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USGS2002

The impact of climatic and seismic events on the short-term evolution of seacliffs based on 3-D mapping: Northern Monterey Bay, California

Coastal cliff retreat along the central California coast is episodic, occurring in response to single large storms or seismic events. Traditional approaches to the study of long-term seacliff retreat utilize historical aerial photography and maps to delineate the landward migration of the top edge of the cliff over periods of tens of years to a century. While these methods yield cumulative retreat amounts, they provide little or no information on the character of the individual retreat events, nor the physical processes of retreat. This study addresses the processes of episodic and short-term coastal cliff retreat through the analysis of seacliff failure styles and retreat magnitudes. The study areas are three, 1-km-long sections of cliffed coast in northern Monterey Bay. The earliest data set is vertical aerial photography from October 18, 1989, taken the day following the magnitude 7.1 Loma Prieta earthquake. More recent photography, collected in late January, early February and early March of 1998, captured seacliff failure in response to the severe storms associated with the 1997-1998 El Nin??o. For each data set, high-resolution digital photogrammetric techniques are used to identify the top edge of the cliff. At each cliff failure location, its position, failure length and character are documented. Results suggest that on a regional scale, the seacliffs respond to seismic and climatic forcing differently. We have found variation in the magnitude of cliff response along the three sections of coast in the study area. Large-scale climatic events such as the 1997-1998 El Nin??o have a greater impact on both the linear extent of seacliff failure and the amount of cliff retreat.

Marine Geology↗

USGS2003

Formation of 'Beach Rock' at Siesta Key, Florida and its influence on barrier island development

Seaward-dipping strata of carbonate-cemented shell debris located along the coast of Siesta Key on the Gulf Coast of the Florida peninsula have long been interpreted to be beachrock equivalent in age to the Pleistocene Anastasia Formation (Stage 5e) of the east coast of Florida. Detailed examination of thin sections along with radiometric dating and isotopic analyses demonstrates clearly that this is a Holocene deposit that is not beachrock but was lithified in a meteoric environment. Whole rock dates, dates from shells only, and from cement only demonstrate that these beach deposits were in place by at least 1800 yr BP and might have been there as long ago as 4300 yr BP. This means that some type of barrier island was in place at that time. Previous investigations have depicted Siesta Key as having a maximum age of 3000 yr with these deposits being located about 2 km landward of the beach deposits. This suggests that the beach deposits might have been the site of the original position of Siesta Key. These data also indicate that sea level must have been near its present position at the time that these foreshore beach deposits were deposited; sometime between 1800 and 4300 yr ago. This scenario indicates that sea level along this coastal reach probably reached its present level at least about 2000 yr ago. ?? 2003 Elsevier B.V. All rights reserved.

Marine Geology↗

USGS1994

The influence of ice on southern Lake Michigan coastal erosion

Coastal ice does not protect the coast but enhances erosion by displacing severe winter wave energy from the beach to the shoreface and by entraining and transporting sediment alongshore and offshore. Three aspects of winter ice in Lake Michigan were studied over a 3-year period and found to have an important influence on coastal sediment dynamics and the coastal sediment budget: (1) the influence of coastal ice on shoreface morphology, (2) the transport of littoral sediments by ice, and (3) the formation of anchor and underwater ice as a frequent and important event entraining and transporting sediment. Coastal lake ice includes a belt of mobile brash (ice blocks) and slush and a dynamic nearshore ice complex consisting of an icefoot, a lakeward sequence of wave-generated ice ridges, and intervening ice lagoons. Our studies indicate that the nearshore ice complex contains a sediment load (0.2 - 1.2 t/m of coast) that is roughly equivalent to the average amount of sand eroded from the coastal bluffs and to the amount sand ice-rafted offshore to the deep lake basin each year. Up to 0.28 t/m of coast can be entrained by ice in a single anchor-ice event, and separate events occurred on 15 days in January 1991. The brash/slush belt is the most important system component responsible for ice-induced sediment transport. Estimates of longshore ice drift, ice volume, and ice-borne sediment load suggest that 0.36 to 4.14 × 10 3 t/d are transported alongshore.

Journal of Great Lakes Research↗

USGS1997

Geochemical characteristics and K-Ar ages of rare-metal bearing pegmatites from the Birimian of southeastern Ghana

The pegmatite-aplite rocks at Mankwadzi (Ejisimanku Hills) in southeastern Ghana are part of the pegmatite district that extends from Cape Coast to Winneba along the Atlantic coastline. The pegmatites are associated with the Cape Coast granite complex and were intruded during the waning phase of the Eburnian Orogeny (∼2.0 Ga). Three muscovite separates from pegmatite give KAr retention ages of 1909 ± 13 Ma , 1965 ± 13 Ma and 2019 ± 14 Ma. A biotite separate from granite yields a KAr age of 1907 ± 13 Ma. These ages are similar to KAr dates previously reported for the Cape Coast granites, indicating that the granites and pegmatites are coeval and probably genetically linked. The pegmatites are enriched in Li, Be, Nb and Sn and considerably impoverished in Rb, Th, Y and REEs. Microscopic examination of quartz from the pegmatites shows a large number of low salinity fluid inclusions that can be divided into two types: (1) one-phase liquid or gas-filled inclusions; and (2) two-phase liquid-vapour inclusions, with the vapour occupying 2–5% of the volume. The homogenisation temperature of the fluid inclusions clusters between 129 and 144°C. These homogenisation temperatures lead to an inferred entrapment temperature of ∼300°C at a pressure of ∼2.5 kbar, which is estimated for the metamorphism of host hornblende schists. The pegmatite fluid inclusions are interpreted as being secondary to the quartz hosts.

Journal of African Earth Sciences↗

USGS2001

Coastline complexity: A parameter for functional classification of coastal environments

To understand the role of the world's coastal zone (CZ) in global biogeochemical fluxes (particularly those of carbon, nitrogen, phosphorus, and sediments) we must generalise from a limited number of observations associated with a few well-studied coastal systems to the global scale. Global generalisation must be based on globally available data and on robust techniques for classification and upscaling. These requirements impose severe constraints on the set of variables that can be used to extract information about local CZ functions such as advective and metabolic fluxes, and differences resulting from changes in biotic communities. Coastal complexity (plan-view tortuosity of the coastline) is a potentially useful parameter, since it interacts strongly with both marine and terrestrial forcing functions to determine coastal energy regimes and water residence times, and since 'open' vs. 'sheltered' categories are important components of most coastal habitat classification schemes. This study employs the World Vector Shoreline (WVS) dataset, originally developed at a scale of 1:250 000. Coastline complexity measures are generated using a modification of the Angle Measurement Technique (AMT), in which the basic measurement is the angle between two lines of specified length drawn from a selected point to the closest points of intersection with the coastline. Repetition of these measurements for different lengths at the same point yields a distribution of angles descriptive of the extent and scale of complexity in the vicinity of that point; repetition of the process at different points on the coast provides a basis for comparing both the extent and the characteristic scale of coastline variation along different reaches of the coast. The coast of northwestern Mexico (Baja California and the Gulf of California) was used as a case study for initial development and testing of the method. The characteristic angle distribution plots generated by the AMT analysis were clustered using LOICZVIEW, a high dimensionality clustering routine developed for large-scale coastal classification studies. The results show distinctive differences in coastal environments that have the potential for interpretation in terms of both biotic and hydrogeochemical environments, and that can be related to the resolution limits and uncertainties of the shoreline data used. These objective, quantitative measures of coastal complexity as a function of scale can be further developed and combined with other data sets to provide a key component of functional classification of coastal environments. ?? 2001 Elsevier Science B.V. All rights reserved.

Conference Paper↗

USGS2001

Deformation across the Pacific-North America plate boundary near San Francisco, California

We have detected a narrow zone of compression between the Coast Ranges and the Great Valley, and we have estimated slip rates for the San Andreas, Rodgers Creek, and Green Valley faults just north of San Francisco. These results are based on an analysis of campaign and continuous Global Positioning System (GPS) data collected between 1992 and 2000 in central California. The zone of compression between the Coast Ranges and the Great Valley is 25 km wide. The observations clearly show 3.8±1.5 mm yr −1 of shortening over this narrow zone. The strike slip components are best fit by a model with 20.8±1.9 mm yr −1 slip on the San Andreas fault, 10.3±2.6 mm yr −1 on the Rodgers Creek fault, and 8.1±2.1 mm yr −1 on the Green Valley fault. The Pacific-Sierra Nevada-Great Valley motion totals 39.2±3.8 mm yr −1 across a zone that is 120 km wide (at the latitude of San Francisco). Standard deviations are one σ. The geodetic results suggest a higher than geologic rate for the Green Valley fault. The geodetic results also suggest an inconsistency between geologic estimates of the San Andreas rate and seismologic estimates of the depth of locking on the San Andreas fault. The only convergence observed is in the narrow zone along the border between the Great Valley and the Coast Ranges.

California↗

USGS2001

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m 3 with local peaks >100 μg/m 3 . The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

Journal of Geophysical Research D: Atmospheres↗

USGS2002

Valley floor climate observations from the McMurdo dry valleys, Antarctica, 1986-2000

Climate observations from the McMurdo dry valleys, East Antarctica are presented from a network of seven valley floor automatic meteorological stations during the period 1986 to 2000. Mean annual temperatures ranged from -14.8??C to -30.0??C, depending on the site and period of measurement. Mean annual relative humidity is generally highest near the coast. Mean annual wind speed increases with proximity to the polar plateau. Site-to-site variation in mean annual solar flux and PAR is due to exposure of each station and changes over time are likely related to changes in cloudiness. During the nonsummer months, strong katabatic winds are frequent at some sites and infrequent at others, creating large variation in mean annual temperature owing to the warming effect of the winds. Katabatic wind exposure appears to be controlled to a large degree by the presence of colder air in the region that collects at low points and keeps the warm less dense katabatic flow from the ground. The strong influence of katabatic winds makes prediction of relative mean annual temperature based on geographical position (elevation and distance from the coast) alone, not possible. During the summer months, onshore winds dominate and warm as they progress through the valleys creating a strong linear relationship (r2 = 0.992) of increasing potential temperature with distance from the coast (0.09??C km-1). In contrast to mean annual temperature, summer temperature lends itself quite well to model predictions, and is used to construct a statistical model for predicting summer dry valley temperatures at unmonitored sites. Copyright 2002 by the American Geophysical Union.

Journal of Geophysical Research D: Atmospheres↗

USGS2009

Northern Monterey Bay upwelling shadow front: Observations of a coastally and surface-trapped buoyant plume

During the upwelling season in central California, northwesterly winds along the coast produce a strong upwelling jet that originates at Point A??o Nuevo and flows southward across the mouth of Monterey Bay. A convergent front with a mean temperature change of 3.77 ?? 0.29??C develops between the warm interior waters and the cold offshore upwelling jet. To examine the forcing mechanisms driving the location and movement of the upwelling shadow front and its effects on biological communities in northern Monterey Bay, oceanographic conditions were monitored using cross-shelf mooring arrays, drifters, and hydrographic surveys along a 20 km stretch of coast extending northwestward from Santa Cruz, California, during the upwelling season of 2007 (May-September). The alongshore location of the upwelling shadow front at the northern edge of the bay was driven by: regional wind forcing, through an alongshore pressure gradient; buoyancy forces due to the temperature change across the front; and local wind forcing (the diurnal sea breeze). The upwelling shadow front behaved as a surface-trapped buoyant current, which is superimposed on a poleward barotropic current, moving up and down the coast up to several kilometers each day. We surmise that the front is advected poleward by a preexisting northward barotropic current of 0.10 m s -1 that arises due to an alongshore pressure gradient caused by focused upwelling at Point A??o Nuevo. The frontal circulation (onshore surface currents) breaks the typical two-dimensional wind-driven, cross-shelf circulation (offshore surface currents) and introduces another way for water, and the material it contains (e.g., pollutants, larvae), to go across the shelf toward shore.Copyright 2009 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗

USGS2023

Global projections of storm surges using high-resolution CMIP6 climate models

In the coming decades, coastal flooding will become more frequent due to sea-level rise and potential changes in storms. To produce global storm surge projections from 1950 to 2050, we force the Global Tide and Surge Model with a ∼25-km resolution climate model ensemble from the Coupled Model Intercomparison Project Phase 6 High Resolution Model Intercomparison Project (HighResMIP). This is the first time that such a high-resolution ensemble is used to assess changes in future storm surges across the globe. We validate the present epoch (1985–2014) against the ERA5 climate reanalysis, which shows a good overall agreement. However, there is a clear spatial bias with generally a positive bias in coastal areas along semi-enclosed seas and negative bias in equatorial regions. Comparing the future epoch (2021–2050) against the historical epoch (1951–1980), we project ensemble-median changes up to 0.1 (or 20%) in the 1 in 10-year storm surge levels. These changes are not uniform across the globe with decreases along the coast of Mediterranean and northern Africa and southern Australia and increases along the south coast of Australia and Alaska. There are also increases along (parts) of the coasts of northern Caribbean, eastern Africa, China and the Korean peninsula, but with less agreement among the HighResMIP ensemble. Information resulting from this study can be used to inform broad-scale assessment of coastal impacts under future climate change.

Earth's Future↗

USGS1996

Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Heat flow and energetics of the San Andreas fault zone

Approximately 100 heat flow measurements in the San Andreas fault zone indicate (1) there is no evidence for local factional heating of the main fault trace at any latitude over a 1000-km length from Cape Mendocino to San Bernardino, (2) average heat flow is high (∼2 HFU, ∼80 mW m −2 ) throughout the 550-km segment of the Coast Ranges that encloses the San Andreas fault zone in central California; this broad anomaly falls off rapidly toward the Great Valley to the east, and over a 200-km distance toward the Mendocino Triple Junction to the northwest. As others have pointed out, a local conductive heat flow anomaly would be detectable unless the frictional resistance allocated to heat production on the main trace were ≲100 bars. Frictional work allocated to surface energy of new fractures is probably unimportant, and hydrologic convection is not likely to invalidate the conduction assumption, since the heat discharge by thermal springs near the fault is negligible. Explanations for the low dynamic friction fall into two intergradational classes: those in which the fault is weak all of the time and those in which it is weak only during earthquakes (possibly just large ones). The first class includes faults containing anomalously weak gouge materials and faults containing materials with normal frictional properties under near-lithostatic steady state fluid pressures. In the second class, weakening is caused by the event (for example, a thermally induced increase in fluid pressure, dehydration of clay minerals, or acoustic fluidization). In this class, unlike the first, the average strength and ambient tectonic shear stress may be large, ∼1 kbar, but the stress allocated to elastic radiation (the apparent stress) must be of similar magnitude, an apparent contradiction with seismic estimates. Unless seismic radiation is underestimated for large earthquakes, it is difficult to justify average tectonic stresses on the main trace of the San Andreas fault in excess of ∼200 bars. The development of the broad Coast Range heat flow anomaly southward from Cape Mendocino suggests that heat flow increases by a factor of 2 within 4 m.y. after the passage of the Mendocino Triple Junction. This passage leaves the San Andreas transform fault zone in its wake; the depth of the anomalous sources cannot be much greater than the depth of the seismogenic layer. Some of the anomalous heat may be supplied by conduction from the warmer mantle that must occur south of the Mendocino transform (where there is no subducting slab), and some might be supplied by shear heating in the fault zone. With no contribution from shear heating, extreme mantle upwelling would be required, and asthenosphere conditions should exist today at depths of only ∼20 km in the northernmost Coast Ranges. If there is an appreciable contribution from shear heating, the heat flow constraint implies that the seismogenic layer is partially decoupled at its base and that the basal traction is in the sense that resists right lateral motion on the fault(s). As a result of these basal tractions, the average shearing stress in the seismogenic layer would increase with distance from the main fault, and the seismogenic layer would offer substantial resistance to plate motion even though resistance on the main fault might be negligible. These speculative models have testable consequences.

Journal of Geophysical Research Solid Earth↗

USGS2012

Geodetic strain measurements in Washington

Two new geodetic measurements of strain accumulation in the state of Washington for the interval 1972–1979 are reported. Near Seattle the average principal strain rates are 0.07 ± 0.03 μstrain/yr N 19°W and −0.13 ± 0.02 μstrain/yr N71°E, and near Richland (south central Washington) the average principal strain rates are −0.02 ± 0.01 μstrain/yr N36°W and −0.04 ± 0.01 μstrain/yr N54°E. Extension is taken as positive, and the uncertainties quoted are standard deviations. A measurement of shear strain accumulation (dilatation not determined) in the epoch 1914–1966 along the north coast of Vancouver Island by the Geodetic Survey of Canada indicates a marginally significant accumulation of right-lateral shear (0.06 ± 0.03 μrad/yr) across the plate boundary (N40°W strike). Although there are significant differences in detail, these strain measurements are roughly consistent with a crude dislocation model that represents subduction of the Juan de Fuca plate. The observed accumulation of strain implies that large, shallow, thrust earthquakes should be expected off the coast of Washington and British Columbia. However, this conclusion is not easily reconciled with either observations of elevation change along the Washington coast or the focal mechanism solutions for shallow earthquakes in Washington.

Journal of Geophysical Research Solid Earth↗

USGS2012

Quaternary and geodetically measured crustal movements in the Tokai District, Central Honshu, Japan

Geodetic measurements made in this century demonstrate that modern movements correlate well with deformation that has occurred during the past 2 m.y. Both show effects of subduction and of the Quaternary collision of Izu Peninsula with Central Honshu. Westward from Suruga Bay, deformation is greatest near the coast, decreases markedly towards the uplifting Akaishi Mountains, and then increases again in the conjugate block-faulted terranes of Chubu district. Quaternary faults, historic earthquakes, and the distribution of horizontal shear straining since ∼1880 all reflect this deformation pattern. Since 1900 the west coast of Suruga Bay has undergone secular subsidence, horizontal contraction and tilting towards Suruga Trough, all typical of the strain buildup process that occurs landward of subduction zones. However, futher westward, active mountain building is occuring, and the Akaishi Ranges are rising 3 mm/a. Unlike the episodic aseismic uplift of the Transverse Ranges of southern California, movements here have been relatively uniform. It seems likely that an increase in the tilt rate of the Suruga coast recognized since ∼1973 is part of a long-term readjustment to the great 1944 Tonankai earthquake. Data show that a monotonic increase has occurred since at least 1949, and similar effects are seen adjacent to the 1944 rupture zone over 200 km southwest along the Nankai Trough.

Journal of Geophysical Research Solid Earth↗

USGS2012

Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington, implications for mechanisms of tectonic rotation

Paleomagnetic and geologic investigations in Eocene volcanic rocks of the southwest Washington Coast Range demonstrate a close relationship between tectonic rotations and the local structural geology. The allochthonous middle Eocene submarine basalt basement of the Crescent Formation consists of several fault‐bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation (20° to perhaps as much as 65°) when compared to the Eocene reference pole for North America. Structural analysis shows the differential rotations postdate middle Eocene folding of the Crescent Formation against the continental margin and predate the unconformably overlying upper Eocene Goble Volcanics, which are rotated about 23° and do not show the same domains of rotation as the underlying Crescent Formation. Post‐Goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults. Major north‐northwest trending faults with several kilometers of dextral displacement form the boundaries of cross‐faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west‐northwest trending sinistral R ′ Riedel shears. Thirty‐five to 100% of the observed post‐late Eocene rotations could have occurred by this shear rotation mechanism. Other paleomagnetic study areas in the Coast Range and western Cascades have a similar fault geometry and may also have undergone significant shear rotations. Long‐term northward oblique subduction of the Farallon plate beneath the Coast Range throughout most of the Tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin. Shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large, rigid plates.

Washington↗

USGS1997

Thermal regime of the San Andreas fault near Parkfield, California

Knowledge of the temperature variation with depth near the San Andreas fault is vital to understanding the physical processes that occur within the fault zone during earthquakes and creep events. Parkfield is near the southern end of the Coast Ranges segment of the San Andreas fault. This segment has higher mean heat flow than the Cape Mendocino segment to the northwest or the Mojave segment to the southeast. Boreholes were drilled specifically for the U.S. Geological Survey's Parkfield earthquake prediction experiment or converted from other uses at 25 sites within a few kilometers of the fault near Parkfield. These holes, which range in depth from 150 to over 1500m, were intended mainly for the deployment of volumetric strain meters, water-level recorders, and other downhole instruments. Temperature profiles were obtained from all the holes, and heat flow values were estimated from 17 of them. For a number of reasons, including a paucity of thermal conductivity data and rugged local topography, the accuracy of individual determinations was not sufficiently high to document local variations in heat flow. Values range from 54 to 92 mW m −2 , with mean and 95% confidence limits of 74±4 mW m −2 . This mean is slightly lower than the mean (83±3) of 39 previously published values from the central Coast Ranges, but it is consistent with the overall pattern of elevated heat flow in the Coast Ranges, and it is transitional to the mean of 68±2 mW m −2 that characterizes the Mojave segment of the San Andreas fault immediately to the south. The lack of a heat flow peak near the fault underscores the absence of a frictional thermal anomaly and provides additional support for a very small resolved shear stress parallel to the San Andreas fault and the nearly fault-normal maximum compressive stress observed in this region. Estimates of subsurface thermal conditions indicate that the seismicaseismic transition for the Parkfield segment corresponds to temperatures in the range of 350°–400°C. Increasing heat flow to the northwest of Parkfield corresponds to a transition from locked to creeping sections and to a shallowing of the base of seismicity and confirms the importance of temperature in controlling the thickness of the seismogenic crust. Lateral variations in heat flow do not appear to have any major role in determining the regularity of M 5.5–6 earthquakes at Parkfield.

Journal of Geophysical Research B: Solid Earth↗

USGS2010

Cenozoic plate motions and the volcano-tectonic evolution of western Oregon and Washington

A refined northeast Pacific plate-motion model provides a framework for analysis of the Tertiary volcanic and tectonic history of western Oregon and Washington. We examine three possible models for the origin of the allochthonous Paleocene and Eocene oceanic basalt basement of the Coast Range: (1) accretion to the continent of hot spot generated linear seamount chains; (2) accretion of thick oceanic crust and seamounts generated during Farallon-Kula spreading reorganizations between 61 and 48 Ma; and (3) eruption of basalt during oblique rifting of the continental margin as it overrode an active Yellowstone hot spot on the Kula-Farallon ridge. The plate model suggests that microplate rotation and accretion of hot spot generated linear aseismic ridges cannot be easily reconciled with rapid northeast motion of the KuIa and Farallon plates and the well-established paleomagnetic rotations. Following emplacement of the Coast Range basement, changes in the character of forearc, back arc and Cascade arc volcanism correlate with a marked decrease in the rate of Farallon-North America convergence between 43 and and 28 Ma. This slowdown may be responsible for (1) westward stepping of the volcanic arc front from the Challis axis to a Cascade axis at about 42 Ma; (2) a subsequent episode of increased ash flow tuff volcanism and extension in the Cascade arc between 37 and 18 Ma that correlates with the “ignimbrite flare-up” in the Basin and Range; and (3) a period of extensional basaltic and alkalic volcanism and intrusion in the Coast Range between 44 and 28 Ma. Reduction of the convergence rate and westward stepping of the flexure in the subducted slab may have reduced the horizontal compressive stress on the continent, allowing increased injection of magma into the crust, development of large, shallow magma chambers, and the outbreak of extensional volcanism over a large area behind the Farallon-North America subduction zone.

Oregon, Washington↗

USGS1991

Radiocarbon test of earthquake magnitude at the Cascadia subduction zone

The Cascadia subduction zone, which extends along the northern Pacific coast of North America, might produce earthquakes of magnitude 8 or 9 ('great' earthquakes) even though it has not done so during the past 200 years of European observation 1–7 . Much of the evidence for past Cascadia earthquakes comes from former meadows and forests that became tidal mudflats owing to abrupt tectonic subsidence in the past 5,000 years 2,3,6,7 . If due to a great earthquake, such subsidence should have extended along more than 100 km of the coast 2 . Here we investigate the extent of coastal subsidence that might have been caused by a single earthquake, through high-precision radiocarbon dating of coastal trees that abruptly subsided into the intertidal zone. The ages leave the great-earthquake hypothesis intact by limiting to a few decades the discordance, if any, in the most recent subsidence of two areas 55 km apart along the Washington coast. This subsidence probably occurred about 300 years ago.

California, Oregon, Washington↗