Resolving the Kinematics and Moment Release of Early Afterslip Within the First Hours Following the 2016 M_w 7.1 Kumamoto Earthquake: Implications for the Shallow Slip Deficit and Frictional Behavior of Aseismic Creep

Abstract

As stresses following rupture are dissipated continuous measurements of postseismic surface deformation provide insight into variations of the frictional strength of faults and the rheology of the lower crust and upper mantle. Due to the difficulty of capturing the earliest phase of afterslip, most analyses have focused on understanding postseismic processes over timescales of weeks to years. Here we investigate the kinematics, moment release, and frictional properties of the earliest phase of afterslip within the first hours following the 2016 M w 7.1 Kumamoto earthquake using a network of 5-minute sampled continuous Global Positioning System (GPS) stations. Using independent component analysis to filter the GPS data, we find that (1) early afterslip contributes only~1% of total moment release within the first hour and 8% after 24 hr. This suggests that the lack of a coseismic slip deficit, which we estimate using standard geodetic data sets (e.g., InSAR, GPS, and pixel offsets) and which span the first 4 days of the postseismic period, is largely reflective of the dynamic rupture process and we can rule out contamination of moment release by early afterslip. (2) Early afterslip shows no evidence of a delayed nucleation or acceleration phase, where instead fault patches transition to immediate deceleration following rupture that is consistent with frictional relaxation under steady state conditions with dependence only on the sliding velocity. (3) There is a close correlation between the near-field aftershocks and afterslip within the first hours following rupture, suggesting afterslip may still be an important possible triggering mechanism during the earliest postseismic period.

Probing Fault Frictional Properties During Afterslip Updip and Downdip of the 2017 Mw 7.3 Sarpol‐e Zahab Earthquake With Space Geodesy

“…The postseismic InSAR observations in this study started 5-7 days after the mainshock, during which the cumulative afterslip is expected to already have greatly exceeded the critical slip distance D c in the full rate-and-state frictional law. The rate-strengthening simplification is also supported by the high-sampling-rate GPS observations shortly after the 2016 Kumamoto earthquake (Milliner et al, 2020). Under the rate-strengthening simplification, the fault slip rate at the onset of the afterslip can be expressed as (e.g., Barbot et al, 2009) follows:…”

Very Early Postseismic Deformation Following the 2015 Mw 8.3 Illapel Earthquake, Chile Revealed From Kinematic GPS

“…In the first few hours after the Illapel earthquake rupture, the rate of afterslip was relatively fast, with frequent aftershocks. Previous studies have suggested that afterslip may be an important trigger mechanism for aftershocks in the earliest period after the mainshock (Hsu et al, 2006;Jiang et al, 2021;Milliner et al, 2020;Perfettini & Avouac, 2004;Tsang et al, 2019). We compare our afterslip estimates with the distribution of aftershocks (USGS, Mw > 2.5) during the first 12 hr immediately following the mainshock event.…”

“…We also calculated the impact of the poroelastic rebound (Okada, 1985) on the early deformation and found that it was almost negligible as well (Table S5 in Supporting Information S1). Therefore, we presumed the afterslip as the predominant mechanism to explain the surface displacements in the very early stage (Cattania et al, 2015;Frank et al, 2017;Helmstetter & Shaw, 2009;Perfettini et al, 2018;Ross et al, 2017;Milliner et al, 2020).…”

“…Such observations are key to investigating the afterslip and the underlying process of fault stress redistribution in the very early postseismic period, as well as to better understand the mechanics of the megathrust fault within a full seismic cycle. At present, only a few studies utilize kinematic GNSS to analyze the early postseismic deformation (e.g., Fukuda et al, 2009;Jiang et al, 2021;Malservisi et al, 2015;Milliner et al, 2020;Miyazaki & Larson, 2008;Munekane, 2012;Tsang et al, 2019;Twardzik et al, 2019).…”

A Unified Framework for Earthquake Sequences and the Growth of Geological Structure in Fold‐Thrust Belts

“…Instead, we suggest partial pooling of these datasets, which is a popular machine learning method to take advantage of datasets that contain similar information on average but differ in statistically coherent ways (Gelman & Hill, 2006). Alternatively, it may also be possible to detect hinge deformation by explicitly inverting for rapid early afterslip in addition to coseismic slip (Milliner et al, 2020;Ragon et al, 2019).…”