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Research Papers

Reversed-Polarity Secondary Deformation Structures Near Fault Stepovers

[+] Author and Article Information
Yehuda Ben-Zion, Shiqing Xu

 Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740

Thomas K. Rockwell, Zheqiang Shi

 Department of Geological Sciences, San Diego State University, San Diego, CA 92182-1020

J. Appl. Mech 79(3), 031025 (Apr 06, 2012) (12 pages) doi:10.1115/1.4006154 History: Received August 11, 2011; Revised October 21, 2011; Posted February 21, 2012; Published April 06, 2012; Online April 06, 2012

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 reversed-polarity 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.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Large-scale volumetric stresses expected in fault stepover regions

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Figure 2

Schematic representation of right-lateral slip (top purple curve) on a finite fault segment bounded by barriers (yellow stars) as superposition of near constant right-lateral slip (bottom left blue curve) and reversed left-lateral slip near the barriers (bottom right red curve)

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Figure 3

A model for 2D in-plane dynamic rupture on a frictional fault at y = 0 with initial stress loading at the remote boundaries. Strong barriers at |x|≥25 km confine the ruptures to the finite segment between the yellow stars. A 3 km wide nucleation zone is centered at x = 0 and x = −15 km for cases producing bilateral crack and unidirectional pulse, respectively.

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Figure 4

Profiles of along-fault slip velocity (top) and slip (bottom) at various times (indicated by the color scale) during propagation of symmetric bilateral crack-type rupture. The star symbols at x = ±25 km indicate locations of strong barriers.

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Figure 5

Contours of differential volumetric strain ΔɛV (t > tref  = 9.380 s) in the case of symmetric bilateral crack-like rupture at (a) tref  + 0.16 s around the right barrier, (b) tref  + 2.32 s around the right barrier, (c) tref  + 0.16 s around the left barrier, and (d) tref  + 2.32 s around the left barrier. The black line at y = 0 indicates the slipping portion of the fault. See text for additional explanations.

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Figure 6

Profiles of along-fault slip velocity (top) and slip (bottom) at various times (indicated by the color scale) during propagation of predominately unilateral pulse-like rupture. The star symbols at x = ±25 km indicate strong barriers.

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Figure 7

Contours of differential volumetric strain ΔɛV (t > tref ) in the case of unilateral pulse-like rupture at (a) tref  + 0.16 s around the right barrier, (b) 17.0 s around the right barrier, (c) tref  + 0.16 s around the left barrier, and (d) 17.0 s around the left barrier. The black line at y = 0 indicates the slipping portion of the fault. Here tref  = 16.256 s for the differential fields around the right barrier ((a), (b)) and tref  = 5.552 s for the left barrier ((c), (d)). See text for additional explanations.

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Figure 8

The southern end of the San Andreas fault with a large transtensive step across the Brawley Seismic Zone to the Imperial fault that continues right lateral shear southward into Mexico. A series of transpressive steps are present to the northwest of the 10 km step and include Durmid Hill, the Mecca Hills, and the Indio Hills.

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Figure 9

The Rose Canyon fault in southern California terminates in the vicinity of San Diego Bay, a depression cause by the 10 km right step to the Descanso fault. Mt. Soledad, which is a large transpressive uplift, and several small pressure ridges exist in the Rose Canyon fault to the northwest of the step. At least one small transpressive step is present just north of the San Diego River. Paleoseismic investigations demonstrate that it is a late Quaternary feature [48].

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Figure 10

The Elsinore fault with 2.5 km releasing step at Lake Elsinore. Several small pressure ridges are present along the Wildomar fault on the southeast side of the step.

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Figure 11

The San Jacinto fault zone near Hemet with a large releasing step referred to as the Hemet stepover. Along the Casa Loma strand, several small pressure ridges are present southeast of Mystic Lake, the locus of deep sedimentary fill.

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Figure 12

The Hog Lake paleoseismic site near Anza associated with a 100 m scale releasing step. A small pressure ridge to the north of the step has produced folding and off-fault thrusting during a subset of the events observed at the site.

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