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A review on slow earthquakes in the Japan Trench
Slow earthquakes are episodic slow fault slips. They form a fundamental component of interplate deformation processes, along with fast, regular earthquakes. Recent seismological and geodetic observations have revealed detailed slow earthquake activity along the Japan Trench—the subduction zone where the March 11, 2011, moment magnitude (Mw) 9.0 Tohoku-Oki earthquake occurred. In this paper, we review observational, experimental, and simulation studies on slow earthquakes along the Japan Trench and their research history. By compiling the observations of slow earthquakes (e.g., tectonic tremors, very-low-frequency earthquakes, and slow slip events) and related fault slip phenomena (e.g., small repeating earthquakes, earthquake swarms, and foreshocks of large interplate earthquakes), we present an integrated slow earthquake distribution along the Japan Trench. Slow and megathrust earthquakes are spatially complementary in distribution, and slow earthquakes sometimes trigger fast earthquakes in their vicinities. An approximately 200-km-long along-strike gap of seismic slow earthquakes (i.e., tectonic tremors and very-low-frequency earthquakes) corresponds with the huge interplate locked zone of the central Japan Trench. The Mw 9.0 Tohoku-Oki earthquake ruptured this locked zone, but the rupture terminated without propagating deep into the slow-earthquake-genic regions in the northern and southern Japan Trench. Slow earthquakes are involved in both the rupture initiation and termination processes of megathrust earthquakes in the Japan Trench. We then compared the integrated slow earthquake distribution with the crustal structure of the Japan Trench (e.g., interplate sedimentary units, subducting seamounts, petit-spot volcanoes, horst and graben structures, residual gravity, seismic velocity structure, and plate boundary reflection intensity) and described the geological environment of the slow-earthquake-genic regions (e.g., water sources, pressure–temperature conditions, and metamorphism). The integrated slow earthquake distribution enabled us to comprehensively discuss the role of slow earthquakes in the occurrence process of the Tohoku-Oki earthquake. The correspondences of the slow earthquake distribution with the crustal structure and geological environment provide insights into the slow-earthquake-genesis in the Japan Trench and imply that highly overpressured fluids are key to understanding the complex slow earthquake distribution. Furthermore, we propose that detailed monitoring of slow earthquake activity can improve the forecasts of interplate seismicity along the Japan Trench.
Journal Article
Reexamination of tsunami source models for the twentieth century earthquakes off Hokkaido and Tohoku along the eastern margin of the Sea of Japan
Large earthquakes around Japan occur not only in the Pacific Ocean but also in the Sea of Japan, and cause both damage from the earthquake itself and from the ensuing tsunami to the coastal areas. Recently, offshore active fault surveys were conducted in the Sea of Japan by the Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan (JSPJ), and their fault models (length, width, strike, dip, and slip angles) have been obtained. We examined the causative faults of M7 or larger earthquakes in the Sea of Japan during the twentieth century, comprising events of 1940, 1964, 1983, and 1993, using seismic and tsunami data. The 1940 off Shakotan Peninsula earthquake (MJMA 7.5) appears to have been caused by the offshore active faults MS01, MS02, ST01, and ST02 as modeled by the JSPJ. The 1993 off the southwest coast of Hokkaido earthquake (MJMA 7.8) likely occurred on the offshore active faults OK03a, OK03b, and OK05, while the 1983 Central Sea of Japan earthquake (MJMA 7.7) probably related to MMS01, MMS04, and MGM01. For these earthquakes, the observed tsunami waveforms were basically reproduced by tsunami numerical simulation from the offshore active faults with the slip amounts obtained by the scaling relation with three stages between seismic moment and source area for inland earthquakes. However, the observed tsunami runup heights along the coast were not reproduced at certain locations, possibly because of the coarse bathymetry data used for the simulation. The 1983 west off Aomori (MJMA 7.1) and the 1964 off Oga Peninsula (MJMA 6.9) earthquakes showed multiple faults near the source area that could be used to reproduce the observed tsunami waveforms; therefore, we could not identify the causative faults. Further analysis using near-field seismic waveforms is required for their identification of their causative faults and their parameters. The scaling relation for inland earthquakes can be used to obtain the slip amounts for offshore active faults in the Sea of Japan and to estimate the coastal tsunami heights and inundation area which can be useful for disaster prevention and mitigation of future earthquakes and tsunamis.
Journal Article
Rapid Estimation of Earthquake Fatalities in Mainland China Based on Physical Simulation and Empirical Statistics—A Case Study of the 2021 Yangbi Earthquake
At present, earthquakes cannot be predicted. Scientific decision-making and rescue after an earthquake are the main means of mitigating the immediate consequences of earthquake disasters. If emergency response level and earthquake-related fatalities can be estimated rapidly and quantitatively, this estimation will provide timely, scientific guidance to government organizations and relevant institutions to make decisions on earthquake relief and resource allocation, thereby reducing potential losses. To achieve this goal, a rapid earthquake fatality estimation method for Mainland China is proposed herein, based on a combination of physical simulations and empirical statistics. The numerical approach was based on the three-dimensional (3-D) curved grid finite difference method (CG-FDM), implemented for graphics processing unit (GPU) architecture, to rapidly simulate the entire physical propagation of the seismic wavefield from the source to the surface for a large-scale natural earthquake over a 3-D undulating terrain. Simulated seismic intensity data were used as an input for the fatality estimation model to estimate the fatality and emergency response level. The estimation model was developed by regression analysis of the data on human loss, intensity distribution, and population exposure from the Mainland China Composite Damaging Earthquake Catalog (MCCDE-CAT). We used the 2021 Ms 6.4 Yangbi earthquake as a study case to provide estimated results within 1 h after the earthquake. The number of fatalities estimated by the model was in the range of 0–10 (five expected fatalities). Therefore, Level IV earthquake emergency response plan should have been activated (the government actually overestimated the damage and activated a Level II emergency response plan). The local government finally reported three deaths during this earthquake, which is consistent with the model predictions. We also conducted a case study on a 2013 Ms7.0 earthquake in the discussion, which further proved the effectiveness of the method. The proposed method will play an important role in post-earthquake emergency response and disaster assessment in Mainland China. It can assist decision-makers to undertake scientifically-based actions to mitigate the consequences of earthquakes and could be used as a reference approach for any country or region.
Journal Article
Surface-Rupturing Historical Earthquakes in Australia and Their Environmental Effects: New Insights from Re-Analyses of Observational Data
We digitize surface rupture maps and compile observational data from 67 publications on ten of eleven historical, surface-rupturing earthquakes in Australia in order to analyze the prevailing characteristics of surface ruptures and other environmental effects in this crystalline basement-dominated intraplate environment. The studied earthquakes occurred between 1968 and 2018, and range in moment magnitude (Mw) from 4.7 to 6.6. All earthquakes involved co-seismic reverse faulting (with varying amounts of strike-slip) on single or multiple (1–6) discrete faults of ≥ 1 km length that are distinguished by orientation and kinematic criteria. Nine of ten earthquakes have surface-rupturing fault orientations that align with prevailing linear anomalies in geophysical (gravity and magnetic) data and bedrock structure (foliations and/or quartz veins and/or intrusive boundaries and/or pre-existing faults), indicating strong control of inherited crustal structure on contemporary faulting. Rupture kinematics are consistent with horizontal shortening driven by regional trajectories of horizontal compressive stress. The lack of precision in seismological data prohibits the assessment of whether surface ruptures project to hypocentral locations via contiguous, planar principal slip zones or whether rupture segmentation occurs between seismogenic depths and the surface. Rupture centroids of 1–4 km in depth indicate predominantly shallow seismic moment release. No studied earthquakes have unambiguous geological evidence for preceding surface-rupturing earthquakes on the same faults and five earthquakes contain evidence of absence of preceding ruptures since the late Pleistocene, collectively highlighting the challenge of using mapped active faults to predict future seismic hazards. Estimated maximum fault slip rates are 0.2–9.1 m Myr−1 with at least one order of uncertainty. New estimates for rupture length, fault dip, and coseismic net slip can be used to improve future iterations of earthquake magnitude—source size—displacement scaling equations. Observed environmental effects include primary surface rupture, secondary fracture/cracks, fissures, rock falls, ground-water anomalies, vegetation damage, sand-blows/liquefaction, displaced rock fragments, and holes from collapsible soil failure, at maximum estimated epicentral distances ranging from 0 to ~250 km. ESI-07 intensity-scale estimates range by ± 3 classes in each earthquake, depending on the effect considered. Comparing Mw-ESI relationships across geologically diverse environments is a fruitful avenue for future research.
Journal Article
The Aftermath of the 2011 east Japan earthquake and tsunami
An insightful study in disaster anthropology, this book takes as its focus the fishing town of Otsuchi in Japan’s Iwate Prefecture, one of the worst damaged areas in the mammoth 2011 tsunami. Here, 1281 of the pre-tsunami population of 15000 were killed and 60% of houses destroyed. To make matters worse, the town’s administrative organs were completely obliterated, and fire ravaged the downtown area for three days, blocking external rescue attempts. Complete with vivid and detailed witness testimony collected by the author, the book traces the course of eighteen months from the day of the disaster, through the subsequent months of community life in the evacuation centers, onto the struggles between the citizens and local governments in formulating reconstruction plans. It particularly addresses community interactions within the post-disaster context, assessing the locals’ varying degrees of success in organizing emergency committees to deal with such tasks as clearing rubble, hunting down food and obtaining fuel, and inquiring into the sociological reasons for these differences. It also casts new light on administrative failings that significantly augmented the loss of human lives in the disaster, and are threatening to bring further damage through insistence on reconstruction centered on enormous sea walls, against local citizens’ wishes.
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