The Mentawai Forearc Sliver (MFS) is characterized by oblique deformation formed as slip partitioned between normal and parallel trench plate convergence. The surge of great earthquakes from 2004 to 2012 along the adjacent Sunda trench left a large unbroken segment known as the Mentawai Seismic Gap. Here, we adopted continuous Global Navigation Satellite System (GNSS) observation data to identify the present regional crustal deformation using geodetic strain rates. The principal strain rate, dilatation rate, and maximum shear strain rate are about 0.13 microstrain/yr, 0.2 microstrain/yr, and 0.1 microstrain/yr, respectively, with the range of its uncertainties between 0.01 and 0.04 microstrain/yr. The dilatation and maximum shear strain rates reveal the spatial coverage of strike-slip duplex and backthrust tectonics along the Mentawai Forearc Sliver.

The Palu M_W7.4 earthquake occurred on September 28, 2018, with the epicenter at 119.86°, 0.72°. The severe shaking caused severe damage in Central Sulawesi, especially in Palu. We conducted a postseismic deformation study to determine the deformation pattern and reduce future earthquakes' impact. Interferometric Synthetic Aperture Radar (InSAR) data were processed using LiCSBAS to get the time series. The time series data were fitted to exponential and logarithmic functions to determine the mechanism of postseismic deformation. The exponential model identified the influence of the viscoelastic mechanism, and the logarithm identified the afterslip mechanism. The Palu earthquake was fitted to logarithmic and exponential, but the logarithmic was more significant than an exponential function. Afterslip mechanism predominates, and viscoelastic mechanisms play a minor role in this postseismic deformation.

The eastern part of Java Island is transversed by major faults such as Cepu, Blumbang, Surabaya, and Waru Segment, part of the Kendeng Fault, Wonsorejo Fault, Pasuruan Fault, and Probolinggo Fault. Due to the major fault, we used decomposition of identified fault from the Global Navigation Satellite System (GNSS) observation data to identify the potential of local deformation. We analyzed surface deformation due to the effect of major fault using scaling law and elastic half-space method. We investigated the possibility of unidentified fault using strain rates based on velocity vector data before and after correcting the effect of a major fault. We found that strain calculation for principal strain value in the eastern part of Java Island is less than one microstrain/year and the dominant one with a compression pattern due to the Sunda subduction zone. The maximum shear strain rate value goes from 0.002 to 0.094 microstrain/year, and the dilatation rate value ranges from −0.141 to 0.038 microstrain/year, which correlates with the reverse of the Kendeng Fault. A higher compression pattern outside the major fault in a differential maximum shear strain rate might indicate a local fault.