Detection of "Rapid" Aseismic Slip at the Izu‐Bonin Trench. Issue 9 (28th August 2021) - Fukao, Yoshio
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. Plain Language Summary: Movement of tectonic plates occurs mainly by relative slip between two plates in frictional contact. Slip can be fast enough to radiate seismic waves as earthquake. Slip can also be slow enough to allow stable sliding at a speed expected from the plate tectonic theory. Relatively recent discovery of slow slip events with source durations ranging from days to years motivated seismologists to think about slip mode in the transitional regime between the unstable seismic regime and the stable sliding regime. Here, we report detection of slip events amid the transitional regime by using an ocean bottom array of absolute pressure gauges. The detected events were four orders of magnitude faster than previously reported slow slip events but four orders of magnitude slower than ordinary earthquakes. Our finding has thus provided a missing link in understanding the whole spectrum of slip modes. Key Points: We deployed an array of seafloor pressure gauges in the Izu‐Bonin Trench, where subduction is believed to occur largely by aseismic slip We detected two aseismic slip events with rise times of ∼1 h with the resultant moment 17 times larger than the preceding M6.0 earthquake These events are interpreted as those amid the transitional regime between the unstable seismic‐slip and stable plate‐sliding regimes