Reversed-Polarity Secondary Deformation Structures Near Fault Stepovers

Abstract

We study volumetric deformation structures in stepover regions using numerical simulations and field observations, with a focus on small-scale features near the ends of rupture segments that have opposite-polarity from the larger-scale structures that characterize the overall stepover region. The reversed-polarity small-scale structures are interpreted to be generated by arrest phases that start at the barriers and propagate some distance back into the rupture segment. Dynamic rupture propagating as a symmetric bilateral crack produces similar (anti-symmetric) structures at both rupture ends. In contrast, rupture in the form of a predominantly unidirectional pulse produces pronounced reversedpolarity structures only at the fault end in the dominant propagation direction. Several observational examples at different scales from strike-slip faults of the San Andreas system in southern California illustrate the existence of reversed-polarity secondary deformation structures. In the examples shown, relatively-small pressure-ridges are seen only on one side of relatively-large extensional stepovers. This suggests frequent predominantly unidirectional ruptures in at least some of those cases, although multisignal observations are needed to distinguish between different possible mechanisms. The results contribute to the ability of inferring from field observations on persistent behavior of earthquake ruptures associated with individual fault sections.

Bimaterial interfaces in the south San Andreas Fault with opposite velocity contrasts NW and SE from San Gorgonio Pass

“…The opposite velocity contrast to the SE of SGP suggests that subshear earthquakes tend to propagate on that section of the SAF to the SE. This is consistent with observed along‐strike asymmetry of aftershocks [ Zaliapin and Ben‐Zion , ] and small reversed‐polarity deformation structures [ Ben‐Zion et al ., ] on the southern SAF. Occasional supershear ruptures on these sections would tend to propagate in the opposite directions.…”

Seismic velocity structures in the southern California plate-boundary environment from double-difference tomography

“…2005; Wechsler et al . 2009; Yang & Zhu 2010), along‐strike asymmetry of aftershocks (Zaliapin & Ben‐Zion 2011) and generation of small‐scale transpressive structures on one side of larger pull‐apart basins (Ben‐Zion et al 2012). We note that the central SJFZ, especially the area associated with the Anza gap, has the largest inferred slip for the past several large earthquakes (Salisbury et al 2011).…”

Bimaterial interfaces at the Karadere segment of the North Anatolian Fault, northwestern Turkey

“…This is relevant for numerous topics including earthquake locations, focal mechanisms, Moho topography below the fault, delay times of teleseismic waves, interseismic strain fields, and near‐fault amplification of ground motion [e.g., Oppenheimer et al , ; Ben‐Zion and Malin , ; Schulte‐Pelkum and Ben‐Zion , ; Ozakin et al , ; Wdowinski et al , ; Kurzon et al , ]. Moreover, bimaterial fault interfaces can significantly affect the mode, dynamic properties, and propagation direction of earthquake ruptures [e.g., Ben‐Zion , ; Ampuero and Ben‐Zion , ; Lengliné and Got , ; Calderoni et al , ], along with space‐time variations of seismicity along the fault [e.g., Rubin and Gillard , ; Zaliapin and Ben‐Zion , ] and generation of small‐scale local structural properties [e.g., Dor et al , ; Wechsler et al , ; Ben‐Zion et al , ].…”