The attenuation of seismic shear waves in quaternary alluvium in Santa Clara Valley, California

1994 ◽  
Vol 84 (1) ◽  
pp. 76-90 ◽  
Author(s):  
James F. Gibbs ◽  
David M. Boore ◽  
William B. Joyner ◽  
Thomas E. Fumal
Keyword(s):  
Attenuation Factor ◽  
Shear Waves ◽  
Ground Surface ◽  
Strong Motion ◽  
Spectral Ratio ◽  
Mean Value ◽  
Depth Range ◽  
Nonlinear Wave Propagation ◽  
Surface Source ◽  
Frequency Spectra

Abstract We used shear waves, generated by an air-powered source at the ground surface and recorded in a borehole, to estimate the shear-wave quality factor at strong-motion station Gilroy no. 2. We find similar values of Q using both the decay of the spectra with depth and the slope of the spectral ratio at two depths; we find no evidence of a frequency dependence of Q. The mean value of Q over the depth range 10 to 115 m is close to 10. The use of this value over the depth of the borehole and the observed travel time of 0.358 sec gives a cumulative attenuation factor t* of 0.036 sec for the upper 180 m of the Quaternary alluvium. This is comparable to the differential decay between Gilroy no. 2 and a rock site 1.9 km away (Gilroy no. 1), as measured from the decay of the high-frequency spectra of accelerograms from large earthquakes, plotted on a log-linear scale: t* = 0.05, 0.04, and 0.03 sec for the 1979 Coyote Lake, 1984 Morgan Hill, and 1989 Loma Prieta earthquakes, respectively. The similarity between the attenuations measured from the low-strain surface source and those from the larger amplitude earthquake sources suggests that increases of damping due to nonlinear wave propagation effects are limited.

Ground Surface Tilts Estimated Based on Acceleration from Wenchuan Earthquake

2011 ◽  
Vol 243-249 ◽  
pp. 3913-3918
Author(s):  
Xiao Bo Peng ◽  
Xiao Jun Li
Keyword(s):  
Low Frequency ◽  
Ground Surface ◽  
Strong Motion ◽  
Spectral Ratio ◽  
Fourier Amplitude ◽  
Surface Rupture ◽  
Fault Surface ◽  
Amplitude Spectra ◽  
The Difference ◽  
Small Change

Dynamic equations of pendulums in strong motion instrument are analyzed. The equations imply horizontal recordings may be significantly influenced by ground surface tilt while the effects can be ignored for vertical recordings. The difference causes anomaly of horizontal to vertical Fourier amplitude spectra ratio, which is used to identify affected low frequency range.. The low frequency filter based on spectral ratio was employed to estimate the coseismic ground surface tilts in Wenchuan Ms8.0 earthquake. Results show the ground surface tilts are generally less than 1 degree and mainly happened within 30 kilometers to fault surface rupture traces, while rarely discovered in area outside 100 km and squared horizontal peak accelerations less than 200 cm/s2. Comparison of ground surface tilts under spectral ratio being 2 and 4, shows ground surface tilting amplitudes usually increased when spectral ratio is decreased, but residual ground surface tilts have small change.

Investigation of Relationship between the Response and Fourier Spectral Ratios Based on Statistical Analyses of Strong-Motion Records

2020 ◽  
pp. 2150008
Author(s):  
Haizhong Zhang ◽  
Yan-Gang Zhao
Keyword(s):  
Seismic Design ◽  
Site Effects ◽  
Epicentral Distance ◽  
Ground Motions ◽  
Scaling Law ◽  
Ground Surface ◽  
Strong Motion ◽  
Spectral Ratio ◽  
Spectral Ratios ◽  
Earthquake Scenario

In both seismic design and probabilistic seismic-hazard analyses, site effects are typically characterized as the ratio of the response spectral ordinate on the ground surface to that on the bedrock based on the scaling law borrowed from the Fourier spectral ordinate. Recent studies have shown that different from the Fourier spectral ratio (FSR), the response spectral ratio (RSR) does not purely reflect the site effects but also depends on the earthquake scenario even for linear analysis. However, previous studies are limited to theoretical analysis. This study statistically compares the two spectral ratios by analyzing many actual seismic ground motions recorded at nearby soil and rock sites. It is observed that the average RSR and FSR have similar overall shapes, and their maximum values occur at approximately the same period; however, the values around the peak are clearly different with FSRs consistently exceeding the RSRs. The RSR–FSR relationship depends on the earthquake scenario and the oscillator damping; their difference at periods longer than the site’s fundamental period decreases as the magnitude and epicentral distance increase, and the RSRs generally approach the FSRs as the oscillator damping decreases.

Ground-motion model for subduction earthquakes in northern South America

2021 ◽  
pp. 875529302110275
Author(s):  
Carlos A Arteta ◽  
Cesar A Pajaro ◽  
Vicente Mercado ◽  
Julián Montejo ◽  
Mónica Arcila ◽  
...  
Keyword(s):  
South America ◽  
Ground Motion ◽  
Ground Motions ◽  
Strong Motion ◽  
Motion Prediction ◽  
Depth Range ◽  
Ground Motion Prediction ◽  
Natural Period ◽  
Nazca Plate ◽  
Northern South America

Subduction ground motions in northern South America are about a factor of 2 smaller than the ground motions for similar events in other regions. Nevertheless, historical and recent large-interface and intermediate-depth slab earthquakes of moment magnitudes Mw = 7.8 (Ecuador, 2016) and 7.2 (Colombia, 2012) evidenced the vast potential damage that vulnerable populations close to earthquake epicenters could experience. This article proposes a new empirical ground-motion prediction model for subduction events in northern South America, a regionalization of the global AG2020 ground-motion prediction equations. An updated ground-motion database curated by the Colombian Geological Survey is employed. It comprises recordings from earthquakes associated with the subduction of the Nazca plate gathered by the National Strong Motion Network in Colombia and by the Institute of Geophysics at Escuela Politécnica Nacional in Ecuador. The regional terms of our model are estimated with 539 records from 60 subduction events in Colombia and Ecuador with epicenters in the range of −0.6° to 7.6°N and 75.5° to 79.6°W, with Mw≥4.5, hypocentral depth range of 4 ≤  Zhypo ≤ 210 km, for distances up to 350 km. The model includes forearc and backarc terms to account for larger attenuation at backarc sites for slab events and site categorization based on natural period. The proposed model corrects the median AG2020 global model to better account for the larger attenuation of local ground motions and includes a partially non-ergodic variance model.

scholarly journals Near-Source Simulation of Strong Ground Motion in Amatrice Downtown Including Site Effects

Geosciences ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 186
Author(s):  
Alessandro Todrani ◽  
Giovanna Cultrera
Keyword(s):  
Ground Motion ◽  
Site Effects ◽  
Central Italy ◽  
Strong Motion ◽  
Seismic Source ◽  
Spectral Ratio ◽  
Intensity Measures ◽  
Standard Spectral Ratio ◽  
Heavy Damage ◽  
Strong Ground

On 24 August 2016, a Mw 6.0 earthquake started a damaging seismic sequence in central Italy. The historical center of Amatrice village reached the XI degree (MCS scale) but the high vulnerability alone could not explain the heavy damage. Unfortunately, at the time of the earthquake only AMT station, 200 m away from the downtown, recorded the mainshock, whereas tens of temporary stations were installed afterwards. We propose a method to simulate the ground motion affecting Amatrice, using the FFT amplitude recorded at AMT, which has been modified by the standard spectral ratio (SSR) computed at 14 seismic stations in downtown. We tested the procedure by comparing simulations and recordings of two later mainshocks (Mw 5.9 and Mw 6.5), underlining advantages and limits of the technique. The strong motion variability of simulations was related to the proximity of the seismic source, accounted for by the ground motion at AMT, and to the peculiar site effects, described by the transfer function at the sites. The largest amplification characterized the stations close to the NE hill edge and produced simulated values of intensity measures clearly above one standard deviation of the GMM expected for Italy, up to 1.6 g for PGA.

Frequency-dependent attenuation of shear waves in the crust of the southern Kanto area, Japan

1994 ◽  
Vol 84 (5) ◽  
pp. 1387-1396
Author(s):  
Shigeo Kinoshita
Keyword(s):  
Shear Waves ◽  
Strong Motion ◽  
Body Waves ◽  
Philippine Sea Plate ◽  
Eurasian Plate ◽  
Philippine Sea ◽  
Motion Data ◽  
Basement Rocks ◽  
Acceleration Data ◽  
Attenuation Characteristics

Abstract The attenuation characteristics of shear waves in the crust of the southern Kanto area, central Japan, were estimated using strong-motion data, including acceleration data recorded in the pre-Tertiary basement rocks by means of downhole observation. The quality factor Qs(f) was determined for a range of discrete frequencies from 0.5 to 16 Hz from the analysis of data from 13 local earthquakes with focal depths of less than about 50 km that occurred in the Philippine Sea plate and in the boundary zone between the lower part of the Eurasian plate and the upper part of the Philippine Sea plate. The estimated 1/Qs(f) shows a peaked structure in this frequency range on the assumption that the geometrical spreading exponent is -1 (body waves). The estimated peak 1/Qs(f) is of the order of 10-2 at 0.8 Hz.

Preliminary microzonation of the Memphis, Tennessee, area

1982 ◽  
Vol 72 (3) ◽  
pp. 1011-1024
Author(s):  
Sunil Sharma ◽  
William D. Kovacs
Keyword(s):  
United States ◽  
Soil Properties ◽  
Damping Ratio ◽  
Ground Surface ◽  
Strong Motion ◽  
Response Analysis ◽  
Borehole Data ◽  
Central United States ◽  
Hazardous Zone ◽  
High Level

abstract The city of Memphis, which is situated very close to the inferred epicenter of one of the three major 1811 to 1812 earthquakes, is in a potentially hazardous zone which will be susceptible to the usual seismic hazards. By recognizing the high level of seismicity in the New Madrid area, this study attempts to microzone the potential hazards by considering the following subjects: (i) the seismicity of the central United States; (ii) design earthquakes; and (iii) response analysis which allows construction of the necessary microzonation maps. The seismicity of the region is evaluated from state-of-the-art literature as there is no recorded strong-motion data available for the central United States. Synthetically generated accelerograms, simulating the design earthquakes, were used to represent the ground motions which were applied at a depth of 45 m, below ground surface, at numerous sites in Memphis. The soil stratigraphy was conceptualized from borehole data, made available by local sources, and dynamic soil properties estimated from available empirical correlations. The results of the response analysis were transformed into microzonation maps depicting: (i) zones showing qualitative estimates of ground response; (ii) zones showing the natural frequency of the soils; (iii) zones showing the peak spectral acceleration for 2 per cent damping ratio; and (iv) zones of liquefaction potential. These maps are useful for preliminary design and are not intended to be used on a quantitative basis. Further investigation is necessary in determining the stratigraphy and soil properties for a site-specific design and analysis.

scholarly journals Event couple spectral ratio Q method for earthquake clusters: application to North-West Bohemia

2018 ◽  
Author(s):  
Marius Kriegerowski ◽  
Simone Cesca ◽  
Matthias Ohrnberger ◽  
Torsten Dahm ◽  
Frank Krüger
Keyword(s):  
Wave Attenuation ◽  
Attenuation Factor ◽  
Source Region ◽  
Spectral Ratio ◽  
Ratio Method ◽  
Earthquake Swarms ◽  
North West ◽  
High Attenuation ◽  
West Bohemia ◽  
Spectral Ratio Method

Abstract. We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q−1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wavefield seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms, but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to North-West Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Qp 

scholarly journals Effects of Underground Cavities on the Frequency Spectrum of Seismic Shear Waves

2014 ◽  
Vol 2014 ◽  
pp. 1-17 ◽  
Author(s):  
G. Lancioni ◽  
R. Bernetti ◽  
E. Quagliarini ◽  
L. Tonti
Keyword(s):  
Frequency Spectrum ◽  
Ad Hoc ◽  
Parametric Design ◽  
Shear Waves ◽  
Scattering Problem ◽  
Ground Surface ◽  
Ground Level ◽  
Seismic Shear ◽  
Underground Cavities ◽  
Free Wave Propagation

A numerical method is proposed to study the scattering of seismic shear waves induced by the presence of underground cavities in homogeneous soils. The method is based on the superposition of two solutions: the solution of the free-wave propagation problem in a uniform half-space, easily determined analytically, and the solution of the wave scattering problem due to the cave presence, evaluated numerically by means of an ad hoc code implemented by using the ANSYS Parametric Design Language. In the two-dimensional setting, this technique is applied to the case of a single cave, placed at a certain depth from the ground level. The frequency spectrum of the seismic shear oscillation on the ground surface is determined for different dimensions and depths of the cave and compared with the spectrum registered without caves. The influence of the cave dimensions and depth on the spectrum amplification is analyzed and discussed.

Seismoelectric and electroseismic modelling in stratified porous media with a shallow or ground surface source

2021 ◽  
Author(s):  
Xu‐Zhen Zheng ◽  
Hengxin Ren ◽  
Karl E. Butler ◽  
Haiming Zhang ◽  
Yao‐Chong Sun ◽  
...  
Keyword(s):  
Porous Media ◽  
Ground Surface ◽  
Surface Source

Continuous measurements of microtremors on sediments and basement in Los Angeles, California

1993 ◽  
Vol 83 (5) ◽  
pp. 1595-1609 ◽  
Author(s):  
Hiroaki Yamanaka ◽  
Marijan Dravinski ◽  
Hiroshi Kagami
Keyword(s):  
Los Angeles ◽  
Continuous Measurement ◽  
Strong Motion ◽  
Spectral Ratio ◽  
Earthquake Ground Motion ◽  
Spectral Amplitude ◽  
Spectral Ratios ◽  
Long Period ◽  
Short Period ◽  
Deep Sediments

Abstract Continuous measurement of microtremors at two sites on basement rock and sediments was carried out in Los Angeles, California, in order to understand the fundamental characteristics of microtremors. A predominant peak with a period of about 6.5 sec was found in the microtremor spectra in both media. The spectral amplitude of the peaks varied gradually with time in a similar manner at the two sites. Their time-variant characteristics are in agreement with change in oceanic swell height observed at an oceanic buoy in the southwest of Los Angeles. This suggests that they originate from an oceanic disturbance. On the other hand, a clear daily variation of spectral amplitudes at a period of 0.3 sec indicates that short-period microtremors are caused by cultural noises. It was found that the spectral ratio of long-period microtremors between the basement and the sediments was repeatable, although the spectral amplitudes at the two sites were time-variant. The spectral ratio of the long-period microtremors was similar to that derived from strong motion records. This suggests the applicability of spectral ratios of microtremors to assess the effects of deep sediments on long-period earthquake ground motion.

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