Great Subduction Earthquakes Research Articles

Are rupture zone limits of great subduction earthquakes controlled by upper plate structures? Evidence from multichannel seismic reflection data acquired across the northern Ecuador–southwest Colombia margin

Subduction of the Nazca plate beneath the Ecuador‐Colombia margin has produced four megathrust earthquakes during the last century. The 500‐km‐long rupture zone of the 1906 (Mw = 8.8) event was partially reactivated by three thrust events, in 1942 (Mw = 7.8), 1958 (Mw = 7.7), and 1979 (Mw = 8.2), whose rupture zones abut one another. Multichannel seismic reflection and bathymetric data acquired during the SISTEUR cruise show evidence that the margin wedge is segmented by transverse crustal faults that potentially correlate with the limits of the earthquake coseismic slip zones. The Paleogene‐Neogene Jama Quininde and Esmeraldas crustal faults define a ∼200‐km‐long margin crustal block that coincides with the 1942 earthquake rupture zone. Subduction of the buoyant Carnegie Ridge is inferred to partially lock the plate interface along central Ecuador. However, coseismic slip during the 1942 and 1906 earthquakes may have terminated against the subducted northern flank of the ridge. We report on a newly identified Manglares crustal fault that cuts transversally through the margin wedge and correlates with the limit between the 1958 and 1979 rupture zones. During the earthquake cycle the fault is associated with high‐stress concentration on the plate interface. An outer basement high, which bounds the margin seaward of the 1958 rupture zone, may act as a deformable buttress to seaward propagation of coseismic slip along a megathrust splay fault. Coseismic uplift of the basement high is interpreted as the cause for the 1958 tsunami. We propose a model of weak transverse faults which reduce coupling between adjacent margin segments, together with a splay fault and an asperity along the plate interface as controlling the seismogenic rupture of the 1958 earthquake.

A revised dislocation model of interseismic deformation of the Cascadia subduction zone

CAS3D‐2, a new three‐dimensional (3‐D) dislocation model, is developed to model interseismic deformation rates at the Cascadia subduction zone. The model is considered a snapshot description of the deformation field that changes with time. The effect of northward secular motion of the central and southern Cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc. Horizontal deformation data, including strain rates and surface velocities from Global Positioning System (GPS) measurements, provide primary geodetic constraints, but uplift rate data from tide gauges and leveling also provide important validations for the model. A locked zone, based on the results of previous thermal models constrained by heat flow observations, is located entirely offshore beneath the continental slope. Similar to previous dislocation models, an effective zone of downdip transition from locking to full slip is used, but the slip deficit rate is assumed to decrease exponentially with downdip distance. The exponential function resolves the problem of overpredicting coastal GPS velocities and underpredicting inland velocities by previous models that used a linear downdip transition. A wide effective transition zone (ETZ) partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model. The pattern of coseismic deformation is expected to be different from that of interseismic deformation at present, 300 years after the last great subduction earthquake. The downdip transition from full rupture to no slip should take place over a much narrower zone.

Frequency of large crustal earthquakes in Puget Sound–Southern Georgia Strait predicted from geodetic and geological deformation rates

Earthquake hazard in the Cascadia subduction zone forearc comes from three sources: great subduction earthquakes, Wadati–Benioff slab events, and earthquakes in the forearc crust. This study deals with a concentration of forearc crustal earthquakes in the Puget–Georgia region of southwestern Canada and northwestern United States. These earthquakes are due to margin‐parallel shortening, rather than compression in the direction of plate convergence. The frequency of large earthquakes has previously been based mainly on extrapolation of the statistics of smaller events from the short instrumental record. In this study, an independent estimate has been obtained through the seismic moment rate required to accommodate current rates of deformation from GPS and geological data. The catalogue statistics to Mx = 7.5 give a moment rate of 4.0 × 1017Nm/yr and a shortening rate of 2.9 mm/yr if all deformation is seismic. GPS data indicate local current N–S shortening of 3 ± 1 mm/yr. For a seismogenic thickness of 12 km, this deformation represents a moment rate of 4.1 × 1017Nm/yr. The predicted occurrences are 0.022/yr (45 years) forM> 6, and 0.0025/yr (400 years) forM> 7, within the uncertainties of the catalogue statistics. Large characteristic earthquakes are not required. Forearc long‐term average motion based upon larger‐scale GPS, paleomagnetic, and geological data ranges from 5 to7 mm/yr. The estimated long‐term occurrence of large events thus is approximately double the current rate, and we infer that the extra earthquakes could follow abrupt N–S shortening in this part of the forearc associated with the oblique rupture motion of great subduction events.

Thermal models of flat subduction and the rupture zone of great subduction earthquakes

In subduction zones, the size of the seismogenic zone that ruptures during a great thrust earthquake may be thermally controlled. We have constructed finite element thermal models of six transects across three different subduction zones in order to determine the temperature distribution along the plate interface and to predict the size of the seismogenic zone. These models incorporate the complex plate geometries (variable dip downsection) necessary to model a flat slab subduction style. We focus on the rupture zones of the great earthquakes of Nankai (SW Japan) 1946 M8.3 and Alaska 1964 M9.2 as well as on the Cascadia margin. Subduction zone segments with moderate to steep dips exhibit rupture zones of 100–150 km downdip width, consistent with earlier elastic dislocation models and thermal models. For shallow dipping flat slab segments, our models predict larger locked zones, of 150–250 km width, in good agreement with aftershock and geodetic studies. The wider seismogenic zone predicted for flat slab segments results from an uncommonly wide, cold, forearc region, as corroborated by surface heat flow observations. A global analysis of great M > 8 interplate earthquakes of the 20th century reveals that more than a third of these events occurred in flat slab segments, whereas these segments represent only 10% of modern convergent margins. This implies substantially higher interplate coupling for flat slab segments, likely due to the increased downdip extent of the seismogenic zone, and suggests that the seismic risk near such regions may be higher than previously thought.

So you want to see a tsunami?

Many people are familiar with the shaking experience of earthquakes, but very few people have experienced a tsunami\---|even a small one. Can we safely watch miniature tsunami waves and learn something about wave propagation? Breaking waves at the beach are a familiar sight and the basis for wave knowledge for most people. Tsunami waves are not a gigantic vertical wall of water that crashes directly onto land, but that persistent notion captures our fascination and awe. While few of us have seen tsunami waves, the pictures of the destruction left behind when the water recedes amply portray the hazards. Public education can mitigate the tsunami hazard from a nearby earthquake: If you are at the beach and you feel earthquake shaking, move away from the water! Great subduction earthquakes can generate tsunami waves that propagate across the ocean and then grow to dangerous heights on distant shores—this hazard underscores the importance of tsunami early warning systems (see http://www.prh.noaa.gov/pr/itc for links to international sites for tsunami warning and information). Of course, one must be at a Pacific Ocean beach to worry about tsunamis, right? While tsunami waves represent a terrible hazard, they also provide an excellent educational vehicle to understand waves. It is a constant challenge to explain seismic wave generation and propagation to students. To help explain basic concepts, it is useful to employ more familiar wave systems such as light waves and sound waves. But our familiarity with seeing and hearing does not translate into an easy grasp of light and acoustic wave propagation. Surface waves on a pond …

Exploring the Ecuador‐Colombia Active Margin and Interplate Seismogenic Zone

Seismic investigations of lithospheric structures associated with subduction megathrusts are critical to understanding the mechanics of the interplate seismogenic zone, where very destructive, great subduction earthquakes occur. Several factors have been proposed as controlling interplate coupling and tectonic regime of the margin, including sea‐floor relief and the possible re‐activation of such oceanic features when subducted, and the subduction or accretion of high‐fluid content sediment.

Stress–strain ‘paradox’, plate coupling, and forearc seismicity at the Cascadia and Nankai subduction zones

In the continental forearcs of the Cascadia and Nankai subduction zones, geodetic strain measurements indicate crustal contraction in the direction of plate convergence (nearly ‘margin-normal’), but earthquake focal mechanisms and other stress indicators show maximum compression along strike (‘margin-parallel’). It is evident that the geodetic strain signals reflect the temporal fluctuations of elastic stress associated with subduction earthquake cycles, not the absolute stresses. The absolute margin-normal stress is not only much less than the margin-parallel stress but also no greater than the lithostatic value, indicating a very weak subduction thrust fault. Weak subduction faults are also consistent with geothermal data which require very low frictional heating along the faults. Great subduction earthquakes occur at very low shear stresses and cause small perturbations to the forearc stress regime. Because these small elastic stress perturbations are relatively fast, they give large strain rates that are detected by geodetic measurements. The present nearly margin-normal contraction in both places is due to the locking of the subduction fault and the consequent increase in elastic stress in the direction of plate convergence. With the margin-parallel compression being dominant, the small increase in margin-normal stress may change the forearc seismicity in Cascadia from a mixture of thrust and strike–slip types into mainly thrust before the next great subduction earthquake. In Nankai, the change should be from strike–slip to quiescence, as observed in the Nankai forearc prior to the 1944–1946 great subduction earthquakes.

The updip and downdip limits to great subduction earthquakes: Thermal and structural models of Cascadia, south Alaska, SW Japan, and Chile

This study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes. Data on past great earthquake seismic limits have been compiled for four continental subduction zones, Cascadia, SW Japan, south Alaska, and Chile. These limits have then been compared to the predictions of several models for what constrains great earthquake rupture. Temperatures on the subduction thrusts have been estimated by finite element numerical models. The landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100°C, that is, depths of about 2 to 10 km for the subduction zones studied. This temperature agrees with the dehydration of stable sliding smectite clays to illite‐chlorite. The temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow. The downdip limits correspond to the depth on the thrust where either (1) the temperature reaches about 350°C, which corresponds to thermally activated stable‐sliding behavior for crustal rocks (with a transition to 450°C), or (2) about 40 km depth if 350°C is reached at greater depth. Depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc Moho, and this downdip limit may be a consequence of stable‐sliding serpentinite or talc and other hydrated forearc mantle rocks. The primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile. Secondary control comes from the thickness of incoming sediment, the convergence rate, and the radioactive heat generation in the overlying forearc. The 100°C updip limit occurs near the trench for young subducting plates with a thick sediment section such as Cascadia (6–8 Ma), and up to 80 km landward for older oceanic crust such as south Alaska (∼50 Ma). The 350°C downdip thermal limit is applicable for young oceanic plates (e.g., Cascadia and Nankai), whereas the forearc mantle limit applies for older plates (e.g., south Alaska and Chile except near the Chile Rise). For the margins studied that have experienced great earthquakes, there is generally good agreement between the postulated thermal and Moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform, tsunami and dislocation modeling. The downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits.

Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision

The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes M w≥7.8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 – from 6°N to 2.5°N with steep ESE-dipping subduction and a narrow volcanic arc; Region 2 – from 2.5°N to 1°S showing an intermediate-depth seismic gap and a broad volcanic arc; Region 3 – from 1°S to 2°S with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 – south of 2°S with flat subduction and no modern volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc. These findings conflict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of the volcanoes in the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Carnegie Ridge supports a local `flat slab' segment similar to that observed in Peru. The impact of the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicity indicates (1) great earthquakes on the northern and southern flanks of the colliding ridge, (2) a slight reduction in observed seismicity at the trench–ridge intersection, (3) increased stress far into the continent, and (4) a NNE displacement of the N. Andes block, to be further effects of the collision.

次世代の史料地震学 文献史料からみた東海・南海巨大地震 １． １４世紀前半までのまとめ

A large volume of historical documents in Japan show that great subduction earthquakes have repeatedly occurred along the Suruga-Nankai trough off southwest Japan since A.D. 684 with an interval of 100-200 years. They occurred as pairs of M8 events, one in the eastern half (Tokai earthquake) and another in the western half (Nankai earthquake), as was the case for the 1854 Ansei earthquakes, while sometimes occurring as single giant events like the 1707 Ho'ei earthquake. Although the space-time pattern of their recurrence is the best-known in the world, we should study more past events in order to understand the tectonophysical bases of their recurrence. In this respect I review the present understanding of historic Tokai and Nankai earthquakes and discuss related problems from the viewpoint of historical seismology. In this paper, the first of the three in all, I review the events until the early half of the 14th century. The keys to identifying older events are strong ground motion and damage in Kyoto, Nara, and Osaka, those in wider area of southwest Japan, tsunamis along the Pacific coasts of southwest Japan, typical coseismic vertical crustal movements of the Kochi plain, the Muroto and Oma'ezaki points, and the Ise and Suruga Bay coasts, temporal inactivity of specific hot springs, and aftershock activities recorded in Kyoto. The 684 Hakuho earthquake was definitely a Nankai event, and possibly included a Tokai event simultaneously (possibly Ho'ei type). The 887 Nin'na earthquake was also a definite Nankai event and was probably a Tokai event as well (Ho'ei type). The 1096 Eicho earthquake was clearly a Tokai event, but the following 1099 Kowa earthquake has some discrepancies that prevent it from being regarded as a M8 Nankai event. It is not clear yet whether great earthquakes occurred or not in the ca. 200 year intervals of 684-887 and 887-1096. It seems probable that great Tokai and Nankai earthquakes took place in the mid-13th century, but a more detailed investigation of historical seismology is required to discover the missing event.

A history of Pacific Northwest earthquakes recorded in Holocene sediments from Lake Washington

Large earthquakes can trigger slumping of the steep walls of lake basins and landslides in the drainage area, resulting in turbidite deposition in the lake and increased detrital flux from inlets. Holocene sediments in piston cores from Lake Washington contain a series of terrigenous layers that were episodically deposited in the lake. Sedimentological, geochemical, and paleomagnetic analyses on nine piston cores show that the detrital layers are temporally and areally correlatable, indicating basinwide disruptions. These layers are opaque on X‐radiographs, are coincident with magnetic susceptibility peaks, have abundant aluminosilicate minerals, are relatively coarse grained, and have low organic carbon and biogenic silica contents. The thicknesses and geographic distributions of the layers suggest that they are not due to floods or delta destabilization. Side scan swath imagery and subbottom profiling show that large slumps, subaqueous landslides, and debris flows are common along the margins of the lake. A detailed chronology, established from 21 radiocarbon ages on five cores, show that a prominent turbidite was deposited about 1000–1100 years ago. This turbidite apparently was triggered by a large earthquake that probably occurred on the Seattle fault. Other depositional events in the sediment record at 1500–1700, 2400–2500, and 2800–3200 years ago coincide with periods of landsliding that have been previously inferred from the dating of drowned trees in the lake. More than 30 depositional events have occurred in the last 12,000 years and 21 disturbances have occurred since the deposition of the Mazama ash about 7,600 years ago. If all the events are due to earthquakes, the Puget Sound region has been subject to major shaking every 300 to 400 years. The strong intensities needed to trigger subaqueous slides may not be generated by just local sources such as the Seattle fault but could also be caused by great subduction earthquakes occurring along the coast.

The rupture zone of Cascadia great earthquakes from current deformation and the thermal regime

An important but poorly known part of the earthquake hazard at near‐coastal cities of western North America from southern British Columbia to northern California is from great thrust earthquakes on the Cascadia subduction zone. Although there have been no such events in the historical record, there is good geological evidence that they have occurred in the past. The downdip landward limit of the seismogenic or seismic rupture zone on the subduction thrust fault has been estimated for the whole Cascadia margin from (1) the locked zone from dislocation modeling of current deformation data, and (2) the thermal regime, taking the downdip limit of seismic behavior on the fault to be controlled by temperature. The geodetic data include ten leveling lines, tide gauges at six locations along the coast, one high precision gravity line, seven horizontal strain arrays, and a continuously recording Global Positioning System (GPS) network. There is present uplift for most of the coast at a rate of a few millimeters per year, decreasing inland, and shortening across the coastal region at about 0.1 µstrain/yr (i.e., 10 mm/yr over a distance of 100 km). The present interseismic uplift is consistent with the great earthquake coseismic subsidence inferred from buried coastal salt marshes and other paleoseismicity data. The modeled width of the locked zone that is taken to be accumulating elastic strain averages 60 km fully locked, plus 60 km transition (90 km fully locked with no transition gives similar interseismic deformation). It is wider off the Olympic Peninsula of northern Washington and narrower off central Oregon to northern California. This unusually narrow downdip extent compared to many other subduction zones is a consequence of high temperatures associated with the young oceanic plate and the thick blanket of insulating sediments on the incoming crust. The variations in the modeled locked zone from geodetic data correspond well to variations along the margin of downdip temperatures on the fault as estimated from numerical thermal models, taking the maximum temperature for the fully locked seismogenic zone to be 350°C with a transition zone to 450°C. The temperatures on the subduction thrust fault and thus the downdip extent of the seismogenic zone depend on five local subduction parameters: (1) the age of the subducting plate, (2) the plate convergence rate, (3) the thickness of insulating sediments on the incoming crust, (4) the dip angle profile of the fault, and (5) the thermal properties of the overlying material. The landward limit to the seismogenic zone, extending little if at all beneath the coast, limits the ground motion from great subduction earthquakes at the larger Cascadia cities that lie 100–200 km inland. The narrow width also limits the earthquake size but events of magnitude well over 8 are still possible; the maximum depends on the along‐margin length. If the whole Cascadia margin seismogenic zone fails in a single event, empirical fault area versus magnitude relations give earthquakes as large asMw=9.

Thermal constraints on the seismogenic portion of the southwestern Japan subduction thrust

For coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault. In this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature. We have developed a transient thermal model for the Nankai subduction zone of southwest Japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from (1) seismicity and tsunami data for two great subduction earthquakes, (2) the coseismic faulting extent of these events estimated from geodetic deformation data, and (3) the interseismic locked zone determined from interseismic geodetic data. The Nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures. It has earthquake, tsunami, and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain. On the Nankaido margin off Shikoku Island, the thermal model indicates that a temperature of 350°C (taken to be the limit for seismic initiation from laboratory and field data) is reached on the subduction thrust fault 150 km from the trench. A transition zone into which rupture may extend with decreasing offset (taken to be 450°C) extends an additional 45 km downdip. The thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 Nankaido Ms = 8.2 earthquake off Shikoku Island and with the downdip extent of the present locked zone. In the region of the 1944 Tonankai Ms = 8.2 earthquake to the northeast, the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent. The seismogenic‐locked zone from earthquake and geodetic data is also narrower. Thus our analysis of the southwest Japan margin indicates that all three constraints on the downdip extent of the seismogenic zone, thermal, coseismic and interseismic geodetic data, are in general agreement. The study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature. The thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip. The subducting plate age and thus heat flow, and the thickness of the insulating sediments on the incoming plate, are also very important to the thermal regime and thus to the seismogenic width. The relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as Cascadia where there have been no great historical events.

Forearc Deformation and Great Subduction Earthquakes: Implications for Cascadia Offshore Earthquake Potential

The maximum size of thrust earthquakes at the world's subduction zones appears to be limited by anelastic deformation of the overriding plate. Anelastic strain in weak forearcs and roughness of the plate interface produced by faults cutting the forearc may limit the size of thrust earthquakes by inhibiting the buildup of elastic strain energy or slip propagation or both. Recently discovered active strike-slip faults in the submarine forearc of the Cascadia subduction zone show that the upper plate there deforms rapidly in response to arc-parallel shear. Thus, Cascadia, as a result of its weak, deforming upper plate, may be the type of subduction zone at which great (moment magnitude approximately 9) thrust earthquakes do not occur.

Great subduction earthquakes studied

Great subduction earthquakes are the largest earthquakes on Earth, and they are associated with the subduction of lithosphere. They rupture the shallow dipping megathrust interfaces between the plates along areas with dimensions up to 1000 km by 200 km. Recent studies on great subduction earthquakes was the focus of the Wadati Conference on Great Subduction Earthquakes, held at the University of Alaska's Geophysical Institute in Fairbanks from September 16 to 19, 1992. About eighty scientists participated in the conference, which was sponsored jointly by the National Science Foundation and the U.S. Geological Survey.

Discordant 14C Ages from Buried Tidal-Marsh Soils in the Cascadia Subduction Zone, Southern Oregon Coast

AbstractPeaty, tidal-marsh soils interbedded with estuarine mud in late Holocene stratigraphic sequences near Coos Bay, Oregon, may have been submerged and buried during great (M > 8) subduction earthquakes, smaller localized earthquakes, or by nontectonic processes. Radiocarbon dating might help distinguish among these alternatives by showing that soils at different sites were submerged at different times along this part of the Cascadia subduction zone. But comparison of conventional 14C ages for different materials from the same buried soils shows that they contain materials that differ in age by many hundreds of years. Errors in calibrated soil ages represent about the same length of time as recurrence times for submergence events (150–500 yr)—this similarity precludes using conventional 14C ages to distinguish buried soils along the southern Oregon coast. Accelerator mass spectrometer 14C ages of carefully selected macrofossils from the tops of peaty soils should provide more precise estimates of the times of submergence events.

Horizontal deformation in the Cascadia subduction zone as derived from serendipitous geodetic data

Data from recent Global Positioning System (GPS) surveys are combined with triangulation/trilateration data to estimate horizontal shear-strain rates for two regions in the Cascadia subduction zone. Near Bellingham, Washington, we estimate that the maximum horizontal shear rate (γ) equals 0.116 ± 0.089 μrad/yr and the direction of maximum horizontal contraction (θ) orients N71° E ± 21° for data spanning the 1905–1985 interval. The corresponding estimates for a region near Portland, Oregon, are 0.057 ± 0.027 μrad/yr and N95° E±14° for data spanning the 1881–1988 interval. These estimates are consistent with estimates from independent geodetic data in the area. Moreover, the estimates for θ are consistent with the N68°E direction of ongoing convergence between the Juan de Fuca plate and the North American plate as predicted by the NUVEL-1 plate motion model. This consistency between θ-estimates and the direction of plate convergence supports the argument for the possibility of a great subduction earthquake occurring in the Cascadia subduction zone. The low shear rates, however, imply that the recurrence interval between such earthquakes would be several centuries long.

Strain Measurements and the Potential for a Great Subduction Earthquake Off the Coast of Washington

Geodetic measurements of deformation in northwestern Washington indicate that strain is accumulating at a rate close to that predicted by a model of the Cascadia subduction zone in which the plate interface underlying the continental slope and outer continental shelf is currently locked but the remainder of the interface slips continuously. Presumably this locked segment will eventually rupture in a great thrust earthquake with a down-dip extent greater than 100 kilometers.

Slab pull and the seismotectonics of subducting lithosphere

This synthesis links many seismic and tectonic processes at subduction zones, including great subduction earthquakes, to the sinking of subducted plate. Earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force. Interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity, providing details on how subduction occurs. Compression is localized across a locked interface thrust zone, because both the ridge push and the slab pull forces are resisted there. The slab pull force increases with increasing plate age; thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone, the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate. The 1960 Chile earthquake (Mw 9.5), the largest earthquake to occur in historic times, began its rupture at an interface bounding oceanic plate <30 m.y. old. However, this rupture initiation was associated with the locally oldest subducting lithosphere (weakest coupling); the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere, terminating near the subducting Chile Rise. Prior to a great subduction earthquake, the sinking subducted slab will cause increased tension at depths of 50–200 km, with greatest tension near the shallow zone resisting plate subduction. Plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260–350 km, provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle. This explains K. Mogi's observations of M ≥ 7 thrust earthquakes at depths of 260–350 km, immediately downdip and within 3 years prior to five great, shallow earthquakes of northern Japan. The slab pull model explains the lower layer of double seismic zones as due to tension from the deeper, sinking plate and the upper layer as due to localized in‐plate compression, as plate motion is resisted by the bounding mantle. Just downdip of the interface thrust zone, there occurs an aseismic 20°–50° dip increase of subducted plate. This slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change. Resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone.