Large Subduction Zone Earthquakes Research Articles

Lessons Learned from the 2004 Sumatra-Andaman Megathrust Rupture

The 2004 Sumatra-Andaman earthquake has been extensively studied because of its great size and devastating consequences. Large amounts of high-quality seismic, geodetic, and geologic data have led to a number of proposed models for its length, duration, fault geometry, rupture velocity, and slip history. The latest of these models vary in their details but now largely agree in their large-scale features, which include significant coseismic slip along the entire 1300- to 1500-km rupture, the bulk of which occurred fast enough to radiate seismic waves. The earthquake's enormous size has challenged conventional processing approaches and stimulated the development of new analysis and inversion methods, including multiple-source inversions, high-frequency body-wave imaging, and satellite observations of tsunami heights and gravity changes. The Sumatra megathrust earthquake was the largest in 40 years and is by far the best documented, but it does not seem fundamentally different in its properties from other large subduction-zone earthquakes.

Fore‐arc motion and Cocos Ridge collision in Central America

We present the first regional surface velocity field for Central America, showing crustal response to interaction of the Cocos and Caribbean plates. Elastic half‐space models for interseismic strain accumulation on the dipping subduction plate boundary fit the GPS data well and show strain accumulation offshore and beneath the Nicoya and Osa peninsulas in Costa Rica but not in Nicaragua. Since large subduction zone earthquakes occur in Nicaragua, we suggest that interseismic locking in Nicaragua and some other parts of Central America occurs but is mainly shallow, <20 km depth, too far offshore to be detected by our on‐land GPS measurements. Our data also show significant trench‐parallel motion for most of the region, generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates. However, trench‐parallel motion is also observed in central Costa Rica, where plate convergence is normal to the trench, and in the Nicaraguan fore arc, where trench‐parallel motion is fast, up to 9 mm a−1, but mechanical coupling is low. A finite element model of collision (as opposed to subduction) involving the aseismic Cocos Ridge also fits the GPS surface velocity field, most significantly reproducing the pattern of trench‐parallel motion. We infer that buoyant, thickened CNS‐2‐Cocos Ridge crust resists normal subduction and instead acts as an indenter to the Caribbean plate, driving crustal shortening in southern Costa Rica and contributing to trench‐parallel fore‐arc motion in Costa Rica and perhaps Nicaragua as a type of tectonic escape.

Long-term hydrogeochemical records in the oceanic basement and forearc prism at the Costa Rica subduction zone

Two sealed borehole hydrologic observatories (CORKs) were installed in two active hydrogeochemical systems at the Costa Rica subduction zone to investigate the relationship between tectonics, fluid flow, and fluid composition. The observatories were deployed during Ocean Drilling Program (ODP) Leg 205 at Site 1253, ~ 0.2 km seaward of the trench, in the upper igneous basement, and at Site 1255, ~ 0.5 km landward of the trench, in the décollement. Downhole instrumentation was designed to monitor formation fluid flow rates, composition, pressure, and temperature. The two-year records collected by this interdisciplinary effort constitute the first co-registered hydrological, chemical, and physical dataset from a subduction zone, providing critical information on the average and transient state of the subduction thrust and upper igneous basement. The continuous records at ODP Site 1253 show that the uppermost igneous basement is highly permeable hosting an average fluid flow rate of 0.3 m/yr, and indicate that the fluid sampled in the basement is a mixture between seawater (~ 50%) and a subduction zone fluid originating within the forearc (~ 50%). These results suggest that the uppermost basement serves as an efficient pathway for fluid expelled from the forearc that should be considered in models of subduction zone hydrogeology and deformation. Three transients in fluid flow rates were observed along the décollement at ODP Site 1255, two of which coincided with stepwise increases in formation pressure. These two transients are the result of aseismic slip dislocations that propagated up-dip from the seismogenic zone over the course of ~ 2 weeks terminating before reaching ODP Site 1255 and the trench. The nature and temporal behavior of strain and the associated hydrological response during these slow slip events may be an analog for the response of the seaward part of the subduction prism during or soon after large subduction zone earthquakes.

Integrated Ground Motion and Tsunami Simulation for the 1944 Tonankai Earthquake Using High-Performance Supercomputers

An integrated simulation of seismic wave and tsunami has been developed for mitigation of earthquake and tsunami disasters associated with large subduction-zone earthquakes occurring in the Nankai Trough. The ground motion due to the earthquake is firstly calculated by solving equation of motions with heterogeneous source-rupture model and 3-D heterogeneous subsurface structural model. Tsunami generation and propagation in heterogeneous bathymetry is then simulated by solving the 3-D Navier-Stokes equation. Ground motion and tsunami simulations are combined through an appropriate dynamic boundary condition at the sea floor. Thanks to supercomputers and efficient parallel computing, we are reproducing strong ground motion and tsunamis caused by the M8.0 Tonankai earthquake in the Nankai Trough in 1944. The visualized seismic wavefield and tsunami derived by integrated simulation provides a direct understanding of disasters associated with Nankai Trough earthquakes with the development of long-period ground motion in highly populated basins such as Tokyo, Osaka, and Nagoya and tsunamis striking along Japan’s Pacific Ocean coast.

Quantifying geomorphology associated with large subduction zone earthquakes

ABSTRACTFor 30 large‐earthquake generating subduction zones, we quantify forearc basin size and subducting seafloor roughness to see if the size and shape of geomorphologic features determine earthquake magnitude. The subduction of geomorphologic features, such as ridges and seamounts, not only influences seismicity, but increases basal erosion and subsidence of the accretionary wedge, resulting in the formation of forearc basins. Many subduction zone ruptures have been associated with these basins, where a great portion of the ruptures' asperities collocate with the basins. First, we attempt to discern a relationship between forearc and subducting geomorphology by quantifying along‐strike variations in the bathymetry associated with 30 different rupture zones and the sections of subducting seafloor adjacent to those rupture zones. Two parameters, determined from theoretical models fit to empirical semivariograms, characterize areas of bathymetry: the sill, which estimates the degree of relief in the terrain, and the range, which is the horizontal scale associated with the sill. The ratio of sill to range provides us with a measure of seafloor roughness at a scale relevant to the mechanics of subduction. We investigate the control that forearc basins and subducting bathymetric highs may have on the occurrence of large thrust earthquakes by separately comparing our rupture zone and subducting seafloor roughness measurements to the moment magnitudes of the 30 events. The trench‐parallel dimension of bathymetrically high features on the subducting seafloor appears to correlate with moment magnitude. Also, we observe in the ratio of sill to range measurements that geomorphology on the subducting seafloor and forearc constrain the earthquake size at a margin.

Implementation of Routine Regional Moment Tensor Analysis in New Zealand

The active tectonics of New Zealand are dominated by three main features (figure 1). Beneath the North Island there is active subduction of the Hikurangi trough where the Pacific plate is undergoing oblique subduction beneath the Australian plate at ∼ 45 mm/yr ( e.g. , Demets et al. 1990; Demets et al. 1994). The Hikurangi trough extends from the Kermadec trench in the north and terminates in the upper South Island. Large subduction zone earthquakes of around Mw 8.0 in the lower North Island and around Mw 6.9 farther north are expected (Reyners 1998). Slow slip events have also been observed on the Hikurangi subduction interface ( e.g. , Wallace and Beavan 2006) similar to those documented at other Pacific Rim subduction margins ( e.g. , Dragert et al. 2001; Obara et al. 2004; Larson et al. 2004). A number of dextral strike-slip faults are also present throughout the North Island and the upper South Island and have produced a number of M 6.0–7.5 earthquakes in the past century (Doser and Webb 2003). The Wellington region is cut by five active dextral faults that have average recurrence intervals of meter-scale surface rupture that range from ∼ 500 to 5,000 years (Van Dissen and Berryman 1996). The Wairarapa fault is the only one of these to have ruptured in historic times, in 1855, in an event estimated at Mw 8.2 (Darby and Beanland 1992). ▴ Figure 1. Overview of the tectonic setting of New Zealand, which is dominated by the interaction between the Pacific plate and the Australian plate. In the North Island the Pacific plate is subducting beneath the Australian plate at the Hikurangi trough and Kermadec trench. The Alpine fault delineates a dextral strike-slip fault through most of the South Island, and south of the South Island the …

A seismological overview of long-period ground motion

Long-period ground motion has become an increasingly important consideration because of the recent rapid increase in the number of large-scale structures, such as high-rise buildings and oil storage tanks. Large subduction-zone earthquakes and moderate to large crustal earthquakes can generate far-source long-period ground motions in distant sedimentary basins with the help of path effects. Near-fault long-period ground motions are generated, for the most part, by the source effects of forward rupture directivity. Far-source long-period ground motions consist primarily of surface waves with longer durations than near-fault long-period ground motions. They were first recognized in the seismograms of the 1968 Tokachi-oki and 1966 Parkfield earthquakes, and their identification has been applied to the 1964 Niigata earthquake and earlier earthquakes. Even if there is no seismogram, we can identify far-source long-period ground motions through the investigation of tank damage by liquid sloshing.

Influence of fore‐arc structure on the extent of great subduction zone earthquakes

Structural features associated with fore‐arc basins appear to strongly influence the rupture processes of large subduction zone earthquakes. Recent studies demonstrated that a significant percentage of the global seismic moment release on subduction zone thrust faults is concentrated beneath the gravity lows resulting from fore‐arc basins. To better determine the nature of this correlation and to examine its effect on rupture directivity and termination, we estimated the rupture areas of a set of Mw 7.5–8.7 earthquakes that occurred in circum‐Pacific subduction zones. We compare synthetic and observed seismograms by measuring frequency‐dependent amplitude and arrival time differences of the first orbit Rayleigh waves. At low frequencies, the amplitude anomalies primarily result from the spatial and temporal extent of the rupture. We then invert the amplitude and arrival time measurements to estimate the second moments of the slip distribution which describe the rupture length, width, duration, and propagation velocity of each earthquake. Comparing the rupture areas to the trench‐parallel gravity anomaly (TPGA) above each rupture, we find that in 11 of the 15 events considered in this study the TPGA increases between the centroid and the limits of the rupture. Thus local increases in TPGA appear to be related to the physical conditions along the plate interface that favor rupture termination. Owing to the inherently long timescales required for fore‐arc basin formation, the correlation between the TPGA field and rupture termination regions indicates that long‐lived material heterogeneity rather than short timescale stress heterogeneities are responsible for arresting most great subduction zone ruptures.

Repeated large subduction zone earthquakes in the Nankai-Suruga trough: Evidence from submarine liquefactions

[1] Subduction zones are generally characterized by large earthquakes that contribute about 90% of the total seismic moment worldwide, and can cause great damage from tsunamis such as the 2004 Sumatra Mw 9.0 earthquake. Here, we report recurrent large subduction zone earthquakes revealed by extensive submarine liquefactions formed in the Nankai-Suruga trough where the Philippine Sea plate is subducting beneath the Eurasian plate. Field evidence and grain-size analysis of liquefied deposits show that the multiple-liquefactions were repeatedly induced by strong earthquake shaking during the Pleistocene in the eastern Nankai-Suruga subduction zone. The recognition and analysis of submarine liquefactions provide reliable paleoseismic evidence in the Nankai-Suruga subduction zone.

Geothermal evidence for fluid flow through the gas hydrate stability field off Central Chile-transient flow related to large subduction zone earthquakes?

Fluids are suspected to play a major role in the nucleation and rupture propagation of earthquakes. In Chile, seismological data were previously interpreted to indicate that fluids captured in the fault zone are released periodically during large underthrust earthquakes, leading to post-seismic fluid flow. In central Chile, heat flow derived from the presence of a bottom simulating reflector (BSR) show a smooth trend across the margin. BSR-derived data are in excellent agreement with thermal subduction zone models. Over the young accretionary prism, both BSR-derived and measured surface heat flow support a common trend. Landwards of the backstop, however, measured heat flow triples over a distance of 20–30 km, producing a profound discrepancy to the BSR-derived data. We suggest that this disparity is related to transient flow of warm fluids through the gas hydrate stability zone possibly caused by fluids released after large underthrust earthquakes. Such flow events may inherently affect the distribution of solid gas hydrates between the seafloor and the BSR.

Performance of Lightly Confined Reinforced Concrete Columns in Long-Duration Subduction Zone Earthquakes

The main goals of this research were to evaluate typical 1950s and 1960s as-built bridge columns in western Washington State in large subduction zone earthquakes and to investigate the dependency of failure mechanisms on loading history. Eight displacement histories were applied to eight nominally identical, half-scale, circular reinforced concrete columns expected to respond primarily in flexure (flexure-dominated). The main design deficiencies were a short longitudinal reinforcement lap splice at the base of the column (35 db) and inadequate transverse reinforcement. Test results showed that the failure mode of reinforced concrete columns was controlled by the column loading history. Three distinct failure mechanisms were observed for columns with an aspect ratio of approximately 4.2, assuming symmetric, double-curvature behavior. Large initial displacements greater than six times the effective yield displacement (Δ y) were likely to result in shear failures. Columns experiencing many displacements less than 4Δ y were likely to fail because of longitudinal reinforcement buckling. Columns subjected to several displacement excursions less than 4Δ y followed by an excursion greater than 6Δ y were likely to fail by longitudinal reinforcement slipping within the splice region. Despite the deficiencies present in circular reinforced concrete bridge columns built before 1975 in western Washington State, this study showed that flexure-dominated columns with a 35 db lap splice in multiple-column bent, three-or four-span bridges were not likely to experience significant damage in the predicted Cascadia Subduction Zone earthquake. However, other components of the bridge need to be assessed to determine whether the global bridge response is acceptable under the predicted Cascadia Subduction Zone earthquake.

Part 5: Seismic Design of Bridges: Performance of Lightly Confined Reinforced Concrete Columns in Long-Duration Subduction Zone Earthquakes

The main goals of this research were to evaluate typical 1950s and 1960s as-built bridge columns in western Washington State in large subduction zone earthquakes and to investigate the dependency of failure mechanisms on loading history. Eight displacement histories were applied to eight nominally identical, half-scale, circular reinforced concrete columns expected to respond primarily in flexure (flexure-dominated). The main design deficiencies were a short longitudinal reinforcement lap splice at the base of the column (35db) and inadequate transverse reinforcement. Test results showed that the failure mode of reinforced concrete columns was controlled by the column loading history. Three distinct failure mechanisms were observed for columns with an aspect ratio of approximately 4.2, assuming symmetric, double-curvature behavior. Large initial displacements greater than six times the effective yield displacement (Δyy) were likely to result in shear failures. Columns experiencing many displacements less ...

Effect of directivity on estimates of radiated seismic energy

To accurately estimate the seismic energy radiated in an earthquake, it is important to use appropriate corrections for path and source effects. Using slip models obtained by inversion of seismic data, we examine the effect of directivity on estimates of radiated energy and develop a method to correct for this source effect. From our calculations we suggest that the directivity correction for the earthquakes we studied is less than a factor of three; we also conclude that the directivity correction for dip‐slip earthquakes with rupture propagating along strike is less than a factor of two at teleseismic distances. Thus teleseismic energy estimates of large subduction zone earthquakes will not be significantly affected by directivity.

Past Earthquake-Induced Rapid Subsidence along the Northern San Andreas Fault: A Paleoseismological Method for Investigating Strike-Slip Faults

Evidence of rapid relative sea level changes preserved in sediment of coastal estuaries along the north coast segment of the San Andreas fault provides information on the dates of past earthquakes. This approach, although successfully used to document large subduction zone earthquakes, has not been used previously to constrain the dates of San Andreas fault or other strike-slip fault earthquakes. Data on prehistoric San Andreas fault earthquakes is needed for development of robust probabilistic hazard assessments in northern California and the San Francisco Bay area. Evidence of earthquake-induced subsidence is preserved in marshes at the northern margin of Bolinas Lagoon and the southeastern margin of Bodega Harbor. These sites occupy structural basins or troughs along the northern San Andreas fault. Radiocarbon dating and identification of the first occurrence of nonnative pollen near abrupt sedimentological changes in cores are used to constrain the dates of San Andreas fault earthquakes over about the last 800 years. Estuarine sediment from northern Bolinas Lagoon preserves evidence of an earthquake that occurred about 750 years ago. Evidence for this earthquake includes a marsh soil that was abruptly buried by tidal flat mud or coarse, poorly sorted deltaic deposits containing a mixture of terrestrial sediment and marine mud. Evaluation of diatom assemblages from above and below the buried soil horizon provides evidence of at least several decimeters of sustained relative sea level rise. Three buried soils in a marsh at the southern end of Bodega Harbor are likely a result of coseismic subsidence. All three buried marsh soils have abrupt upper contacts, although diatom evidence for decimeters of sustained subsidence is not as strong as it is for the buried soil in Bolinas Lagoon. Existing paleoseismic data on the northern San Andreas fault is compared with results of this study in order to evaluate the dates of prehistoric large earthquakes and length of past ruptures. These data indicate that an earthquake occurred about 400 years ago and another earthquake occurred about 700 years ago. The data allow for 1906-type ruptures of the fault from the Santa Cruz Mountains to at least Point Arena. However, a sequence of closely timed, smaller earthquakes would produce a similar set of paleoseismic data.

Seismic cycle and plate margin deformation in Costa Rica: GPS observations from 1994 to 1997

Global Positioning System (GPS) observations in Costa Rica from 1994 to 1997 reveal a complex pattern of motion consistent with the superposition of seismic cycle and secular plate margin deformation. In the south, velocity vectors are consistent with motion of the Panama Block plus postseismic deformation following the 1991 Limon earthquake and interseismic strain due to partial locking of the Middle America Trench (MAT) thrust. In the northwest, sites west of the volcanic arc are moving to the NW as a forearc sliver. Superimposed on this sliver motion are vertical and horizontal interseismic deformations from the adjacent Nicoya segment of the MAT. We apply two different inverse methods to understand the source of the seismic strain in NW Costa Rica. We compare fault‐locking models derived using a singular value decomposition inversion with that of a simulated annealing global optimization approach. Both methods yield similar models for partial locking of the thrust interface beneath the Nicoya Peninsula. Our results define an area of nearly fully locked fault beneath the outer coast of the southern portion of the peninsula, with somewhat lower coupling beneath the northern half and with low coupling elsewhere. These initial results show the promise for detailed imaging of the locked portion of a thrust interface responsible for future large subduction zone earthquakes.

Noncharacteristic behavior and complex recurrence of large subduction zone earthquakes

The last few years have been remarkable with respect to the number of large underthrusting earthquakes in subduction zones that reruptured plate boundary segments that failed in previous great events. Availability of modern seismic data for two consecutive large earthquakes rupturing the same portion of the plate interface provides the opportunity to compare the spatial distribution of moment release for both events. Such comparisons have been made for the plate boundary segments that failed in (1) the 1957 (Mw = 8.6), 1986 (Mw = 8.0), and 1996 (Mw = 7.9) Aleutian Islands earthquakes; (2) the 1963 (Mw = 8.5) and 1995 (Mw = 7.9) Kuril Islands earthquakes; (3) the 1971 (Mw = 8.0) and 1995 (Mw = 7.7) Solomon Islands earthquakes; and (4) the 1968 (Mw = 8.2) and 1994 (Mw = 7.7) northern Honshu earthquakes. The spatial distribution of moment release for all four of the initial great earthquakes and two of the repeat events has been determined in previous studies. Here, slip distributions for the three most recent events are determined from inversion of source time functions, derived by empirical Greens function analysis of long‐period surface waves and broadband body waves. Comparisons of the spatial distribution of moment release for sequential earthquake ruptures reveal considerable differences in the pattern of recurrent fault slip. The 1994 northern Honshu and 1995 Solomon Islands earthquakes primarily fill in areas of slip deficit left by their preceding events rather than rerupture identical asperities. The 1995 Kuril Islands and the 1996 Aleutian Islands earthquakes both rerupture portions of an asperity distribution defined by preceding events but with variable amounts of slip. This study provides the first direct evidence that recurrence of large circum‐Pacific plate boundary events is more complex than repeat rupture of dominant asperities. Recurrent fault slip for the four plate boundary segments studied does not support characteristic slip models either where failure on an entire fault segment occurs repeatedly in events with nearly identical rupture lengths, locations, and slip magnitudes or where failure of individual asperities occurs with identical slip functions through consecutive earthquake cycles. These sequential slip patterns are not consistent with physical models of earthquake rupture where slip complexity is exclusively controlled by invariant geometric and/or material heterogeneity but suggest that dynamic considerations are also important.

Geologic Setting, Field Survey and Modeling of the Chimbote, Northern Peru, Tsunami of 21 February 1996

Whereas the coast of Peru south of 10°S is historically accustomed to tsunamigenic earthquakes, the subduction zone north of 10°S has been relatively quiet. On 21 February 1996 at 21:51 GMT (07:51 local time) a large, tsunamigenic earthquake (Harvard estimate M w =7.5) struck at 9.6°S, 79.6°W, approximately 130 km off the northern coast of Peru, north of the intersection of the Mendana fracture zone with the Peru-Chile trench. The likely mechanism inferred from seismic data is a low-angle thrust consistent with subduction of the Nazca Plate beneath the South American plate, with relatively slow rupture characteristics. Approximately one hour after the main shock, a damaging tsunami reached the Peruvian coast, resulting in twelve deaths. We report survey measurements, from 7.7°S to 11°S, on maximum runup (2–5 m, between 8 and 10°S), maximum inundation distances, which exceeded 500 m, and tsunami sediment deposition patterns. Observations and numerical simulations show that the hydrodynamic characteristics of this event resemble those of the 1992 Nicaragua tsunami. Differences in climate, vegetation and population make these two tsunamis seem more different than they were. This 1996 Chimbote event was the first large (M W>7) subduction-zone (interplate) earthquake between about 8 and 10°S, in Peru, since the 17th century, and bears resemblance to the 1960 (M w 7.6) event at 6.8°S. Together these two events are apparently the only large subduction-zone earthquakes in northern Peru since 1619 (est. latitude 8°S, est. M w 7.8); these two tsunamis also each produced more fatalities than any other tsunami in Peru since the 18th century. We concur with Pelayo and Wiens (1990, 1992) that this subduction zone, in northern Peru, resembles others where the subduction zone is only weakly coupled, and convergence is largely aseismic. Subduction-zone earthquakes, when they occur, are slow, commonly shallow, and originate far from shore (near the tip of the wedge). Thus they are weakly felt, and the ensuing tsunamis are unanticipated by local populations. Although perhaps a borderline case, the Chimbote tsunami clearly is another wake-up example of a “tsunami earthquake.”

The effect of seamount subduction on seismic coupling

It has long been speculated that the subduction of seamounts may have profound mechanical consequences, particularly on the generation of large subduction zone earthquakes. We consider this problem as a special case of the seismic coupling model of Scholz and Campos. The model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling. In the case of coupled arcs, this increased coupling will result in a large increase in the recurrence intervals of earthquakes. Such events will therefore appear rare in the instrumental record, as is observed. For decoupled arcs, the subduction of a large seamount should cause them to become locally coupled. Three of the latter localities have been found with satellite altimetry data, two on the Tonga-Kermadec Trench and one on the Izu-Bonin Trench. The diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate. In each case, the small to moderate plate-interface seismicity typical of decoupled seismic zones is almost completely absent in a 100–200 km arc segment, indicating that the subduction zone is locally locked, as predicted by the model. The occurrence of large interplate earthquakes at both Tonga sites confirms that they are seismically coupled.

Southeastern Caribbean sea level variability and viscoelastic relaxation

Viscoelastic modeling of earthquakes in the southeast Caribbean shows that distant dip‐slip earthquakes in the Lesser Antilles and the geometry of the subduction zone play a significant role in vertical deformation observed along the southern Caribbean plate margin. We find that dip‐slip earthquakes yield vertical displacements that are 1–2 orders of magnitude greater than those generated by strike‐slip earthquakes along the Caribbean‐South American plate boundary. In addition, postseismic vertical displacements are at least as great or greater at distances of several hundred kilometers than the coseismic counterpart depending upon the geometry of the source. Tide gage data from Venezuela suggest that these vertical displacement rates may still be observed several decades after the occurrence of large subduction zone earthquakes (Ms 7 or greater). In addition, the uplift rate observed at the tide gages largely follows the anticipated flexure of the Caribbean plate due to the subduction process.

Late Holocene Tectonics and Paleoseismicity, Southern Cascadia Subduction Zone

Holocene deformation indicative of large subduction-zone earthquakes has occurred on two large thrust fault systems in the Humboldt Bay region of northern California. Displaced stratigraphic markers record three offsets of 5 to 7 meters each on the Little Salmon fault during the past 1700 years. Smaller and less frequent Holocene displacements have occurred in the Mad River fault zone. Elsewhere, as many as five episodes of sudden subsidence of marsh peats and fossil forests and uplift of marine terraces are recorded. Carbon-14 dates suggest that the faulting, subsidence, and uplift events were synchronous. Relations between magnitude and various fault-offset parameters indicate that earthquakes accompanying displacements on the Little Salmon fault had magnitudes of at least 7.6 to 7.8. More likely this faulting accompanied rupture of the boundary between the Gorda and North American plates, and magnitudes were about 8.4 or greater.