An estimate of tidal and non-tidal modulations of plate subduction speed in the transition zone in the Tokai district

Abstract

Non-volcanic tremors and slow slip events in subduction zones have been found to be triggered by small external stress disturbances, as demonstrated by the synchronization of temporal variations in tremor rate with diurnal and semi-diurnal tides. Therefore, long-term variations in tremor rate might be predicted by amplitude modulations of diurnal and semi-diurnal tides at decadal time scales. Given that tremors and slow slips are shear slip on the plate boundary, their long-term variations must be associated with fluctuations in plate subduction speed below the seismogenic zone. In previous work, we showed a good correlation between long-term seismicity and empirically calculated tremor rate based on observed tidal levels in the Nankai region, western Japan. Here, we present an improved method of modeling long-term slip rate fluctuation based on the calculation of Coulomb stress due to ocean and solid earth tides on the plate interface. We also include the effects of non-tidal ocean variations, such as the Pacific Decadal Oscillation and the Kuroshio Current, employing an ocean model developed by the Japan Meteorological Agency. We apply the method to the Tokai district, where the effects of the Kuroshio Current are large, and demonstrate the importance of considering non-tidal effects. Our calculated slip rate fluctuations could amount to 1 mm/year in decadal scales, and periods with faster rates partly corresponded to variations in seismicity. Slow slip events in the study region weakly corresponded to times of higher stress.

Two long-term slow slip events around Tokyo Bay found by GNSS observation during 1996–2011

“…Finally, we compared the obtained slow slip history with external stresses acting on Faults A and B, based on the method presented by Tanaka et al (2015), to investigate whether external stresses could trigger or drive SSEs. Tanaka et al (2015) assumed an exponential relationship between the Coulomb stress change CFS and the fault slip velocity V : V = V 0 exp CFS A , where V 0 and A denote a steady-state velocity under no applied external stress and a constant prescribing slip response to stress, respectively.…”

“…In this simple model, interactions between the locked zone and the slow slip area are neglected. We computed CFS for Faults A and B due to body and ocean tides and non-tidal variations in the ocean bottom pressure, as described by Tanaka et al (2015). These stress changes are shown in Additional file 1: Fig.…”

“…Tanaka et al (2015) assumed an exponential relationship between the Coulomb stress change CFS and the fault slip velocity V : V = V 0 exp CFS A , where V 0 and A denote a steady-state velocity under no applied external stress and a constant prescribing slip response to stress, respectively. In this simple model, interactions between the locked zone and the slow slip area are neglected.…”

Possible correlation between annual gravity change and shallow background seismicity rate at subduction zone by surface load

“…Recently, on the basis of seismicity analysis (of tremors) about tidal responses, Ide and Tanaka (2014) and Tanaka et al (2015a) suggested exponential dependence of stress changes on fault slip rates. This corresponds to the rate and state friction framework (Dieterich 1994) based on laboratory rock experiment.…”

“…Annual changes of atmospheric pressure can be on the order of 1 kPa (Ohtake and Nakahara 1999), comparable to that by the water mass movements in this study. Annual changes of ocean bottom pressure can be also as large as 1 kPa (Tanaka et al 2015a). These load effects may also modulate seismicities.…”