Ryoko Nakata

A slip pulse model with fault heterogeneity for low‐frequency earthquakes and tremor along plate interfaces

[1] Deep low-frequency earthquakes (LFEs) and nonvolcanic tremor have distinctive characteristics unlike those of regular earthquakes, including strong anisotropy in their migration velocity and source spectra displaying 1/f decay. We show that a physical model can explain these features in a simple framework with slip pulses originating on fault heterogeneity and triggered by slow-slip events. LFE/tremor source areas in the model consist of unstable patches sparsely and heterogeneously distributed following a Gaussian distribution. The difference in their migration speeds along dip and along strike was reproduced, without anisotropic rheological properties, by introducing alignments of their sources similar to observed streaks of LFEs/tremor. The key to reproducing inverse linear spectral decay is that the slip pulse has a constant mean moment rate. This model provides new insights into the physical source process of LFEs and tremor and should find practical use in assessing properties of deep plate interfaces.

Migration process of very low-frequency events based on a chain-reaction model and its application to the detection of preseismic slip for megathrust earthquakes

In order to reproduce slow earthquakes with short duration such as very low frequency events (VLFs) migrating along the trench direction as swarms, we apply a 3-D subduction plate boundary model based on the slowness law of rate- and state-dependent friction, introducing close-set numerous small asperities (rate-weakening regions) at a depth of 30 km under high pore pressure condition, in addition to a large asperity. Our simulation indicates that swarms of slip events occur repeatedly at the small asperities, and these events are similar to the observed slow earthquake group, especially to VLF, on the basis of the relation between characteristic duration and seismic moment. No slip events occur there without the small asperities, which mean that the close-set numerous small asperities may be one of the necessary conditions for generating the short-duration slow earthquakes such as VLFs. In the preseismic stage of the megathrust earthquakes that occur at the large asperity, the swarms of VLFs have higher migration speeds and higher moment release rate as well as shorter recurrence interval. Thus, monitoring the migration of slow earthquakes may be useful in imaging the preseismic slip of megathrust earthquakes.

The detectability of shallow slow earthquakes by the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) in Tonankai district, Japan

In order to understand the characteristics of shallow very low-frequency (VLF) events as revealed by recent ocean-floor observation studies, we perform a trial simulation of earthquake cycles in the Tonankai district by taking the characteristics of the 1944 Tonankai earthquake and assuming that slow earthquakes occur on numerous small asperities. Our simulation results show that the increase of moment release rate of shallower VLF events in the pre-seismic stage of a megathrust earthquake is higher than that of deeper VLF events. This increase may make leveling change due to VLF swarms detectable at Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET). We also introduce the time series of hydraulic pressure data at DONET, comparing with the leveling change expected from our numerical simulation. Since leveling change due to shallower VLF swarms is so local as to be incoherent, removal of the moving-averaged data from the data stacked by four nearby observation points in the same node may be useful to detect the short-term local leveling change.

Possible slip history scenarios for the Hyuga‐nada region and Bungo Channel and their relationship with Nankai earthquakes in southwest Japan based on numerical simulations

The Nankai earthquakes are great interplate earthquakes with recurrence intervals of 100–200 years, occurring along the Nankai Trough, southwestern Japan. New geological evidences suggest that the rupture area of the 1707 earthquake (one of the largest) extended farther westward than previously thought, and similar magnitude events may have recurrence intervals of several hundred years. West of the Nankai earthquake segments, in the Bungo Channel region, slow slip events (SSEs) have occurred with recurrence intervals of several years since 1980. Farther westward, no M ≥ 7.5 interplate earthquakes have occurred except for one in 1968 in the northern Hyuga-nada region, near the Bungo Channel. We numerically simulated the generation cycles of Nankai earthquakes, Hyuga-nada earthquakes, and Bungo Channel SSEs to obtain possible slip histories in these regions, particularly during 1707–1980. We showed that the Nankai and Hyuga-nada earthquakes occur mostly independently, with average recurrence intervals of 151 and 117 years, respectively. An SSE patch slipped coseismically during both the Nankai and Hyuga-nada earthquakes. The rupture of the Nankai earthquakes extended to the Hyuga-nada region every ~500–600 years and was followed by quiescence of SSEs for more than 200 years. Subsequently, SSEs occurred repeatedly every 8 years. Current observational findings are generally consistent with the characteristics obtained from our scenarios, except regarding the recurrence intervals of Hyuga-nada earthquakes, for which few observations are available. Further evidence of the slip history in these regions will be required to use information from such simulations to forecast future earthquakes.

A trial derivation of seismic plate coupling by focusing on the activity of shallow slow earthquakes

To understand the effect of plate coupling on very low-frequency event (VLFE) activity resulting from megathrust earthquakes, we performed long-term multiscale earthquake cycle simulations (including a megathrust earthquake and slow earthquakes) on a 3-D subduction plate boundary model, based on a rate- and state-dependent friction law. Our simulation suggests that quiescence of shallow VLFEs off Miyagi may be explained by the location in the shallow central part of the 2011 Tohoku earthquake because of the locally strong coupling, while observed activation of VLFEs off Iwate (northern part of Tohoku district), Fukushima (southern part of Tohoku district), and Ibaraki (northern part of Kanto district) is explained by the location on the outer rim. The area and duration of the quiescence off Miyagi may be a new clue to evaluate the potential for plate coupling strong enough to cause the next megathrust earthquake.

Discontinuous boundaries of slow slip events beneath the Bungo Channel, southwest Japan

The down-dip limit of the seismogenic zone and up-dip and down-dip limits of the deep low-frequency tremors in southwest Japan are clearly imaged by the hypocentre distribution. Previous studies using smooth constraints in inversion analyses estimated that long-term slow slip events (L-SSEs) beneath the Bungo Channel are distributed smoothly from the down-dip part of the seismogenic zone to the up-dip part of the tremors. Here, we use fused regularisation, a type of sparse modelling suitable for detecting discontinuous changes in the model parameters to estimate the slip distribution of L-SSEs. The largest slip abruptly becomes zero at the down-dip limit of the seismogenic zone, is immediately reduced to half at the up-dip limit of the tremors, and becomes zero near its down-dip limit. Such correspondences imply that some thresholds exist in the generation processes for both tremors and SSEs. Hence, geodetic data inversion with sparse modelling can detect such high resolution in the slip distribution.

Possible scenarios for occurrence of M ~ 7 interplate earthquakes prior to and following the 2011 Tohoku-Oki earthquake based on numerical simulation

We show possible scenarios for the occurrence of M ~ 7 interplate earthquakes prior to and following the M ~ 9 earthquake along the Japan Trench, such as the 2011 Tohoku-Oki earthquake. One such M ~ 7 earthquake is so-called the Miyagi-ken-Oki earthquake, for which we conducted numerical simulations of earthquake generation cycles by using realistic three-dimensional (3D) geometry of the subducting Pacific Plate. In a number of scenarios, the time interval between the M ~ 9 earthquake and the subsequent Miyagi-ken-Oki earthquake was equal to or shorter than the average recurrence interval during the later stage of the M ~ 9 earthquake cycle. The scenarios successfully reproduced important characteristics such as the recurrence of M ~ 7 earthquakes, coseismic slip distribution, afterslip distribution, the largest foreshock, and the largest aftershock of the 2011 earthquake. Thus, these results suggest that we should prepare for future M ~ 7 earthquakes in the Miyagi-ken-Oki segment even though this segment recently experienced large coseismic slip in 2011.

Geodetic inversion for spatial distribution of slip under smoothness, discontinuity, and sparsity constraints

In geodetic data inversion, insufficient observational data and smoothness constraints for model parameters make it difficult to clearly resolve small-scale heterogeneous structures with discontinuous boundaries. We therefore developed a novel regularization scheme for the inversion problem that uses discontinuity, sparsity, and smoothness constraints. In order to assess its usefulness and applicability, the proposed method was applied to synthetic displacements calculated by a ring-shaped and sharply varying afterslip distribution on a plate interface. The afterslip was obtained from reasonable numerical simulation of earthquake generation cycle with a rate- and state- dependent friction law and realistic three-dimensional plate geometry. The obtained afterslip distribution was heterogeneous, and the discontinuous boundary was sharper than that obtained by using smoothness constraint only. The same inversion test was conducted with a smoothly varying circular slip distribution with large slips inside the ring-shaped distribution. The method accurately reproduces the smooth distribution of the slip area as well as the ring-shaped distribution. Therefore, the method could be applied to any slip distribution, with both discontinuous and continuous boundaries. Adopting this method for measured data will make it possible to obtain detailed heterogeneous distributions of physical structures on fault planes. The proposed method is therefore applicable to various geophysical inversion problems that exhibit discontinuous heterogeneity.

Geodetic inversion for spatial distribution of slow earthquakes under sparsity constraints

In geodetic data inversion, insufficient observational data and smoothness constraints for model parameters make it difficult to clearly resolve small-scale heterogeneous structures with discontinuous boundaries. We therefore applied sparse modelling to geodetic data inversion. In this paper, we reported two examples; one is developed a novel regularization scheme for the inversion problem that uses discontinuity, sparsity, and smoothness constraints. In order to assess its usefulness and applicability, the proposed method was applied to synthetic displacements calculated by a ring-shaped and sharply varying afterslip distribution on the plate interface beneath the Hyuga-nada region in southwest Japan. The discontinuous boundary was sharper than that obtained by using smoothness constraint only. The other is used the fused regularization, a type of sparse modelling suitable for detecting discontinuous changes in the model parameters. We estimated spatial distribution of the long-term slow slip events beneath the Bungo channel in southwest Japan. We found that the largest slip abruptly becomes zero at the down-dip limit of the seismogenic zone, and is immediately reduced to half at the up-dip limit of the deep low-frequency tremors, and becomes zero near its down-dip limit. Such correspondences imply that some thresholds exist in the generation processes for both tremors and SSEs. These results suggest that geodetic data inversion with sparse modelling can detect such abrupt changes and discontinuous boundaries in the slip distribution of slow earthquakes.