Daisuke Inazu

Tsunami-generated turbidity current of the 2011 Tohoku-Oki earthquake

We show the first real-time record of a turbidity current associated with a great earthquake, the Mw 9.0, 2011 Tohoku-Oki event offshore Japan. Turbidity current deposits (turbidites) have been used to estimate earthquake recurrence intervals from geologic records. Until now, however, there has been no direct evidence for large-scale earthquakes in subduction plate margins. After the 2011 Tohoku-Oki earthquake and tsunami, an anomalous event on the seafloor consistent with a turbidity current was recorded by ocean-bottom pressure recorders and seismometers deployed off Sendai, Japan. Freshly emplaced turbidites were collected from a wide area of seafloor off the Tohoku coastal region. We analyzed these measurements and sedimentary records to determine conditions of the modern tsunamigenic turbidity current. We anticipate our discovery to be a starting point for more detailed characterization of modern tsunamigenic turbidites, and for the identification of tsunamigenic turbidites in geologic records.

Dispersion and nonlinear effects in the 2011 Tohoku‐Oki earthquake tsunami

This study reveals the roles of the wave dispersion and nonlinear effects for the 2011 Tohoku-Oki earthquake tsunami. We conducted tsunami simulations based on the nonlinear dispersive equations with a high-resolution source model. The simulations successfully reproduced the waveforms recorded in the offshore, deep sea, and focal areas. The calculated inundation area coincided well with the actual inundation for the Sendai Plain, which was the widest inundation area during this event. By conducting sets of simulations with different tsunami equations, we obtained the followings insights into the wave dispersion, nonlinear effects, and energy dissipation for this event. Although the wave dispersion was neglected in most studies, the maximum amplitude was significantly overestimated in the deep sea if the dispersion was not included. The waveform observed at the station with the largest tsunami height (∼2 m) among the deep-ocean stations also verified the necessity of the dispersion. It is well known that the nonlinear effects play an important role for the propagation of a tsunami into bays and harbors. Additionally, nonlinear effects need to be considered to accurately model later waves, even for offshore stations. In particular, including nonlinear terms rather than the inundation was more important when precisely modeling the waves reflected from the coast.

Simulation of distant tsunami propagation with a radial loading deformation effect

A simple parameterization of the loading deformation of the seafloor is incorporated into a tsunami simulation model in order to realistically calculate tsunami travel time, especially at regions far from the source. The parameterization uses one scalar parameter that is optimized effectively by far-field, deep-sea records of recent giant tsunamis: the 2011 Tohoku and the 2010 Chilean tsunamis. Using this parameterization with the optimal values, the observed tsunamis are realistically simulated in both near and far fields. The optimal values seem equivalent for both giant tsunamis, and are relatively smaller than those previously verified for ocean tide modeling, which is reasonable because of the shorter wavelengths of tsunamis.

Tsunami source uncertainty estimation: The 2011 Japan tsunami

This article was published in the Journal of Geophysical Research: Solid Earth [©2016 American Geophysical Union] and the definite version is available at : http://dx.doi.org/10.1002/2015JB012764 The Journals website is at: http://onlinelibrary.wiley.com/doi/10.1002/2015JB012764/abstract;jsessionid=BF5813B61924FB094F31BC8F213840A1.f02t04

Gravity and uplift rates observed in southeast Alaska and their comparison with GIA model predictions

gravity change rates (unit: mGal/yr, 1 mGal = 10 8 ms 2 ) over the 6 sites are estimated to be 4.50 0.76 and 4.30 0.92 by only using our data and also using the 1987 data, respectively. We computed the uplift and gravity rates predicted by ice load models for three different time intervals: Last Glacial Maximum (LGM), Little Ice Age (LIA) and Present-Day (PD). Except for 1–2 examples, the predictions recover the observed rates within the observation errors. We also estimated the viscous portion of the ratio (unit: mGal/mm) of the observed gravity rate to the uplift rate by correcting for the effects of the Present-Day Ice Mass Change (PDIMC). Two PDIMC models are compared, which are called here as UAF05 and UAF07. Mean ratios are estimated to be 0.205 0.089 and 0.183 0.052 for the cases using UAF05 and UAF07, respectively. The predicted mean ratios are 0.166 0.001 and 0.171 0.002 for the cases using both the LGA and LIA ice models and only using the LIA ice model, respectively. We have confirmed that our AG and GPS observations detect the rates and ratios reflecting an early stage of viscoelastic relaxation mainly due to the unloading effects after the LIA.

Two subevents across the Japan Trench during the 7 December 2012 off Tohoku earthquake (Mw 7.3) inferred from offshore tsunami records

A tsunamigenic earthquake with a moment magnitude of 7.3 occurred near the Japan Trench, off Tohoku, northeast Japan, on 7 December 2012. Operational seismic monitoring inferred that the earthquake was composed of doublet sources of comparable magnitudes: the first event was reverse faulting and the second event, which occurred 10–20 s later, was normal faulting. An associated tsunami was observed at offshore stations, having an amplitude of 10−1 to 101 cm. Inverse modeling using the observed tsunami records reveals possible vertical seafloor deformations. We can then constrain, in terms of the source location, that the tsunamigenic earthquake involves two subfaults across the Japan Trench: one is the outer trench reverse faulting as the first event and the second is the inner trench normal faulting. The present study shows that tsunami analysis with seismograph analyses effectively constrains the epicenter locations, even in the case of complex earthquakes like doublets.

Time-Series Modeling of Tide Gauge Records for Monitoring of the Crustal Activities Related to Oceanic Trench Earthquakes Around Japan

Tide gauge observations along the coastline of Japan have recorded the land sinking due to the continuous subduction of the oceanic plates, indicating that stress and strain energies have been accumulating at the plate boundary, which would eventually cause large oceanic trench earthquakes like the 2011 Great East Japan Earthquake. The proposed method extracts such long-term activities of the Earths crust together with rapid displacements related to earthquakes, even before the establishment of the global positioning system, from monthly mean data of the sea levels. A state space model decomposes the tide gauge time series into trend, seasonal, autoregressive and observation noise components, each of which are estimated using the particle filter algorithm. The spatial and temporal distributions of the extracted trend component clearly indicate high-risk regions, near which giant earthquakes have occurred or are predicted to occur. A multivariate analysis of the observatories located at the northeast coast of Japan successfully determines the past crustal displacement in the case of the 1978 Off-Miyagi Earthquake. The proposed method has the potential application for monitoring crustal activities related to the accumulation of earthquake energy.

Preparing for the Future Nankai Trough Tsunami: A Data Assimilation and Inversion Analysis From Various Observational Systems

The future Nankai Trough tsunami is one of the imminent threats to the Japanese coastal communities that could potentially cause a catastrophic event. As a part of the countermeasure efforts for such an occurrence, this study analyzes the efficacy of combining tsunami data assimilation (DA) and waveform inversion (WI). The DA is used to continuously refine a wave field model whereas the WI is used to estimate the tsunami source. We consider a future scenario of the Nankai Trough tsunami recorded at various observational systems, including ocean bottom pressure (OBP) gauges, global positioning system (GPS) buoys, and ship height positioning data. Since most of the OBP gauges are located inside the source region, the recorded tsunami signals exhibit significant offsets from surface measurements due to coseismic seafloor deformation effects. Such biased data are not applicable to the standard DA, but can be taken into account in the WI. On the other hand, the use of WI for the ship data may not be practical because a considerably large precomputed tsunami database is needed to cope with the spontaneous ship locations. The DA is more suitable for such an observational system as it can be executed sequentially in time and does not require precomputed scenarios. Therefore, the combined approach of DA and WI allows us to concurrently make use of all observational resources. Additionally, we introduce a bias correction scheme for the OBP data to improve the accuracy, and an adaptive thinning of observations to determine the efficient number of observations.

Investigation on the Postseismic Deformation Associated with the 2011 Tohoku Earthquake Based on Terrestrial and Seafloor Geodetic Observations: To Evaluate the Further Seismic Hazard Potential on the Plate Interface Beneath the Northeastern Japanese Islands

The 2011 Tohoku Earthquake (M9.0), which occurred on the plate boundary between the subducting Pacific plate and continental plate has been associated with postseismic deformation, including aseismic slip at the plate interface (postseismic slip). In order to evaluate the potential for further seismic activity, we investigated the spatial and temporal evolution of the postseismic slip based not only on terrestrial GPS data but also on seafloor geodetic data. We estimated the displacements due to the postseismic slip by subtracting the displacements due to large aftershocks and viscoelastic relaxation from the original displacement time series data and used a time-dependent inversion method to estimate the postseismic slip distributions. The resultant postseismic slip distributions depend strongly on the assumed value of the viscosity. However, the following two features are independent of the viscosity assumption: (1) large postseismic slip has been occurring at a very shallow ( ≤ 20 km in depth) portion of the plate interface south of the area of huge coseismic slip and (2) significant postseismic slip has occurred at a deep (approximately 50 km in depth) portion of the plate interface. The results suggest that the elastic strain and the stress concentrated at the plate interface at a depth of approximately 30 km in the segment off the Boso Peninsula have not yet been released and continue to generate large aftershocks.