Early postseismic deformation of the 2018 Lombok, Indonesia, earthquake sequence constrained by GPS data

Global Positioning System (GPS) stations in Lombok, Indonesia showed postseismic displacement following the 2018 Lombok earthquake sequence which occurred on 28 July 2018 (Mw 6.4), 5 August 2018 (Mw 6.9), 9 August 2018 (Mw 5.9), and the 19 August 2018 (Mw 6.3 and Mw 6.9). This research has investigated the contribution of afterslip and viscoelastic relaxation during the one-month period after the mainshock on 19 August 2018. This study utilizes continuous GPS data available from 20 August 2018–19 September 2018. The analysis reveals that the afterslip was distributed at the shallower portion of the fault, complementary to the deeper coseismic slip of the mainshock. Viscoelastic relaxation, involving a gravitational Maxwell viscoelastic response with viscosity of 9.0 × 10¹⁷ Pa s, was estimated. This research also found that the depth of elastic upper crust in this region is 50 km. Observations made during the one-month period after the mainshock revealed that afterslip was responsible for most of the ongoing postseismic deformation. Although the viscoelastic relaxation contributed only insignificantly during the one-month period, this research shows that viscoelastic relaxation is expected to increase with time.