Detection of earthquake swarms in world's subduction zones and its insight into SSE activity

Abstract:

Earthquake swarms in subduction zones are likely to be related with slow slip events (SSEs) and locking on the plate interface. In the Boso-Oki region in central Japan, swarms repeatedly occur accompanying SSEs (e.g, Hirose et al., 2012). It is pointed out that ruptures of great earthquakes tend to terminate in regions with recurring swarm activity because of reduced shear stress due to heterogeneous locking (Holtkamp and Brudzinsiki, 2014). Given these observations, we may be able to infer SSEs and spatial variations in locking on the plate interface by investigating swarm activity in subduction zones.

It is known that swarms do not follow Omori’s law and have much higher seismicity rates than predicted by the ETAS model (e.g., Llenos et al., 2009). Here, we devised a statistical method to detect unexpectedly frequent earthquakes using the space-time ETAS model (Zhuang et al., 2002). We applied this method to world’s subduction zones and detected swarms in JMA catalog (M ≥ 3) from 2001 to 2010 and ANSS catalog (M ≥ 4.5) from 1995 to 2009.

We detected recurring swarm activities as expected in the Boso-Oki region and also in many other regions worldwide. In particular, we found recurring swarm activities and intensive foreshock activity, which is defined as events followed by large events (M ≥ 6.5) within 5 days and 50km, in the Ibaraki-Oki region (the south of Tohoku), New Hebrides, and Kurile. Given that both foreshocks and swarms are triggered by SSEs (e.g., Kato et. al, 2012; Bouchon et al., 2013), these results suggest that the regions with intensive foreshock activity and recurring swarm activity such as the Ibaraki-Oki region are characterized by extensive occurrences of SSEs just like the Boso-Oki region. On the other hand, swarms are rare in Nankai, where great earthquakes have repeatedly occurred. These variations in swarm activity may reflect variations in SSE activity among subduction zones.

Figures show hypocenters of swarms (left) and ratio of the number of swarm events to all events near each point (< 30km) (right) in the Ibaraki-Boso-oki region. In the right figure, red and black colors mean regions with intensive swarm activity.