Shallow slip deficit due to large strike-slip earthquakes in dynamic rupture simulations with elasto-plastic off-fault response

Abstract

SUMMARY Slip inversions of geodetic data from several large (magnitude ∼7) strike‐slip earthquakes point to coseismic slip deficit at shallow depths (<3–4 km), that is, coseismic slip appears to decrease towards the Earth surface. While the inferred slip distribution may be consistent with laboratory‐derived rate and state friction laws suggesting that the uppermost brittle crust may be velocity strengthening, there remains a question of how the coseismic slip deficit is accommodated throughout the earthquake cycle. The consequence of velocity‐strengthening fault friction at shallow depths is that the deficit of coseismic slip is relieved by post‐seismic afterslip and interseismic creep. However, many seismic events with inferred shallow slip deficit were not associated with either resolvable shallow interseismic creep or robust shallow afterslip. Hence, the origin of shallow ‘slip deficit’ remains uncertain. In this study, we investigate whether inelastic failure in the shallow crust due to dynamic earthquake rupture can explain the inferred deficit of shallow slip. Evidence for such failure is emerging from geologic, seismic and geodetic observations. We find that the amount of shallow slip deficit is proportional to the amount of inelastic deformation near the Earth surface. Such deformation occurs under a wide range of parameters that characterize rock strength in the upper crust. However, the largest magnitude of slip deficit in models accounting for off‐fault yielding is 2–4 times smaller than that inferred from kinematic inversions of geodetic data. To explain this discrepancy, we further explore to what extent assumptions in the kinematic inversions may bias the inferred slip distributions. Inelastic deformation in the shallow crust reduces coseismic strain near the fault, introducing an additional ‘artificial’ deficit of up to 10 per cent of the maximum slip in inversions of geodetic data that are based on purely elastic models. The largest magnitude of slip deficit in our models combined with the bias in inversions accounts for up to 25 per cent of shallow slip deficit, which is comparable, but still smaller than 30–60 per cent deficit inferred from kinematic inversions. We discuss potential mechanisms that may account for the remaining discrepancy between slip deficit predicted by elasto‐plastic rupture models and that inferred from inversions of space geodetic data.

Complex multifault rupture during the 2016 M w 7.8 Kaikōura earthquake, New Zealand

“…Forward stressing of fault segments along the Kaik ura coast may have played an instrumental role in allowing the rupture to propagate along such a great length of the plate boundary. Zones of permanent deformation are well documented in the geological record at step-overs in strike-slip fault zones at spatial scales of hundreds of meters or more (36)(37)(38). Large misfits to geodetic data observed following large earthquakes have also been explained by the occurrence of inelastic deformation and coseismic ground damage (39).…”

Landers 1992 "reloaded": Integrative dynamic earthquake rupture modeling

“…While Xu et al [2016] argue that the majority of inferred SSD is a result of poor resolution of near-fault surface data in slip inversions, it is often attributed to coseismically occurring plastic deformation at shallow depths [e.g., Kaneko and Fialko, 2011;Milliner et al, 2015]. Numerical models show that shallow slip is already reduced by 18.6 % in simulations with purely elastic material properties [supplemental material of Roten et al, 2017].…”

“…Recent advances in processing high-resolution aerial photographs of near-fault deformation patterns reveal that off-fault deformation primarily correlates with fault complexity [Milliner et al, 2015]. A significant slip reduction towards the shallow part of the faults is inferred, known as shallow slip deficit (SSD), which is often attributed to plastic deformation [Fialko et al, 2005;Kaneko and Fialko, 2011;Milliner et al, 2015;Gombert et al, 2018]. Simulations on a single, planar fault plane reveal that purely elastic simulations underpredict the SSD [Roten et al, 2017] as well as ground motions [Roten et al, 2014[Roten et al, , 2015.…”

Shallow Rupture of the 2011 Tarlay Earthquake (Mw 6.8), Eastern Myanmar

“…Figure 7c illustrates the comparison of normalized slip potency as a function of depth between the Tarlay event and other studied earthquakes (e.g., Simons et al, 2002;Fialko et al, 2005;Kaneko and Fialko, 2011). We find the shallow slip deficit of the Tarlay earthquake resembles that of the 1992 Landers earthquake and the 2010 Baja earthquake.…”