Tidal triggering of shallow very-low-frequency earthquakes in the Ryukyu Trenc

Abstract:

We analyzed tidal response of shallow very-low-frequency earthquakes (VLFEs), which occur in Ryukyu Trench (RT), and showed that the activity of VLFEs responds to ocean tide loading.

We used the epicenter catalog of the VLFEs from 2002–2014, the epicenter determination procedure of which is the same as that described by Nakamura & Sunagawa [2015]. The minimum magnitude of the used events was 3.4, which was determined from the magnitude–frequency relationship.

We computed the tidal sensitivity and phase lag between the occurrence of VLFEs and the ocean tide for each point along the RT. We then predicted the fault normal stress (FNS) and updip shear stress (UDSS) on the plate interface and compared them with the excess value (N_ex) of the VLFEs. The stress components at a depth of 15 km were predicted using Earth tide strain [Matsumoto et al., 2001] and ocean tide [Matsumoto et al., 2000].

The distribution of tidal sensitivity shows that the sensitivity and phase change gradually from northeast to southwest along the RT. High sensitivity (0.20–0.42) is evident at central RT, but the sensitivity is < 0.2 at southwestern RT. The N_ex in the FNS ranges 0.22–0.28 at central RT and 0.04–0.11 at southwestern RT. The N_ex in the UDSS ranges 0.17–0.22 at central RT and 0.04–0.11 at southwestern RT.

The comparison of N_ex with stress components shows that the VLFEs are activated by the thrust-encouraging shear stress or maximum extensional normal stress of the ocean tide corresponding to the low-tide period. The tidal sensitivity is highest in the central portion of the RT, where the thin sediment layer is subducting, whereas it is lowest in the southwestern portion of the RT, where a thick sediment layer exists and an accretionary prism is developed. Differences in the sediment–rock properties of the overriding plate, which cause lateral heterogeneity in the extremely high pore-fluid pressure in the plate interface, would control the sensitivity of the VLFEs to ocean tides.