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USGS2026

How to determine an earthquake rate budget? A comparison of geodetic, geologic, and seismologic moment rates

How much tectonic geodetic strain is accommodated as earthquakes? Can estimates of tectonic geodetic strain be used in probabilistic seismic hazard analyses? Understanding the balance of moment accumulation (i.e., geodetic moment) and accommodation (i.e., geologic and seismologic moment) is a necessary step toward leveraging corrected geodetic strain rates to quantify earthquake budgets. We investigate these questions using updated geodetic, geologic, and seismologic datasets compiled for the western United States (WUS) portion of the 2023 U.S. National Seismic Hazard Model (NSHM23‐WUS) to provide new views of moment rates across the WUS. We compare moment rates using a floating footprint approach across the extent of the NSHM23‐WUS fault sections database. We make these comparisons using “moment rate deviation (MRD),” which is defined as a surplus or deficit of geodetic moment rate relative to the sum of geologic and seismologic moment rates. We find that, in general, higher strain rate areas of the WUS (i.e., plate boundary) have smaller values of MRD, meaning that geodetic moment rates approximate the earthquake record preserved by geologic and seismologic moment rates well. In contrast, the lower strain rate, eastern portions of our analysis domain (i.e., intraplate) have very high values of MRD, with geodetic moment rates exceeding measured geologic and seismologic moment rates. We find that geodetic moment rates appear to be representative of the earthquake budget in relatively well‐characterized, interplate regions, and geodetic estimates of off‐fault deformation could be considered in seismic hazard analyses where MRD is minimized.

western United States↗

USGS2007

A record of large earthquakes on the southern Hayward fault for the past 1800 years

This is the second article presenting evidence of the occurrence and timing of paleoearthquakes on the southern Hayward fault as interpreted from trenches excavated within a sag pond at the Tyson's Lagoon site in Fremont, California. We use the information to estimate the mean value and aperiodicity of the fault's recurrence interval (RI): two fundamental parameters for estimation of regional seismic hazard. An earlier article documented the four most recent earthquakes, including the historic 1868 earthquake. In this article we present evidence for at least seven earlier paleoruptures since about A.D. 170. We document these events with evidence for ground rupture, such as the presence of blocky colluvium at the base of the main trace fault scarp, and by corroborating evidence such as simultaneous liquefaction or an increase in deformation immediately below event horizons. The mean RI is 170 ?? 82 yr (1??, standard deviation of the sample), aperiodicity is 0.48, and individual intervals may be expected to range from 30 to 370 yr (95.4% confidence). The mean RI is consistent with the recurrence model of the Working Group on California Earthquake Probabilities (2003) (mean, 161 yr; range, 99 yr [2.5%]; 283 yr [97.5%]). We note that the mean RI for the five most recent events may have been only 138 ?? 58 yr (1??). Hypothesis tests for the shorter RI do not demonstrate that any recent acceleration has occurred compared to the earlier period or the entire 1800-yr record, principally because of inherent uncertainties of the event ages.

Bulletin of the Seismological Society of America↗

USGS2021

Onshore flow characteristics of the 1755 CE Lisbon tsunami: Linking forward and inverse numerical modeling

The 1755 CE Lisbon earthquake triggered the largest historical tsunami ever impacting the Atlantic coasts of Europe. Despite recent efforts to better understand this event, there are still unanswered questions about the location of its epicenter and whether physical and historical evidences are in agreement. Inverse modeling using tsunami sediments can be applied to quantify onshore flow characteristics. Forward numerical modeling is also a powerful tool capable of simulating tsunami hydrodynamics and the induced sediment transport. This work presents novel results from a combination of inverse and forward modeling to assess tsunami characteristics onshore. The study site is located on the Portuguese southern coast, at the Salgados lowland where inverse modeling was performed using TsuSedMod ( Jaffe and Gelfenbaum, 2007 ) based on data retrieved from sediment samples. Forward modeling, including tsunami generation and propagation, was performed using the FLOW module of Delft3D suite model. Onshore topography was corrected for the 1755 CE scenario based on extensive tsunami sedimentary deposit thickness data. The tsunami source was chosen based on recent results from the authors that pointed to a good correlation between modeled and field tsunami data for the Marques de Pombal Fault (MPF), Horseshoe Fault (HSF) and a hypothetical scenario represented by a simple combination between Gorringe Bank and Horseshoe Fault (Scenario 1 - SC1). Results from inverse model show tsunami onshore average speed varying from 7.3 up to 9.3 m/s and shear velocities from 0.52 up to 0.66 m/s. Forward modeling results show a wide variation according to the seismic source and tsunami onshore velocities can range from around 7 m/s when considering MPF to even an absence of inundation (SC1). The good agreement between both modeling approaches estimating tsunami velocity confirms the potential of numerical modeling coupled with geological records to improve the understanding of historical events.

Marine Geology↗

USGS2020

Local earthquake Vp and Vs tomography in the Mount St. Helens region with the iMUSH broadband array

We present new 3-D P wave and S wave velocity models of the upper 20 km of the Mount St. Helens (MSH) region. These were obtained using local-source arrival time tomography from earthquakes and explosions recorded at 70 broadband stations deployed as part of the imaging Magma Under St. Helens (iMUSH) project and augmented by several data sets. Principal features of our models include (1) low P wave and S wave velocities along the St. Helens seismic zone to depths of at least 20 km corresponding to high conductivity imaged by iMUSH magnetotelluric studies. This delineates a zone of weakness that magma can exploit at the location of MSH; (2) a 5- to 7-km diameter, 6–15 km deep, 3–6% negative P wave and S wave velocity anomaly beneath MSH, consistent with previous estimates of the source region for recent eruptions. We interpret this as a magma storage region containing up to 15–20 km 3 of partial melt, which is about 5 times more than the largest documented eruption at MSH; (3) a broad region of low P wave velocity below 10-km depth extending between Mount Adams and Mount Rainier along and to the east of the main Cascade arc, which is likely due to high-temperature arc crust and possible presence of fluids or melt; (4) several anomalies associated with surface-mapped features, including high-velocity igneous units such as the Spud Mountain and Spirit Lake plutons and low velocities in the Chehalis sedimentary basin and the Indian Heaven volcanic field. Our results place further constraints on the geometry of these features at depth.

Washington↗

USGS2012

Seismic guided waves trapped in the fault zone of the Landers, California, earthquake of 1992

A mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the M 7.4 Landers earthquake of June 28, 1992, with a maximum offset of 1 km from the trace. We found a distinct wave train with a relatively long period following the S waves that shows up only when both the stations and the events are close to the fault trace. This wave train is interpreted as a seismic guided wave trapped in a low‐velocity fault zone. To study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone, we eliminated source and recording site effects by the coda normalization method. The normalized amplitudes of guided waves show a spectral peak at 3–4 Hz, which decays sharply with distance from the fault trace. Spectral amplitudes at high frequencies (8–15 Hz) show an opposite trend, increasing with distance from the fault trace. The normalized amplitudes of guided waves at 3–4 Hz also show a systematic decrease with hypocentral distance along the fault zone, from which we infer an apparent Q of 50. In order to confirm the existence of the guided waves, a dense array of 31 stations was deployed at one of the 11 sites. The resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone. By modeling the waveforms as S waves trapped in a low‐velocity waveguide sandwiched between two homogeneous half‐spaces with velocity V s = 3.0 km/s, we infer a waveguide width of about 180 m, a shear velocity of 2.0–2.2 km/s, and a Q of ∼50. Hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low‐velocity fault zone in three dimensions. We find that the zone extends to a depth of at least 10 km. This zone apparently continues to the south across the Pinto Mountain fault because guided waves are observed at stations north of the Pinto Mountain fault for earthquakes with epicenters south of it. On the other hand, the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter; guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides.

California↗

USGS1980

Driving- stress waveform and the determination of rock internal friction by the stress-strain curve method.

Harmonic distortion in the stress-time function applied to rock specimens affects the measurement of rock internal friction in the seismic wave periods by the stress-strain hysteresis loop method. If neglected, the harmonic distortion can cause measurements of rock internal friction to be in error by 3O% in the linear range. The stress-time function therefore must be recorded and Fourier analysed for correct interpretation of the experimental data. Such a procedure would also yield a value for internal friction at the higher harmonic frequencies.-Author

Geophysical Journal of the Royal Astronomical Soci↗

USGS2020

Short and long-term movement of mudflows of the Mississippi River Delta Front and their known and potential impacts on oil and gas infrastructure

Mudflows on the Mississippi River Delta Front (MRDF) are recognized hazards to oil and gas infrastructure in the shallow (20 – 300 m water depth) Gulf of Mexico. Preconditioning of the seafloor for failure results from high sedimentation rates coupled with slope over-steepening, under-consolidation, and abundant biogenic gas production. Catastrophic failure of production platforms and pipelines due to seafloor displacement during infrequent large hurricanes such as Camille in 1969 and Ivan in 2004, point to cyclical loading of the seafloor by waves as a primary movement trigger. Due to data limitations, the role of smaller storms and background oceanographic processes in driving seafloor movement have remained largely unconstrained, but are thought to contribute to significant seafloor change. With the aid of new high-resolution multibeam mapping and seismic reflection profiling across sections of the MRDF, several moving features within the deforming delta-front environment are investigated and potential hazards to infrastructure installed and adjacent to the region are discussed. Via repeat mapping surveys of selected areas and records of changing shipwreck locations, we highlight significant seafloor displacement across annual to decadal timescales. For example, individual blocks mapped within mudflow gullies adjacent to Southwest Pass show downslope transport of more than 80 m in a single year, while the S.S. Virginia, a 153 m-long oil tanker sunk in 1942 has been relocated and found to have moved downslope more than 400 m in 14 years, without a major hurricane (> Category 2) passing through the region.

Louisiana↗

USGS2011

Some possible causes of and corrections for STS-1 response changes in the Global Seismographic Network

The Global Seismographic Network (GSN) (Figure 1) plays a key role in providing seismic data for global earthquake monitoring ( e.g. , Benz et al. 2005), earthquake science ( e.g. , Tsai et al. 2005), and studies of Earth structure ( e.g. , Dalton et al. 2008). One of the key GSN design goals is to "provide high fidelity digital recordings of all teleseismic ground motions (adequate to resolve at or near ambient noise up to the largest teleseismic signals over the bandwidth from free oscillations (10 -4 Hz) to teleseismic body waves (up to approximately 15 Hz))" (GSN ad hoc Design Goals Subcommittee 2002). To help meet this goal, Streckeisen STS-1 seismometers were deployed at 80 GSN stations. Some of the GSN sensors have been deployed for more than 25 years. Several recent studies (Davis et al. 2005; Ekström et al. 2006; Davis and Berger 2007) have examined the question of overall calibration of the GSN. Ekström et al. (2006) indicated that a number of sites showed anomalous responses and suggested a gradual decay in the sensitivity. We have investigated the anomalous responses at several GSN sites. At least some of the problems observed by Ekström et al. (2006) may be attributed to humid air leaking into the feedback electronics of the STS-1 seismometers, which produces lower than normal sensitivities near the long-period corner of the instrument (360 seconds period). It appears that even though the feedback electronics boxes are designed to be sealed, water vapor can penetrate their interior after they have been exposed to highly humid seismometer vault air for extended periods. Highly humid air was also found to be present inside some STS-1 bell-jars (especially horizontal instruments) after loss of vacuum, resulting in corrosion and leakage between electrical conductors in connectors.

Seismological Research Letters↗

USGS2001

Vertical tectonics in northern Escanaba Trough as recorded by thick late Quaternary turbidites

Escanaba Trough, the southernmost segment of the Gorda Ridge, is filled by as much as 500 m of late Quaternary turbidite and hemipelagic sediment. Coring at Deep Sea Drilling Project Site 35 and Ocean Drilling Program (ODP) Sites 1037 and 1038 together with 4.5-kHz deep-tow and 3.5-kHz surface-ship seismic reflection profiles enable a distinct pattern of reflections to be mapped throughout Escanaba Trough in the upper part of this sediment fill. The uppermost 80 m of turbidite sediment, which includes at least 11 turbidity current events, were deposited in <7 kyr. Nine of these turbidites are found throughout Escanaba Trough at water depths >3200 m. The turbidity currents were trapped upon entering Escanaba Trough, resulting in all of the sediment in suspension in the flows being deposited. The thickness of the turbidite layers reflects both the flow thickness and the vertical grain concentration within the flow that deposited the layer. Variations in the turbidite thickness with respect to water depth can be used to estimate the degree of relative vertical movement within the floor of Escanaba Trough. In the area of hydrothermal activity near ODP Site 1038, uplift of as much as 140 m has occurred over the past 8 kyr.

California↗

USGS2016

Adjusting central and eastern North America ground-motion intensity measures between sites with different reference-rock site conditions

Adjustment factors are provided for converting ground‐motion intensity measures between central and eastern North America (CENA) sites with different reference‐rock site conditions ( V S 30 =760, 2000, and 3000 m/s) for moment magnitudes ranging from 2 to 8, rupture distances ranging from 2 to 1200 km, Fourier amplitude spectra (FAS) for frequencies ranging from 0.01 to 100 Hz, response spectra for periods ranging from 0.01 to 10.0 s, peak ground acceleration, and peak ground velocity. The adjustment factors are given for a wide range of the site diminution parameters ( κ 0 ) for sites with V S 30 =760 m/s and for a κ 0 of 0.006 s for two harder rock sites. Fourteen CENA velocity profiles with V S 30 values within a factor of 1.1 of 760 m/s were used to derive average FAS amplification factors as a function of frequency, which were then used in simulations of peak ground‐motion parameters and response spectra to derive the adjustment factors. The amplification function differs from that used in western North America (e.g., Campbell and Boore, 2016 ) in having a peak near 9 Hz, due to the resonance of motions in the relatively thin low‐velocity material over hard rock that characterizes many CENA sites with V S 30 near 760 m/s. We call these B/C sites, because this velocity marks the boundary between National Earthquake Hazards Reduction Program site classes B and C ( Building Seismic Safety Council, 2004 ). The adjustments for short‐period motions are sensitive to the value of κ 0 , but there are very few if any determinations of κ 0 for CENA B/C sites. For this reason, we determined κ 0 from multiple recordings at Pinyon Flat Observatory (PFO), California, which has a velocity‐depth profile similar to those of CENA B/C sites. The PFO and other results from the literature suggest that appropriate values of κ 0 for CENA B/C sites are expected to lie between 0.01 and 0.03 s.

Bulletin of the Seismological Society of America↗

USGS1996

Implications of the Northridge earthquake for strong ground motions from thrust faults

The peak accelerations recorded on alluvial sites during the Northridge earthquake were about 50% larger than the median value predicted by current empirical attenuation relations at distances less than about 30 km. This raises the question of whether the ground motions from the Northridge earthquake are anomalous for thrust events or are representative of ground motions expected in future thrust earthquakes. Since the empirical data base contains few strong-motion records close to large-thrust earthquakes, it is difficult to assess whether the Northridge ground motions are anomalous based on recorded data alone. For this reason, we have used a broadband strong-motion simulation procedure to help assess whether the ground motions were anomalous. The simulation procedure has been validated against a large body of strong-motion data from California earthquakes, and so we expect it to produce accurate estimates of ground motions for any given rupture scenario, including blind-thrust events for which no good precedent existed in the strong-motion data base until the occurrence of the Northridge earthquake. The ground motions from the Northridge earthquake and our simulations of these ground motions have a similar pattern of departure from empirical attenuation relations for thrust earthquakes: the peak accelerations are at about the 84th percentile level for distances within 20 to 30 km and follow the median level for larger distances. This same pattern of departure from empirical attenuation relations was obtained in our simulations of the peak accelerations of an Elysian Park blind-thrust event prior to the occurrence of the Northridge earthquake. Since we are able to model this pattern with broadband simulations, and had done so before the Northridge earthquake occurred, this suggests that the Northridge strong-motion records are not anomalous and are representative of ground motions close to thrust faults. Accordingly, it seems appropriate to include these recordings in strong-motion data sets that are used to develop empirical ground-motion attenuation relations for thrust faults and to use this augmented data set as the basis for evaluating the need for modifications in design coefficients in the seismic provisions of building codes.

California↗

USGS1991

Initiation processes and flow evolution of turbidity currents: Implications for the depositional record

Interpretations of sea-level change and source-area tectonism from the character of turbidite deposits require knowledge of the sediment source, the flow-initiation process, how the turbidity current evolved during flow, and what flow phases were associated with deposition. The flow responds to shape and size characteristics of both the erosional pathway and the morphology of previous deposits; these, in turn, reflect factors such as basin size, shape, and tectonic activity. The initiation of turbidity currents generally involves either transformation from mass failures or one of a variety of fluid-flow events involving ignitive flow. The variability of these initiating processes with geologic setting is illustrated by analysis of three case studies involving 1) initiation by storm surge on a carbonate-reef slope, 2) a seismically triggered failure of a line source within midslope sediment, and 3) a bedload delta showing both hyperpycnal flow of river bedload and failure of unstable prodelta sediment. General criteria for the initiation of turbidity currents are given. Depositional and erosional features of the turbidite deposits themselves allow recognition of changes in flow processes as the turbidity currents traverse the basin. High-concentration basal flow beneath the main turbulent turbidity current will leave a distinctive depositional record from more energetic flow, particularly in proximal and channel settings. Turbidity currents of different sizes and sediment distribution interact quite differently with local fan morphology. Many large, muddy turbidity currents are much thicker than levee-channel relief; the upper parts of such flows are "stripped off" across levees and can become rechannelized farther downslope. in restricted basins, the basin margin acts like a channel wall for larger flows, thus maintaining competence over long distances. Thinner, sandier flows flush out channel systems, and a large proportion of sediment in some turbidite deposits may be derived from bed erosion. A broad range of local bedforms, especially sediment waves and scours, reflects changes in the character of turbulence and competence within quasi-steady flows. This paper emphasizes the broad range of size, sediment distribution, and depositional processes in turbidity currents that largely reflects the variety of initiation processes. This variability means that caution is needed when interpreting depositional conditions of ancient turbidite sediment with ill-defined morphologic setting.

Book chapter↗

USGS1979

Preliminary catalog of earthquakes in northern Imperial Valley, California, October 1978-December 1978

The northern section of the Imperial Valley region in southern California is an area of known geothermal resources and high seismicity. To investigate the relationship between geothermal areas and earthquakes in the Imperial Valley, a 16-station seismic network was established in early 1973 by the U. S. Geological Survey. Six more stations were added to the network in November 1976. This catalog contains a description of the network and a list of preliminary data on earthquakes detected and recorded by the network from October 1978 through December 1973.

California↗

USGS1979

Preliminary catalog of earthquakes in northern Imperial Valley, California, July 1978 - September 1978

California↗

USGS2018

Damage accumulation for a two-story wood-frame building in sequences of induced earthquakes

In this study, a nonlinear model of a two-story multifamily wood-frame residential structure is subjected to recordings of sequences of induced earthquakes in order to quantify changes in fragility and accumulation of damage throughout multiple earthquake loadings. Initial efforts consisting of ground motion selection, building design, numerical modeling, and preliminary results are presented. Damage is quantified through a seismic loss estimation procedure that accounts for damage to nonstructural and structural components of the building. Examining damage accumulation from sequential earthquake shaking enables us to explore how the occurrence of damage in an earthquake, even to relatively small levels, may increase a structure’s susceptibility to collapse or damage in subsequent ground shaking. Ultimately, the goal is to compare damage fragilities and seismic losses as a function of the building’s initial damage state.

Kansas, Oklahoma↗

USGS2006

Ground deformation associated with the precursory unrest and early phases of the January 2006 eruption of Augustine volcano, Alaska

On January 11, 2006 Augustine Volcano erupted after nearly 20 years of quiescence. Global Positioning System (GPS) instrumentation at Augustine, consisting of six continuously recording, telemetered receivers, measured clear precursory deformation consistent with a source of inflation or pressurization beneath the volcano's summit at a depth of around sea level. Deformation began in early summer 2005, and was preceded by a subtle, but distinct, increase in seismicity, which began in May 2005. After remaining more or less constant, deformation rates accelerated on at least three stations beginning in late November 2005. After this date, GPS data suggest the upward propagation of a small dike into the edifice, which, based on the style of deformation and high levels of gas emission, appears to have ascended to shallow levels by mid-December 2005, about four weeks before the eruption began.

Alaska↗

USGS2017

Miocene−Pleistocene deformation of the Saddle Mountains: Implications for seismic hazard in central Washington, USA

The Yakima fold province, located in the backarc of the Cascadia subduction zone, is a region of active strain accumulation and deformation distributed across a series of fault-cored folds. The geodetic network in central Washington has been used to interpret large-scale N-S shortening and westward-increasing strain; however, geodetic data are unable to resolve shortening rates across individual structures in this low-strain-rate environment. Resolving fault geometries, slip rates, and timing of faulting in the Yakima fold province is critically important to seismic hazard assessment for nearby infrastructure and population centers. The Saddle Mountains anticline is one of the most prominent Yakima folds. It is unique within the Yakima fold province in that the syntectonic strata of the Ringold Formation are preserved and provide a record of deformation and drainage reorganization. Here, we present new stratigraphic columns, U-Pb zircon tephra ages, U-series caliche ages, and geophysical modeling that constrain two line-balanced and retrodeformed cross sections. These new constraints indicate that the Saddle Mountains anticline has accommodated 1.0−1.3 km of N-S shortening since 10 Ma, that shortening increases westward along the anticline, and that the average slip rate has increased 6-fold since 6.8 Ma. Provenance analysis suggests that the source terrane for the Ringold Formation was similar to that of the modern Snake River Plain. Using new slip rates and structural constraints, we calculate the strain accumulation time, interpretable as a recurrence interval, for earthquakes on the Saddle Mountains fault and find that large-magnitude earthquakes could rupture along the Saddle Mountains fault every 2−11 k.y.

Washington↗

USGS2012

Deep-Sea Turbidites as Guides to Holocene Earthquake History at the Cascadia Subduction Zone—Alternative Views for a Seismic-Hazard Workshop

This report reviews the geological basis for some recent estimates of earthquake hazards in the Cascadia region between southern British Columbia and northern California. The largest earthquakes to which the region is prone are in the range of magnitude 8-9. The source of these great earthquakes is the fault down which the oceanic Juan de Fuca Plate is being subducted or thrust beneath the North American Plate. Geologic evidence for their occurrence includes sedimentary deposits that have been observed in cores from deep-sea channels and fans. Earthquakes can initiate subaqueous slumps or slides that generate turbidity currents and which produce the sedimentary deposits known as turbidites. The hazard estimates reviewed in this report are derived mainly from deep-sea turbidites that have been interpreted as proxy records of great Cascadia earthquakes. The estimates were first published in 2008. Most of the evidence for them is contained in a monograph now in press. We have reviewed a small part of this evidence, chiefly from Cascadia Channel and its tributaries, all of which head offshore the Pacific coast of Washington State. According to the recent estimates, the Cascadia plate boundary ruptured along its full length in 19 or 20 earthquakes of magnitude 9 in the past 10,000 years; its northern third broke during these giant earthquakes only, and southern segments produced at least 20 additional, lesser earthquakes of Holocene age. The turbidite case for full-length ruptures depends on stratigraphic evidence for simultaneous shaking at the heads of multiple submarine canyons. The simultaneity has been inferred primarily from turbidite counts above a stratigraphic datum, sandy beds likened to strong-motion records, and radiocarbon ages adjusted for turbidity-current erosion. In alternatives proposed here, this turbidite evidence for simultaneous shaking is less sensitive to earthquake size and frequency than previously thought. Turbidites far below a channel confluence, instead of representing the merged flows from two tributaries, monitor the dominant tributary only. Sandy beds low in the turbidites, instead of matching from channel to channel, permit divergent stratigraphic correlations; and rather than approximating strong-motion seismograms, the sandy beds more likely record processes internal to the generation and transformation of subaqueous mass movements. The age adjustments, instead of supporting other evidence that all the northern ruptures were long, are uncertain enough to accord with variation in rupture mode, and this variation improves agreement with onshore paleoseismology. Many of the turbidites counted as evidence for frequent earthquakes on the southern Cascadia plate boundary may instead reflect nearness to steep slopes. This report is meant to aid in the updating of national maps of seismic hazards in Canada and the United States. It offers three main conclusions for consideration at a U.S. hazard-map workshop slated for March 21-22, 2012: If giant earthquakes are the norm for the plate boundary offshore southern Washington, the strongest paleoseismic evidence for this rupture mode is the average earthquake-recurrence interval of about 500 years that is evidenced both offshore in lower Cascadia Channel and onshore at estuaries of southern Washington and northernmost Oregon. The plate boundary offshore southern British Columbia and northern Washington may be capable of producing great earthquakes at an average interval as short as 300 years that is evidenced mainly onshore. Review of more of the turbidite evidence now in press may clarify implications for the hazard maps. Further work on the deep-sea turbidites could target sedimentary processes and chronological uncertainties that may affect the turbidites' sensitivity to fault-rupture lengths and recurrence rates.

Washington;Oregon;California↗