Slow Slip Events in Subduction Zones

Recent GPS observations show that slow slip events in south central Alaska are segmented along-strike. In this project, we review several mechanisms that might contribute to this kind of segmentation, suggesting that the subducted Yakutat Plateau might be main reason. We then use numerical simulations in the framework of rate-and-state friction with a non-planar fault geometry, aiming to understand which parameters are required to vary along-strike to recreate the segmentation. The simulations reproduced key observations such as the interval, duration, magnitude, propagation speed, and moment-area scaling of slow slip events in this region. In most cases, these two segments rupture independently but occasionally rupture together. Results show that the segmentation is most likely related to the along-strike variation of effective normal stress on the fault plane. Receiver function modeling based on seismic data show little along-strike variation of Vp/Vs ratio on the fault zone, suggesting little along-strike change of pore-pressure. Then the difference in effective normal stress would come from difference of normal stress from overburden structure and the extra buoyancy from the subducted Yakutat Plateau, with a larger contribution from the later. Our work implies that a structural anomaly will have a long-lived effect on the slip behavior of a subduction zone and might be a driving factor on slow slip event segmentation.

Figure 1. Map of south central Alaska, including the epicenter (white star) and the rupture area (white solid line region) of the 1964 Mw 9.2 earthquake (Plafker, 1965; Furumoto, 1965), two SSE patches (black ellipses) (Wei et al., 2012; Fu et al., 2015; Li et al. 2016), estimated boundaries of the subducted Yakutat plateau (black dashed line) (Eberhart-Phillips et al., 2006), and deep tremor (white ellipses) (Wech, 2016). The blue dashed lines indicate the contours of the plate interface depth from Li et al. (2013). The white arrows show the plate motion for Pacific and Yakatut slabs are also shown (Li et al., 2013). The black star shows the epicenter of the 2002 Mw 7.8 Denali earthquake, the white star shows the epicenter of the 1964 Mw 9.2 Alaska earthquake, and the white square is the city of Anchorage (CoA).