Explore the vast range of titles available.

We investigated temperature records associated with seafloor pressure observations at eight stations that experienced the 2011 Mw 9 Tohoku earthquake near its epicenter. The temperature data were based on the temperature measured inside the pressure transducer. We proposed a method to estimate ambient water temperature from the internal temperature using an equation of heat conduction. The estimated seafloor water temperature showed remarkable anomalies, especially increases several hours after the Mw 9 earthquake. A station of P03 (sea depth of 1.1 km) showed an abrupt temperature increase of + 0.19 °C that occurred ~ 3 h after the earthquake, which lasted for several hours. At stations of GJT3 (sea depth of 3.3 km) and TJT1 (sea depth of 5.8 km), there were abrupt temperature anomalies of + 0.20 °C and + 0.10 °C that began to occur 3–4 h after the earthquake. These anomalies both decayed to their original levels over a few tens of days. During the decay processes, only TJT1 showed several intermittent temperature rises. A water temperature anomaly within + 0.03 °C was found up to ~ 500 m above TJT1 2 weeks after the earthquake. There was no significant anomaly at the other five stations. Processes to cause these seafloor temperature anomalies were discussed. The temperature anomaly of P03 was reasonably caused by a tsunami-generated turbidity current, as also suggested by a previous study. Meanwhile, we proposed a scenario that the abrupt temperature anomalies of GJT3/TJT1 and the intermittent anomalies of TJT1 were caused by warm water discharges from the subseafloor. The pathways of the warm water were probably composed of the branch normal fault between GJT3 and TJT1, the reverse fault near TJT1, the backstop interface, and perhaps reverse faults at the frontal prism. The proposed scenario was almost compatible with other studies based on epicentral observations. We estimated the heat properties of the initial temperature anomalies of GJT3/TJT1. The estimated heat source might be explained by that most of the geothermal fluids trapped in those fault pathways were discharged to the seafloor immediately after the earthquake. The onsets of the subsequent intermittent anomalies of TJT1 were possibly activated by low or falling ocean tidal loading.

This paper summarizes the results of 10 years of research on models of the megathrust earthquake cycles and crustal deformation associated with the 2011 Tohoku-oki earthquake. Several earthquake cycle models have been proposed for the northeast Japan subduction zone to elucidate why megathrust earthquakes occur at intervals of approximately 600 years and why large slips occurred in the shallow subduction zone. A model that considers a strong asperity in the shallow plate interface, and a hierarchical asperity model that considers the scale dependence of the critical displacement of the rate- and state-dependent friction law have been proposed. Modeling with dynamic weakening of faults has also been proposed. In the model using the shallow friction characteristics obtained by the Japan Trench Fast Drilling Project, rupture from depth can propagate to the trench, resulting in shallow large slips. Submarine crustal deformation has been observed for the first time in addition to dense observations of the inland crustal deformation. The observation of the seafloor deformation near the trench showed that viscoelastic relaxation played an important role in short-term postseismic deformation near the trench. The effects of the low-viscosity region at the oceanic lithosphere and asthenosphere boundary, and the cold forearc mantle wedge (cold nose) have been discussed. Simulations using the nonlinear flow law of rock in the mantle, where a power–law relationship holds between stress and strain rate, and the fault friction law at the plate boundary, show that the Tohoku-oki earthquake caused large stress fluctuations, resulting in a sudden viscosity decrease and rapid flow in the asthenosphere below the oceanic lithosphere. The simulations of the crustal deformation associated with the Tohoku-oki earthquake cycle also indicate that in the later stage of the earthquake cycle, the Pacific coastal region begins to subside due to the increasing slip deficit rate on the deeper parts of the plate interface. These results explain the subsidence of the Pacific coast of northeast Japan observed for about 100 years prior to the Tohoku-oki earthquake. In the future, a model that explains the long-term crust and mantle deformation during the entire Tohoku-oki earthquake cycle must be constructed.

The 2011 Tohoku earthquake was observed by dense strong motion, teleseismic, geodetic, and tsunami networks. We first inverted each of the datasets obtained by the networks separately, for the rupture process of the earthquake. We then performed checkerboard resolution tests for assessing the resolving power of these datasets. In order to overcome the limited resolutions of the separate inversions and differences in their results, we performed a quadruple joint inversion of all these data to determine a source model most suitable for explaining all the datasets. In the obtained source model, the maximum coseismic slip was approximately 35 m, and the total seismic moment was calculated to be 4.2 × 1022 Nm, which yielded Mw = 9.0. The main rupture propagated not only in the strike direction but also in the dip direction and included both the deep area called the Miyagi‐oki region and the compact shallow area near the Japan Trench. Key Points We made the source model reflecting all aspects of the 2011 Tohoku earthquake We performed joint inversion of seismic, geodetic, and tsunami datasets The main rupture included both deep Miyagi‐oki area and compact shallow area

After the 2011 Mw 9.0 Tohoku earthquake, seismicity became extremely active throughout Japan. Despite enormous efforts to detect the large number of earthquakes, microearthquakes (M < 2 inland, M < 3 offshore) were not always cataloged and many have remained undetected, making it difficult to understand the detailed seismicity after the 2011 Tohoku earthquake. We developed an automatic hypocenter determination method combined with machine learning to detect microearthquakes. Machine learning was used for phase classification with convolutional neural networks and ensemble learning to remove false detections. We detected > 920,000 earthquakes from March 2011 to February 2012, triple the number of the conventional earthquake catalog (~ 320,000). This represents a great improvement in earthquake detection, especially in and around the Tohoku region. Detailed analysis of our merged catalog more clearly revealed features such as (1) swarm migrations, (2) small foreshock activity, and (3) increased microseismicity preceding repeating earthquakes. This microseismic catalog provides a magnifying glass for understanding detailed seismicity.

After the 2011 Mw 9.1 Tohoku earthquake, numerous intraplate earthquakes occurred beneath the outer slope of the Japan Trench. Based on ocean bottom seismograph observations, these earthquakes occurred in the oceanic crust and uppermost mantle of the Pacific plate at depths shallower than about 40 km and had normal‐faulting focal mechanisms at all depths. Before the 2011 earthquake, normal‐faulting earthquakes beneath the outer trench slope occurred only at depths shallower than 20 km, whereas those at depths of around 40 km had reverse‐faulting mechanisms. These observations suggest that the stress regime in the Pacific plate was changed by the 2011 earthquake. The tensional stresses that now extend to depths of about 40 km may play an important role not only in the occurrence of large normal‐faulting earthquakes but also in hydration of the uppermost mantle of the incoming Pacific plate prior to the subduction. Key Points OBS observations for outer trench slope earthquakes after the 2011 Tohoku EQ Normal‐faulting earthquakes in oceanic crust and mantle of the incoming plate Stress regime in the Pacific plate was changed by the 2011 Tohoku earthquake

The 2011 Tohoku-Oki earthquake generated a surprisingly large near-trench slip, and earth scientists have devoted significant attention to understanding why. Some studies proposed special rupture mechanisms, such as extensive dynamic frictional weakening; others simulated this near-trench slip behavior without supposing the extensive dynamic weakening. However, we have not reached a decisive conclusion for this question due to limited spatial near-trench slip resolution. Hence, in this study we use new tsunami data recorded just above the large slip area in addition to offshore and onshore geodetic data to improve the spatial resolution of stress release in the Tohoku-Oki earthquake and quantitatively examine the mechanical state of the plate interface. A maximum slip of 53 m reaching the trench and an insignificant stress drop (< 3 MPa) at the shallowest portion of the fault were estimated. Based on our modeling results and the past experimental studies, it is suggested that friction at the shallow near-trench portion should be inherently low both before and during the earthquake. This result provides perspectives on the shallow slip behavior along the plate boundary, in which the strain energy accumulation at the deep portion of the fault accounts for the anomalous large shallow slip, but shallow mechanical coupling does not. A large shallow slip has been considered as a result of the release of sufficiently large strain energy at the shallow portion of the plate interface, but we suggest that shallow slips similar to that during the 2011 Tohoku-Oki earthquake may occur in any subduction zones where the energy sufficiently accumulates only in the deeper portion.

The 2011 Tohoku Earthquake caused a devastating tsunami along the shoreline from the Tohoku to Kanto districts. Although many of the tide gauge stations along the Tohoku coast were saturated or damaged due to the tsunami, two cabled ocean-bottom tsunami sensors installed off Kamaishi successfully recorded the tsunami waveform just above the source rupture area. The records indicated a characteristic two-stage tsunami development sequence: a smoothly increasing tsunami amplitude from 0 to 2 m during the first 800 s from the earthquake origin time, and a short-period impulsive tsunami with a peak of more than 5 m in the following 200 s. Such observations strongly suggest the lack of any sea floor upheaval at the stations during the earthquake, and the occurrence of an extremely large slip in the shallow portion of the subducting Pacific Plate near the trench axis. The source model derived from the offshore tsunami records indicates that a very large slip of 57 m occurred off Miyagi near the trench axis, south of the rupture area of the 1896 Meiji Sanriku tsunami earthquake, and was the major source of the highly destructive tsunami that subsequently developed.

The precursory atmospheric gravity wave (AGW) activity in the stratosphere has been investigated in our previous paper by studying an inland Kumamoto earthquake (EQ). We are interested in whether the same phenomenon occurs or not before another major EQ, especially an oceanic EQ. In this study, we have examined the stratospheric AGW activity before the oceanic 2011 Tohoku EQ (Mw 9.0), while using the temperature profiles that were retrieved from ERA5. The potential energy (EP) of AGW has enhanced from 3 to 7 March, 4–8 days before the EQ. The active region of the precursory AGW first appeared around the EQ epicenter, and then expanded omnidirectionally, but mainly toward the east, covering a wide area of 2500 km (in longitude) by 1500 km (in latitude). We also found the influence of the present AGW activity on some stratospheric parameters. The stratopause was heated and descended; the ozone concentration was also reduced and the zonal wind was reversed at the stratopause altitude before the EQ. These abnormalities of the stratospheric AGW and physical/chemical parameters are most significant on 5–6 March, which are found to be consistent in time and spatial distribution with the lower ionospheric perturbation, as detected by our VLF network observations. We have excluded the other probabilities by the processes of elimination and finally concluded that the abnormal phenomena observed in the present study are EQ precursors, although several potential sources can generate AGW activities and chemical variations in the stratosphere. The present paper shows that the abnormal stratospheric AGW activity has also been detected even before an oceanic EQ, and the AGW activity has obliquely propagated upward and further disturbed the lower ionosphere. This case study has provided further support to the AGW hypothesis of the lithosphere-atmosphere-ionosphere coupling process.

The structure of the incoming plate is an important element that is often considered to be related to the occurrence of great earthquakes in subduction zones. In the Japan Trench, where the 2011 Tohoku earthquake occurred, we collected seismic profiles along survey lines separated by 2–8 km to examine the structural characteristics of the incoming Pacific plate in detail. The average thickness of the incoming sediments was < 500 m along most of the Japan Trench, and it was < 300 m at ~ 38° N, where the large shallow megathrust slip occurred during the 2011 Tohoku earthquake. We mapped bending-related normal faults, including their dip direction and amount of throw. The numbers of eastward (oceanward) and westward (trenchward) dipping normal faults were generally comparable in the Japan Trench. Eastward dipping normal faults were dominant in the northern and southern parts of the Japan Trench, whereas westward dipping normal faults were more numerous in the central part. Graben-fill sediments deposited at the landward edge of the graben were bounded by eastward dipping normal faults. Trench-fill sediments were also observed along the trench axis. The sediment fills locally increased the thickness of the input sediments where they were deposited. The along-axis variation in input sediment and sediment fill distribution, and the variations in normal fault dip direction between the central and other parts of the Japan Trench may correspond to different slip styles along the plate boundary.

A quantitative understanding of paleotsunamis is a significant issue in tsunami sedimentology. Onshore tsunami deposits, which are geological records of tsunami inundation, are used to reconstruct paleotsunami events. Numerical models of tsunami hydrodynamics and tsunami-induced sediment transport are utilized in such reconstructions to connect tsunami deposit characteristics, flow conditions, and (paleo-) tsunami sources. Recent progress in tsunami numerical modeling has increased the possibility of developing a methodology to estimate paleotsunami sources from tsunami deposits. Several previous studies have estimated paleotsunami sources using tsunami sediment transport simulations. However, the accuracy of paleotsunami source estimation has not yet been explored. Thus, to bridge this research gap, in this study, we showed the potential and limitations of deposit-based tsunami source estimation based on the 2011 Tohoku-oki tsunami deposit data on the southernmost part of the Sendai Plain, northeastern Japan. The tsunamigenic megathrust along the Japan Trench was divided into ten subfaults having similar lengths and widths. The hypothetical source models with varying slips on each subfault were examined by comparing the depositional volume and sediment source of onshore tsunami deposits. Due to limited information on the depositional area of the tsunami deposits used in the modeling, slips only in some parts of the entire tsunami source region could be estimated. The fault slip was slightly overestimated but could be compared with previous well-constrained source models. Thus, these results indicated that vast high-quality datasets of tsunami deposits can improve the accuracy of paleotsunami source estimation. It is also suggested that the amplitude of the receding wave affects the erosion pattern from the shoreface to the nearshore area. Although sufficient data for paleotsunami source estimation are lacking, an effective combination of tsunami deposit data and sediment transport simulations potentially improves the accuracy of the source estimation. The results will contribute to developing a framework of deposit-based paleotsunami source modeling and assessing its accuracy.