Detection of “Rapid” Aseismic Slip at the Izu‐Bonin Trench

Abstract

No great earthquakes have been historically documented at the Izu‐Bonin Trench, where subduction is believed to occur largely by aseismic slip, although the details are poorly understood. We deployed an array of ocean bottom pressure gauges here for a year from May 2015. The array recorded the coseismic seafloor uplift/subsidence and tsunamis generated by the nearby Mw6.0 thrust earthquake. In association with this event, we detected two much larger aseismic slip events with rise times around 1 h. The total moment of these two aseismic events was 17 times larger than that of the mainshock. Such aseismic, yet still rapid, slip can be interpreted as one amid the transitional regime. This regime is expected to host slow slip events near its boundary with the stable sliding regime and, possibly, tsunami earthquakes and very low frequency earthquakes near the boundary with the unstable seismic slip regime. Slip in the transitional regime may be a prevalent mode of subduction in the Izu‐Bonin trench, where effective normal force on the frictional plate interface is tectonically reduced.

Earthquake Rupture and Tsunami Generation of the 2015 Mw 5.9 Bonin Event Revealed by In Situ Pressure Gauge Array Observations and Integrated Seismic and Tsunami Wave Simulation

“…The APGs recorded large pressure fluctuations of up to ∼0.05 MPa. Small pressure fluctuations, observed ∼1 min before the focal time, are due to the foreshock (the epicenter determined by Fukao et al, 2021 is shown by small white star in Figure 1a). We also show the waveforms from the broadband ocean-bottom seismometer (BBOBS) and the differential pressure gauge (DPG) at B05 in Figure S1a.…”

“…We estimated the coseismic slip distribution of the mainshock by inverting the APG data (Fukao et al, 2021; see Text S2 in Supporting Information S1 for details). To calculate the pressure changes from each subfault (Green's function), we assume a planar fault with a dimension of 40 km length × 28 km width and divide it into 4 × 4 km subfaults.…”

“…We installed an array of stations B01 to B10, inward of the northern Bonin Trench (Figure 1a and Table S1; Fukao et al, 2021). We use the APGs at B01-B03 and B06-B10 for which data were available.…”

“…They used Generic Mapping Tools version 6.2.0 (Wessel et al, 2019) to generate the figures. The station locations of the APGs are listed in Table S1 and are presented in Fukao et al (2021). The APG data are available on the Zenodo repository (https://doi.org/10.5281/zenodo.5139450), which is the supplementary data set of Fukao et al (2021).…”

“…The Global CMT (GCMT) and U.S. Geological Survey (USGS) solutions and the USGS epicenter are shown in gray. The small white star denotes the foreshock epicenter (Fukao et al., 2021). The black arrow denotes the plate convergence direction (DeMets et al., 2010).…”