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

• Published in: Journal of geophysical research. Solid earth 2020-11, Vol.125 (11), p.n/a
• Main Authors: Xie, Surui, Dixon, Timothy H., Malservisi, Rocco, Jiang, Yan, Protti, Marino, Muller, Cyril
• Format: Article
• Language: English
• Subjects: Accumulation; Costa Rica; Deformation; Earth crust; Earthquake damage; Earthquakes; Geophysics; Global Positioning System; Global positioning systems; GPS; Locking; megathrust earthquake; Offshore; Oscillations; Plate convergence; Plate motion; Plate tectonics; Rupture; Rupturing; Seismic activity; Seismic hazard; Slip; slow slip event; Spatial data; Spectrum analysis; Step functions; Strain; subduction zone; Time series
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2020JB020503

We 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. Plain Language Summary Slow slip events (SSEs) release strain energy in Earth's crust that accumulates due to plate motion and frictional locking on the boundaries between plates. SSEs are similar to earthquakes but are less damaging, since strain is released slowly, over weeks or months. We used GPS measurements with millimeter precision to study SSEs in Northwestern Costa Rica between 2002 and 2020 and compared them to the pattern of strain accumulation over the same period. We find that these events happened about every 22 months, and the repeat time is unchanged by the 5 September 2012 Mw 7.6 Costa Rica earthquake. Locking patterns also remain similar before and after this earthquake. However, some locked zones did not rupture in the 2012 earthquake or earthquakes in the last few decades and do not participate in S