Geotechnical Characterization of TriNet Sites: A Status Report (original) (raw)
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The Carquinez Bridge geotechnical arrays are operated by the California Strong Motion Instrumentation Program (CSMIP) and recorded a peak ground acceleration (PGA) of approximately 1.0g at ground surface during the 2014 South Napa earthquake. The recorded PGA was significantly larger than those at the nearby surface sites. This study considers surface and downhole recordings from the additional 28 earthquakes recorded at the same arrays to understand the effects of wave propagation and site response at these arrays. Several site response analyses are performed to understand soil nonlinearity using the observed ground accelerations during the 2014 South Napa sequence. Apparent shear wave velocities are calculated from downhole records, which show clear reduction as ground motion intensity increases. Empirical transfer functions (EFTs) are also calculated in which the resonance frequencies became lower during strong shaking during the 2014 South Napa main shock. The in-situ critical damping ratio appears to be frequency dependent in the soft clay deposits. Lower damping at frequencies greater than about 5 Hz may have contributed to the observed PGA at Array #1 during the main shock.
Seismological Research Letters, 2015
A borehole that penetrated 585 m of soil deposits and terminated 9 m into Paleozoic bedrock was completed near the New Madrid Seismic Zone of the central United States. A vertical array of strongmotion accelerometers and broadband seismometers-the Central United States Seismic Observatory (CUSSO) was installed. CUSSO provides a test site for verification and calibration of seismic-wave propagation analysis and near-surface seismic exploration. Preliminary results show that velocity models produced from the P-wave walkaway soundings, P-wave arrival of earthquake recordings, and downhole P-wave suspension measurements at CUSSO are comparable; however, the S-wave suspension log model underestimates the velocity compared to the models derived from S-wave walkaway soundings and S-wave arrival of earthquake recordings. Significant sediment disturbance in the borehole annulus is speculated to have affected the accuracy of the S-wave suspension log, whereas the saturated condition of the soft sediment at depth made the downhole P-wave measurements less susceptible to the sediment disturbance.
Seismic, geotechnical, and earthquake engineering site characterization
SEG Technical Program Expanded Abstracts 2006, 2006
We determined the seismic model of the soil column within a residential project site in Istanbul, Turkey. Specifically, we conducted refraction seismic survey at 20 locations and estimated the P-and S-wave velocity-depth profiles down to a depth of 30 m. We then combined the seismic velocities with the geotechnical borehole information regarding the lithology of the soil column and determined the site-specific geotechnical earnthquake engineering parameters for the site. Specifically, we computed the maximum soil amplification ratio, maximum surfacebedrock acceleration ratio, depth interval of significant acceleration, maximum soil-rock response ratio, and design spectrum periods TA-TB.
Near-fault seismic site response
2000
The understanding of ground motions in the near-fault region is important for seismic risk assessment in a number of populated areas that overlie fault zones. Understanding the effect of local site conditions is important for a complete characterization of near-fault ground motions. A geotechnically-based site classification scheme that includes soil depth is proposed for the evaluation of site effects. This classification scheme is used to evaluate site response for the 1989 Loma Prieta and 1994 Northridge earthquakes. Regression analyses resulted in estimates of ground motion with lower uncertainties than those made with the simpler classification schemes typically used in attenuation relationships. These results emphasize the need to better define the baseline site category used in attenuation relationships. Site amplification factors for each site category are proposed. These factors, however, are based on data recorded at distances generally greater than 10 km from the fault and are not directly applicable to pulse-like near-fault forward-directivity ground motions. Thus, a separate database of near-fault forward-directivity ground motions is studied. Each record in the database is characterized by a pulse period and its peak ground velocity (PGV). Regression analyses
Soil Dynamics and Earthquake Engineering, 2000
Three studies of site amplification factors, based on the recorded aftershocks, and one study based on strong motion data, are compared one with another and with the observed distribution of damage from the Northridge, CA, earthquake of 17 January 1994 M L 6:4: In the epicentral area, when the peak ground velocities are larger than v m Ϸ 15 cm=s; nonlinear response of soil begins to distort the amplification factors determined from small amplitude (linear) wave motion. Moving into the area of near-field and strong ground motion v m Ͼ 30 cm=s; the site response becomes progressively more affected by the nonlinear soil response. Based on the published results, it is concluded that site amplification factors determined from small amplitude waves (aftershocks, small earthquakes, coda waves) and their transfer-function representation may be useful for small and distant earthquake motions, where soils and structures respond to earthquake waves in a linear manner. However in San Fernando Valley, during the Northridge earthquake, the observed distribution of damage did not correlate with site amplification determined from spectra of recorded weak motions. Mapping geographical distribution of site amplification using other than very strong motion data, therefore appears to be of little use for seismic hazard analyses. ᭧
2009
For one week during September 2007, we deployed a temporary network of field recorders and accelerometers at four sites within two deep, seismically active mines. The ground-motion data, recorded at 200 samples/sec, are well suited to determining source and ground-motion parameters for the mining-induced earthquakes within and adjacent to our network. Four earthquakes with magnitudes close to 2 were recorded with high signal/noise at all four sites. Analysis of seismic moments and peak velocities, in conjunction with the results of laboratory stick-slip friction experiments, were used to estimate source processes that are key to understanding source physics and to assessing underground seismic hazard. The maximum displacements on the rupture surfaces can be estimated from the parameter Rv, where v is the peak ground velocity at a given recording site, and R is the hypocentral distance. For each earthquake, the maximum slip and seismic moment can be combined with results from laboratory friction experiments to estimate the maximum slip rate within the rupture zone. Analysis of the four M 2 earthquakes recorded during our deployment and one of special interest recorded by the in-mine seismic network in 2004 revealed maximum slips ranging from 4 to 27 mm and maximum slip rates from 1.1 to 6:3 m=sec. Applying the same analyses to an M 2.1 earthquake within a cluster of repeating earthquakes near the San Andreas Fault Observatory at Depth site, California, yielded similar results for maximum slip and slip rate, 14 mm and 4:0 m=sec.