Fluid migration before and during slow earthquakes in the shallow Nankai subduction zones

Abstract:

Fluid migration in subduction zones is a key controlling factor of slow and megathrust earthquakes at plate boundaries. If such migration occurs, seismic velocity and heterogeneous structures in its pathways are temporarily varied, preferably triggering slow earthquakes. However, there have been few observations on fluid migration in shallow subduction zones, and the temporal relationship between fluid migration and shallow slow earthquakes remain elusive. In this study, we applied ambient noise correlation to very long-term (5-9 y) records observed by a permanent cabled-network of 49 ocean bottom seismometers deployed in the Nankai subduction zone, and explored temporal variations of structure in terms of seismic velocity (dv/v) and heterogeneity. Here, we evaluated seismic heterogeneity changes by estimating the cross correlation coefficient (CC) between a reference cross-correlation function (CCF) and individual CCFs. We retrieved ocean acoustic coupled Rayleigh (ACR) waves at 0.5–2.0 Hz, which are sensitive to the entire accretionary prism.

The resulting dv/v shows an increasing trend in the eastern Nankai, which responded to fluid drainage to sea water, and a sudden decrease at the timing of local earthquake, indicating that dv/v does not respond other phenomena, including slow earthquakes. On the other hand, in the eastern Nankai, we found multiple CC drops 2-9 months before slow slip events (SSEs) that were detected by pore pressure variation at borehole site (Araki et al. 2017) (Fig. 1). In the western Nankai, a CC drop corresponded to the timing of shallow very low frequency earthquakes. These temporal changes are presumably attributed to dynamic fluid migration. Our results possibly indicate the pore pressure difference at the source regions between eastern and western Nankai. When fluids are supplied to source regions, slow earthquakes do not occur immediately but with time delay of at most 9 months with low pore pressure in eastern Nankai, and they occur immediately with high pore pressure in western Nankai. This study suggests that seismic heterogeneous structure is possibly changed by fluid migration before slow earthquakes in the Nankai subduction zone.