Rupture‐Depth‐Varying Seismicity Patterns for Major and Great (M_w ≥ 7.0) Megathrust Earthquakes

Abstract

Large earthquakes on subduction zone plate boundary megathrusts result from intervals of strain accumulation and release. The mechanism diversity and spatial distribution of moderate‐size aftershocks is influenced by the mainshock rupture depth extent. Mainshocks that rupture across the shallow megathrust to near the trench have greater intraplate aftershock faulting diversity than events with rupture confined to deeper portions of the megathrust. Diversity of intraplate aftershock faulting also increases as the size of the mainshock approaches the largest size event to have ruptured that region of the megathrust. Based on these tendencies, we identify “breakthrough” ruptures as those involving shallow rupture of the megathrust with volumetrically extensive elastic strain drop around the plate boundary that allows activation of diverse intraplate faulting influenced by long‐term ambient deformation stresses. In contrast, homogeneity of the aftershock faulting mechanisms indicates only partial release of elastic strain energy and remaining potential for another large rupture.

Subduction zone megathrust earthquakes

“…Normal faulting aftershocks have been observed after many large megathrust ruptures (Fig. 18), particularly those that rupture very shallowly near a trench (e.g., Kanamori, 1971;Christensen and Ruff, 1988;Lin and Stein, 2004;Ammon et al, 2008;Lay et al, 2009;Asano et al, 2011;El Hariri and Bilek, 2011;Bilek et al, 2011;Ide et al, 2011;Kato et al, 2011;Lange et al, 2012;Rietbrock et al, 2012;Yue et al, 2014b;Wetzler et al, 2017;Sladen and Trevisan, 2018). Some of these intraplate extensional events are within the outer rise, seaward of the trench, or on plate-bending related faults below the megathrusts, while others are found within the upper plate adjacent to the zone of high slip (e.g., Farías et al, 2011;Hicks and Rietbrock, 2015).…”

“…We set M c = 5.2 for both populations, based on prior estimates of the GCMT catalog completeness (Ekström et al, 2012;Wetzler et al, 2017). This choice appears to be adequate for all of the cases shown here, and use of a higher M c value did not change the relative values significantly.…”

“…Some of these intraplate extensional events are within the outer rise, seaward of the trench, or on plate-bending related faults below the megathrusts, while others are found within the upper plate adjacent to the zone of high slip (e.g., Farías et al, 2011;Hicks and Rietbrock, 2015). Events with rupture deeper on the mega thrust activate relatively few intraplate aftershocks (Wetzler et al, 2017;Sladen and Trevisan, 2018), but in some cases, they have large afterslip on the updip region of the megathrust (Fig. 18).…”

Enhancing Tsunami Warning UsingPWave Coda

“…The two new events involve initially deep ruptures (the 2016 Solomon Islands event likely begins as an intraplate rupture) that propagate or trigger updip, and while the finite‐fault models do suggest that some slip may extend to near the trench, the resolution is limited and the amount of shallow slip varies among the finite‐fault models. The 2016 Solomon Islands event produced outer rise normal faulting aftershocks along part of the rupture, a common indicator of shallow coseismic slip (e.g., Sladen & Trevisan, ; Wetzler et al, ), while the 2015 Papua had shallow thrust‐faulting aftershocks all the way to the trench (Wetzler et al, ). Such events, with relatively deep moment tensor centroid depths, present particular difficulties for tsunami warning.…”

Coseismic Rupture Geometry and Slip Rupture Process During the 2018 Mw 7.1 Anchorage, South‐Central Alaska Earthquake: Intraplate Normal Faulting by Slab Tear Constrained by Geodetic and Teleseismic Data

“…Many major earthquakes/slip deficits associated with occasional thrust faulting have been observed in the subduction zones no deeper than 30-50 km Tape et al, 2018). In shallow megathrust interface of subduction zones, they yield to typical intervals of strain accumulation and release driven by plate converge and are capable of >Mw 9 destructive ruptures (e.g., Wetzler et al, 2017;Hayes et al, 2018). Numerous studies related to seismic gap, earthquake recurrence period, and earthquake nucleation process have been carried out for fault slip behavior in these subduction regions (e.g., Freymueller & Beavan, 1999;Li & Freymueller, 2018;Tape et al, 2018).…”