Outer trench-slope faulting and the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake (original) (raw)
Related papers
Earth, Planets and Space, 2013
Coulomb stress triggering is examined using well-determined aftershock focal mechanisms and source models of the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake. We tested several slip distributions obtained by inverting onshore GPS-derived coseismic displacements under different a priori constraints on the initial fault parameters. The aftershock focal mechanisms are most consistent with the Coulomb stress change calculated for a slip distribution having a center of slip close to the trench. This demonstrates the capability of the Coulomb stress change to help constrain the slip distribution that is otherwise difficult to determine. Coulomb stress changes for normal-fault aftershocks near the Japan Trench are found to be strongly dependent on the slip on the shallow portion of the fault. This fact suggests the possibility that the slip on the shallow portion of the fault can be better constrained by combining information of the Coulomb stress change with other available data. The case of normal-fault aftershocks near some trench segment which are calculated to be negatively stressed shows such an example, suggesting that the actual slip on the shallow portion of the fault is larger than that inverted from GPS-derived coseismic displacements.
Possible large near-trench slip during the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake
Earth, Planets and Space, 2011
The 11 March 2011 off the Pacific coast of Tohoku (M w 9.0) Earthquake ruptured a 200 km wide megathrust fault, with average displacements of ∼15-20 m. Early estimates of the co-seismic slip distribution using seismic, geodetic and tsunami observations vary significantly in the placement of slip, particularly in the vicinity of the trench. All methods have difficulty resolving the up-dip extent of rupture; onshore geodetic inversions have limited sensitivity to slip far offshore, seismic inversions have instabilities in seismic moment estimation as subfault segments get very shallow, and tsunami inversions average over the total region of ocean bottom uplift. Seismic wave estimates depend strongly on the velocity structure used in the model, which affects both seismic moment estimation and inferred mapping to slip. We explore these ideas using a least-squares inversion of teleseismic P-waves that yields surprisingly large fault displacements (up to ∼60 m) at shallow depth under a protrusion of the upper plate into the trench. This model provides good prediction of GPS static displacements on Honshu. We emphasize the importance of poorly-constrained rigidity variations with depth for estimating fault displacement near the trench. The possibility of large slip at very shallow depth holds implications for up-dip strain accumulation and tsunamigenic earthquake potential of megathrusts elsewhere.
2016
of at least 40 m. The resulting tsunami devastated the Honshu coast southwest of regions struck by earthquake-generated tsunami in 1611, 1896 and 1933. The 1896 Meiji-Sanriku earthquake was also an underthrusting earthquake, but the 1933 Sanriku-oki earthquake was a trench-slope normal faulting event; both generated inundation heights of 10 to 25 m along the coast of Iwate prefecture. Possible occurrence of a great outer trench-slope earthquake seaward of the 2011 Tohoku Earthquake along a southwestward extension of the 1933 fault zone is a concern. The second largest 2011 aftershock, an outer rise Mw 7.7 normal faulting earthquake occurred near the southern end of the 1933 rupture. Additional aftershock activity has been distributed along a trend below the trench and diffusely spread in the outer rise, seaward of the megathrust region where the largest slip occurred. Coulomb stress perturbations of at least 5–10 bars are calculated for outer rise normal fault geometries for mainsho...
Potential tsunamigenic faults of the 2011 off the Pacific coast of Tohoku Earthquake
Earth, Planets and Space, 2011
Faults related to the tsunamigenic 2011 Tohoku-Oki Earthquake (M w 9.0) were investigated by using multichannel seismic reflection data acquired in 1999 and submersible seafloor observations from 2008. The location of the fault system interpreted in the seismic reflection profile is distributed around the area with largest slip and tsunami induction of the 2011 event. Cold-seep communities along the trace of the branch reverse fault and a high scarp associated with the trace of a normal fault suggest current activity on these faults. We interpret the fault system in the seismic profile as a shallow extension of the seismogenic fault that may have contributed to the resulting huge tsunami.
Dynamic Wedge Failure and Along‐Arc Variations of Tsunamigenesis in the Japan Trench Margin
Geophysical Research Letters
Elastic dislocation models require large near-trench slip to explain large tsunamigenesis, which is probably best exemplified in the 2011 M9 Tohoku earthquake. However, it is puzzling that the largest Tohoku tsunami heights occurred about 100 km north of the largest slip zone, where bathymetric surveys indicate no large slip at the trench or submarine landslides. Here we show that coseismic yielding of plentiful sediments in the northern Japan Trench margin can induce large inelastic uplift landward from the trench and diminish slip near the trench. The scarcity of sediments in the south leads to nearly elastic response with large slip at the trench and mostly horizontal seafloor displacement. Thus, the variations of sediments along the Japan Trench and sediment yielding can explain the puzzling variations of tsunamigenesis and near-trench slip in this earthquake. Inelastic wedge deformation can be an important mechanism of tsunamigenesis in accretionary and other sediment-filled plate margins. Plain Language Summary The general consensus about the devastating 2011 Tohoku tsunami is that it was due to large slip at the trench. This can be somewhat misleading because the largest tsunami heights occurred about 100 km north of where the largest trench slip occurred. Using elastic dislocation theory, inversions of tsunami data require large trench slip~30 m in the north (north of 39°N). However, the bathymetry data before and after the earthquake suggest no large trench slip or submarine landslides in the north. Here we present a mechanism that involves the yielding of sediments in the overriding wedge. Our inelastic wedge deformation model can explain the puzzling observations of large tsunami heights along the Sanriku coast in the north with no or small trench slip and small tsunami in the south with large trench slip. The amount of sediments in a subduction margin can be very important in evaluating tsunami hazard. An important characteristic of the Japan Trench margin is that the amount of the sediments increases systematically from south to north (see Figure 17 of Tsuru et al., 2002, and supporting information Figure S1). South of 38°N, sediments subduct along the plate interface in thin sedimentary channels. However, ©2019. American Geophysical Union. All Rights Reserved.
Journal of Geophysical Research: Solid Earth, 1993
Coseismic slip distribution on the fault plane, particularly in the downdip direction, associated with large subduction earthquakes can be estimated by joint inversion of geodetic and tsunami data. Two large earthquakes, the 1944 Tonankai earthquake (Mw=8.1) and the 1946 Nankaido earthquake (Mw=8.3), occurred on the Nankai trough, southwestern Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate. The source areas of these events extended over both land and ocean. Coseismic crustal movements on land were measured by leveling, while those in ocean were recorded as tsunami waveforms on tide gauges. The coseismic slip distribution inverted from these data shows that the slip on the shallower part of the fault plane is comparable to that on the deeper parts. This indicates that large coseismic slip can occur beneath an accretionary wedge where current seismicity is low. The result has implications for other subduction zones having a similar tectonic environment such as for the Pacific Northwest region of the United States. Although the possibility of a large earthquake there is still debated, should a large subduction earthquake occur in this region, coseismic slip on the shallow part could be large, and the potential for large tsunamis is high. IKIRODUCrION The slip distribution of large subduction earthquakes has been studied by many researchers mainly using long-period body waves recorded at far-field stations [e.g., Ruff and Kanamori, 1983; Kikuchi and Fukao, 1987]. These studies show that the coseismic slip distributions of large earthquakes are rarely uniform and slip is concentrated on small areas, called asperities, on the fault plane. Further, the asperity distribution varies from one subduction zone to another and appears to be related to the geological nature of subduction zones [Lay and Kanamori, 1981; Beck and Ruff, 1989; Thatcher, 1990]. Body wave data can resolve temporal and spatial variations of rupture process, particularly along the strike direction. The resolution in the downdip, or depth, direction, however, is relatively poor. Another difficulty in using body waves is that a high-quality data set with good azimuthal coverage is seldom available. Hence most of such studies have used WorldWide Standard Seismic Network (WWSSN) records which are available only since the 1960's. [Satake, 1987, 1989; Satake and Kanamori, 1991]. The use of tsunami data tums out to be powerful for offshore earthquakes,
Physics of the Earth and Planetary Interiors, 1986
The fault model of the 1940 Shakotan-oki, Japan, earthquake of Aug. 1, 1940 is reexamined on the basis of magnitude, aftershock distribution and tsunami data. From a careful examination of the S-P time distribution of aftershocks and comparison of the tide gauge records with numerical simulation of tsunami, the fault area is estimated to be 100 km x 35 km, the slip 1.5 m and the seismic moment 2.4 x 1027 dyn cm. The fault parameters estimated in this study are significantly different from those of Fukao and Furumoto and do not support their conclusion that this event has a very long duration and is a tsunami earthquake. The source process time is estimated to be 30 s, which is normal for an earthquake of this size. Fault parameters of six earthquakes along the eastern margin of the Japan Sea including the Shakotan-oki event are compiled and compared with those of inter-plate earthquakes in the Pacific Ocean. Different relations from well-known scaling laws of the inter-plate shocks are found for the earthquakes in the Japan Sea. Dip angles, aspect ratios of the fault, and the average stress drop are larger in the Japan Sea than those of the Pacific events, although the seismic moment release per unit fault length is the same. The differences can be interpreted in terms of a recent plate boundary model in which the young, immature boundary between the North American and the Eurasian plates lies at the eastern margin of the Japan Sea. They also partially reflect the different excitation of the tsunami in the Japan Sea and the Pacific Ocean.
A review of the rupture characteristics of the 2011 Tohoku-oki Mw 9.1 earthquake
Tectonophysics
The 2011 March 11 Tohoku-oki great (Mw 9.1) earthquake ruptured the plate boundary megathrust fault offshore of northern Honshu with estimates of shallow slip of 50 m and more near the trench. Non-uniform slip extended~220 km across the width and~400 km along strike of the subduction zone. Extensive data provided by regional networks of seismic and geodetic stations in Japan and global networks of broadband seismic stations, regional and global ocean bottom pressure sensors and sea level measurement stations, seafloor GPS/ Acoustic displacement sites, repeated multi-channel reflection images, extensive coastal runup and inundation observations, and in situ sampling of the shallow fault zone materials and temperature perturbation, make the event the best-recorded and most extensively studied great earthquake to date. An effort is made here to identify the more robust attributes of the rupture as well as less well constrained, but likely features. Other issues involve the degree to which the rupture corresponded to geodetically-defined preceding slip-deficit regions, the influence of re-rupture of slip regions for large events in the past few centuries, and relationships of coseismic slip to precursory slow slip, foreshocks, aftershocks, afterslip, and relocking of the megathrust. Frictional properties associated with the slip heterogeneity and in situ measurements of frictional heating of the shallow fault zone support low stress during shallow sliding and near-total shear stress drop of~10-30 MPa in large-slip regions in the shallow megathrust. The roles of fault morphology, sediments, fluids, and dynamical processes in the rupture behavior continue to be examined; consensus has not yet been achieved. The possibility of secondary sources of tsunami excitation such as inelastic deformation of the sedimentary wedge or submarine slumping remains undemonstrated; dislocation models in an elastic continuum appear to sufficiently account for most mainshock observations, although afterslip and viscoelastic processes remain contested.