Lettis Consultants International Inc.

Walnut Creek, CA, United States

Lettis Consultants International Inc.

Walnut Creek, CA, United States
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News Article | April 17, 2017
Site: www.eurekalert.org

There is a 43% probability that the Wasatch Front region in Utah will experience at least one magnitude 6.75 or greater earthquake, and a 57 % probability of at least one magnitude 6.0 earthquake, in the next 50 years, say researchers speaking at the 2017 Seismological Society of America's (SSA) Annual Meeting. In their report released in 2016, the Working Group on Utah Earthquake Probabilities, established by the Utah Geological Survey and the U.S. Geological Survey, presented their first forecast for large earthquakes along faults in the Wasatch region, running roughly from Nephi, Utah north to the Utah-Idaho border. (A map of the region is available from the USGS.) The Working Group's project is the first comprehensive study of large earthquake risk in the U.S. West outside of California. At the SSA Annual Meeting, Ivan Wong of Lettis Consultants International and colleagues will discuss the detailed forecast from the 2016 report, including their findings that at least 22 large earthquakes have ruptured parts of the Wasatch fault zone between Nephi and Brigham City, Utah in the past 6000 years. The data also suggest that some segments of the fault may be more likely to rupture than others, based on the average time between earthquakes. For instance, the segment of the fault around Brigham City ruptures on average every 1100 years, but has not experienced an earthquake in 2500 years.


Zandieh A.,Lettis Consultants International Inc. | Campbell K.W.,CoreLogic | Pezeshk S.,University of Memphis
Bulletin of the Seismological Society of America | Year: 2016

The spectral-decay parameter κ0 is often used to account for the reduction of the high-frequency amplitude of ground motion caused by attenuation within the site profile. In this study, we used the inverse random vibration theory approach described by Al Atiket al.(2014) to calculate Fourier amplitude spectra from predicted values of response-spectral acceleration for all five Next Generation Attenuation (NGA)-West2 ground-motion prediction equations (GMPEs). We used these spectra to estimate κ0 using the spectral-decay method. Each GMPE was evaluated for a National Earthquake Hazard Reduction Program B/C site condition and for default estimates of depth to the top of rupture, hypocentral depth, and sediment (basin) depth. We derived estimates of κ0 for magnitudes ranging from 3.5 to 8.0 and distances ranging from 5 to 20 km and used a mixed-effects model to derive equations for these estimates as a function of magnitude. We also calculated κ0 from the geometric mean of the response-spectral accelerations of the GMPEs to check the sensitivity of the results to the two approaches and found that the values of κ0 derived in this study using a mixed-effects model are in good agreement with these estimates. The values of κ0 obtained in this study do not necessarily represent the physical high-frequency damping within the site profiles used to develop the NGA-West2 GMPEs. Instead, they are intended to represent the high-frequency shape of the median predicted spectral accelerations from the GMPEs. The κ0 model developed in this study can be used in inversions to develop stochastic models that are intended to mimic the predictions from the NGA-West2 GMPEs. © 2016, Seismological Society of America. All rights reserved.


Singh N.M.,National Institute of Technology Silchar | Rahman T.,National Institute of Technology Silchar | Wong I.G.,Lettis Consultants International Inc.
Bulletin of the Seismological Society of America | Year: 2016

In this study, a stochastic point-source model is used to develop a ground-motion prediction model for northeastern India (NEI) crustal earthquakes calibrated using region-specific source, path, and site parameters. The NEI strong ground motion database is too limited to derive an empirical ground-motion prediction model valid for all magnitudes and distances. In the present study, we simulated 30,000 ground motions to develop the horizontal ground-motion prediction model for NEI. These simulations are for 1000 earthquakes of moment magnitudes (Mw) 4–8.5 and epicentral distances of 1–300 km. In simulating ground motions, quality factor, stress drop, and kappa derived from available NEI strong-motion records have been used. The model has been validated by comparing it with the available but sparse database of recorded events. The best fit between the model and observed strong ground motion records indicate that the new model is consistent with the available data and can be used in seismic-hazard analyses until more data are collected and the model is improved. © 2016, Seismological Society of America. All rights reserved.


Unruh J.,Lettis Consultants International Inc. | Humphrey J.,Double Inc.
Geology | Year: 2017

The Sierran microplate is a northwest-translating block entrained in distributed motion east of the Pacific plate. To the north, the Oregon coast block (OCB) moves northward within the hanging wall of the Cascadia subduction zone, above the obliquely converging Juan de Fuca plate. Analysis of GPS velocity data indicates that relative motion between the rigid Sierran and OCB microplates is characterized by several millimeters per year of dextral shear directed ~N70°W, which is distinct from and counterclockwise to macroscopic dextral shear in the Walker Lane east of the Sierran microplate. We present a new analysis of focal mechanisms from small earthquakes in an 80-km-wide zone that spans the geodetically defined Sierran-OCB boundary to evaluate patterns of distributed deformation. We find that the direction of macroscopic dextral shear in this region is parallel to Sierran-OCB motion derived from GPS data. The seismogenic deformation is consistent with postulated dextral shear within an incompletely studied west-northwest-trending zone of faults and lineaments that traverses the northern Sierra Nevada; the faults and lineaments terminate westward against Quaternary folds in the northern Sacramento Valley. Active deformation at the Sierran-OCB boundary accommodates the relative motion of the bounding microplates and probably does not represent discrete transfer of Walker Lane motion to the Cascadia subduction zone in a restraining left step across the northern Sierra and Sacramento Valley. © 2017 Geological Society of America.


Kwong N.S.,University of California at Berkeley | Chopra A.K.,University of California at Berkeley | Mcguire R.K.,Lettis Consultants International Inc.
Earthquake Engineering and Structural Dynamics | Year: 2015

In this short communication, we respond to the comments made by Dr Brendon A. Bradley and provide additional context to our paper under discussion. © 2015 John Wiley & Sons, Ltd.


Kwong N.S.,University of California at Berkeley | Chopra A.K.,University of California at Berkeley | Mcguire R.K.,Lettis Consultants International Inc.
Earthquake Engineering and Structural Dynamics | Year: 2015

This study presents a novel approach for evaluating ground motion selection and modification (GMSM) procedures in the context of probabilistic seismic demand analysis. In essence, synthetic ground motions are employed to derive the benchmark seismic demand hazard curve (SDHC), for any structure and response quantity of interest, and to establish the causal relationship between a GMSM procedure and the bias in its resulting estimate of the SDHC. An example is presented to illustrate how GMSM procedures may be evaluated using synthetic motions. To demonstrate the robustness of the proposed approach, two significantly different stochastic models for simulating ground motions are considered. By quantifying the bias in any estimate of the SDHC, the proposed approach enables the analyst to rank GMSM procedures in their ability to accurately estimate the SDHC, examine the sufficiency of intensity measures employed in ground motion selection, and assess the significance of the conditioning intensity measure in probabilistic seismic demand analysis. © 2015 John Wiley & Sons, Ltd.


Kwong N.S.,University of California at Berkeley | Chopra A.K.,University of California at Berkeley | Mcguire R.K.,Lettis Consultants International Inc.
Earthquake Engineering and Structural Dynamics | Year: 2015

This study develops a framework to evaluate ground motion selection and modification (GMSM) procedures. The context is probabilistic seismic demand analysis, where response history analyses of a given structure, using ground motions determined by a GMSM procedure, are performed in order to estimate the seismic demand hazard curve (SDHC) for the structure at a given site. Currently, a GMSM procedure is evaluated in this context by comparing several resulting estimates of the SDHC, each derived from a different definition of the conditioning intensity measure (IM). Using a simple case study, we demonstrate that conclusions from such an approach are not always definitive; therefore, an alternative approach is desirable. In the alternative proposed herein, all estimates of the SDHC from GMSM procedures are compared against a benchmark SDHC, under a common set of ground motion information. This benchmark SDHC is determined by incorporating a prediction model for the seismic demand into the probabilistic seismic hazard analysis calculations. To develop an understanding of why one GMSM procedure may provide more accurate estimates of the SDHC than another procedure, we identify the role of 'IM sufficiency' in the relationship between (i) bias in the SDHC estimate and (ii) 'hazard consistency' of the corresponding ground motions obtained from a GMSM procedure. Finally, we provide examples of how misleading conclusions may potentially be obtained from erroneous implementations of the proposed framework. © 2014 John Wiley & Sons, Ltd.


Kwong N.S.,University of California at Berkeley | Chopra A.K.,University of California at Berkeley | Mcguire R.K.,Lettis Consultants International Inc.
Earthquake Engineering and Structural Dynamics | Year: 2015

This paper develops a procedure to select unscaled ground motions for estimating seismic demand hazard curves (SDHCs) in performance-based earthquake engineering. Currently, SDHCs are estimated from a probabilistic seismic demand analysis, where several ensembles of ground motions are selected and scaled to a user-specified scalar conditioning intensity measure (IM). In contrast, the procedure developed herein provides a way to select a single ensemble of unscaled ground motions for estimating the SDHC. In the context of unscaled motions, the proposed procedure requires three inputs: (i) database of unscaled ground motions, (ii) IM, the vector of IMs for selecting ground motions, and (iii) sample size, n; in the context of scaled motions, two additional inputs are needed: (i) a maximum acceptable scale factor, SFmax, and (ii) a target fraction of scaled ground motions, γ. Using a recently developed approach for evaluating ground motion selection and modification procedures, the proposed procedure is evaluated for a variety of inputs and is demonstrated to provide accurate estimates of the SDHC when the vector of IMs chosen to select ground motions is sufficient for the response quantity of interest. © 2015John Wiley & Sons, Ltd.


Lienkaemper J.J.,U.S. Geological Survey | Baldwin J.N.,Lettis Consultants International Inc. | Turner R.,William Lettis and Associates | Sickler R.R.,U.S. Geological Survey | Brown J.,U.S. Geological Survey
Bulletin of the Seismological Society of America | Year: 2013

We document evidence for surface-rupturing earthquakes (events) at two trench sites on the southern Green Valley fault, California (SGVF). The 75-80 km long dextral SGVF creeps ~1-4 mm/yr. We identify stratigraphic horizons disrupted by upward-flowering shears and infilled fissures unlikely to have formed from creep alone. The Mason Rd site exhibits four events from ~1013 CE to the present. The Lopes Ranch site (LR, 12 km to the south) exhibits three events from 18 BCE to present including the most recent event (MRE), 1610 ± 52 yr CE (1σ) and a two-event interval (18 BCE-238 CE) isolated by a millennium of low deposition. Using OxCal to model the timing of the four-event earthquake sequence from radiocarbon data and the LR MRE yields a mean recurrence interval (RI or μ) of 199 ± 82 yr (1σ) and ±35 yr (standard error of the mean), the first based on geologic data. The time since the most recent earthquake (open window since MRE) is 402 yr ± 52 yr, well past μ ~ 200 yr. The shape of the probability density function (PDF) of the average RI from OxCal resembles a Brownian passage time (BPT) PDF (i.e., rather than normal) that permits rarer longer ruptures potentially involving the Berryessa and Hunting Creek sections of the northernmost GVF The model coefficient of variation (cv, σ/μ) is 0.41, but a larger value (cv ~ 0.6) fits better when using BPT. A BPT PDF with μ of 250 and cv of 0.6 yields 30 yr rupture probabilities of 20%-25% versus a Poisson probability of 11%-17%.


Madugo C.M.,Earth Consultants International | Dolan J.F.,University of Southern California | Hartleb R.D.,University of Southern California | Hartleb R.D.,Lettis Consultants International Inc.
Bulletin of the Seismological Society of America | Year: 2012

New paleoseismic investigations on the western segment of the Garlock fault at Twin Lakes, California, reveal evidence for up to six surface ruptures in the past ~5600 years. Calibrated radiocarbon dates from accelerator mass spectrometer analysis of detrital charcoal constrain the timing of three well-defined events at Twin Lakes to post-A.D. 1450 (event A), 720-395 B.C. (event G), and 3425-2200 B.C. (event I); two probable events are also constrained to 625-1525 A.D. (event C), and 155 B.C.-A.D. 615 (event E), and a possible additional event to 3425-2200 B.C. and prior to event I. Our findings offer new insights into mid- to late-Holocene behavior of the western and central segments of the Garlock fault, and regional fault interactions. The timing of the most recent event (MRE) on the western segment likely correlates with the MRE at paleoseismic sites on the central segment, suggesting that both segments do sometimes rupture together during large earthquakes. Evidence for events during periods of seismic quiescence on adjacent segments demonstrates that the western and central segments also sometimes rupture independently of one another. The occurrence of event G during a lull in seismic strain release at 2-5 ka on faults in the eastern California shear zone (ECSZ), contrasts with other studies that suggest the Garlock fault ruptures in phase with ECSZ faults. Our data suggest that seismic strain release on the Garlock fault may actually be more in phase with moment release on the Mojave section of the San Andreas fault and the Transverse Ranges faults of the Los Angeles region.

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