Rate and extent of shallow interseismic slip and a recent Mw 6.5 earthquake on the creeping segment of the Philippine Fault

In the Philippine archipelago, the oblique convergence between the Eurasian and Philippine Sea plates is mostly partitioned between subduction zones and the 1,200-km long, left-lateral, strike-slip Philippine Fault. Campaign GPS data suggest that a portion of the Philippine Fault slips aseismically along its central segment in Leyte island. However, the lack of a dense geodetic network prevents careful analysis of the extent of shallow creep. The 2017 M_w 6.5 earthquake ruptured the same segment at 8 km-depth, with discontinuous surface ruptures. While other parts of the Philippine Fault have a history of major earthquakes (M>7), this is the largest instrumented event along the creeping segment, previously considered as aseismic. This provides an opportunity to examine the relationship between seismic and aseismic slip across different stages of the seismic cycle. We use InSAR data covering most of Leyte island to measure ground displacements and estimate the spatial extent and rate of shallow aseismic slip from 2007-2011 with ALOS-1 PALSAR. We measure coseismic deformation using Sentinel-1 InSAR data and estimate the extent of seismic slip. NSBAS time-series analysis and stacking of differential interferograms reveal the signature of aseismic slip in the satellite line-of sight velocities along a 100-km section of the fault extending from northern to central Leyte. At the surface, we estimate around 2-3 cm/yr slip rate, similar to estimates from GPS campaigns in 1991-1995. 6-day interferograms resolve coseismic deformation related to the 2017 mainshock with coseismic slip occurring where the shallow creep rate appears to be lower, separately from a M_w 5.8 aftershock that happened four days after. The largest coseismic displacements from InSAR also correlate well with reported surface displacements in the field. We model the along-strike and depth extent of aseismic slip using a Bayesian framework to discuss the complementarity of seismic and aseismic slip along the fault plane. We also observe subsidence during the interseismic period due to geothermal energy production adjacent to the fault in the vicinity of the coseismic rupture. These preliminary results, from geodetic investigation, provide an important baseline for further exploration of fault zone rheology and the physics of fault slip.