave, whereas the P-SV-wavefield shows dominant contributions of the higher modes of Rayleigh waves. These differences in wave composition lead, in general, to different dispersion characteristics and attenuation phenomena for SH- and P-SV-waves. A parametric study demonstrates the sensitivity of the computer ground motion to small changes in the subsurface topography of the sedimentary basin, and the velocity and quality factor of the sediments. The results obtained for one- and two-dimensional structural models show that only two-dimensional structural models are suitable for the prediction of complete seismic ground motions in sedimentary basins, because they can account for the generation of local surface waves. To establish the validity of the numerical results, they are compared with observed ground motion. The relative amplitudes, durations, and frequency content of the different components of the synthetic signals agree well with the observations.

A three-dimensional simulation of seismic waves in the Santa Clara Valley, California, from a Loma Prieta aftershock

Abstract The finite-difference method is used to propagate elastic waves through a 3-D model of the Santa Clara Valley, an alluvium-filled basin that underlies the city of San Jose, California. The model was based on depth to bedrock information from water wells in the area. The simulation corresponded to a region 30 (east-west) by 22 (north-south) by 6 (depth) km and contained about 4 million grid points. Synthetic seismograms from the simulation are accurate at frequencies up to 1 Hz. Motions from a magnitude 4.4 aftershock of the Loma Prieta earthquake were modeled. Snapshots of ground motion and synthetic seismograms from the simulation are presented. The simulation illustrates S -to-surface-wave conversion at the edges of the basin and the large amplitude and long duration of ground motion in the basin compared to the surrounding rock. Love waves produced at the edge of the basin are the largest arrivals in the transverse synthetics. Because of the slow group velocity of the Love waves, sites near the center of the basin have longer durations of significant motions than basin sites near the valley edges. Sites near the center of the basin also show larger peak amplitudes on the transverse component. Array analysis of the synthetic seismograms indicates that Love waves tend to propagate parallel to the eastern and western edges of the valley. Rayleigh waves are produced along the southern margin of the basin from incident S waves. Large radial motions occur where a Rayleigh wave impinges on the northeast margin of the valley. Some Rayleigh waves travel westward across the basin, after being scattered from the eastern edge of the valley. Synthetic seismograms from the simulation have similar peak amplitudes as seismograms recorded by the Sunnyvale dense array for this aftershock, although the duration of the tranverse component is not matched by the synthetic seismogram, using this basin model. The simulation indicates that the Love waves observed on the actual seismograms were produced by conversion of incident S waves at the southern margin of the Santa Clara Valley. 2-D simulations show how the S -to-Love-wave conversion is affected by the angle of incidence of the S wave and the sharpness of the velocity transition between the alluvium and bedrock. As more accurate basin models are developed, 3-D simulations should become valuable for predicting ground motions in sedimentary basins for future large earthquakes.