Interseismic Strain Research Articles

Tectonic deformation in western Washington from continuous GPS measurements

The analysis of 3 years of continuous data from 7 permanent GPS stations along the western Washington section of the Cascadia Subduction Zone indicates that the direction of the observed horizontal velocities (with respect to station DRAO, nominally representing the stable North American continent) is roughly parallel to the relative plate convergence direction of the Juan de Fuca and North America plates and that their magnitude decreases away from the trench. Most of this deformation can be attributed to inter‐seismic strain accumulation due to the locking of the thrust interface. When the dislocation model predictions are subtracted from the observed velocities, there is evidence for an additional N‐S oriented contraction at a rate of ∼4 mm/yr over a distance of 250 km. This signal presumably represents a more long‐term deformation pattern than the periodic accumulation and release of elastic strain connected with subduction earthquakes and is most likely related to the occurrence of shallow earthquakes in western Washington that are characterized by predominantly N‐S oriented maximum principal stress.

Interplate coupling in the Tokai District, central Japan, deduced from continuous GPS data

I invert GPS displacement rates in the Tokai district to investigate strain accumulation associated with the anticipated Tokai earthquake. I apply a representation of interseismic strain accumulation at subduction zones by Savage [1983] and estimate a distribution of virtual normal fault slip (back‐slip) on the plate boundary surface. Estimated back‐slip takes its maximum value of 30–40mm/year under the Sea of Enshu. This value is nearly equal to the relative plate motion there, implying almost complete coupling. Back‐slip is smaller under Suruga Bay, and its direction is different from the relative motion between the Eurasian and the Philippine Sea plates. Localized deformation at the northern tip of the Philippine Sea plate may be responsible for this discrepancy, partitioning tectonic strain due to the plate convergence into faults on both sides of the Izu peninsula and also distributed strain inside the peninsula.

Role of fluids in faulting inferred from stress field signatures

The stress orientation signature of weak faults containing high-pressure fluids has been observed for segments of the San Andreas fault system in southern California. The inferred lithostatic fluid pressures extend into the surrounding relatively intact rock in a zone scaling with the width of the interseismic strain accumulation. Repeated strain-related fracturing and crack sealing may have created low-permeability barriers that seal fluids into the network of currently active fractures.

Convergent-margin deformation of Pleistocene strata on the Olympic coast of Washington, USA

Abstract Quaternary tectonism on the central Olympic coast is dominated by the Kalaloch syncline, a broad (>32 km wavelength), low-amplitude ( c. 45 m) fold deforming the Pleistocene glacial-nonglacial stratigraphic sequence. The stratigraphy includes two sequences of glacial and nonglacial sediments, one overlying and one underlying a prominent wave-cut surface. Stratigraphic and chronological data indicate that the buried wave-cut surface formed during isotope stage 7 or 5 sea-level highstands, probably during substage 5e or 5c. The elevation pattern of the wave-cut surface expresses the syncline most clearly. The outcrop patterns of overlying strata also reflect the syncline, as does outcrop-scale deformation in the underlying strata. Geological uplift rates for the wave-cut surface on the syncline’s north limb contrast in magnitude with, but are similar in pattern to, geodetically measured 1931–1974 uplift rates. Geological uplift rates range from −0.03 to 0.7 mm/year. The difference at each location between geological and geodetic uplift rates ranges from 0.4 to 1.0 mm/year, and is inferred to reflect interseismic strain accumulation. For each assumed age for the wave-cut surface, the inferred interseismic strain accumulation is nearly constant across the syncline’s north limb. Thus, the syncline accounts for most of the spatial variability in the 1931–1974 uplift rates. Comparison of the geological uplift rates with alternative representations of 1931–1974 uplift does not result in the same patterns, but does reveal a significant component of interseismic strain accumulation.

Moment tensor solutions of the 1994 to 1996 Double Spring Flat, Nevada, earthquake sequence and implications for local tectonic models

AbstractThe 12 September 1994 Mw 5.8 Double Spring Flat, Nevada, earthquake initiated at the intersection of a northeast- and northwest-striking set of conjugate faults within an overlapping zone between the Genoa and Antelope Valley fault zones, of the eastern Sierra Nevadan range frontal fault system. The mainshock ruptured on the northeast-striking fault plane. Eight days after the mainshock, the aftershock activity migrated from the mainshock fault plane to the northwest-striking conjugate fault. Over the next 2 years, aftershocks migrated southward onto another set of conjugate faults and then onto the Antelope Valley fault zone. The focal mechanisms of 17 M > 4 aftershocks were estimated from a time-domain moment tensor inversion using regional broadband data. The T axis (minimum stress direction) is oriented east-west (N80°E to N100°E) for the (M > 4) events as is, commonly observed along the eastern Sierra Nevadan range front in northwestern Nevada. From these results, we make some general points that can be considered in seismic hazard assessment. The maximum magnitude in overlapping normal fault zone is limited to the size of the overlapping zone. This makes small- to moderate-size (M < 6) strike-slip earthquakes more likely than large range-front (M > 7) earthquakes. The seismicity within this overlapping zone may indicate interseismic strain accumulation from east-west extension mainly through strike-slip deformation. The apparent scarcity of modern normal-faulting earthquakes along the Sierran range-front faults suggests a characteristic model, while a Gutenberg and Richter model for the recurrence behavior of earthquakes applies to the overlap zones between the normal faults. The pattern of seismicity and principle stress directions from the aftershock fault-plane solutions suggest a tectonic model of changing fault geometry for the overlapping zone between the Genoa and Antelope Valley fault zones. Two plausible long-term tectonic outcomes may develop with this model: a normal fault growth model where the overlapping segments of the Genoa and Antelope Valley faults eventually become “hard linked” (form a throughgoing fault) or a normal fault growth model where the overlapping segment of the Genoa fault system grows southward while the Antelope Valley fault is isolated in the formation of new basins and ranges.

Radar interferometry and its application to changes in the Earth's surface

Geophysical applications of radar interferometry to measure changes in the Earth's surface have exploded in the early 1990s. This new geodetic technique calculates the interference pattern caused by the difference in phase between two images acquired by a spaceborne synthetic aperture radar at two distinct times. The resulting interferogram is a contour map of the change in distance between the ground and the radar instrument. These maps provide an unsurpassed spatial sampling density (∼100 pixels km−2), a competitive precision (∼1 cm), and a useful observation cadence (1 pass month−1). They record movements in the crust, perturbations in the atmosphere, dielectric modifications in the soil, and relief in the topography. They are also sensitive to technical effects, such as relative variations in the radar's trajectory or variations in its frequency standard. We describe how all these phenomena contribute to an interferogram. Then a practical summary explains the techniques for calculating and manipulating interferograms from various radar instruments, including the four satellites currently in orbit: ERS‐1, ERS‐2, JERS‐1, and RADARSAT. The next chapter suggests some guidelines for interpreting an interferogram as a geophysical measurement: respecting the limits of the technique, assessing its uncertainty, recognizing artifacts, and discriminating different types of signal. We then review the geophysical applications published to date, most of which study deformation related to earthquakes, volcanoes, and glaciers using ERS‐1 data. We also show examples of monitoring natural hazards and environmental alterations related to landslides, subsidence, and agriculture. In addition, we consider subtler geophysical signals such as postseismic relaxation, tidal loading of coastal areas, and interseismic strain accumulation. We conclude with our perspectives on the future of radar interferometry. The objective of the review is for the reader to develop the physical understanding necessary to calculate an interferogram and the geophysical intuition necessary to interpret it.

Models of rapid relative sea-level change in Washington and Oregon, USA

Much geological evidence points to repeated plate-boundary earthquakes on the Cas-cadia subduction zone, Pacific Northwest USA. Current land motions, calculated. using shortterm (<100 yr) tide gauge, repeat levelling and other geodetic data, record deformation during the present interseismic period. These short-term data form the basis of efforts to model longer-term plate-boundary deformation and assess seismic hazard in this region. In this paper we use relative sea-level (RSL) data from the last 4000 cal. yr BP to examine four aspects of these plate-boundary deformation models over several earthquake cycles: rates of isostatic rebound, the spatial pattern and magnitude of coseismic deformation, and rates of interseismic strain accumulation. Age/altitude plots of RSL data from a 300-km-long string of sites stretching from Copalis River (central Washington) to Alsea Bay (central Oregon) reveal a systematic northsouth decline in the rate of isostatic rebound equal to c. 0.25 ± 0.02 mm yr-1 100 krn’. High precision microfossil and RSL data from Johns River (Washington) and Netarts Bay (Oregon) register repeated episodes of late-Holocene coseismic submergence. This is contrary to the coseismic uplift expected, based on differences in uplift rates calculated using late-Quaternary marine terrace data from the region. Moreover, the magnitudes of coseismic submergence inferred from these RSL data are typically half the modelled predictions based on current deformation rates. Lastly, we identify evidence from Netarts Bay for pre-seismic RSL rise of up to 0.4 mm yr’ in the decades immediately preceding each episode of marsh burial. Together, our findings shine new light on models of rapid relative sea-level rise in the Pacific Northwest, and highlight the need to strengthen further the dialogue between long-term geological and short-term geodetic studies in this region.

TIDAL MARSH STRATIGRAPHY, SEA-LEVEL CHANGE AND LARGE EARTHQUAKES—II: SUBMERGENCE EVENTS DURING THE LAST 3500 YEARS AT NETARTS BAY, OREGON, USA

This is the second investigation where we apply the methods and scientific framework common to sea-level investigations in northwest Europe to test a model of coseismic land subsidence followed by land uplift during interseismic strain accumulation, used to explain alternating peat–mud couplets of late Holocene age found in many of the estuaries of the Pacific Northwest of the USA and Canada. Recent studies in this region interpret such couplets as the product of repeated large (magnitude 8 or 9) earthquakes on the Cascadia subduction zone. Stratigraphic investigations of the intertidal sediments at Netarts Bay, Oregon, using lithological, pollen and diatom analyses reveal different types of submergence associated with peat–mud couplets during the last 3500yr. Three peat-mud couplets record gradual sedimentation changes within an infilling body of water. Four other couplets record rapid submergence, burial of tidal marshes and their replacement by low marsh environments. In three of the events rapid submergence is small, 0–0.5m. The most recent marsh submergence event, around AD 1700, was 0.4±0.3m. Most of the buried peats contain microfossil evidence for a gradual increase in marine influence before the rapid change from organic to minerogenic sedimentation. Similar gradual changes are common in comparable palaeoenvironments on non-seismic coasts and are strong evidence against a simple interseismic strain accumulation-coseismic subsidence model. One hypothesis is that the gradual increase in marine influence before each submergence represents pre-seismic relative sea-level rise caused by a reduction in the rate of land uplift. The microfossil analyses provide evidence of three stages within an earthquake deformation cycle: coseismic subsidence; interseismic land uplift; and pre-seismic reduction in the rate of land uplift. At present the microfossil data do not differentiate between rapid post-seismic land uplift and rapid sedimentation.

Interseismic and coseismic motions in GPS series related to the Ms 7.3 July 13, 1994, Malekula Earthquake, Central New Hebrides Subduction Zone

On July 13, 1994, an earthquake Ms=7.3 occurred at Malekula, in the New Hebrides archipelago. The GPS data collected across the New Hebrides trench between 1990 and 1996 were processed in order to separate the interseismic and coseismic motions from the drifts related to the convergence with the Australian plate. The GPS‐derived coseismic displacements at the GPS site in Malekula are 49 ± 15 mm southward, 230 ± 30 mm westward and 170 ± 37 mm downward, when the CMT‐derived displacements are 50 mm southward, 210 mm westward and 150 mm downward. Taking into account the interseismic strain accumulation (25 mm/yr at the source established from historical seismicity, 7.5 mm/yr at the GPS site), the strain‐free convergence rate at Malekula is 49 ± 3 mm/yr. Other GPS‐derived convergence rates are 95 ± 1 mm/yr at Efate and 37 ± 2 mm/yr at Santo. These rates imply a regional right‐lateral motion between the Efate and the Santo‐Malekula segments. In contrast, the focal mechanism of the earthquake mostly indicates a left‐lateral motion. Therefore, we hypothesize that the earthquake is related to variations in the interplate coupling along the converging boundary of the Santo‐Malekula segment.

Geodetic observations of interseismic strain segmentation at the Sumatra Subduction Zone

Deformation above the Sumatra subduction zone, revealed by Global Positioning System (GPS) geodetic surveys, shows nearly complete coupling of the forearc to the subducting plate south of 0.5°S and half as much to the north. The abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 (Mw>8) thrust earthquakes. The rupture boundary appears as an abrupt change in strain accumulation well into the interseismic cycle, suggesting that seismic segmentation is controlled by properties of the plate interface that persist through more than one earthquake cycle. Structural evidence indicates that differences in basal shear stress may be related to elevated pore pressure in the north.

Crustal deformation at the Nankai Subduction Zone, southwest Japan, derived from GPS measurements

The interseismic strain accumulation process at the Nankai subduction zone, the plate‐boundary region between the Philippine Sea and the Eurasian plates off Southwest Japan, has been revealed by GPS measurements spanning five years. Large strains characterized by a contraction of 2.2 ∼ 3.4 × 10−7/yr have accumulated in a narrow region extending parallel to the strike of the plate boundary. The contraction axes lie in a northwest‐southeast direction nearly parallel to the direction of relative motion of the Philippine Sea plate with respect to the Eurasian plate. GPS‐derived deformation data, including both rate and direction, are consistent with the expected earthquake deformation cycle at the Nankai subduction zone. Current interplate coupling in this zone is strong; much of the relative plate motion has been taken up by deformation of the marginal parts of the Eurasian plate.

The 1992 M = 7 Cape Mendocino, California, earthquake: Coseismic deformation at the south end of the Cascadia megathrust

We invert geodetic measurements of coseismic surface displacements to determine a dislocation model for the April 25, 1992, M = 7 Cape Mendocino, California, earthquake. The orientation of the model slip vector, which nearly parallels North America‐Juan de Fuca relative plate convergence, and the location and orientation of the model fault relative to the offshore Cascadia megathrust, suggest that the 1992 Cape Mendocino earthquake is the first well‐recorded event to relieve strain associated with the Cascadia subduction zone. We use data from three geodetic techniques: (1) the horizontal and vertical displacements of 13 monuments surveyed with the Global Positioning System, corrected for observed horizontal interseismic strain accumulation, (2) 88 section‐elevation differences between leveling monuments, and (3) the uplift of 12 coastal sites observed from the die‐off of intertidal marine organisms. Maximum observed displacements are 0.4 m of horizontal movement and 1.5 m of uplift along the coast. We use Monte Carlo techniques to estimate an optimal uniform slip rectangular fault geometry and its uncertainties. The optimal model using all the data resolves 4.9 m of slip on a 14 by 15 km fault that dips 28° SE. The fault extends from 1.5 to 8.7 km in depth and the main‐shock hypocenter is close to the downdip projection of the fault. The shallowly dipping fault plane is consistent with the observed aftershock locations, and the estimated geodetic moment is 3.1 × 1019 N m, 70% of the seismic moment. Other models that exclude leveling data collected in 1935 and 1942 are more consistent with seismological estimates of the fault geometry. If the earthquake is characteristic for this segment, the estimated horizontal slip vector compared with plate convergence rates suggests a recurrence interval of 140 years, with a 95% confidence range of 100–670 years. The coseismic uplift occurred in a region that also has high Quaternary uplift rates determined from marine terrace studies. If repeated ruptures of this southernmost segment of the Cascadia megathrust are responsible for the Quaternary uplift, a comparison of the coseismic uplift with coastal uplift rates suggests a recurrence interval of 200–400 years. Thus comparing horizontal and vertical coseismic to long‐term deformation suggests a recurrence interval of about 100–300 years for M = 7 events at the south end of the Cascadia megathrust.

Evolution of moderate seismicity in the San Francisco Bay region, 1850 to 1993: Seismicity changes related to the occurrence of large and great earthquakes

The rate of seismic activity of moderate‐size (M > 5.5) earthquakes in the San Francisco Bay (SFB) region has varied considerably during the past 150 years. As measured by the rate of seismic moment release, seismic activity in the SFB region is observed to accelerate prior to M > 7.0 earthquakes in 1868, 1906, and 1989, and then to decelerate following them. We examine these seismicity changes in the context of the evolution of the stress field in the SFB region as a result of strain accumulation and release using a model of dislocations in an elastic halfspace. We use a Coulomb failure function (CFF) to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the SFB region. We find that the occurrence of a large or great earthquake creates a “stress shadow”: a region where the stress driving earthquake deformation is decreased. Interseismic strain accumulation acts to reverse this process, gradually bringing faults in the SFB region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failure is possible. As the stress shadow generated by a large or great earthquake disappears, it migrates inward toward the fault associated with that large or great event. The observed changes in the rate of occurrence of moderate earthquakes in the SFB region are broadly consistent with this model. In detail, the decrease in seismicity throughout most of the SFB region and a localized increase in the Monterey Bay region following the great 1906 earthquake is consistent with our predicted stress changes. The timing and location of moderate‐size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by strain accumulation in the SFB region. Those earthquakes that are most inconsistent with our stress evolution model, including the 1911 earthquake southeast of San Jose, are found to occur in regions where dip‐slip faulting is common in addition to strike‐slip. The 1906 earthquake brought that zone of dip‐slip faulting closer to failure, suggesting that the 1911 event may have been a reverse faulting earthquake rather than a strike‐slip one similar to the 1984 Morgan Hill earthquake. The occurrence of activity on faults very close to the San Andreas, such as the Lake Elsman earthquakes of 1988 and 1989, appear to be associated with the last disappearence of the stress shadow on the Loma Prieta segment of the San Andreas fault. Thus events of that type may represent an intermediate‐term precursor to a large earthquake, such as the 1989 Loma Prieta event. Much of the moderate‐size earthquake activity in the SFB region appears to be modulated in time by the buildup and release of stress in large and great earthquakes. A tensorial approach to earthquake prediction, i.e., taking into account changes in the components of the stress tensor, has several advantages over examining scalar changes such as those in seismic activity and moment release rates. This tensorial approach allows for both activation and quiescence (but in different subregions) prior to as well as after large earthquakes.

Tidal marsh stratigraphy, sea-level change and large earthquakes, i: a 5000 year record in washington, U.S.A.

Many of the estuaries of the Pacific Northwest of the U.S.A. and Canada contain stratigraphic sequences typified by alternating peat-mud couplets. Recent studies in this region interpret such couplets as the product of repeated large (magnitude S or 9) earthquakes on the Cascadia subduction zone. The resultant pattern of land-level movements is described by a model, the ‘earthquake deformation cycle’, of coseismic land subsidence followed by land uplift during interseismic strain accumulation. However, peat-mud couplets similar to those recorded in the Pacific Northwest are found on other less tectonically active temperate-latitude coasts, such as northwest Europe and the Atlantic coast of the U.S.A., where they have been interpreted as the product of non-seismic coastal processes. In this paper we apply the methods and scientific framework common to sea-level investigations in northwest Europe to a sequence of peat-mud couplets recorded in the lower Johns River, an estuary in southern Washington, to provide a test of the ‘earthquake deformation cycle’. Stratigraphic investigations of the intertidal sediments along the lower Johns River, using lithological, pollen, diatom and foraminiferal data, show evidence for eight coastal submergence events during the last 5000 years. To evaluate the ‘earthquake deformation cycle’ we assess the lateral extent of peat-mud couplets, the synchroneity of submergence, the presence of tsunami deposits accompanying submergence, and the suddenness and amount of submergence. Each submergence is shown to be accompanied by changes in coastal sedimentation broadly commensurate with those predicted by the ‘earthquake deformation cycle’, demonstrating the continued intermittent seismic activity of the Cascadia subduction zone throughout the mid and late-Holocene. Quantitative analyses of contemporary and fossil biostratigraphic data, using TWINSPAN and Detrended Correspondence Analysis, enable us to estimate the magnitude of submergence accompanying each peat-mud couplet. One event was accompanied by submergence of about 1.5 m or more, four events by intermediate submergence of about 1±0.5 m, and a further three events by submergence of <0.5 m. There is evidence for non-seismic relative sea-level rise prior to two of the eight submergence events, but for at least the last 3500 years the magnitude of relative sea-level rise has been less than the combined influence of sediment accretion following submergence and interseismic land uplift.

Interseismic strain accumulation on the Himalayan crustal ramp (Nepal)

The Departement of Mines and Geology has been monitoring the seismicity of the Central Himalayas of Nepal since 1985. Intense microseismicity and frequent medium‐size earthquakes (mL&lt;4) tend to cluster beneath the topographic front of the Higher Himalaya. This 10–20km deep seismicity also correlates with a zone of localized uplift that has been evidenced from geodetic data. Both microseismic and geodetic data indicate strain accumulation on a mid‐crustal ramp that had been previously inferred from geological and geophysical evidence. This ramp connects a flat decollement under the Lesser and Sub‐Himalaya with a deeper decollement under the Higher Himalaya, and probably acts as a geometric asperity where strain and stress build up during the interseismic period. The large Himalayan earthquakes could nucleate there and probably activate the whole flat‐and‐ramp system up to the blind thrusts of the Sub‐Himalaya.

Topographic form of the Coast Ranges of the Cascadia Margin in relation to coastal uplift rates and plate subduction

The Coast Ranges of the Cascadia margin are overriding the subducted Juan de Fuca/Gorda plate. We investigate the extent to which the latitudinal trend in average topography of the Coast Ranges is a function of the latitudinal change in attributes related to the subduction process. These attributes include the variable age of the subducted slab that underlies the Coast Ranges and average vertical crustal velocities of the western margin of the Coast Ranges for two markedly different time periods, the last 45 years and the last 100 kyr. These vertical crustal velocities are computed from the resurveying of highway bench marks and from the present elevation of shore platforms that have been uplifted in the late Quaternary, respectively. Topography of the Coast Ranges is in part a function of the age and buoyancy of the underlying subducted plate. This is evident in the fact that the two highest topographic elements of the Coast Ranges, the Klamath Mountains and the Olympic Mountains, are underlain by youngest subducted oceanic crust. The subducted Blanco Fracture Zone in southernmost Oregon is responsible for an age discontinuity of subducted crust under the Klamath Mountains. The northern terminus of the topographically higher Klamaths is offset to the north relative to the position of the underlying Blanco Fracture Zone, the offset being in the direction of migration of the fracture zone, as dictated by relative plate motions. Vertical crustal velocities at the coast, derived from bench mark surveys, are as much as an order of magnitude greater than vertical crustal velocities derived from uplifted shore platforms. This uplift rate discrepancy indicates that strain is accumulating on the plate margin, to be released during the next interplate earthquake. In a latitudinal sense, average Coast Range topography is relatively high where bench mark‐derived, short‐term vertical crustal velocites are highest. Because the shore platform vertical crustal velocites reflect longer‐term, permanent uplift, we infer that a small percentage of the interseismic strain that accumulates as rapid short‐term uplift is not recovered by subduction earthquakes but rather contributes to rock uplift of the Coast Ranges. The conjecture that permanent rock uplift is related to interseismic uplift is consistent with the observation that those segments of the subduction zone subject to greater interseismic uplift rates are at approximately the same latitudes as those segments of the Coast Ranges that have higher magnitudes of rock uplift over the long term.

Present‐day vertical deformation of the Cascadia Margin, Pacific Northwest, United States

We estimate present‐day uplift rates along the Cascadia Subduction Zone in California, Oregon, and Washington in the Pacific Northwest, United States, by utilizing repeated leveling surveys and tide gauge records. These two independent data sets give similar profiles for latitudinal variation of contemporary uplift rates along the coast. Uplift rates are extended inland through east‐west leveling lines that connect the north‐south line along the coast to the north‐south line along the inland valleys just west of the Cascades. The results are summarized as a contour map of present‐day uplift rates for the western Pacific Northwest. We find that rates of present‐day uplift vary latitudinally along the coast and inland valleys, as well as longitudinally along transects connecting the coast to the inland valleys. Long‐term tidal records of Neah Bay, Astoria, and Crescent City indicate uplift of land relative to sea level of 1.6±0.2, 0.0±0.2, and 0.9±0.2 mm/yr, respectively (±1 standard error). Unlike previous estimates of relative sea level change at Astoria, we adjust for discharge effects of the Columbia River, including human management influences. After approximating an absolute framework by using 1.8±0.1 mm/yr to compensate for global sea level rise, results indicate that much of the western Pacific Northwest is rising at rates between 0 and 5 mm/yr. The most rapid uplift rates are near the coast, particularly near the Olympic Peninsula, the mouth of the Columbia River, Cape Blanco, and Cape Mendocino. Two axes of uplift are identified: one trends northeast from the southwest Oregon coast, and the other trends south‐southeasterly from the Olympic Peninsula to the Columbia River. The Puget Sound vicinity and a small east‐west region from the north central Oregon coast to the inland Willamette Valley are subsiding at rates up to 1 mm/yr. We interpret the overall pattern of rapid present‐day uplift to be generated by interseismic strain accumulation in the subduction zone. This interseismic elastic strain accumulation implies significant seismic hazard.

Site velocities before and after the Loma Prieta and Gulf of Alaska earthquakes determined from VLBI

We use geodetic data from VLBI to determine the pre‐ and postseismic velocities of two sites. We then place limits on variations in interseismic strain buildup. The 1987 and 1988 Gulf of Alaska earthquakes (each Ms=7.6) broke the Pacific plate interior. During the earthquakes the Cape Yakataga site moved 78 mm toward southwest. During the 1989 Loma Prieta earthquake (Ms=7.1) the Fort Ord site moved 48 mm toward north. Baselines (a) from Fairbanks to Cape Yakataga and (b) from Mojave to Fort Ord change at nearly the same rate before and after the earthquakes. Postseismic transients, which we determine from differences between post‐ and preseismic rates, are minor: at Cape Yakataga the transient is 3±4 mm in a postseismic interval of 23 months, and at Fort Ord the transient is 6±5 mm in 21 months. The slip beneath the Loma Prieta rupture needed to generate the Fort Ord transient is 0.22±0.19 m, one‐tenth the coseismic slip (2 m). We analyze elastic lithosphere‐viscous asthenosphere models to determine that the characteristic time describing exponential decay in deep fault slip is longer than 6 years. The VLBI measurements are consistent with uniform interseismic strain buildup. They disagree with fast postseismic rates caused by an asthenosphere with very low viscosity.

Current deformation and the width of the seismogenic zone of the northern Cascadia subduction thrust

Evidence has been obtained for the accumulation of elastic strain across the northern Cascadia subduction zone that may be released in a future very large subduction thrust earthquake. Vertical and horizontal strain rates across the southern Vancouver Island region have been determined through (1) long‐term trends in tide gauge data, (2) changes in repeated accurate leveling surveys, (3) changes in repeated high‐accuracy gravity profiles, and (4) horizontal shortening observed in repeated precise positioning surveys. The outer coast is uplifting at a rate of a few millimeters per year decreasing landward, and shortening is occurring across the 100‐km‐wide coastal region at a rate of about 0.1 microstrain per year (mm km−1yr−1). The results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault. The location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data. Most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern Vancouver Island and northern Washington, provided the locked portion is restricted to a 60‐km‐wide band offshore beneath the continental shelf and slope. This conclusion also results from modeling the coseismic subsidence on the outer coast of Vancouver Island about 300 years ago deduced from paleoseismicity data. The unusually narrow downdip extent of the subduction thrust seismogenic zone, that extends little if at all beneath the coast, is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments. The high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km. The distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of Vancouver and Victoria from this source. The narrow width may also limit the earthquake size; however, events of magnitude well over 8 are possible.

Episodic tectonic subsidence of Late Holocene salt marshes, northern Oregon Central Cascadia Margin

Salt marsh subsurface deposits (0–4 m depth) in Netarts Bay, a small coastal lagoon of northern Oregon, record six events of marsh burial in the last several thousand years. Five of the buried marsh surfaces show sharp, nonerosional upper contacts with either anomalous sand layers (tsunami deposits) or tidal flat mud deposits. These sequences indicate episodic, abrupt subsidence of the marsh surfaces to low intertidal levels. In contrast, lower marsh contacts with underlying intertidal muds are gradational, indicating gradual uplift and development of the marsh. Three independent measures of deposit elevation relative to mean tidal level (percent organics, diatom assemblages, and percent eolian sand) have been used to estimate vertical displacements of marsh surfaces. Abrupt subsidence displacements of 1–1.5 m alternate with gradual uplift displacements of the order of 0.5–1.0 m. The vertical tectonic movements are interpreted to reflect coseismic strain release (abrupt subsidence) following interseismic strain accumulation (gradual uplift), associated with interplate coupling between the Juan de Fuca Plate and the North American plate in the Cascadia subduction zone. Recurrence intervals between subsidence events range from possibly less than 300 years to at least 1000 years, with the last dated event likely occurring 300–400 radiocarbon years before present (RCYBP). Significant 14C age overlaps of at least four subsidence events recorded at Netarts and reported for southern Washington and other northern Oregon bays (at 300–500, 1000–1300, 1400–1800, and 3000–3300 RCYBP) suggest the potential for event synchroneity over at least 200 km of the central part of the Cascadia subduction zone. Additional work is needed to test the synchroneity of these episodic events of coseismic subsidence.