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[SCG45-43] A year continued sluggish slip at the Izu-Bonin Trench triggered by a great deep shock
Keywords:Izu-Bonin Trench, fault friction, Ocean bottom pressure gauge, great earthquake
(1)Izu-Bonin is a typical subduction zone (Fig.1), yet little is known about what mode of slip is dominated in this region. We analyzed the one-year records (2015-2016) of the array of ocean bottom pressure gauges deployed to the west of the Bonin Trench, where we detected a year-continued slow slip, This aseismic event produced at the end of the 1 year observation an uplift-subsidence difference of 25 cm on the 5000 m-deep seafloor across a distance of 10km (Figs.2).
(2)As will be shown later, the magnitude of this event was estimated as 7.3. Unlike ordinary earthquakes with comparable magnitude, however, the source time function is logarithmic with a long time constant of several months. Our detection of slip of this magnitude implies that the fault friction system at the Bonin trench can still accumulate strain energy to generate Mw7.3-class slip events but cannot release it through processes in the velocity-weakening regime but only through the velocity-strengthening regime.
(3)Logarithmic growth of slow slip is just the first order feature of the event. This complex slip contains many local events with different spacio-temporal scales, including the Mw6.0 seismic rupture (E4) and the rapid aseismic slip events (E4 and A1, A2) with durations of approximately an hour (Fig.3). Figs.2 and 3 show the one-year trend of seafloor movement consistent with the short transient movements (E4, A1 and A2). This consistency implies their common tectonic origin. We call this type of complex event “sluggish” slip which is an integral of various slip modes ranging from the velocity-weakening to velocity-strengthening regimes. The name of sluggish implicitly presume that the fundamental mode is velocity-strengthening.
(4)We inverted the data to a preliminary source model assuming an infinite rupture velocity (Fig4). which shows an asperity surrounded by aseismic slip. Interaction of the asperity with the surrounding aseismic slip is observable at the preseismic stage (Fig.3). The moment magnitude of this sluggish slip was Mw=7.3 (Fig.4). To our knowledge, this is the largest thrust event ever documented in the Bonin Trench. The slip model we obtained can explain the observations of both differential pressures between station pairs and the tide-corrected pressures at single stations (Fig.5).Sluggish slip may be a dominant mode of subduction along the Izu-Bonin-Mariana .
(2)As will be shown later, the magnitude of this event was estimated as 7.3. Unlike ordinary earthquakes with comparable magnitude, however, the source time function is logarithmic with a long time constant of several months. Our detection of slip of this magnitude implies that the fault friction system at the Bonin trench can still accumulate strain energy to generate Mw7.3-class slip events but cannot release it through processes in the velocity-weakening regime but only through the velocity-strengthening regime.
(3)Logarithmic growth of slow slip is just the first order feature of the event. This complex slip contains many local events with different spacio-temporal scales, including the Mw6.0 seismic rupture (E4) and the rapid aseismic slip events (E4 and A1, A2) with durations of approximately an hour (Fig.3). Figs.2 and 3 show the one-year trend of seafloor movement consistent with the short transient movements (E4, A1 and A2). This consistency implies their common tectonic origin. We call this type of complex event “sluggish” slip which is an integral of various slip modes ranging from the velocity-weakening to velocity-strengthening regimes. The name of sluggish implicitly presume that the fundamental mode is velocity-strengthening.
(4)We inverted the data to a preliminary source model assuming an infinite rupture velocity (Fig4). which shows an asperity surrounded by aseismic slip. Interaction of the asperity with the surrounding aseismic slip is observable at the preseismic stage (Fig.3). The moment magnitude of this sluggish slip was Mw=7.3 (Fig.4). To our knowledge, this is the largest thrust event ever documented in the Bonin Trench. The slip model we obtained can explain the observations of both differential pressures between station pairs and the tide-corrected pressures at single stations (Fig.5).Sluggish slip may be a dominant mode of subduction along the Izu-Bonin-Mariana .