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[SCG44-08] The anomalous structure along the slow earthquake fault zone, inferred from short signal duration tremors
Keywords:tremor, Nankai trough, slow earthquake
It has been known that two different types of earthquakes occur in a shallow part of plate subduction zones; one is an ordinary earthquake, and the other is a tremor. Compared to ordinary earthquakes of a similar magnitude, tremors have been characterized by a long signal duration, a depletion of signals above 10 Hz, and unclear signal onset.
Here, we report on anomalous tremors found in records of Dense Oceanfloor Network system for Earthquakes and Tsunamis1 (DONET1), deployed in eastern Nankai Trough. These tremors exhibit an anomalously short duration signal only when observed closely above the source, while they exhibit the typical long-duration signal of tremors when recorded slightly ( ~10km) away from the source. The observations infer that any tremor sources could be located within a thin layer with an anomalous structure that strongly scatters the seismic wave to generate the long signal duration of tremors. In order to examine this hypothesis, we performed simple numerical simulations on elastic wave propagations. The results qualitatively show that a localized structure consisting of small-sized inclusions placed around the seismic source can partly be responsible for typical tremors' long signal duration.
It has been documented that slow earthquakes of different sizes occur along the same fault zone. Therefore, the anomalous structure around tremor sources should be the very structure of the slow earthquake fault zone and can be the controlling factor of slow earthquakes. Also, we note that our proposed structure is consistent with the image of the slow earthquake fault zone obtained based on the Nankai Trough drilling project, in which multiple high pore pressure zones are distributed in a patchy manner.
Here, we report on anomalous tremors found in records of Dense Oceanfloor Network system for Earthquakes and Tsunamis1 (DONET1), deployed in eastern Nankai Trough. These tremors exhibit an anomalously short duration signal only when observed closely above the source, while they exhibit the typical long-duration signal of tremors when recorded slightly ( ~10km) away from the source. The observations infer that any tremor sources could be located within a thin layer with an anomalous structure that strongly scatters the seismic wave to generate the long signal duration of tremors. In order to examine this hypothesis, we performed simple numerical simulations on elastic wave propagations. The results qualitatively show that a localized structure consisting of small-sized inclusions placed around the seismic source can partly be responsible for typical tremors' long signal duration.
It has been documented that slow earthquakes of different sizes occur along the same fault zone. Therefore, the anomalous structure around tremor sources should be the very structure of the slow earthquake fault zone and can be the controlling factor of slow earthquakes. Also, we note that our proposed structure is consistent with the image of the slow earthquake fault zone obtained based on the Nankai Trough drilling project, in which multiple high pore pressure zones are distributed in a patchy manner.