Slow Slip and Inter‐transient Locking on the Nicoya Megathrust in the Late and Early Stages of an Earthquake Cycle

Abstract

AbstractWe analyzed continuous GPS data collected from 2002–2020 to characterize slow slip events (SSEs) in and near the Nicoya Peninsula, Costa Rica. These data are bisected by the 5 September 2012 Mw 7.6 earthquake. The displacement time series contain multiple signals, including plate convergence, plate interface locking, coseismic and postseismic deformation, seasonal oscillations, SSEs, and noise. GPS‐measured coseismic and postseismic displacements associated with the Mw 7.6 earthquake are modeled and removed by a step function plus multiple timescale relaxation processes with four characteristic times: 11, 94, 470, and 1,865 days. Seasonal oscillations are eliminated using a multichannel singular spectrum analysis (M‐SSA). Ten major SSEs (Mw > 6.6) are observed in the remaining time series, with a constant recurrence interval of 21.7 ± 2.6 months. SSEs occur in both shallow (~10 km) and deep (~35 km) portions of the plate interface, but the latter last longer and have larger magnitudes. There is minimum to no slow slip in the Mw 7.6 seismic rupture area and a persistent slow slip patch beneath the Nicoya Gulf entrance. Despite strong earthquake‐related stress perturbations, the inter‐SSE locking status on the megathrust is very similar between the late and early stages of the earthquake cycle and includes locked patches that ruptured in the 2012 earthquake or continue to rupture via SSEs. Some locked patches offshore south of the Nicoya Peninsula did not rupture in 2012, do not participate in SSEs, and may be indicative of supercycle behavior, that is, strain accumulation over several seismic cycles. These areas warrant heightened monitoring.