Can the Updip Limit of Frictional Locking on Megathrusts Be Detected Geodetically? Quantifying the Effect of Stress Shadows on Near‐Trench Coupling

Abstract

The updip limit of the seismogenic zone of megathrusts is poorly understood. The relative absence of observed microseismicity in such regions, together with laboratory studies of friction, suggests that the shallow fault is mostly velocity strengthening, and likely to creep. Inversions of geodetic data commonly show low to zero coupling at the trench, reinforcing this view. We show that the locked, downdip portion of the megathrust creates an updip stress shadow that prevents the shallow portion of the fault from creeping at a significant rate, regardless of its frictional behavior. Our models demonstrate that even if the shallowest 40% of the fault is frictionally unlocked, the expected creep at the fault tip is at most 30% of the plate rate, often within the uncertainties of surface geodetic measurements, and below current resolution of seafloor measurements. We conclude that many geodetic models significantly underestimate the degree of shallow coupling on megathrusts, and thus seismic and tsunami hazard.

The Accumulation of Slip Deficit in Subduction Zones in the Absence of Mechanical Coupling: Implications for the Behavior of Megathrust Earthquakes

“…The fault slip recovers more gradually updip of the locked zone because of the effects at shallow depths of the nearby free surface. As a result, the area of the megathrust between the updip edge of the locked patch and the trench does not slide at the relative plate motion rate despite being free to slide (similar to the result from Almeida et al, ). In the reference model, the magnitude of fault slip at the trench updip of the locked zone is 0.51 m.…”

“…Although onshore geodetic observations do not provide information about slip on the shallow megathrust, tsunamis generated by recent great earthquakes (e.g., in the 2004 Sumatra, Lay et al, ; 2010 Maule, Moreno et al, ; and 2011 Tohoku, Mori et al, , events), widespread geological evidence of coseismic slip in the accretionary wedge sections of subduction décollements (Hubbard et al, ), and slip deficit modeling (Almeida et al, ; this study) suggest that large coseismic shallow slip is possible at many subduction zones globally. We compare results from the models in this study to observations from the tsunami source region of the 2011 Mw 9.0 Tohoku earthquake to explore the role of pseudo‐coupling effects in tsunami‐generation processes.…”

Active Convergence of the India‐Burma‐Sunda Plates Revealed by a New Continuous GPS Network

“…The lack of surface fault creep is not in conflict with observations of fluid overpressure and low friction at shallow depths (Steckler et al, ; Zahid & Uddin, ). The absence of creep is an observation of strong interplate coupling, but it does not imply that the shallow megathrust is frictionally locked (we refer the reader to Table 1 in Almeida et al, , for definitions). A down‐dip locked zone is expected to create stress shadows on the shallow megathrust that will limit the creep rate to a small fraction of the plate rate, irrespective of the frictional properties (Almeida et al, ).…”

“…The absence of creep is an observation of strong interplate coupling, but it does not imply that the shallow megathrust is frictionally locked (we refer the reader to Table 1 in Almeida et al, , for definitions). A down‐dip locked zone is expected to create stress shadows on the shallow megathrust that will limit the creep rate to a small fraction of the plate rate, irrespective of the frictional properties (Almeida et al, ). This results in accumulation of a slip deficit during the interseismic period, even on fault patches that are frictionally unlocked or unclamped.…”

Earthquake‐Related Structures Beneath the Southernmost Portion of the Ryukyu Arc and Forearc

“…Previous studies of subduction zones mostly reveal the features related to the updip and downdip limits of seismogenic zones (Almeida et al, 2018;Herrendörfer et al, 2015;Hsu et al, 2006;Hyndman et al, 1997;Hyndman, 2013;Kinoshita et al, 2017;Moore & Saffer, 2001;Wang & Hu, 2006). Studying the seismogenic zone features of subduction zones is thus important to understand mechanisms of generating megathrust earthquakes along the subducting plate interfaces.…”