Analysis on Crustal Deformation of Slow Slip Events Occurred in the Southwestern Ryukyu Arc in 2010-2014
Abstract:
We analyze GNSS data in the southwestern Ryukyu Arc, and estimate fault slip of slow slip events (SSEs). The analyzed area (shown in lower right of Fig. 1 (a)) is characterized by back-ark spreading and fast relative plate motion (12 cm/year). In this region, SSEs are reported to occur every 6 month by Heki and Kataoka (2008) on the subducted plate interface beneath Iriomotejima Island. We set up 4 new GNSS stations in Iriomotejima Island and neighbor islands, and analyze the data of thirteen GNSS stations in total. They include eight GNSS stations operated by the Geospatial Information Authority of Japan, and one station by the Japan Coast Guard. First, we estimate displacements of each SSE by fitting formula (1) to daily GNSS coordinates. Observation period is from April 2010 to January 2015.X(t) = at + b + Σ8j=1AjH (t - Tj) [1 – exp{- (t - Tj) /τj}] + Σ1k=1BkH (t - Tk) (1)
H: Heaviside function
, where a is velocity of a linear trend, b is offset, third term expresses transient deformation of SSEs, and fourth one expresses instantaneous deformation of earthquake. A and B are amplitude of an event, Tj and Tkare occurrence time, and τj is a time constant for duration of an event. Then, we obtain displacement of eight SSEs and one earthquake occurred in April 17, 2013 at all stations, and estimate fault slip using the displacement data. We divide a slip area lying on the plate interface into rectangular fault patches and estimate each dislocation on each patch in an elastic half-space (Okada, 1992). We adopt constraints on smoothness of slip distribution and non-negative slip. We fix a slip azimuth to 141°from north , that is parallel to an estimated relative plate motion, and we estimate a translation component common for all stations together with fault slip. The geometry of the plate interface is based on the slab 1.0 model (Hayes et al., 2012).
Figure 1 shows displacement and slip distribution of SSE occurred on May 10, 2011. Fig. 1(a) shows observed (black) and calculated (white) horizontal displacement with a reference station on Irabujima Island marked as square. Fig. 1(b) shows slip amount on the fault patches. We calculate moment magnitude from slip amount on all patches, and find that for each event ranges from 6.7 to 6.9 in the observation period.