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

Abstract

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 as stresses following rupture are dissipated. However, 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 Mw 7.1 Kumamoto earthquake using a network of five-minute sampled continuous 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 hours. This suggests that the slip model of the mainshock, which we estimate using standard geodetic datasets (e.g., InSAR, GPS and pixel offsets), and which span the first four 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: …”

Coevolving early afterslip and aftershock signatures of a San Andreas fault rupture

“…The concurrent evolution of afterslip and aftershocks have been observed in some subduction zones and crustal faults, typically over time scales longer than hours or spatial scales larger than tens of kilometers (1,41,48,49). Questions remain on whether the two processes reflect similar or common statistical fault properties, physical mechanisms, or both.…”

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).…”