Streak and hierarchical structures of the Tohoku–Hokkaido subduction zone plate boundary

Abstract

Here, we investigated four families of moderate-size (M ~ 4.8) repeating earthquakes and nearby smaller events that occurred in the Naka-Oki, Kushiro-Oki, Urakawa-Oki, and Kamaishi-Oki regions of the Tohoku-Hokkaido subduction zone, Japan, between 2002 and 2017. We employed a hypocenter relocation method based on waveform crosscorrelations of the above four families, as well as a waveform inversion method using the empirical Green's function for the Naka-Oki and Kushiro-Oki earthquakes, where every moderate-size repeating earthquake showed complex and variable rupture processes. However, their rupture areas overlapped significantly and were elongate parallel to the slip direction. Some parts of these areas were also repeatedly ruptured by smaller earthquakes, with these smaller earthquakes and moderate events sometimes following almost identical rupture processes. The results indicate the existence of a characteristic structure that is represented by several elliptical patches that are elongate in the slip direction of the plate interface. It is likely that many streak structures have developed over a 1000 km along-strike section of the plate interface that extends from the Kushiro-Oki region to the Naka-Oki region of the subduction zone. These patches may be distributed hierarchically, with the smaller patches rupturing independently. The rupture of a smaller patch may either stop, producing a small earthquake, or it may grow into a large earthquake, with this rupture process depending on subtle differences in the physical conditions that we are unable to observe in a practical sense. Earthquakes are random processes, but they may not be completely random, as the preexisting characteristic structure of the seismic region may control the rupture process to some degree.

The Dynamics of Elongated Earthquake Ruptures

“…Elongated ruptures can happen also in moderate earthquakes ( Figure 2) and even in small earthquakes, as suggested by spectra with double corner frequencies [Imanishi and Ellsworth, 2006;Uchide and Imanishi, 2016] and source inversion studies [Okuda and Ide, 2018]. The rupture width of moderate and small earthquakes may be confined by other constraints such as heterogeneities of initial stresses and fault materials.…”

The Dynamics of Elongated Earthquake Ruptures

“…Elongated ruptures can happen also in moderate earthquakes (Figure ) and even in small earthquakes, as suggested by spectra with double corner frequencies (Imanishi & Ellsworth, ; Uchide & Imanishi, ) and source inversion studies (Okuda & Ide, ). The rupture width of moderate and small earthquakes may be confined by other constraints such as heterogeneities of initial stresses and fault materials.…”

Toward comparable relative locations between the mainshock slip and aftershocks via empirical approaches

“…1) that is used for the convolution in the relocation analysis. We invert for the spatiotemporal slip rate distribution �u(ξ , τ ) on an assumed fault plane, which is expressed as follows (Ide and Takeo 1997;Okuda and Ide 2018):…”

“…Here we assume the basis functions to be triangle functions with a 40-km length and 20-s duration. We utilize bootstrapping to regularize the inversion solutions, as in Okuda and Ide (2018), and retrieve the most robust features of the slip inversion results instead of applying commonly implemented smoothing constraints. We randomly select the same number of waveform data from the available dataset, which include those from different stations and components, with replacement, and perform a slip inversion, with this process run 300 times.…”

“…We randomly select the same number of waveform data from the available dataset, which include those from different stations and components, with replacement, and perform a slip inversion, with this process run 300 times. We then take the mean of the model parameters and examine various features, such as the waveform recovery and slip distributions given by this averaged solution, and estimate the model error for each feature based on the standard deviation of each feature produced in each realization (Hartzell et al 2007;Hayes 2011;Okuda and Ide 2018). We implement the non-negative constraint by applying the non-negative least squares algorithm (Lawson and Hanson 1974), as well as the causality constraint, which only allows initial slip expansion in space and time from the hypocenter after an assumed rupture velocity of 3 km/s.…”