The Juan de Fuca plate is a small oceanic plate between the Pacific and North America plates. In the southernmost region, referred to as the Gorda deformation zone, the maximum compressive stress σ 1 constrained by earthquake focal mechanisms is N‐S. Off Oregon, and possibly off Washington, NW trending left‐lateral faults cutting the Juan de Fuca plate indicate a σ 1 in a NE‐SW to E‐W direction. The magnitude of differential stress increases from north to south; this is inferred from the plastic yielding and distribution of earthquakes throughout the Gorda deformation zone. To understand how tectonic forces determine the stress field of the Juan de Fuca plate, we have modeled the intraplate stress using both elastic and elastic‐perfectly plastic plane‐stress finite element models. We conclude that the right‐lateral shear motion of the Pacific and North America plates is primarily responsible for the stress pattern of the Juan de Fuca plate. The most important roles are played by a compressional force normal to the Mendocino transform fault, a result of the northward push by the Pacific plate and a horizontal resistance operating against the northward, or margin‐parallel, component of oblique subduction. Margin‐parallel subduction resistance results in large N‐S compression in the Gorda deformation zone because the force is integrated over the full length of the Cascadia subduction zone. The Mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike‐normal compressive stresses. Internal failure of the Gorda deformation zone potentially places limits on the magnitude of the fault‐normal stresses being transmitted and correspondingly on the magnitude of strike‐parallel subduction resistance. Transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner.
Abstract Static stress drop distribution and its average value over the rupture area contain important information on the mechanics of large earthquakes. Here we derive static stress drop distributions from 40 published rupture models for the 2011 M w 9 Tohoku‐oki earthquake that are based on various multidisciplinary observations. Average stress drop value over the fault area encompassed by the 5 m coseismic slip contour is not unusually large for each rupture model; the mean for the 40 models is 2.3 ± 1.3 MPa, assuming a uniform rigidity 40 GPa. The value for the entire rupture zone and with a more realistic rigidity structure will be even lower. In the majority of the models, local stress drop in parts of the rupture zone well exceeds 20 MPa. The heterogeneous stress change distribution, with large stress drop being accompanied by large stress increase, leads to the small average for the earthquake.
We test hypothetical tsunami scenarios against a 4,600‐year record of sandy deposits in a southern Oregon coastal lake that offer minimum inundation limits for prehistoric Cascadia tsunamis. Tsunami simulations constrain coseismic slip estimates for the southern Cascadia megathrust and contrast with slip deficits implied by earthquake recurrence intervals from turbidite paleoseismology. We model the tsunamigenic seafloor deformation using a three‐dimensional elastic dislocation model and test three Cascadia earthquake rupture scenarios: slip partitioned to a splay fault; slip distributed symmetrically on the megathrust; and slip skewed seaward. Numerical tsunami simulations use the hydrodynamic finite element model, SELFE, that solves nonlinear shallow‐water wave equations on unstructured grids. Our simulations of the 1700 Cascadia tsunami require >12–13 m of peak slip on the southern Cascadia megathrust offshore southern Oregon. The simulations account for tidal and shoreline variability and must crest the ∼6‐m‐high lake outlet to satisfy geological evidence of inundation. Accumulating this slip deficit requires ≥360–400 years at the plate convergence rate, exceeding the 330‐year span of two earthquake cycles preceding 1700. Predecessors of the 1700 earthquake likely involved >8–9 m of coseismic slip accrued over >260 years. Simple slip budgets constrained by tsunami simulations allow an average of 5.2 m of slip per event for 11 additional earthquakes inferred from the southern Cascadia turbidite record. By comparison, slip deficits inferred from time intervals separating earthquake‐triggered turbidites are poor predictors of coseismic slip because they meet geological constraints for only 4 out of 12 (∼33%) Cascadia tsunamis.
Numerous observations pertaining to the magnitude 9.0 2011 Tohoku-oki earthquake (offshore Japan) have led to new understanding of subduction zone earthquakes. By synthesizing published research results and our own findings, we explore what has been learned about fault behavior and Earth rheology from the observation and modeling of crustal deformation before, during, and after the earthquake. Before the earthquake, megathrust locking models based on land-based geodetic observations correctly outlined the along-strike location of the future rupture zone. Their incorrect definition of the locking pattern in the dip direction demonstrates the need to model the effects of interseismic viscoelastic stress relaxation and stress shadowing. The observation of decade-long accelerated slip downdip of the future rupture zone raises new questions on fault mechanics. During the earthquake, seafloor geodetic measurements revealed huge coseismic displacements (up to 31 m). Modeling of bathymetry difference before and after the earthquake suggests >60 m of coseismic slip of the most seaward 40 km of the fault in the main rupture area, with the slip peaking at the trench. Large differences in shallow slip between published rupture models are due mainly to the near absence of near-trench deformation measurements, but model simplifications in fault and seafloor geometry also bear large responsibility. After the earthquake, seafloor geodetic measurements provided unambiguous evidence for the dominance of viscoelastic relaxation in short-term postseismic deformation. There is little deep afterslip in the fault area where the decade-long pre-earthquake slip acceleration is observed. Investigating the physical processes responsible for the complementary spatial distribution of pre-slip and afterslip calls for new scientific research.
The Lithosphere-Asthenosphere Boundary (LAB) beneath oceanic plates is generally imaged as a sharp seismic velocity reduction, suggesting the presence of partial melts. However, the fate of a melt-rich LAB is unclear after these plates descend into the mantle at subduction zones. Recent geophysical studies suggest its persistence with down-going old and cold slabs, but whether or not it is commonly present remains unclear, especially for young and warm slabs such as in the Cascadia subduction zone. Here we provide evidence for its presence at Cascadia in the form of a large (9.8
The redistribution of heat by fluid circulation in subducting igneous crust generates thermal anomalies that can affect the alteration of material both within a subduction zone and in the incoming plate prior to subduction. This hydrothermal circulation mines heat from subducted crust and transports it seaward, resulting in anomalously high temperatures in material seaward of the trench and anomalously low temperatures in the subduction zone. Anomalously high temperatures on the incoming plate are spatially limited; for example, on the Nankai margin of southern Japan, a zone of high temperatures is within ∼30 km of the accretionary prism deformation front. The incoming plate (Shikoku Basin) undergoes the high-temperature anomaly for less than 2 million years; so the alteration of clay minerals in Shikoku basin sediments advances only slightly because of the thermal anomaly. In contrast, subducted material is cooled by hydrothermal circulation, and therefore alteration of subducted sediment and igneous rock is shifted farther landward (i.e., delayed); in the Cascadia and Nankai margins, this includes the seismically inferred locations of the basalt-to-eclogite transition in the subducting crust. In very hot margins, hydrothermal circulation cools the subducting slab and affects where, and if, subducting material may melt. In southern Chile, this cooling helps explain the lack of a basaltic melt signature in arc lavas despite the young subducting lithosphere. Finally, the cooling of the subducting slab via hydrothermal circulation shifts fluid sources from dehydration reactions farther landward, delays metamorphic reactions that tend to reduce permeability, and increases fluid viscosity. The responses to hydrothermal circulation in subducting crust are most pronounced in the hottest subduction zones, where the lateral heat exchange in the subducting basement aquifer is greatest.
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