Detection of “rapid” aseismic slip that bridges a gap between seismic slip and slow slip

Abstract:

Unstable seismic slip and quasi-stable slow slip are two contrasted phenomena of frictional faulting. Here we report our finding of aseismic, yet still “rapid”, slip that may bridge a gap between these two contrasted phenomena. We deployed an array of absolute pressure gauges on the seafloor at depths of about 5000 m to the west of the Bonin Trench, where no great earthquakes have been historically documented.

Removal of background tide from the records revealed two events of aseismic seafloor uplifting, one immediately after and the other 3.5 days after the nearby M6.0 thrust earthquake of 2015 (see Kubota et al. 2020 for this earthquake, submitted to S018 Session). The records were quasi-statically inverted for the model of slip distributions along the plate boundary as a function of time. The agreement between the observed pressure changes and those calculated for the aseismic slip model was in general remarkable. The slip history of each fault segment followed a typical slip-growth curve (Brune, 1970) with a rise time on the order of 1 h.

Fig.1 shows the estimated source parameters (red dots) obtained for each fault segment of the two aseismic events compared with those for ordinary earthquakes and those for slow slips (Gao et al., 2012; Gomberg, 2016). Fig.1A is a plot of the source duration vs seismic moment. Fig.2B is a plot of the rupture velocity vs stress drop. In either Fig.1A or B, our observations fell neatly into the observational gap between slow slips and earthquakes.

In the framework of rate- and state-dependent friction law, there is a transitional regime between the stable sliding regime and the unstable seismic slip regime. Our finding of an aseismic, yet still “rapid”, slip has filled the observational gap between the near-boundary phenomena of the transitional regime (slow slips in one end and very-low-frequency earthquakes and tsunami earthquakes in the other end) and has thus provided a missing link in understanding the whole spectrum of slip modes. The transitional slip may be a prevalent mode of subduction at the Bonin Trench and other Mariana-type trenches, where the effective normal stress at the frictional interface is tectonically reduced (Scholz and Campos, 1995).