Scaling relationship between the duration and the amplitude of non-volcanic deep low-frequency tremors

金沢大学大学院自然科学研究科自然計測金沢大学理学部We investigate a duration-amplitude relation of non-volcanic deep low-frequency (DLF) tremors in the Tokai region, southwest Japan, to constrain the source process of the tremors. We apply two models to the distribution, one is an exponential model as a scale bound distribution and the other a power law model as a scale invariant distribution. The exponential model shows a better fit to the duration-amplitude distribution of the tremors than a power law model, implying that the DLF tremors are caused by a scale-bound source process. The source process of the DLF tremors, therefore, differs from those for earthquakes. We suggest that the non-volcanic DLF tremor is possibly caused by a fixed source dimension with variable excess pressure of fluid or variable stress drop. Copyright 2007 by the American Geophysical Union

Deep low-frequency tremors as a proxy for slip monitoring at plate interface

金沢大学理工研究域自然システム学系We propose a new method to monitor slip at the plate interface using non-volcanic deep-low frequency (DLF) tremors. We assume that a DLF tremor is the superposition of frequently excited intermittent events, meaning that the envelope of the reduced displacement of the DLF tremor provides an apparent moment rate function. We estimate a conversion factor from the apparent moment to the seismic moment with an assumption that a total size of DLF tremors of an episode is proportional to the size of corresponding slow slip event (SSE). The cumulative seismic moment estimated by DLF tremors is consistent with that estimated from geodetic methods and provides appropriate slip and slip rate at the plate interface. This proves our assumptions and demonstrates that DLF tremors are useful tool for realtime monitoring of the slip at the plate interface. Copyright 2008 by the American Geophysical Union

Identifying the Recurrence Patterns of Nonvolcanic Tremors Using a 2‐D Hidden Markov Model With Extra Zeros

Nonvolcanic tremor activity has been observed in many places worldwide. In some regions, their activity was observed to accompany slow slip events. Before examining whether and how nonvolcanic tremor activity is related to slow slip, it is essential to understand quantitatively the spatiotemporal migration patterns of nonvolcanic tremors. We developed a 2‐D hidden Markov model to automatically analyze and forecast the spatiotemporal behavior of tremor activity in the regions Kii and Shikoku, southwest Japan. This new automated procedure classifies the tremor source regions into distinct segments in 2‐D space and infers a clear hierarchical structure of tremor activity, where each region consists of several subsystems and each subsystem contains several segments. The segments can be quantitatively categorized into three different types according to their occurrence patterns: episodic, weak concentration, and background, extending earlier knowledge gained from handpicked tremor swarms. The Kii region can be categorized into four different subsystems, with two often linked to each other. The Shikoku region can be divided into six subsystems, with two in central Shikoku linked to each other. Moreover, a significant increase in the proportion of tremor occurrence was detected in a segment in southwest Shikoku before the 2003 and 2010 long‐term slow slip events in the Bungo channel. This highlights the possible correlation between nonvolcanic tremor and slow slip events. The model can be used to analyze tremor data from other regions.Peer Reviewe

Cascading elastic perturbation in Japan due to the 2012 Mw 8.6 Indian Ocean earthquake

Since the discovery of extensive earthquake triggering occurring in response to the 1992 Mw (moment magnitude) 7.3 Landers earthquake, it is now well established that seismic waves from earthquakes can trigger other earthquakes, tremor, slow slip, and pore pressure changes. Our contention is that earthquake triggering is one manifestation of a more widespread elastic disturbance that reveals information about Earth’s stress state. Earth’s stress state is central to our understanding of both natural and anthropogenic-induced crustal processes. We show that seismic waves from distant earthquakes may perturb stresses and frictional properties on faults and elastic moduli of the crust in cascading fashion. Transient dynamic stresses place crustal material into a metastable state during which the material recovers through a process termed slow dynamics. This observation of widespread, dynamically induced elastic perturbation, including systematic migration of offshore seismicity, strain transients, and velocity transients, presents a new characterization of Earth’s elastic system that will advance our understanding of plate tectonics, seismicity, and seismic hazards.UTokyo Research掲載「スマトラ地震により東北日本の地殻の状態が次々と変化」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/sumatra-earthquake-caused-changes-in-Earths-crust-in-japan.htmlUTokyo Research "Sumatra earthquake caused changes in Earth’s crust in Japan" URI: http://www.u-tokyo.ac.jp/en/utokyo-research/research-news/sumatra-earthquake-caused-changes-in-Earths-crust-in-japan.htm