Effects of Shallow-Velocity Reductions on 3D Propagation of Seismic Waves

2020 ◽  
Vol 91 (6) ◽  
pp. 3313-3322
Author(s):  
Alan Juarez ◽  
Yehuda Ben-Zion
Keyword(s):  
Los Angeles ◽  
Seismic Waves ◽  
Cross Correlation ◽  
Ground Motions ◽  
Velocity Model ◽  
Los Angeles Basin ◽  
Relative Time ◽  
Velocity Changes ◽  
Reference Community ◽  
Strong Ground

Abstract We perform 3D simulations of seismic wavefields to clarify effects of strong reductions of shallow velocities on long-period seismic waves. The simulations use a reference Community Velocity Model of southern California and a modified version with strong velocity reductions in the top 500 m of the Los Angeles basin. Differences between wavefields generated by 10 earthquakes in the reference and perturbed models are analyzed. Velocity changes are estimated by measuring relative time shifts between reference and perturbed seismograms using wavelet cross-correlation spectra. The results indicate that strong localized temporal velocity drops near the surface, such as those observed during strong ground motions, may generate regional perturbations of wavefields at periods up to 20 s. These perturbations may be misinterpreted as generated by temporal changes at seismogenic depths. The results also have important implications for waveform tomography studies.

Long-period ground motions and dynamic strain field of Los Angeles basin during large earthquakes

1996 ◽  
Vol 86 (5) ◽  
pp. 1417-1433
Author(s):  
T. L. Teng ◽  
J. Qu
Keyword(s):  
Los Angeles ◽  
Displacement Field ◽  
Seismic Moment ◽  
Strain Field ◽  
San Andreas Fault ◽  
Ground Motions ◽  
Dynamic Strain ◽  
Los Angeles Basin ◽  
Long Period ◽  
San Andreas

Abstract During a big earthquake along the San Andreas fault in southern California, high excitation and low attenuation of long-period (3 to 10 sec) strong ground motions will cause wave motions to propagate efficiently far from the epicentral area. These ground motions could potentially be destructive to large-dimension structures in the Los Angeles basin. We performed calculations using the surface-wave Gaussian beam method for a 3D southern California crustal structure. Displacement field as well as the associated dynamic strain field produced by large propagating ruptures along the San Andreas fault are evaluated. Results indicate that in the presence of lateral heterogeneity, focusing and multipathing interference contribute significantly to a complex pattern of the displacement field and the associated dynamic strain field. For a big event on the San Andreas fault with a seismic moment of 1.8 × 1028 dyne-cm, long-period displacement in the Los Angeles basin could reach a maximum amplitude of meters in places. Since this calculation is fast, we have evaluated the displacement field for a dense grid of points; a differentiation gives the corresponding effective horizontal dynamic strain field. At times, the maximum effective dynamic strains may reach mid-10−3 to even 10−3—high enough to be of engineering concern. This computational result probably gives the upper bound values due to the large source assumed. For events of smaller seismic moment release along less extensive ruptures, these results can easily be scaled down proportionally. Different scenarios are considered in this study with different slip distributions. It is found that with a given seismic moment, a more evenly distributed fault slip over the rupture surface will result in lower peak values on both displacements and dynamic strains. Our displacement results give similar values to those obtained by Kanamori using empirical Green's functions but substantially higher than Bouchon and Aki's results.

Basin waves on a seafloor recording of the 1990 Upland, California, earthquake: Implications for ground motions from a larger earthquake

1999 ◽  
Vol 89 (1) ◽  
pp. 317-324
Author(s):  
David M. Boore
Keyword(s):  
Time Series ◽  
Wave Propagation ◽  
Los Angeles ◽  
Surface Waves ◽  
Ground Motions ◽  
Response Spectra ◽  
Theoretical Models ◽  
Regression Equations ◽  
Los Angeles Basin ◽  
Basin Response

Abstract The velocity and displacement time series from a recording on the seafloor at 74 km from the 1990 Upland earthquake (M = 5.6) are dominated by late-arriving waves with periods of 6 to 7 sec. These waves are probably surface waves traveling across the Los Angeles basin. Response spectra for the recording are in agreement with predictions from empirical regression equations and theoretical models for periods less than about 1 sec but are significantly larger than those predictions for longer periods. The longer-period spectral amplitudes are controlled by the late-arriving waves, which are not included in the theoretical models and are underrepresented in the data used in the empirical analyses. When the motions are scaled to larger magnitude, the results are in general agreement with simulations of wave propagation in the Los Angeles basin by Graves (1998).

scholarly journals Source-related variations of ground motions in 3-D media: application to the Newport-Inglewood fault, Los Angeles Basin

2008 ◽  
Vol 175 (1) ◽  
pp. 202-214 ◽  
Author(s):  
Haijiang Wang ◽  
Heiner Igel ◽  
František Gallovič ◽  
Alain Cochard ◽  
Michael Ewald
Keyword(s):  
Los Angeles ◽  
Ground Motions ◽  
Los Angeles Basin

The Whittier Narrows, California Earthquake of October 1, 1987—Note on Peak Accelerations during the 1 and 4 October Earthquakes

1988 ◽  
Vol 4 (1) ◽  
pp. 101-113 ◽  
Author(s):  
M. D. Trifunac
Keyword(s):  
Wave Propagation ◽  
Los Angeles ◽  
Ground Motion ◽  
Strong Ground Motion ◽  
Strong Dependence ◽  
Three Dimensional ◽  
Physical Processes ◽  
Los Angeles Basin ◽  
Dense Arrays ◽  
Strong Ground

Attenuation patterns of the recorded peak accelerations during two moderate earthquakes (ML = 5.9 and 5.3) in Los Angeles, California are described. It is shown that the recording of earthquake motions by dense arrays of accelerographs can yield a detailed and deterministic picture of the physical processes which are involved in shaping the observed variations of strong ground motion. For the two earthquakes the observed changes of peak amplitudes with respect to the azimuth and distance are slowly and continuously changing functions showing strong dependence of amplitudes on the radiation patterns of the two earthquakes and on the effects of wave propagation through irregular three-dimensional geology of the Los Angeles basin.

The Whittier Narrows, California Earthquake of October 1, 1987—Early Results of Isoseismal Studies and Damage Surveys

1988 ◽  
Vol 4 (1) ◽  
pp. 1-10 ◽  
Author(s):  
E. V. Leyendecker ◽  
L. M. Highland ◽  
M. Hopper ◽  
E. P. Arnold ◽  
P. Thenhaus ◽  
...  
Keyword(s):  
Los Angeles ◽  
Field Survey ◽  
Ground Motions ◽  
Geographical Area ◽  
Mail Survey ◽  
Strong Motion ◽  
Peak Acceleration ◽  
Early Results ◽  
Acceleration Data ◽  
Strong Ground

Preliminary isoseismals for Modified Mercalli intensities are presented for the Whittier Narrows Earthquake. Isoseismals for intensities VI and lower are based on responses to a mail survey. Intensity VII and larger are based on a field survey of damage described in this paper. The maximum observed intensity of VIII was confined to Whittier. The shapes of the intensity contours compare favorably with the distribution of average peak acceleration data from the strong motion array in the greater Los Angeles area. The damage assessments appeared consistent with earthquake magnitude. However, the accelerations were higher than expected for the magnitude. The building classification and survey strategies developed were tested and found usable and adequate for describing damage. With further refinement this system can be used to describe damage within a limited geographical area and in a format useful for correlations with strong ground motions and the Modified Mercalli Intensity scale.

Liquid Sloshing of Oil Storage Tanks and Long-Period Strong Ground Motions in the 2003 Tokachi-Oki Earthquake

2004 ◽  
Author(s):  
Shinsaku Zama
Keyword(s):  
Seismic Waves ◽  
Ground Motions ◽  
Liquid Sloshing ◽  
Response Analysis ◽  
Storage Tanks ◽  
Severe Damage ◽  
Long Period ◽  
Oil Storage ◽  
Wave Heights ◽  
Strong Ground

The 2003 Tokachi-oki earthquake caused the severe damage to oil storage tanks by liquid sloshing. Especially at Tomakomai, two tank fires broke out and six floating roofs sank. Seismograms showed that long-period motions were predominant and duration became longer when the seismic waves propagated into the Yufutsu Plain, where Tomakomai is located. Sloshing wave heights (Wh) of all tanks were calculated by two-dimensional response analysis. It was found that estimated Wh exceeded 3 m at periods 5 and 7.5 sec, and exceeded 2m from 3.5 to 9 sec of sloshing period and that severe damaged tanks had the highest Wh at each period in general.

scholarly journals Evaluation of the Strong Ground Motions in the Area Close to the Surface Faults

2018 ◽  
Vol 12 (04) ◽  
pp. 1841002
Author(s):  
Kiyoshi Irie ◽  
Dorjpalam Saruul ◽  
Kazuo Dan ◽  
Haruhiko Torita
Keyword(s):  
Seismic Waves ◽  
Ground Motions ◽  
Strong Motion ◽  
Strike Slip ◽  
Reverse Fault ◽  
Surface Layers ◽  
Period Range ◽  
Strong Ground Motions ◽  
Seismogenic Layer ◽  
Strong Ground

In Japan, the seismic waves radiated from the fault in the surface layers above the seismogenic layer are not considered in the usual strong motion prediction. However, in the inland crustal earthquakes, the strong ground motions in the areas close to the surface faults could be influenced by the seismic waves radiated from the fault in the surface layers. Hence, we evaluated the seismic waves radiated from vertical strike-slip and dipping reverse faults in the surface layers to investigate their influence on the strong motions. The results of the strike-slip fault showed that the seismic waves of the fault normal (FN) component were larger than those of the fault parallel (FP) component in the period range of 0.5–5 s. At least, 80–90% of the FN component was attributed to the seismic wave radiated from the fault in the seismogenic layer. Almost 100% of the FP component was attributed to the seismic waves radiated from the fault in the surface layers. On the other hand, the results of the reverse fault showed that the seismic waves were not attributed to those from the fault in the surface layers.

Characterization of spall from observed strong ground motions on Pahute Mesa

1990 ◽  
Vol 80 (5) ◽  
pp. 1326-1345 ◽  
Author(s):  
Howard J. Patton
Keyword(s):  
Water Table ◽  
Seismic Waves ◽  
Ground Motions ◽  
Coupling Efficiency ◽  
Cube Root ◽  
Cylindrically Symmetric ◽  
Acceleration Data ◽  
Apparent Attenuation ◽  
Strong Ground

Abstract Ground motion velocity and acceleration data recorded within the spall region on over 30 Pahute Mesa explosions have been analyzed for characterization of the spall source. These data provide observations of peak spall velocities and the spatial extent of spall from which estimates of the volume and mass of spalled material were obtained, along with the yield scaling. Spall velocities at ground zero are higher, and the apparent attenuation of velocities is lower for explosions below the water table compared to shots above the water table in tuff or rhyolite. As a result, the lateral and depth extents of spall are larger for shots below the water table, and the estimated spall mass is three times larger than the estimate for explosions above the water table. The lateral and depth extents were found to scale as the quarter root of the yield (W) as opposed to the familiar cube-root scaling, and assuming a cylindrically symmetric volume, the mass scales as W0.77±.05. At 150 kt, the estimated mass for a shot below the water table is 15 times greater than the first estimates made by Viecelli. While this estimate suggests a much larger spall source than originally believed, not all of the mass is necessarily involved in generating seismic waves recorded in the far field, and the coupling efficiency of spall remains an outstanding, unresolved problem.

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