14:15 〜 14:30
[SCG50-15] Prevailing updip ruptures in small interplate earthquakes along the Japan Trench revealed from land-based and offshore observations
キーワード：破壊伝播指向性、プレート境界地震、S-net
A physical understanding of earthquakes involves not only the static characteristics of the ruptures but also the dynamics. Rupture directivity is a fundamental characteristic of earthquake growth and is essential for improving our current physical understanding of earthquakes. The recent development of seafloor seismic observations facilitates reliable estimation of the rupture directivities of offshore earthquakes. The present study used seismic waveforms obtained by a new seafloor seismic network (S-net) and onland stations to systematically examine the rupture directivities of interplate earthquakes along the Japan trench.
We first selected interplate earthquakes and estimated the apparent moment rate functions (AMRFs) by using the empirical Green’s functions. We then estimated the rupture directions by applying the simple unilateral rupture model of Haskell (1964) to the duration of the AMRFs. As a result, the rupture directions were derived for 206 Mw 3.5–5 events, most of which occurred near the base of the seismogenic zone.
We found that most earthquake ruptures (>80 %) were directional. They primarily propagated in the updip direction, which is the opposite of the prediction from the bimaterial effect. On the contrary, deep, steady creep and upward fluid migration along the plate interface can facilitate rupture initiation near the deeper edge of the seismic patch and propagation in the updip direction, given that the nucleation size is sufficiently smaller than the final rupture area. The predominance of updip rupture in interplate earthquakes suggests that other effects, such as deep creep and upward fluid migration, can overtake the bimaterial effect in affecting earthquake ruptures in the subduction zone.
The updip ruptures redistributed the accumulated shear stress from the base of the seismogenic zone to the shallow large seismic patches. The updip ruptures may also open up ways for deeper fluids to migrate further upward along the plate interface. Both effects facilitate the occurrence of shallow megathrust earthquakes.
We first selected interplate earthquakes and estimated the apparent moment rate functions (AMRFs) by using the empirical Green’s functions. We then estimated the rupture directions by applying the simple unilateral rupture model of Haskell (1964) to the duration of the AMRFs. As a result, the rupture directions were derived for 206 Mw 3.5–5 events, most of which occurred near the base of the seismogenic zone.
We found that most earthquake ruptures (>80 %) were directional. They primarily propagated in the updip direction, which is the opposite of the prediction from the bimaterial effect. On the contrary, deep, steady creep and upward fluid migration along the plate interface can facilitate rupture initiation near the deeper edge of the seismic patch and propagation in the updip direction, given that the nucleation size is sufficiently smaller than the final rupture area. The predominance of updip rupture in interplate earthquakes suggests that other effects, such as deep creep and upward fluid migration, can overtake the bimaterial effect in affecting earthquake ruptures in the subduction zone.
The updip ruptures redistributed the accumulated shear stress from the base of the seismogenic zone to the shallow large seismic patches. The updip ruptures may also open up ways for deeper fluids to migrate further upward along the plate interface. Both effects facilitate the occurrence of shallow megathrust earthquakes.