Toshitaka Baba

The 1946 Nankai earthquake and segmentation of the Nankai Trough

Abstract We perform a simple subevent analysis of seismic P-waveforms from historical records of the 1946 Nankai earthquake. The results of this analysis establish that the earthquake rupture consisted of two main subevents, one lying within the 1-day aftershock area off the Kii Peninsula, while the other lies much farther to the west in the area of pronounced crustal deformation of western Shikoku. These conform to two segments of the Nankai Trough which have long been referred to in studies of historical earthquakes and geology of the Nankai Trough, but to our knowledge have not previously been established as having distinct megathrust rupture behavior. We show that these two subevents are bounded by pronounced features in the geometry of the subducting Philippine Sea Plate (PSP): a tear in the plate to the east beneath the Kii Peninsula, and a subducting seamount farther to the west. We speculate that these features may be associated with low-strength parts of the upper and/or lower plates that may be susceptible to permanent, anelastic deformation which prevents the accumulation of the elastic strain energy necessary to sustain seismic rupture.

Tsunami source of the 2004 off the Kii Peninsula earthquakes inferred from offshore tsunami and coastal tide gauges

Tsunamis from the 2004 off the Kii Peninsula earthquakes (M 7.1 and 7.4) were recorded on offshore tsunami gauges, a GPS tsunami gauge and eight bottom-pressure gauges, as well as coastal tide gauges located south of Honshu and Shikoku. The maximum amplitudes on the GPS and bottom-pressure gauges were several to ten cm, while those on tide gauges were up to 0.9 m. We first computed tsunami waveforms from the earthquake source models proposed Yamanaka (2004) and Yagi (2004) from seismic waveform analysis, and compared them with the observed waveforms. For the first event (foreshock), both models produce similar waveforms with the observations. For the second event (mainshock), the waveforms computed from the Yamanaka model is closer to the observed waveforms, but there are still discrepancies between the observed and computed waveforms. We then performed tsunami waveform inversions to estimate the water height distributions in the source area. The foreshock source is ≈1600 km2 with the maximum water height of 0.2 m. The estimated tsunami source area for the mainshock, ≈3600 km2 with the maximum of 0.6 m, extends ≈60 km toward northwest and ≈40 km southwest from the epicenter along the aftershock distribution, suggesting that multiple faulting was involved in the mainshock.

Compound fault rupture during the 2004 off the Kii Peninsula earthquake (M 7.4) inferred from highly resolved coseismic sea-surface deformation

For a tsunami inversion analysis, we incorporated a new technique that uses many Green’s functions to improve model spatial resolution. Since this method precisely reproduces observed tsunami waveforms, we can obtain a better model of the tsunami source process. We applied the proposed method to the 2004 off the Kii Peninsula earthquake to determine what fault or faults ruptured to cause the earthquake. The estimated coseismic sea-surface deformation extended to two directions, and it was consistent with the distributions of the two aftershock swarms. Hence, we concluded that the earthquake resulted from the rupture of two faults.

Deformation of a seamount subducting beneath an accretionary prism: Constraints from numerical simulation

We examined the process of seamount subduction via a numerical simulation using the finite element method, applying a frictional force on the plate interface that is proportional to the normal stress. We calculate the incremental stress due to infinitesimal deformation of the seamount associated with subduction, and consider the implications for stress buildup and fracturing of the seamount itself. Our results show that the maximum shear stress concentrates at both flanks of the seamount, which suggests that fracturing will start there. We can surmise that, eventually, the seaward flank may be more apt to break than the landward flank at shallow depth if the confining pressure there is sufficiently low. We consider this to be a possible scenario for the generation of a thrust fault imaged at the seaward flank of the Muroto seamount, which is subducting under the Nankai trough accretionary prism.

Tsunami waveform inversion for sea surface displacement following the 2011 Tohoku earthquake: Importance of dispersion and source kinematics

This paper considers the importance of model parameterization, including dispersion, source kinematics, and source discretization, in tsunami source inversion. We implement single and multiple time window methods for dispersive and nondispersive wave propagation to estimate source models for the tsunami generated by the 2011 Tohoku-Oki earthquake. Our source model is described by sea surface displacement instead of fault slip, since sea surface displacement accounts for various tsunami generation mechanisms in addition to fault slip. The results show that tsunami source models can strongly depend on such model choices, particularly when high-quality, open-ocean tsunami waveform data are available. We carry out several synthetic inversion tests to validate the method and assess the impact of parameterization including dispersion and variable rupture velocity in data predictions on the inversion results. Although each of these effects has been considered separately in previous studies, we show that it is important to consider them together in order to obtain more meaningful inversion results. Our results suggest that the discretization of the source, the use of dispersive waves, and accounting for source kinematics are all important factors in tsunami source inversion of large events such as the Tohoku-Oki earthquake, particularly when an extensive set of high-quality tsunami waveform recordings are available. For the Tohoku event, a dispersive model with variable rupture velocity results in a profound improvement in waveform fits that justify the higher source complexity and provide a more realistic source model.

Tsunami Inundation Modeling of the 2011 Tohoku Earthquake Using Three-Dimensional Building Data for Sendai, Miyagi Prefecture, Japan

In conventional modeling of tsunami inundation, based on nonlinear shallow water theory in a finite-difference scheme, the effect of buildings and structures is represented by a bottom friction parameter rather than by three-dimensional (3D) building shapes. But large, strong buildings should offer direct protection against an incoming tsunami, like seawalls. In this study, therefore, we incorporated 3D building data obtained from lidar measurements in modeling the tsunami from the 2011 Tohoku earthquake at the port of Sendai, Miyagi Prefecture, Japan, and compared the results from conventional modeling based on a digital elevation model. In the model incorporating 3D building data, the maximum inundation height was greater than in the conventional model at the front of coastal buildings and structures and smaller behind them. High-velocity currents appeared in the corridors between these buildings, and the tsunami inundation area was smaller in the residential zone because of the obstacles that buildings presented to the tsunami. These results mean that solid buildings and structures have a significant influence on the propagation of tsunamis on land. The effects of the 3D shapes of buildings and structures should be further investigated for detailed tsunami hazard assessments in urban areas.

Development and application of an advanced ocean floor network system for megathrust earthquakes and tsunamis

Japan is prone to great earthquakes because of its position near two different subduction zones. The Philippine Sea plate subducts from the southeast, and the Pacific plate subducts from the east. The former was the source of a series of great earthquakes, of which the Tonankai earthquake of 1944 and the Nankaido earthquake of 1946 are the latest events. The latter was the source of the 2011 earthquake off the Pacific coast of Tohoku (Tohoku earthquake) of 11 March 2011 (M9).

Effect of elastic inhomogeneity on the surface displacements in the northeastern Japan: Based on three-dimensional numerical modeling

In geodetic inversions such as estimation of coseismic slip and/or afterslip distribution on faults, the displacements on the surface calculated under an assumption of homogeneous elastic half space have been mostly used as the Green’s functions (GF’s). However, this seems not adequate for better estimations of such slip distribution, because the subsurface structures are more or less inhomogeneous, especially those in and around Japan where the structure must be much complicated. In this study, to examine how much the inhomogeneous subsurface structure affects on the surface displacements, we conduct some 3-D finite element calculations with a grid for the region of 1400 km (EW) × 1200 km (NS) × 200 km (depth) including the Tohoku and Hokkaido, northeastern Japan. Assuming homogeneous and inhomogeneous elastic models with various values for the Young’s modulus and Poisson’s ratio, we calculated the surface displacements due to a dip-slip type dislocation of 1 m on many cell-like subfaults assumed on the interface between the Pacific and land side plates. Comparing the results, we find a large discrepancy in the surface displacements between the homogeneous and inhomogeneous elastic models and less dependency of the surface displacements on the Poisson’s ratio. The discrepancy is found to be more than 20% and can be as large as ~40% in some cases. Such a large discrepancy indicates that the surface displacements calculated for inhomogeneous elastic medium with realistic subsurface structure, unlike as in usual cases, should be used as the GF’s for better geodetic inversions.

Near-field tsunami amplification factors in the Kii Peninsula, Japan for Dense Oceanfloor Network for Earthquakes and Tsunamis (DONET)

We investigated the correlation between coastal and offshore tsunami heights by using data from the Dense Oceanfloor Network for Earthquakes and Tsunamis (DONET) observational array of ocean-bottom pressure gauges in the Nankai trough off the Kii Peninsula, Japan. For near-field earthquakes, hydrostatic pressure changes may not accurately indicate sea surface fluctuations, because ocean-bottom pressure gauges are simultaneously displaced by crustal deformation due to faulting. To avoid this problem, we focused on the average waveform of the absolute value of the hydrostatic pressure changes recorded at all the DONET stations during a tsunami. We conducted a Monte Carlo tsunami simulation that revealed a clear relationship between the average waveforms of DONET and tsunami heights at the coast. This result indicates the possibility of accurate real-time prediction of tsunamis by use of arrays of ocean-bottom pressure gauges.

The possibility of deeper or shallower extent of the source area of Nankai Trough earthquakes based on the 1707 Hoei tsunami heights along the Pacific and Seto Inland Sea coasts, southwest Japan

To validate the abundance of scenarios of large earthquakes in the Nankai Trough, we examined the effects of both lateral and vertical expansions of the source areas on maximum tsunami heights along the Pacific coast and Seto Inland Sea. The recently proposed Nankai Trough earthquake scenario (Mw = 9) has a maximum slip of 20 m near the trough axis. However, the predicted tsunami heights exceeded those obtained from historical records of damage caused by the 1707 Hoei tsunami event at Tosa Bay and along the Pacific coastlines near the Kii Channel, owing to the large slip on the up-dip extension of fault segments off Shikoku Island. Such discrepancy indicates that for segments off Shikoku Island, the slip near the trough axis was unremarkable, even for the 1707 Hoei earthquake event, which is considered one of the larger historical Nankai Trough earthquake events. For segments east of the Kii Peninsula, the large slip on the up-dip end might be ineffective. While the proposed Mw9-class scenario also includes large slip of several meters on the down-dip side (down to about 35-km depth), coseismic crustal subsidence reached further landward than is usual for Nankai Trough earthquakes. For the Seto Inland Sea region, this resulted in maximum subsidence of about 1 m, and such crustal subsidence effectively increased the height of the tsunamis. Furthermore, simulated tsunami heights, corrected for crustal subsidence, were in good agreement with those obtained from historical records of the damage caused in the Seto Inland Sea region.