Geometrical and Frictional Effects on Incomplete Rupture and Shallow Slip Deficit in Ramp‐Flat Structures

Abstract

Several recent moderate‐magnitude (Mw > 7) earthquakes, such as the 2015 Gorkha, Nepal, ruptured only the deep (>15 km depth) portions of megathrust faults, leaving the updip sections unbroken. Here we investigate the effects of geometrical and frictional variations at depth on the stress accumulation and release in ramp‐flat structures using 2‐D finite element models. Our results show that ramp‐flat structures allow for faster but lower shear stress accumulation with increasing dip of the deep ramp section while increasing frictional strength of the faults allows more stress accumulation. These factors lead to earlier yet smaller failures of the ramp followed by larger and less frequent failures affecting the shallow section. Our models thus suggest that the dynamics of strain reservoirs are related to both the frictional strength and dips of ramp‐flat megathrust structures, and the failure time of the shallow fault section is affected by the stress regime at the deep fault segment.

Complex earthquake behavior on simple faults

“…Other events, including the 2014 M w 8.2 Iquique, Chile earthquake (Schurr et al, 2014) ruptures a fraction of a locked patch, arresting along strike where earlier ruptures propagated. Partial ruptures have been attributed to geometrical or frictional fault heterogeneity (Qiu et al, 2016;Moreno et al, 2018;Li et al, 2018;Dal Zilio et al, 2019). Other studies invoke, as I do here, gradients in the stress field due to nonuniform loading (Herrendorfer et al, 2015;Michel et al, 2017), implying that partial ruptures may be rather common for sufficiently large velocity weakening regions.…”

“…The discrepancy between the predominantly characteristic and periodic behavior of continuum models and the rich complexity of discrete models led to the view that the statistics of seismicity on a regional scale is controlled by the discrete nature of faults (Ben-Zion & Rice, 1995;Ben-Zion, 2008) or by frictional/geometrical fault heterogeneity (e.g. Hillers et al (2007); Aochi and Ide (2009); Kaneko et al (2010); Dublanchet et al (2013)); the latter is also understood to be responsible for partial ruptures on megathrust faults (Li et al, 2018;Qiu et al, 2016;Dal Zilio et al, 2019). An important question is then: if fault roughness, segmentation and the interaction between separate fault segments are responsible for earthquake statistics on a regional scale, are relatively smooth, isolated faults more likely to exhibit characteristic quasi-periodic behavior?…”

Impacts of Topographic Relief and Crustal Heterogeneity on Coseismic Deformation and Inversions for Fault Geometry and Slip: A Case Study of the 2015 Gorkha Earthquake in the Central Himalayan Arc

“…An important geometrical feature of MHT that hosted the 2015 Gorkha earthquake is the downdip portion of the MHT in a region encompassing along‐dip geometric changes (i.e., fault dip angle ∼30°) in the middle crust, named “ramp‐flat” structure (e.g., Hubbard et al., 2016). The presence of a ramp fault segment is thought to influence the interseismic strain accumulation (e.g., Pandey et al, 1995) and earthquake occurrence time (Li, Barnhart et al., 2018). Since a ramp‐flat geometry essentially increases both fault dip angle and depth in the downdip portion of the fault in comparison to a flat fault without a ramp segment, our inversion results suggest that the ramp‐flat structure requires more geodetically inverted slip on the ramp segment.…”

Complex Earthquake Sequences On Simple Faults

“…Other events, including the 2014 M w 8.2 Iquique, Chile earthquake (Schurr et al, ) ruptured a fraction of a locked patch, arresting along strike where earlier ruptures propagated. Partial ruptures have been attributed to geometrical or frictional fault heterogeneity (Dal Zilio et al, ; Li et al, ; Moreno et al, ; Qiu et al, ). Other studies invoke, as I do here, gradients in the stress field due to nonuniform loading (Herrendorfer et al, ; Michel et al, ), implying that partial ruptures may be rather common for sufficiently large VW regions.…”

“…The discrepancy between the predominantly characteristic and periodic behavior of continuum models and the rich complexity of discrete models led to the view that the statistics of seismicity on a regional scale is controlled by the discrete nature of faults (Ben‐Zion, ; Ben‐Zion & Rice, ) or by frictional/geometrical fault heterogeneity (e.g., Aochi & Ide, ; Dublanchet et al, ; Hillers et al, ; Kaneko et al, ); the latter is also understood to be responsible for partial ruptures on megathrust faults (Dal Zilio et al, ; Li et al, ; Qiu et al, ). An important question is then as follows: if fault roughness, segmentation, and the interaction between separate fault segments are responsible for earthquake statistics on a regional scale, are relatively smooth, isolated faults more likely to exhibit characteristic quasiperiodic behavior?…”