William Lettis and Associates

Walnut Creek, CA, United States

William Lettis and Associates

Walnut Creek, CA, United States
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Kozaci O.,University of Southern California | Kozaci O.,William Lettis and Associates | Dolan J.F.,University of Southern California | Yonlu O.,Eskiehir Osmangazi University | And 2 more authors.
Lithosphere | Year: 2011

Paleoseismologic trenches excavated across the eastern part of the North Anatolian fault at Yaylabeli, Turkey, provide evidence for five surface ruptures during the past 2000 yr. We interpret these events as: (1) the historical 1939 Mw 7.9 earthquake; (2) the historical 1254 A.D. earthquake; (3) the historical 1045 A.D. earthquake; (4) an earthquake that occurred between 660 A.D. and 1020 A.D., most probably between 717 A.D. and 844 A.D.; and (5) an earthquake that occurred between 302 A.D. and 724 A.D., possibly the historical 499 A.D. event. Although one of the interevent intervals we document is 685 yr long (between the 1254 A.D. and 1939 A.D. earthquakes), the other three intervals are between 200 and 350 yr long. Our results, which facilitate a rare opportunity to test the completeness of the paleoseismologic record at multiple sites, are generally similar to those from the nearby Çukurçimen trench site, located 2 km to the east, demonstrating reproducibility of the paleoearthquake record. However, the eighthto ninth-century event (E4) that we document at Yaylabeli was not observed at Çukurçimen. The addition of this event facilitates the recognition of a previously unnoticed North Anatolian fault earthquake cluster, during which at least the eastern and central parts of the fault appear to have ruptured during a brief sequence in the eighth and ninth centuries. Addition of this possible cluster suggests that the North Anatolian fault commonly ruptures in brief, systemwide sequences, although the individual earthquakes in each sequence differ from cluster to cluster in terms of location, magnitude, and rupture sequence. These paleoearthquake data reinforce the idea of relatively regular recurrence of infrequent, large-magnitude earthquakes on the eastern section of the North Anatolian fault. We attribute this relatively simple behavior to the structural maturity of the North Anatolian fault and its relative isolation from other major seismic sources within the Anatolia-Eurasia plate boundary. © 2011 Geological Society of America.

Amos C.B.,William Lettis and Associates | Kelson K.I.,William Lettis and Associates | Rood D.H.,Lawrence Livermore National Laboratory | Rood D.H.,University of California at Santa Barbara | And 2 more authors.
Lithosphere | Year: 2010

The Kern Canyon fault represents a major tectonic and physiographic boundary in the southern Sierra Nevada of east-central California. Previous investigations of the Kern Canyon fault underscore its importance as a Late Cretaceous and Neogene shear zone in the tectonic development of the southern Sierra Nevada. Study of the late Quaternary history of activity, however, has been confounded by the remote nature of the Kern Canyon fault and deep along-strike exhumation within the northern Kern River drainage, driven by focused fl uvial and glacial erosion. Recent acquisition of airborne lidar (light detection and ranging) topography along the ~140 km length of the Kern Canyon fault provides a comprehensive view of the active surface trace. High-resolution, lidar-derived digital elevation models (DEMs) for the northern Kern Canyon fault enable identifi cation of previously unrecognized offsets of late Quaternary moraines near Soda Spring (36.345°N, 118.408°W). Predominately north-striking fault scarps developed on the Soda Spring moraines display west-side-up displacement and lack a signifi cant sense of strike-slip separation, consistent with detailed mapping and trenching along the entire Kern Canyon fault. Scarp-normal topographic profi ling derived from the lidar DEMs suggests normal displacement of at least 2.8 +0.6/-0.5 m of the Tioga terminal moraine crest. Cosmogenic 10Be exposure dating of Tioga moraine boulders yields a tight age cluster centered around 18.1 ± 0.5 ka (n = 6), indicating a minimum normal-sense fault slip rate of ~0.1-0.2 mm/yr over this period. Taken together, these results provide the fi rst clear documentation of late Quaternary activity on the Kern Canyon fault and highlight its role in accommodating internal deformation of the southern Sierra Nevada. © 2010 Geological Society of America.

Niedoroda A.W.,URS Corporation | Resio D.T.,U.S. Army | Toro G.R.,William Lettis and Associates | Divoky D.,AECOM Technology Corporation | And 2 more authors.
Ocean Engineering | Year: 2010

Following the extreme flooding caused by Hurricane Katrina, the Federal Emergency Management Agency (FEMA) commissioned a study to update the Mississippi coastal flood hazard maps. The project included development and application of new methods incorporating the most recent advances in numerical modeling of storms and coastal hydrodynamics, analysis of the storm climatology, and flood hazard evaluation. This paper discusses the methods that were used and how they were applied to the coast of the State of Mississippi. © 2009 Elsevier Ltd.

Toro G.R.,William Lettis and Associates | Resio D.T.,U.S. Army | Divoky D.,AECOM Technology Corporation | Niedoroda A.Wm.,URS Corporation | Reed C.,URS Corporation
Ocean Engineering | Year: 2010

The Joint-Probability Method (JPM) was adopted by federal agencies for critical post-Katrina determinations of hurricane surge frequencies. In standard JPM implementations, it is necessary to consider a very large number of combinations of storm parameters, and each such combination (or synthetic storm) requires the simulation of wind, waves, and surge. The tools used to model the wave and surge phenomena have improved greatly in recent years, but this improvement and the use of very large high-resolution grids have made the computations both time-consuming and expensive. In order to ease the computational burden, two independent approaches have been developed to reduce the number of storm surge simulations that are required. Both of these so-called JPM-OS (JPM-Optimal Sampling) methods seek to accurately cover the entire storm parameter space through optimum selection of a small number of parameter values so as to minimize the number of required storm simulations. Tests done for the Mississippi coast showed that the accuracy of the two methods is comparable to that of a full JPM analysis, with a reduction of an order of magnitude or more in the computational effort. © 2009 Elsevier Ltd.

Berger G.W.,Desert Research Institute | Sawyer T.L.,Piedmont Geosciences Inc. | Unruh J.R.,William Lettis and Associates
Bulletin of the Seismological Society of America | Year: 2010

Near urban areas and extending ~60 km along the eastern margin of the Livermore Valley, the Greenville fault is the easternmost right-lateral strike-slip fault of the San Andreas system in the greater San Francisco Bay area. Notwithstanding the 1980 Livermore earthquake sequence (mainshock ML 5.9) on the Greenville fault, there is no record of recency or of Holocene rates of activity on the Greenville fault, yet this fault exhibits clear geomorphic evidence of late Quaternary faulting. In trenches parallel and normal to the fault through alluvial fan deposits at the Laughlin Road site only pedogenic carbonate was available for 14C dating. Therefore, we applied several photon-stimulated luminescence (PSL) sediment-dating procedures to the silt and sand fractions of six samples. The polymineral-fine-silt multi-aliquot age estimates are generally inaccurate, but the single-grain quartz (SGQ) and multigrain quartz single-aliquot regenerative-dose (SAR) ages from sand grains are in stratigraphic sequence. In trench 3A these SAR ages range from 125 11 yrs (before 2007) within the topmost unit L to 13:45 0:79 ka in the base of the lowermost channel-fill unit G. The SAR PSL results demonstrate the importance of the use of SGQ dating for such sediments and provide the first numerical ages used to constrain the slip rate on the Greenville fault. Trench exposures reveal that unit G is an alluvial sequence infilling a paleochannel offset in a right-lateral sense along the northern Greenville fault. Age estimates from upper and middle unit G bracket deposition of subunit Gb are between 11:12 0:55 ka and 10:64 0:85 ka and those from middle and lower unit G bracket deposition of subunit Go are between 11:12 0:55 ka and 13:45 0:79 ka. These SAR PSL age estimates and measurements of the lateral offset constrain a preliminary slip-rate estimate to about 2 mm=yr or higher for the northern Greenville fault zone.

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%.

Denlinger R.P.,U.S. Geological Survey | O'Connell D.R.H.,William Lettis and Associates
Bulletin of the Geological Society of America | Year: 2010

Using a flow domain that we constructed from 30 m digital-elevation model data of western United States and Canada and a two-dimensional numerical model for shallow water flow over rugged terrain, we simulated outburst floods from Pleistocene Glacial Lake Missoula. We modeled a large, but not the largest, flood, using initial lake elevation at 1250 m instead of 1285 m. Rupture of the ice dam, centered on modern Lake Pend Oreille, catastrophically floods eastern Washington and rapidly fills the broad Pasco, Yakima, and Umatilla Basins. Maximum flood stage is reached in Pasco and Yakima Basins 38 h after the dam break, whereas maximum flood stage in Umatilla Basin occurs 17 h later. Drainage of these basins through narrow Columbia gorge takes an additional 445 h. For this modeled flood, peak discharges in eastern Washington range from 10 to 20 × 106 m3/s. However, constrictions in Columbia gorge limit peak discharges to <6 × 106 m3/s and greatly extend the duration of flooding. We compare these model results with field observations of scabland distribution and high-water indicators. Our model predictions of the locations of maximum scour (product of bed shear stress and aver age flow velocity) match the distribution of existing scablands. We compare model peak stages to high-water indicators from the Rathdrum-Spokane valley, Walulla Gap, and along Columbia gorge. Though peak stages from this less-than-maximal flood model attain or exceed peak-stage indicators along Rathdrum-Spokane valley and along Columbia gorge, simulated peak stages near Walulla Gap are 10-40 m below observed peak-stage indicators. Despite this discrepancy, our match to field observations in most of the region indicates that additional sources of water other than Glacial Lake Missoula are not required to explain the Missoula floods. © 2010 Geological Society of America.

Frost E.,University of Southern California | Frost E.,William Lettis and Associates | Dolan J.,University of Southern California | Ratschbacher L.,TU Bergakademie Freiberg | And 2 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011

Structural analysis of two key exposures reveals the architecture of the brittle-ductile transition (BDT) of the subvertical, strike-slip Salzachtal fault. At Lichtensteinklamm, the fault zone is dominantly brittle, with a ∼70 m wide, high-strain fault core highlighted by a 50 m thick, highly foliated gouge zone. In contrast, at Kitzlochklamm, deformation is dominantly ductile, albeit with relatively low strain indicated by weak lattice-preferred orientations (LPOs). The marked contrast in structural style indicates that these sites span the BDT. The close proximity of the outcrops, coupled with Raman spectroscopy indicating similar maximum temperatures of ∼400C, suggests that the difference in exhumation depth is small, with a commensurately small difference in total downdip width of the BDT. The small strains indicated by weak LPOs at Kitzlochklamm, coupled with evidence for brittle slip at the main fault contact and along the sides of a 5 m wide fault-bounded sliver of Klammkalk exposed 30 m into the Grauwacken zone rocks, suggest the possibility that this exposure may record hybrid behavior at different times during the earthquake cycle, with ductile deformation occurring during slow interseismic slip and brittle deformation occurring during earthquakes, as dynamic coseismic stresses induced a strain rate-dependent shift to brittle fault behavior within the nominally ductile regime in the lower part of the BDT. A key aspect of both outcrops is evidence of a high degree of strain localization through the BDT, with high-strain fault cores no wider than a few tens of meters. Copyright © 2011 by the American Geophysical Union.

Toro G.R.,William Lettis and Associates | Niedoroda A.W.,URS Corporation | Reed C.W.,URS Corporation | Divoky D.,AECOM Technology Corporation
Ocean Engineering | Year: 2010

The Joint Probability Method (JPM) has been used for hurricane surge frequency analysis for over three decades, and remains the method of choice owing to the limitations of more direct historical methods. However, use of the JPM approach in conjunction with the modern generation of complex high-resolution numerical models (used to describe winds, waves, and surge) has become highly inefficient, owing to the large number of costly storm simulations that are typically required. This paper describes a new approach to the selection of the storm simulation set that permits reduction of the JPM computational effort by about an order of magnitude (compared to a more conventional approach) while maintaining good accuracy. The method uses an integration scheme called Bayesian or Gaussian-process quadrature (together with conventional integration methods) to evaluate the multi-dimensional joint probability integral over the space of storm parameters (pressure, radius, speed, heading, and any others found to be important) as a weighted summation over a relatively small set of optimally selected nodes (synthetic storms). Examples of an application of the method are shown, drawn from the recent post-Katrina study of coastal Mississippi. © 2009 Elsevier Ltd.

O'Connell D.R.H.,William Lettis and Associates | Turner J.P.,William Lettis and Associates
Bulletin of the Seismological Society of America | Year: 2011

Refraction microtremor (ReMi) and multichannel analysis of surface waves (MASW) are effective approaches to estimate shallow shear-wave velocity (VS) structure often needed to estimate ground motions using recent ground motion prediction relations. Interferometric MASW (IMASW) uses slowness-frequency slantstack analyses combined with interferometric time-domain dispersion analyses to improve resolution of lower-frequency Rayleigh-wave dispersion to better constrain VS. Cross-correlation interferometry is used to obtain deterministic correlation Green's function (CGF) IMASW seismograms from ambient-noise and/or activesource wave fields contained in ReMi and/or MASW data. The CGFs are processed using the multiple-filter technique to estimate phase and group dispersion. In the IMASW approach, active seismic sources ensure that the stationary-phase contributions to cross correlations dominate CGF responses. In a single IMASW profile, each geophone represents a virtual source, and the IMASW approach stacks CGF commonoffset data from all virtual sources to obtain a single averaged forward-and reverserecord section. CGF time-domain and slowness-frequency phase-slowness estimates are combined with CGF time-domain group slowness estimates for a consistency check on dispersion picks. A multistate Monte Carlo approach is used to estimate mean slowness depth and slowness uncertainties. IMASW is evaluated with passive ReMi data from two sites and active-source IMASW at six sites with independent downhole velocity-depth logs. Comparison of six P-S suspension log-IMASW profile pairs across the Van Norman Complex in northern San Fernando Valley shows that, on average, 30-m-depth shear-wave velocity estimates between the two methods differed by <1%. At two sites where P-S suspension log measurements of VS were made at the IMASW profile midpoint, the IMASW VS depth inversions resolve 3-m thickness VS variations accurately to the bottom of one borehole at 40-m depth and to 100-m depth at a >200-m-deep borehole site.

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