Geological Observations of Damage Asymmetry in the Structure of the San Jacinto, San Andreas and Punchbowl Faults in Southern California: A Possible Indicator for Preferred Rupture Propagation Direction

Abstract

We present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the San Andreas, San Jacinto, and Punchbowl faults in southern California. The observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters, a fault zone scale of meters to tens of meters and related geomorphologic features. The observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters. In all of those scales, more damage is found on the side of the fault with faster seismic velocities at seismogenic depths. The observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks. The data are consistent with a preferred northwestward propagation direction for ruptures on all three faults. If our results are supported by additional observations, asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section. The property of a preferred rupture direction can explain anomalous behavior of historic rupture events, and may have profound implications for many aspects of earthquake physics on large faults.

Internal structure of the San Jacinto fault zone at Blackburn Saddle from seismic data of a linear array

“…This is in agreement with the observations summarized in Fig. 13 and geological mapping near Hog Lake southeast of the array (Dor et al 2006). Stations at larger distance from the fault may record, in addition to the phases analysed in this work, also P and S body waves reflected within a low velocity fault zone layer (Yang et al 2014) and waves reflected from bimaterial fault interfaces to off-fault stations (Najdahmadi et al 2016).…”

“…Internal structural components of the SJFZ have been studied using various seismic arrays that cross the fault at different locations (e.g. Li & Vernon 2001;Lewis et al 2005;Yang et al 2014;Ben-Zion et al 2015;Li et al 2015;Hillers et al 2016), along with geological mapping of rock damage and analysis of geomorphologic signals (Dor et al 2006;Wechsler et al 2009).…”

“…We focus on these structural components because they contain information on likely properties of past and future earthquake ruptures and associated ground motion (e.g. Andrews & Ben-Zion 1997;Dor et al 2006Dor et al , 2008Brietzke et al 2009;Shlomai & Fineberg 2016).…”

Postseismic quasi‐static fault slip due to pore pressure change on a bimaterial interface

“…Such contrast may also exist at boundary between continental plate and subducting oceanic plate [e.g., Kodaira et al, 2002]. We can also mention a possibility that repeated occurrences of earthquakes on a fault form asymmetrical mechanical property across the fault [e.g., Yamashita, 2000;Dor et al, 2006]. Theoretical works indicate that mode II slip along a frictional bimaterial interface between linear elastic solids produces a change of normal stress on the interface that depends on the slip function, material properties and extension direction of slip zone [e.g., Comninou, 1977aComninou, , 1977bWeertman, 1980;Shi and Ben-Zion, 2006].…”

Initiation propagation and termination of elastodynamic ruptures associated with segmentation of faults and shaking hazard

“…An additional way of examining this in the field would be through damage asymmetry; while the relationship to directivity may be difficult to quantitatively develop, qualitatively if damage occurs primarily on the side of decreasing normal stress, one would expect segments with high directivity fractions to have more damage on the extensional side. Indeed, damage asymmetry has been observed along faults, and ascribed to such rupture direction asymmetries [Dor et al, 2006].…”