Reflection and critically refracted seismic methods use traveltime measurements of body waves propagating between a source and a series of receivers on the ground surface to calculate subsurface velocities. Body wave energy is refracted or reflected at boundaries where there is a change in seismic impedance, defined as the product of material density and seismic velocity. This article provides practical guidance on the use of horizontally propagating shear wave (SH-wave) refraction and reflection methods to determine shear wave velocity as a function of depth for near-surface seismic site characterizations. Method principles and the current state of engineering practice are reviewed, along with discussions of limitations and uncertainty assessments. Typical data collection procedures are described using basic survey equipment, along with information on more advanced applications and emerging technologies. Eight case studies provide examples of the techniques in real-world seismic site characterizations performed in a variety of geological settings.

ABSTRACT We analyze multimethod shear (SH)-wave velocity (VS) site characterization data acquired at three permanent and 25 temporary seismograph stations in Oklahoma that recorded M 4+ earthquakes within a 50 km hypocentral distance of at least one of the 2016 M 5.1 Fairview, M 5.8 Pawnee, or M 5.0 Cushing earthquakes to better constrain earthquake ground-motion modeling in the region. We acquired active-source seismic data for time-averaged VS to 30 m depth (VS30) at 28 seismograph stations near the Fairview, Pawnee, and Cushing epicentral areas. The SH-wave refraction travel times coupled with Rayleigh- and Love-wave phase velocity dispersion were extracted and modeled in a nonlinear least-squares (L2) joint inversion to obtain a best-fit 1D VS versus depth profile for each site. At a subset of sites where the preferred L2 inverse model did not optimally fit each of the Love, Rayleigh, and SH travel-time datasets, we explore application of simulated annealing in a joint inversion to find a more global solution. VS30 values range from 262 to 807 m/s for the preferred measured (in situ) VS profiles, or National Earthquake Hazards Reduction Program (NEHRP) site class D to B, and are broadly comparable with estimates from previous data reports in the region. Site amplification estimates were calculated next from 1D SH transfer functions of the preferred VS profiles and then compared against observed horizontal-to-vertical spectral ratios (HVSRs) from nearby seismograph stations. We generally see good agreement between the predicted in situ model and the observed HVSR resonant frequencies, with nominal amplifications between 2 and 10 within the 2–15 Hz frequency band. Next, using 40 known in situ VS30 measurements in the region, we demonstrate that the in situ VS30 values improve the fit for selected suites of ground-motion models (GMMs) for M 4+ earthquakes within a 50 km hypocentral distance when compared with proxy methods, arguing for future development of GMMs implementing in situ VS profiles.

The New Madrid seismic zone has been the source of multiple major (M ~7.0–7.5) earthquakes in the past 2 ka, yet the surface expression of recent deformation remains ambiguous. Crowleys Ridge, a linear ridge trending north‐south for 300+ km through the Mississippi Embayment, has been interpreted as either a fault‐bounded uplift or a nontectonic erosional remnant. New and previously published seismic reflection and shallow resistivity data show discontinuities at the ridge margins in Plio‐Pleistocene strata, yet the timing of most recent faulting and the lateral extent of these faults remain unknown. To assess Pleistocene‐to‐recent tectonic activity of Crowleys Ridge, we perform landscape‐scale geomorphic analyses, such as relief, slope, hypsometry, and drainage basin shape, on a 10‐m digital elevation model (DEM). North‐to‐south variations in geomorphic indices indicate Pleistocene‐to‐recent tectonic uplift of the southern ridge. Moreover, mapping on a <1‐m lidar‐derived DEM reveals scarps on late Pleistocene geomorphic surfaces. The scarps are primarily located along the southern ridge, trend parallel to the ridge margin discontinuously for 0.1–1 km, and vertically offset <56 ka surfaces 0.4 m with up to 6 m of tilting. These landscape‐scale patterns and scarps, integrated with discontinuities in the seismic reflection and resistivity data, provide evidence of low‐rate (<0.2 mm/year) late Quaternary tectonic activity along the southern segment of Crowleys Ridge. The interpretations agree with recent tectonic models suggesting southern Crowleys Ridge is a compressional step over in a right‐lateral fault system within the Reelfoot Rift.

Three sequences of well‐documented, major ~M7+ earthquakes (1811–1812, ~1450, and ~900 CE) in the New Madrid seismic zone, USA, contribute significantly to seismic hazard in the region. However, it is unknown whether this <550‐year recurrence interval has been constant throughout the Holocene given limited geomorphic evidence of prior earthquakes. We extend the record of paleoearthquakes along the Reelfoot fault via investigation of ridgetop gravitational failure features, interpreted as sackungen. The sackungen occur in bluffs along the eastern margin of the Mississippi River floodplain and are concentrated near (<15 km) the southwest dipping Reelfoot reverse fault. A paleoseismic trench excavated across sackungen at the Paw Paw site exposed four packages of colluvial sediment that postdate 30‐ to 11‐ka Peoria Loess. We interpret the colluvial packages to have been deposited following episodic failure of the sackungen as a result of strong ground motions from the following sequence of earthquakes: event 4, 1640 ± 1730 BCE; event 3, 270 ± 670 CE; event 2, 1430 ± 380 CE; and event 1, 1810 ± 50 CE (2 sigma). Event timing corresponds to previously documented earthquakes and represents the longest archive of paleoearthquakes on the Reelfoot fault. If the trenched sackungen record all major Reelfoot fault earthquakes, our observations in combination with prior investigations indicate a period of quiescence from at least 11 to 4.4 ka, followed by four major seismic events culminating in the 1811–1812 CE sequence. This clustered earthquake recurrence pattern helps place bounds on seismic hazard and geodynamic models in the New Madrid seismic zone.

Editorial| February 28, 2018 Integrate Urban‐Scale Seismic Hazard Analyses with the U.S. National Seismic Hazard Model M. P. Moschetti; M. P. Moschetti aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.govfU.S. Geological Survey Working Group on Urban Seismic Hazard Maps. Search for other works by this author on: GSW Google Scholar N. Luco; N. Luco aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar A. D. Frankel; A. D. Frankel bUniversity of Washington, Department of Earth and Space Sciences, ATG‐228, Seattle, Washington 98195 U.S.A. Search for other works by this author on: GSW Google Scholar M. D. Petersen; M. D. Petersen aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar B. T. Aagaard; B. T. Aagaard cU.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025‐3591 U.S.A. Search for other works by this author on: GSW Google Scholar A. S. Baltay; A. S. Baltay cU.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025‐3591 U.S.A. Search for other works by this author on: GSW Google Scholar M. L. Blanpied; M. L. Blanpied dU.S. Geological Survey, 12201 Sunrise Valley Drive, MS 905, Reston, Virginia 20192 U.S.A. Search for other works by this author on: GSW Google Scholar O. S. Boyd; O. S. Boyd aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar R. W. Briggs; R. W. Briggs aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar R. D. Gold; R. D. Gold aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar R. W. Graves; R. W. Graves eU.S. Geological Survey, 525 South Wilson Avenue, Pasadena, California 91106 U.S.A. Search for other works by this author on: GSW Google Scholar S. H. Hartzell; S. H. Hartzell aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar S. Rezaeian; S. Rezaeian aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar W. J. Stephenson; W. J. Stephenson aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar D. J. Wald; D. J. Wald aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar R. A. Williams; R. A. Williams aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar K. B. Withers K. B. Withers aU.S. Geological Survey, P.O. Box 25046, MS 966, Denver, Colorado 80225 U.S.A., mmoschetti@usgs.gov Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2018) 89 (3): 967–970. https://doi.org/10.1785/0220170261 Article history first online: 23 Apr 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. P. Moschetti, N. Luco, A. D. Frankel, M. D. Petersen, B. T. Aagaard, A. S. Baltay, M. L. Blanpied, O. S. Boyd, R. W. Briggs, R. D. Gold, R. W. Graves, S. H. Hartzell, S. Rezaeian, W. J. Stephenson, D. J. Wald, R. A. Williams, K. B. Withers; Integrate Urban‐Scale Seismic Hazard Analyses with the U.S. National Seismic Hazard Model. Seismological Research Letters 2018;; 89 (3): 967–970. doi: https://doi.org/10.1785/0220170261 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search For more than 20 yrs, damage patterns and instrumental recordings have highlighted the influence of the local 3D geologic structure on earthquake ground motions (e.g., M 6.7 Northridge, California, Gao et al., 1996; M 6.9 Kobe, Japan, Kawase, 1996; M 6.8 Nisqually, Washington, Frankel, Carver, and Williams, 2002). Although this and other local‐scale features are critical to improving seismic hazard forecasts, historically they have not been explicitly incorporated into the U.S. National Seismic Hazard Model (NSHM, national model and maps), primarily because the necessary basin maps and methodologies were not available at the national scale. Instead,... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

This article describes the U.S. Geological Survey (USGS) 2018 one-year probabilistic seismic hazard forecast for the central and eastern United States from induced and natural earthquakes. For consistency, the updated 2018 forecast is developed using the same probabilistic seismicity-based methodology as applied in the two previous forecasts. Rates of earthquakes across the United States M >= 3.0 grew rapidly between 2008 and 2015 but have steadily declined over the past 3 years, especially in areas of Oklahoma and southern Kansas where fluid injection has decreased. The seismicity pattern in 2017 was complex with earthquakes more spatially dispersed than in the previous years. Some areas of west-central Oklahoma experienced increased activity rates where industrial activity increased. Earthquake rates in Oklahoma (429 earthquakes of M >= 3 and 4 M >= 4), Raton basin (Colorado/New Mexico border, six earthquakes M >= 3), and the New Madrid seismic zone (11 earthquakes M >= 3) continue to be higher than historical levels. Almost all of these earthquakes occurred within the highest hazard regions of the 2017 forecast. Even though rates declined over the past 3 years, the short-term hazard for damaging ground shaking across much of Oklahoma remains at high levels due to continuing high rates of smaller earthquakes that are still hundreds of times higher than at any time in the state's history. Fine details and variability between the 2016-2018 forecasts are obscured by significant uncertainties in the inputmodel. These short-term hazard levels are similar to active regions in California. During 2017, M >= 3 earthquakes also occurred in or near Ohio, West Virginia, Missouri, Kentucky, Tennessee, Arkansas, Illinois, Oklahoma, Kansas, Colorado, New Mexico, Utah, and Wyoming.

The Evergreen basin is a 40-km-long, 8-km-wide Cenozoic sedimentary ­basin that lies mostly concealed beneath the northeastern margin of the Santa Clara Valley near the south end of San Francisco Bay (California, USA). The basin is bounded on the northeast by the strike-slip Hayward fault and an approximately parallel subsurface fault that is structurally overlain by a set of west-verging reverse-oblique faults which form the present-day southeastward extension of the Hayward fault. It is bounded on the southwest by the Silver Creek fault, a largely dormant or abandoned fault that splays from the active southern Calaveras fault. We propose that the Evergreen ­basin formed as a strike-slip pull-apart basin in the right step from the Silver Creek fault to the Hayward fault during a time when the Silver Creek fault served as a segment of the main route by which slip was transferred from the central California San Andreas fault to the Hayward and other East Bay faults. The dimensions and shape of the Evergreen basin, together with palinspastic reconstructions of geologic and geophysical features surrounding it, suggest that during its lifetime, the Silver Creek fault transferred a significant portion of the ∼100 km of total offset accommodated by the Hayward fault, and of the 175 km of total San Andreas system offset thought to have been accommodated by the entire East Bay fault system. As shown previously, at ca. 1.5–2.5 Ma the Hayward-Calaveras connection changed from a right-step, releasing regime to a left-step, restraining regime, with the consequent effective abandonment of the Silver Creek fault. This reorganization was, perhaps, preceded by development of the previously proposed basin-bisecting Mount Misery fault, a fault that directly linked the southern end of the Hayward fault with the southern Calaveras fault during extinction of pull-apart activity. Historic seismicity indicates that slip below a depth of 5 km is mostly transferred from the Calaveras fault to the Hayward fault across the Mission seismic trend northeast of the Evergreen basin, whereas slip above a depth of 5 km is transferred through a complex zone of oblique-reverse faults along and over the northeast basin margin. However, a prominent groundwater flow barrier and related land-subsidence discontinuity coincident with the concealed Silver Creek fault, a discontinuity in the pattern of seismicity on the Calaveras fault at the Silver Creek fault intersection, and a structural sag indicative of a negative flower structure in Quaternary sediments along the southwest basin margin indicate that the Silver Creek fault has had minor ongoing slip over the past few hundred thousand years. Two earthquakes with ∼M6 occurred in A.D. 1903 in the vicinity of the Silver Creek fault, but the available information is not sufficient to reliably identify them as Silver Creek fault events.

Short-term seismic hazard in parts of the central and eastern U.S. (CEUS) has decreased from 2016 to 2017 after significantly increasing in the previous 5 years (Figure 1, 2 and 3). In Texas, 1.6 million people are facing much lower likelihood of damaging shaking than in 2016. However, for north-central Oklahoma and southern Kansas, the 2017 forecast shows that about 3 million people live with continuing increased potential for damaging shaking from induced seismicity that is similar to the 2016 forecast. The chance of damage in the next year from induced earthquakes in parts of Oklahoma is still similar to that of natural earthquakes in high-hazard areas of California. Here we review the 2016 and 2017 U.S. Geological Survey (USGS) one-year seismic hazard forecasts in the CEUS due to natural and human-induced earthquakes (Petersen et al., 2017; Petersen et al., 2016). The forecasts have been used to make decisions about assets and infrastructure by government and regulatory agencies. The reduced hazard in some parts of the CEUS in 2017 is apparently the result of successful partnerships between industry and government at the state and federal level to reduce wastewater injection volumes in regions of high shaking hazard (Murray, 2016; Langenbruch and Zoback, 2016; Yeck et al., 2016; Weingarten, et al., 2016). Presentation Date: Tuesday, September 26, 2017 Start Time: 9:20 AM Location: 362D Presentation Type: ORAL

We produce a one-year 2017 seismic-hazard forecast for the central and eastern United States from induced and natural earthquakes that updates the 2016 one-year forecast; this map is intended to provide information to the public and to facilitate the development of induced seismicity forecasting models, methods, and data. The 2017 hazard model applies the same methodology and input logic tree as the 2016 forecast, but with an updated earthquake catalog. We also evaluate the 2016 seismic-hazard forecast to improve future assessments. The 2016 forecast indicated high seismic hazard (greater than 1% probability of potentially damaging ground shaking in one year) in five focus areas: Oklahoma-Kansas, the Raton basin (Colorado/New Mexico border), north Texas, north Arkansas, and the New Madrid Seismic Zone. During 2016, several damaging induced earthquakes occurred in Oklahoma within the highest hazard region of the 2016 forecast; all of the 21 moment magnitude (M) >= 4 and 3 M >= 5 earthquakes occurred within the highest hazard area in the 2016 forecast. Outside the Oklahoma-Kansas focus area, two earthquakes with M >= 4 occurred near Trinidad, Colorado (in the Raton basin focus area), but no earthquakes with M >= 2.7 were observed in the north Texas or north Arkansas focus areas. Several observations of damaging ground-shaking levels were also recorded in the highest hazard region of Oklahoma. The 2017 forecasted seismic rates are lower in regions of induced activity due to lower rates of earthquakes in 2016 compared with 2015, which may be related to decreased wastewater injection caused by regulatory actions or by a decrease in unconventional oil and gas production. Nevertheless, the 2017 forecasted hazard is still significantly elevated in Oklahoma compared to the hazard calculated from seismicity before 2009.

We present probabilistic and deterministic seismic and liquefaction hazard maps for the densely populated St. Louis metropolitan area that account for the expected effects of surficial geology on earthquake ground shaking. Hazard calculations were based on a map grid of 0.005 degrees, or about every 500 m, and are thus higher in resolution than any earlier studies. To estimate ground motions at the surface of the model (e.g., site amplification), we used a new detailed near-surface shear-wave velocity model in a 1D equivalent- linear response analysis. When compared with the 2014 U.S. Geological Survey (USGS) National Seismic Hazard Model, which uses a uniform firm-rock-site condition, the new probabilistic seismic-hazard estimates document much more variability. Hazard levels for upland sites (consisting of bedrock and weathered bedrock overlain by loess-covered till and drift deposits), show up to twice the ground-motion values for peak ground acceleration (PGA), and similar ground-motion values for 1.0 s spectral acceleration (SA). Probabilistic ground-motion levels for lowland alluvial floodplain sites (generally the 20-40-m-thick modern Mississippi and Missouri River floodplain deposits overlying bedrock) exhibit up to twice the ground-motion levels for PGA, and up to three times the ground-motion levels for 1.0 s SA. Liquefaction probability curves were developed from available standard penetration test data assuming typical lowland and upland water table levels. A simplified liquefaction hazard map was created from the 5%-in-50-year probabilistic ground-shaking model. The liquefaction hazard ranges from low (<40% of area expected to liquefy) in the uplands to severe (>60% of area expected to liquefy) in the lowlands. Because many transportation routes, power and gas transmission lines, and population centers exist in or on the highly susceptible lowland alluvium, these areas in the St. Louis region are at significant potential risk from seismically induced liquefaction and associated ground deformation.

We collected new high-resolution P-wave seismic-reflection data to explore for possible faults beneath a roughly linear cluster of early to mid-Holocene earthquake-induced sand blows to the south of Marianna, Arkansas. The Daytona Beach sand blow deposits are located in east-central Arkansas about 75 km southwest of Memphis, Tennessee, and about 80 km south of the southwestern end of the New Madrid seismic zone (NMSZ). Previous studies of these sand blows indicate that they were produced between 10,500 and 5350 yr B.P. (before A.D. 1950). The sand blows are large and similar in size to those in the heart of the NMSZ produced by the 18111812 earthquakes. The seismic-reflection profiles reveal a previously unknown zone of near-vertical faults imaged in the 100-1100-m depth range that are approximately coincident with a cluster of earthquake-induced sand blows and a near-linear surface lineament composed of air photo tonal anomalies. These interpreted faults are expressed as vertical discontinuities with the largest displacement fault showing about 40 m of west-side-up displacement at the top of the Paleozoic section at about 1100 m depth. There are about 20 m of folding on reflections within the Eocene strata at 400 m depth. Increasing fault displacement with depth suggests long-term recurrent faulting. The imaged faults within the vicinity of the numerous sand blow features could be a causative earthquake source, although it does not rule out the possibility of other seismic sources nearby. These newly located faults add to a growing list of potentially active Pleistocene-Holocene faults discovered over the last two decades that are within the Mississippi embayment region but outside of the historical NMSZ.

In October 2014 two moderate-sized earthquakes (Mw 4.0 and 4.3) struck south of Cushing, Oklahoma, below the largest crude oil storage facility in the world. Combined analysis of the spatial distribution of earthquakes and regional moment tensor focal mechanisms indicate reactivation of a subsurface unnamed and unmapped left-lateral strike-slip fault. Coulomb failure stress change calculations using the relocated seismicity and slip distribution determined from regional moment tensors, allow for the possibility that the Wilzetta-Whitetail fault zone south of Cushing, Oklahoma, could produce a large, damaging earthquake comparable to the 2011 Prague event. Resultant very strong shaking levels (MMI VII) in the epicentral region present the possibility of this potential earthquake causing moderate to heavy damage to national strategic infrastructure and local communities.

Earthquake damage is often increased due to local ground-motion amplification caused by soft soils, thick basin sediments, topographic effects, and liquefaction. A critical factor contributing to the assessment of seismic hazard is detailed information on local site response. In order to address and quantify the site response at seismograph stations in the eastern United States, we investigate the regional spatial variation of horizontal:vertical spectral ratios (HVSR) using ambient noise recorded at permanent regional and national network stations as well as temporary seismic stations deployed in order to record aftershocks of the 2011 Mineral, Virginia, earthquake. We compare the HVSR peak frequency to surface measurements of the shear-wave seismic velocity to 30 m depth (Vs30) at 21 seismograph stations in the eastern United States and find that HVSR peak frequency increases with increasing Vs30. We use this relationship to estimate the National Earthquake Hazards Reduction Program soil class at 218 ANSS (Advanced National Seismic System), GSN (Global Seismographic Network), and RSN (Regional Seismograph Networks) locations in the eastern United States, and suggest that this seismic station–based HVSR proxy could potentially be used to calibrate other site response characterization methods commonly used to estimate shaking hazard.

New seismic hazard maps have been developed for the conterminous United States using the latest data, models, and methods available for assessing earthquake hazard. The hazard models incorporate new information on earthquake rupture behavior observed in recent earthquakes; fault studies that use both geologic and geodetic strain rate data; earthquake catalogs through 2012 that include new assessments of locations and magnitudes; earthquake adaptive smoothing models that more fully account for the spatial clustering of earthquakes; and 22 ground motion models, some of which consider more than double the shaking data applied previously. Alternative input models account for larger earthquakes, more complicated ruptures, and more varied ground shaking estimates than assumed in earlier models. The ground motions, for levels applied in building codes, differ from the previous version by less than ±10% over 60% of the country, but can differ by ±50% in localized areas. The models are incorporated in insurance rates, risk assessments, and as input into the U.S. building code provisions for earthquake ground shaking.

The sharp increase in seismicity over a broad region of central Oklahoma has raised concerns regarding the source of the activity and its potential hazard to local communities and energy-industry infrastructure. Efforts to monitor and characterize the earthquake sequences in central Oklahoma are reviewed. Since early 2010, numerous organizations have deployed temporary portable seismic stations in central Oklahoma to record the evolving seismicity. A multiple-event relocation method is applied to produce a catalog of central Oklahoma earthquakes from late 2009 into early 2015. Regional moment tensor (RMT) source parameters were determined for the largest and best-recorded earthquakes. Combining RMT results with relocated seismicity enabled determination of the length, depth, and style of faulting occurring on reactivated subsurface fault systems. It was found that the majority of earthquakes occur on near-vertical, optimally oriented (northeast-southwest and northwest-southeast) strike-slip faults in the shallow crystalline basement. In 2014, 17 earthquakes occurred with magnitudes of 4 or larger. It is suggested that these recently reactivated fault systems pose the greatest potential hazard to the region.

Archaeological data provide a critical perspective on the emergent relationships between Melanesians and Europeans during the mid-nineteenth century. Particularly important to the discussion are the native landscapes within which Europeans settled. As part of a larger exploration of early missionary settlement in southern Vanuatu, archaeologists working closely with local fieldworkers surveyed the native villages at Kwaraka and Anuikaraka, south Tanna Island. These archaeological settlements are notable for their well-preserved stone architecture, rare on Tanna, and their association with local oral traditions concerning inter-island exchange and early Melanesian engagements with Christianity. Archaeological research at Kwaraka has begun to explore the long-term settlement dynamics of these sites, as initial excavations have indicated features dating from the seventeenth through the mid-twentieth century. While direct evidence of nineteenth-century habitation was limited, preliminary results suggest European goods were rare at these sites, while also revealing information about more prominent local exchange networks that persisted through the colonial era. Archaeological approaches that span precolonial and colonial periods can challenge orthodox models for the emergence of modernity, while also providing important long-term perspectives on local historical trajectories.

The late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley illustrates the dynamic nature of the North American-Pacific plate boundary and its effect on basin and landscape development. Prior to early Miocene time, the area that became Santa Clara Valley consisted of eroding Franciscan complex basement structurally interleaved in places with Coast Range ophiolite and Mesozoic Great Valley sequence, and locally overlapped by Paleogene strata. During early to middle Miocene time, this landscape was flooded by the sea and was deformed locally into deeper depressions such as the Cupertino Basin in the southwestern part of the valley. Marine deposition during the middle and late Miocene laid down thin deposits in shallow water and thick deeper-water deposits in the Cupertino Basin. During this sedimentation, the San Andreas fault system encroached into the valley, with most offset partitioned onto the San Andreas fault southwest of the valley and the southern Calaveras-Silver Creek-Hayward fault system in the northeastern part of the valley. A 6-km-wide right step between the Hayward and Silver Creek faults formed the 40-km-long Evergreen pullapart basin along the northeastern margin of the valley, leaving a basement ridge between it and the Cupertino Basin. The Silver Creek fault was largely abandoned ca. 2.5 Ma in favor of a compressional left step between the Calaveras and Hayward fault, although some slip continued to at least mid-Quaternary time. Gravity, seismic, stratigraphic, and interferometric synthetic aperture radar (InSAR) data indicate no other major San Andreas system faults within the central block between the present-day range-front faults bounding the valley and the Silver Creek fault. Sometime between 9 and 4 Ma (9 and 1 Ma for the central block), the area rose above sea level, and a regional surface of erosion was carved into the Mesozoic and Tertiary rocks. Alluvial gravels were deposited on this surface along the margins of the valley beginning ca. 4 Ma, but they may not have prograded onto the central block until ca. 1 Ma, because no older equivalents of the Pliocene-Quaternary Santa Clara gravels have been found there. Thus, either the central block was high enough relative to the surrounding areas that Santa Clara gravels were never deposited on it, or any Santa Clara gravels deposited there were stripped away before ca. 1 Ma. Analysis of alluvium on the central block implies a remarkably uniform, piston-like, subsidence of the valley of similar to 0.4 mm/yr since ca. 0.8 Ma, possibly extending north to northern San Francisco Bay. Today, the central block continues to subside, the range-front reverse faults are active, and the major active faults of the San Andreas system are mostly outside the valley.

A thick Quaternary alluvial section fills a sedimentary basin beneath the Santa Clara Valley, California, located within the San Andreas Fault system at the south end of San Francisco Bay. This section consists of an upper sequence about 1,000 feet thick containing eight sedimentary cycles and a lower fine-grained unit as thick as several hundred feet. Together these constitute the Quaternary Santa Clara Basin. The section overlies an irregular unconformity with more than 1,200 feet of relief cut into the underlying bedrock. This stratigraphy is determined through study of new wells and seismic reflection profiles, together with a sample of the many thousands of water wells in the valley. It represents a major change and improvement in understanding of the basin, particularly with regard to the upper cyclic sequence, which forms a large groundwater system that is an important resource in the San Francisco Bay region. Each of the eight sedimentary cycles consists of a coarse-grained bottom interval overlain by a fine-grained top, with the coarse bottom forming a permeable sheet that is more or less continuous around the basin and the fine top forming a similarly extensive, relatively impermeable confining layer. This stratigraphic organization contrasts with most previous views, which have considered the coarse sediment in the basin to occur as scattered, discrete lenses and (or) sinuous channel sands, all embedded in a predominantly fine-grained section. Temperature logs in several wells demonstrate that the fine cycle tops do limit vertical movement of groundwater, although this may not be the case where those tops are thin to perhaps locally absent around parts of the basin margin. Age control has been obtained from previous work, in which the sedimentary cycles were correlated with the marine oxygen isotope record and the ages of two deeper Quaternary unconformities were estimated, and from detailed paleomagnetic study of cores from the new wells by E.A. Mankinen. Despite careful search of the cores, very few fossils were found, and none that are helpful in subdividing the section. No tephra (volcanic ash) was recovered, and the few carbon samples found and dated radiometrically are limited to the upper 120 feet of the section. The upper cyclic section ranges in age from 0 to somewhat older than 718 thousand years (ka), and the lower fine-grained section lies between unconformities with estimated ages of 950 and 1500 ka. Reflections in the seismic profiles indicate that layering in the basin is subparallel to the ground surface, and this fact, together with the continuous stratigraphic detail provided by geophysical logs of the new wells, allows the confident interwell correlation required to delineate the sedimentary cycles. The sequence of layers within any one cycle tends to persist laterally between the wells in the dataset, which are spaced 1 to 3 km apart, with most changes occurring gradually. The eight cycles, in contrast, tend to differ from each other in the details of their internal organization. Maps and cross sections show the elevations of cycle boundaries and the underlying bedrock surface, the varying thicknesses of the cycles and of their fine tops and coarse bottoms, and the aggregate thickness of coarse layers in those bottom intervals. Coarse sediment is more abundant toward some parts of the basin margin and in the southern part of the basin. Cycle boundary surfaces are relatively smooth, and their shapes are consistent with having been intercycle topographic surfaces. The underlying bedrock surface has a relief of more than 1,200 feet and deepens toward the center of the basin and the west edge of the fault-bounded Evergreen Basin, which is concealed beneath the east side of the Quaternary basin. The absence of consistent abrupt changes in thicknesses or boundary elevations across the basin or in cross section indicates that the interior of the basin is largely unfaulted, with the Silver Creek strand of the San Andreas system at the west edge of the Evergreen Basin being the sole exception. The east and west margins of the Santa Clara Basin, in contrast, are marked by reverse and thrust fault systems.