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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B10304, doi:10.1029/2006JB004764, 2007
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High localization of primary slip zones in large earthquakes from
paleoseismic trenches: Observations and implications for earthquake
physics
Thomas K. Rockwell1 and Yehuda Ben-Zion2
Received 21 September 2006; revised 23 July 2006; accepted 3 May 2007; published 16 October 2007.
[1] Paleoseismic exposures excavated across relatively straight sections of major faults in
southern California display a high degree of localization at depths of only a few meters
below the surface. In some cases, the width of the slip zone in events with multimeter
displacement is on the order of 1–2 mm to a cm, which is the resolution of the
observations. Repetitive slip events in the same zone increase the observed width of the
faulting, as expressed at trench depths, but the superposed slip again tends to be highly
localized. These observations are probably representative of >80% of the length of the
faults studied. Based on these results, combined with the expected tendency for narrower
slip zone with depth, observations from exhumed faults, and high localization of
seismicity along large faults, we hypothesize that the majority of slip carried by large
faults occurs in very narrow zones. If correct, the emerging integrated view of high slip
localization during earthquake ruptures places important constraints on many issues of
earthquake dynamics.
Citation: Rockwell, T. K., and Y. Ben-Zion (2007), High localization of primary slip zones in large earthquakes from paleoseismic
trenches: Observations and implications for earthquake physics, J. Geophys. Res., 112, B10304, doi:10.1029/2006JB004764.
1. Introduction
[2] The geometrical properties of fault structures and
earthquake slip zones have fundamental implications for
earthquake and fault mechanics [e.g., Ben-Zion and
Sammis, 2003]. In general, fault zones have high apparent
geometrical complexity with nested hierarchy of damage
zones and slip localization surfaces that spread at places to
branching structures. This apparent complexity, however,
reflects the entire geological history of the fault zone and it
typically diminishes at depth. Figure 1 schematically depicts
several structural elements of a large continental strike-slip
fault. At a scale of several kilometers, there is a broad
damage zone that is largely a relic structure of the progressive coalescence and localization of the active fault
zone over time. This damage zone produces broad anomalies of low gravity and seismic velocity across the fault [e.g.,
Stierman, 1984; Mooney and Ginzburg, 1986], and may
also be manifested by a crustal deformation response that is
larger than that of the surrounding rocks [Fialko et al.,
2002]. A zone of higher damage at a smaller scale of about
1 km can produce local seismic anisotropy with faultparallel cracks [e.g., Cochran et al., 2003; Liu et al.,
2004; Peng and Ben-Zion, 2004] and elevated seismic
scattering [e.g., Revenaugh, 2000; Rubinstein and Beroza,
1
Department of Geological Sciences, San Diego State University, San
Diego, California, USA.
2
Department of Earth Sciences, University of Southern California, Los
Angeles, California, USA.
Copyright 2007 by the American Geophysical Union.
0148-0227/07/2006JB004764$09.00
2005; Peng and Ben-Zion, 2006]. This in turn may contain a
narrower zone of intense damage at a scale of about 100 m
that can trap seismic waves [e.g., Li and Leary, 1990;
Korneev et al., 2003; Ben-Zion et al., 2003] and may be
manifested at the surface by a belt of pulverized fault zone
rocks [Dor et al., 2006a, 2006b].
[3] Geological studies of deeply exhumed fault structures
[e.g., Sibson, 2003], systematic analyses of large seismic
data sets [e.g., Ben-Zion et al., 2003; Peng and Ben-Zion,
2004; Lewis et al., 2005; Peng and Ben-Zion, 2006], and
theoretical simulations of damage generation [e.g., BenZion and Shi, 2005; Rice et al., 2005] suggest that these
damage zones are typically limited to the top few (3 – 5) km
of the crust and are parts of a hierarchical flower structure.
This is also supported by the collapse of high-resolution
seismicity along large faults to zones with width comparable
to the location error (i.e., the smallest resolvable dimension), which at places is as small as a few tens of meters
[e.g., Poupinet et al., 1984; Ito, 1985; Nadeau et al., 1994;
Schaff et al., 2002; McGuire and Ben-Zion, 2005; Thurber
et al., 2006]. Similar types of hierarchical flower structures
are documented clearly in oil industry data [Wilcox et al.,
1973; Sylvester and Smith, 1976; Sylvester, 1988].
[4] The belts of damaged fault zone rock contain narrow
zones at a scale of 1 – 10 cm that accommodate virtually all
the large scale tectonic motion on the fault. This has been
documented in several exhumed fault structures [e.g.,
Chester et al., 1993; Evans et al., 2000; Sibson, 2003;
Wibberley and Shimamoto, 2003] but was not studied
sufficiently in known earthquake rupture zones. Heermance
et al. [2003] observed in a borehole site that the width of the
slip zone of the 1999 Chi-Chi earthquake at a depth of about
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Figure 1. Schematic representations of a fault zone at crustal scale (a), kilometer scale (b) and trench
scale (c). At the crustal scale, the structure consists of a broad damage zone with width and damage
intensity that generally decrease with depth. Pending further study of faults exhumed from 3 to 12 km
depths, uncertainty remains about the degree of localization at seismogenic depths. Older inactive fault
strands may be embedded within this zone. At the kilometer scale, major faults usually have a damage
zone with increasing intensity toward the active slip zone. At the hundred-meter scale, there is a zone of
highly shattered or powdered or pulverized rock that produces trapped waves and is usually asymmetric
with respect to the principal slip zone. At the trench scale, the fault zone can be a single or multiple
strands that converge downward. In areas of rapid deposition, it is common to see upward branching of
fault splays toward the ground surface or paleo-ground surface for buried events.
250 m is localized on slip surfaces that range from 50 to 350
microns. Wibberley and Shimamoto [2003] documented at
several surface exposures of the Median Tectonic Line in
Japan slip localization zones with widths of 10– 100 mm.
[5] In the present work we provide evidence for a high
degree of localization in shallow paleoseismic trenches of
earthquake ruptures (inset of Figure 1) along several large
strike-slip faults of the San Andreas system in southern
California. Like the other structural elements discussed
above, the slip zones in the paleoseismic trenches show
some apparent complexity with hierarchical branching
structures. In all cases, however, the complexity for individual ruptures decreases with depth, and the bulk of the
slip is accommodated by very narrow primary slip zones.
Further, the most recent ruptures tend to reoccupy the same
narrow slip zone as ancient rupture surfaces, implying that
the narrowness of the slip zone at the surface reflects its
localization at depth, a property that is generally observed
for exhumed faults. Based on the similarity between the
observed localized slip zones in the shallow paleoseismic
trenches and the related observations in exhumed faults,
combined with the observed localization of seismicity in
large faults and theoretical expectations, we infer that
localization in large fault zones likely occurs throughout
the seismogenic zone.
2. Observations From Paleoseismic Studies
[6] Considerable effort has been expended on mapping
surface ruptures associated with moderate and large earthquakes, in an effort to clarify the location and timing of past
earthquakes [e.g., Clark et al., 1972; Sieh, 1978; Rockwell
et al., 1986], as well as fault zone complexity and slip
distribution [e.g., Tchalenko, 1970; Clark, 1972; Sharp et
al., 1982; Sieh et al., 1993; Treiman et al., 2002]. Many of
these studies have made the observation that the overall
width of surface faulting can be broad, up to several
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slip could be demonstrated to have occurred in a narrow
zone. In all of the discussed examples, we believe that the
results are representative for most of the length of the fault.
Although the trench exposures are relatively shallow, we
argue that most slip is accommodated by the faults exposed
in the trench studies used in this paper. We generally ignore
the broadly distributed fractures and minor faults that
constitute the damage zone of the fault, as slip along these
minor interfaces contributes little to the long term motion
along the fault.
Figure 2. A map of major faults in southern California
with the four study sites indicated as dotted circles.
hundred meters, as observed for portions of the 1992
Landers ruptures [Johnson et al., 1997]. However, these
studies have also shown that most of the slip typically
occurs in a narrow zone, usually less than a few meters
wide, and it is this zone that is commonly trenched in
paleoseismic investigations.
[7] Typically, trench excavations extend downward only
a few meters, although some trenches have been excavated
to depths of 6 – 8 m [Fumal et al., 1993; Rockwell et al.,
1986, 2000, 2003]. In most trench studies, the width of
faulting varies between narrow zones of a meter or less at
the surface to broad zones of tens or hundreds of meters.
However, even in areas of distributed faulting, most of the
slip commonly occurs within a narrow zone. Many paleoseismic studies were conducted specifically in areas of fault
zone complexity, as these areas are often better in resolving
long earthquake records [McCalpin, 1996; Weldon et al.,
1996]. However, areas of fault complexity typically occupy
a relatively small portion of the fault trace and numerous
sites have also been developed in areas of relatively straight
simple fault geometry. For this study, we used only sites
along relatively simple reaches of faults where most of the
2.1. Site Selection and Description of Observations
[8] We chose four paleoseismic exposures along four
faults in southern California to illustrate the narrowness of
the active slip zones during large earthquakes (Figure 2). The
four sites are: (1) the San Jacinto fault near Anza along the
northern ‘‘Anza Seismicity Gap’’ at Hog Lake [Rockwell
et al., 2003, 2005]; (2) the Superstition Mountain fault at the
shoreline site [Gurrola and Rockwell, 1996]; (3) the Rose
Canyon fault in San Diego north of Mission Valley [Lindvall
and Rockwell, 1995]; and (4) The Hondo Road site along
the southern Johnson Valley fault that ruptured in 1992
[Rockwell et al., 2000]. The criteria for site selection are
based on the ability to recognize and distinguish the discrete
slip surface associated with the most recent event, and on
the presence of evidence for multiple events through which
the cumulative effects of multiple ruptures can be addressed.
We also chose a variety of faults from the perspective of slip
rate and recurrence interval (San Jacinto and Superstition
Mountain are relatively fast, while Johnson Valley and Rose
Canyon are relatively slow), although all exhibit relatively
large slip in the most recent event varying from 2 to 4 m.
Each site is briefly described below and a complete description for most can be found in the indicated references.
2.1.1. Hog Lake Site
[9] The Hog Lake paleoseismic site lies along the San
Jacinto fault in what is termed the ‘‘Anza Seismic Gap’’
[Sanders and Kanamori, 1984]. The fault in this area is
simple and straight [Sharp, 1967] with a slip rate of about
15 mm/yr for the past 5 ka [Merifield et al., 1991], the
background seismicity is depressed [Sanders and Kanamori,
1984], and exposures in bedrock show a narrow (<0.5 m)
gouge zone along the active strand [Dor et al., 2006a]. For the
75 km of the San Jacinto fault from Bautista Canyon southeast to Clark Valley (which includes the Hog Lake site),
between 82 and 89% of this length of fault displays a narrow
active zone at the surface based on bedrock mapping by
Sharp [1967] and geomorphology by Sharp et al. [1982] and
Middleton [2006]. Based on these general surface mapping
observations, we infer that our shallow subsurface observations on fault zone width at Hog Lake are representative of
most of the central San Jacinto fault zone.
[10] Paleoseismic investigations were conducted at Hog
Lake on the Ramona Indian Reservation along the northern
portion of the Anza Gap [Rockwell et al., 2003, 2005],
exposing the fault to a depth of about 6.5 m in middle to late
Holocene alluvium. Multiple trenches provided the ability
to compare exposures where slip occurred on discrete
strands as well as repeatedly on the same strand. Figure 3a
shows a photomosaic of one of the moderately deep
exposures where slip is concentrated in a narrow zone.
Seven surface ruptures have occurred on this fault in the
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Figure 3. (a) Photomosaic of a trench exposure of the San Jacinto fault at Hog Lake near Anza,
California. Note that the main strand is the only currently active strand, and that the detail (box) shows
the only strand active in the past three surface ruptures. Here, the active fault zone is about a centimeter in
width but almost all slip has occurred across the millimeter-wide fracture indicated by the arrow. Sixteen
surface ruptures have occurred after deposition of the lowest exposed strata. (b) Detail from another
trench exposure at Hog Lake showing slip in only the past two events, which occurred on separate
strands. Note that slip from the most recent event occurred on a very narrow, distinct crack. Also note that
unit 80 is up on the right, whereas unit 120 is down to the right indicating substantial lateral slip (>4 m
average slip per event for Anza events). The width of the crack is on the order of a millimeter at the base
of this 1.2 m-deep exposure.
past 1000 years, and 16 events in the past 3500 years, yet
the entire width of the main zone at the base of the trench is
less than a half meter. Figure 3b shows a parallel exposure
(8 m north of that given in Figure 3a) where the most recent
and penultimate events ruptured discrete strands about 0.5 m
apart. Note that for the most recent event, slip is concentrated in a very narrow zone no more than a couple
millimeters in width. Also note that unit 80 shows vertical
separation that is up on the right (southwest) whereas units
120 and below show the opposite sense of separation of down
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Figure 4. (a) Exposure of the Superstition Mountain fault at the shoreline site of Gurrola and Rockwell
[1996]. The most recent event, shown in the detail in (b), broke along a very narrow zone that is on the
order of a few millimeters or less. An older strand exposed deeper in the trench is shown on the right (c)
with a similarly narrow zone of shear.
to the right. This observation argues for substantial slip
during the most recent event, consistent with that estimated
from the geomorphology at 3.5 –4 m [Middleton, 2006].
2.1.2. Superstition Mountain Fault
[11] The Superstition Mountain fault was exposed at the
Shoreline site by Gurrola and Rockwell [1996]. At the
location indicated in Figure 2, they found displaced lacustrine clayey strata, shoreline sandy strata and fluvial alluvial
deposits (Figure 4a) across a 2 – 3 m-wide fault zone.
However, the most recent event, which was determined to
have slipped about 2.2 m between AD 1440 and 1640
[Gurrola and Rockwell, 1996], occurred on a very narrow
slip surface of 1 – 2 mm width (Figures 4a and 4b). Other
strands that ruptured in earlier events at the same site, and
are buried deeper in the section, also show a similarly
narrow slip zone (Figure 4c). The overall width of the fault
becomes narrower with depth, although is much broader
than slip in discrete events. Part of this appears to be upward
splaying of fractures as individual slip events approach the
ground surface. Gurrola and Rockwell [1996] made several
parallel exposures, and all displayed a similar width of
rupture for the most recent event.
[12] Observations on the width of the active slip zone at
the Shoreline site are corroborated by new exposures at
Carrizo Wash [Rockwell et al., 2000; Ragona, 2003; Verdugo
et al., 2005], and from mapping of the geomorphology at
Superstition Mountain [Faneros, 2005]. Based on these other
observations and the overall structure of the fault zone, we
estimate that greater than 70% of the Superstition Mountain
fault is localized to a similar degree as in the Shoreline site
exposures in Figure 4.
2.1.3. Rose Canyon Fault
[13] The Rose Canyon fault was studied in Rose Creek in
the City of San Diego [Lindvall and Rockwell, 1995;
Rockwell and Murbach, 1999]. The excavations were done
on an abandoned early Holocene stream terrace to Rose
Creek (Figure 2), so the site was not the best for timing of
past events. However, the trenches exposed a buried gravelfilled channel that provided an early Holocene to present
slip rate. Further, the most recent event broke along a new
strand (in the meter-scale dimensions of the fault zone
width), so 3 m of strike-slip in the most recent event could
be resolved [Rockwell and Murbach, 1999]. Figure 5 shows
the trench log and a photograph of the strand active in the
most recent event, with all 3 m of slip accommodated across
a very narrow (<1 cm width) shear. The entire fault zone
here is about 1 meter in width at the base of the trench but
individual strands are distinct at the millimeter scale. Again,
there is typically a flowering upward or broadening of the
rupture width as the surface is approached. Although most
of the fault is offshore to the north, our exposures in Rose
Creek are typical of numerous geotechnical exposures of the
fault zone in San Diego and probably provide an adequate
characterization of the average properties of the fault zone
in general.
2.1.4. Southern Johnson Valley Fault
[14] The southern Johnson Valley fault ruptured as part of
the 1992 Landers earthquake sequence and was subse-
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Figure 5. (a) Log of trench T4 from Lindvall and Rockwell [1995], showing the NW wall. Note that the
strand that broke in the most recent event is very localized at the base of the trench. (b) Photographic log
of a trench exposure in the Rose Canyon fault in San Diego. The fault trace (indicated by white arrow)
produced 3 m of slip in the most recent earthquake [Rockwell and Murbach, 1999] across a cm-wide zone
of shear, and was the only strand active in this event. The width of this fault strand at the base of the
trench was resolvable as a knife-width shear. A, E, Bt and BC refer to their respective soil horizons, K =
krotovina, or animal burrows.
quently studied for timing of past events [Rockwell et al.,
2000]. In that study, exposures at several trench sites
displayed the same general trend of upward broadening of
fault splays toward the ground surface, as is shown in
Figure 6 at the Hondo Road site (Figure 2). There are two
salient issues here. First, the overall width of faulting in this
area was tens of meters but almost all (>95%) of the slip
occurred along the main strand exposed in the trench.
Second, the 1 – 2 m-wide surface fault zone narrows to less
than a centimeter in width at the base of the 3 m-deep
trench, and this zone represents many Holocene and late
Pleistocene events. This observation is similar to most other
sites in that individual events display an upward splaying of
the width of faulting, especially within a few meters of the
ground surface, and most slip in the most recent event (1992
in this case) occurs across a very narrow zone, no more than
a centimeter at this site. It should be noted that the actual
width of the 1992 slip zone could be appreciably less than a
centimeter, as this was the resolution at the base of the
trench and the centimeter width actually represents the
width of faulting from many events. Similar observations
were made for the Kickapoo fault which also ruptured in
1992 [Rockwell et al., 2000].
2.2. Generalized Observations, Exceptions, and
Ambiguities
[15] The above results demonstrate that the width of slip
zones along major earthquake-producing faults tends to
decrease rapidly with depth and that multimeter displacements in large individual earthquakes often occur, even at a
depth of only 1 m or so, in slip zones with widths of
millimeters to a centimeter. We limited our study to the
relatively straight portions of the faults, so we expect
broader distribution of slip in areas of surface geometric
complexity as observed in recent surface ruptures [e.g.,
Johnson et al., 1997]. Nevertheless, the majority of fault
area during large earthquakes (e.g., nearly 90% of the Clark
strand of the San Jacinto fault) is relatively straight, and for
that portion we infer that the slipping zone is very narrow at
depth, perhaps on the order of a few millimeters or less.
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Figure 6
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Figure 7. Schematic representations of two types of superposed structures. (a) Where the thickness of
deposition is large between surface ruptures, it may be possible to see the width of the slip zone collapse
downward into a narrow zone. (b) Where the thickness of deposition is small between surface ruptures, it
is likely that the upward flowering of slip toward the free surface will give the appearance of an overall
broader zone.
Similar observations of high slip localization to those shown
in Figures 3 –6 are evident in many other paleoseismic
trenches along the North Anatolian fault [Rockwell et al.,
2001; Hartleb et al., 2003], the Dead Sea Transform [Marco
et al., 2005], and sections of other faults. Below we
summarize the observations on the width of active slip
zones in the discussed examples and other paleoseismic
studies.
[16] At many paleoseismic sites excavated where the fault
is relatively straight at the surface, narrow slip zones tend to
be the rule, especially concerning the accommodation of
most of the slip [Clark et al., 1972; Lindvall and Rockwell,
1995; Gurrola and Rockwell, 1996; Fumal et al., 2002;
Rockwell et al., 2000, 2003, 2005]. In these cases, it is
common for individual or discrete shear zones to be as
narrow as 1 –2 mm in width at the base of the trench,
especially where the trench has been excavated fairly deeply
into well-bedded material. We have shown from the examples presented here that discrete slip in the most recent event
may occur over widths of only 1 – 2 millimeters to a
centimeter (commonly the resolution is less for massively
bedded material), whereas a broader slip zone of several
centimeters to a meter or two is common after many events.
It is also typical for ruptures to splay upward close to the
ground surface such that a 1 – 2 m-wide surface rupture zone
often narrows dramatically within a few meters below the
surface.
[17] The general observation that faults typically have
upward branching structures in the top few meters (and
commonly only the upper 1 – 2 m) is largely due to the
rupture approaching the free surface. If deposition between
events is less than the depth that branching begins, say <1 m
of new sediment between events, then superposition of the
branching expressed after multiple events will appear as a
broader fault zone, as is exposed in many trenches. This is
illustrated in Figure 7, where we show schematically a
sequence of events separated by moderate and low levels
of sedimentation. In cases with several meters of deposition
between discrete surface ruptures (Figures 1c and 7a), such
as for very long recurrence interval faults (low slip rate) or
in areas of very high sedimentation, it may be possible to
see the slip width from each event collapsing downward
Figure 6. (top) Log of the fault zone exposed in the Hondo trench across the 1992 Landers earthquake surface rupture
[from Rockwell et al., 2000]. The primary rupture and nearly all of the slip (>95%) was limited to the 2 m-wide fault zone
on the left. Distributed cracking occurred on an older fault strand on the right. Several more cracks with very minor
displacement (few cm) occurred farther west (see Rockwell et al., 2000 for details and unit explanations). (bottom)
Photograph of the fault at the base of the trench.
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onto a narrow slip surface. In most cases, and certainly the
ones that are best for paleoseismic studies (enough sedimentation to separate events but not so much as to require
very deep exposures to acquire long records), there may
only be a half meter or so of sedimentation between surface
ruptures. In these cases, and probably at all sites that have
less than 2 – 3 m of sedimentation between events, the
upward branching of the fault will be superposed for each
successive event, as shown in Figure 7b. The result is the
appearance of a broad (1 – 2 m wide) zone even where slip
in individual events is highly localized at depth.
[18] The superposition of shallow branching structures is
shown nicely in the examples presented in Figures 3– 6,
where slip on the most recent event is very narrow but the
width of the zone containing several events may be significantly broader. Furthermore, in some cases such as the
Hondo trench along the 1992 rupture, it is known that other
minor fractures occurred at distances of tens of meters from
the main rupture. In some areas, the overall width of the
1992 Landers rupture was observed as many hundreds of
meters in width, leading Johnson et al. [1997] to describe a
hierarchy of shear zones within broader shear zones. In fact,
most of these secondary faults experienced very minor
displacements. For instance at Hondo Road along the
southern Johnson Valley fault, nearly all slip occurred on
the main zone (295 cm was measured crossing Hondo Road
hours after the 1992 earthquake on the main fault), even
though secondary cracks were mapped tens to hundreds of
meters to the west. From the perspective of the main motion
accommodated by an earthquake fault, the secondary fractures at shallow depth have negligible contribution.
[19] Finally, it is common to open paleoseismic exposures
in relatively wet environments, as they commonly have the
best records for resolving a paleoearthquake history. However, saturated sediments also tend to accommodate warping and deform plastically, as demonstrated after the Izmit
earthquake where an average of 15% of the slip was nonbrittle at the surface [Rockwell et al., 2002]. In extreme
cases, shear may even be absent at the surface. This,
however, is a near-surface phenomenon and probably
extends downward no deeper than the unconsolidated sediment layer.
3. Discussion
[20] The observations discussed in this paper on high
localization in shallow paleoseismic trenches contribute to
the body of evidence that the majority of slip carried by
large faults occurs in very narrow zones. The localized
shallow structures are especially striking since various nearsurface phenomena tend to increase the width of the slip
localization zone. The observations are consistent with
highly localized slip documented in exhumed faults
[Chester et al., 1993; Evans et al., 2000; Sibson, 2003;
Wibberley and Shimamoto, 2003] and shallow drill cores
[Heermance et al., 2003].
[21] A key difficulty with inferring properties of earthquake ruptures from exhumed fault zones is the ambiguity
on whether the structures were formed by earthquake
ruptures or stable slip. This ambiguity does not exist in
the observations described in this paper. While our observations are done in shallow trenches, we argue that it is
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unlikely that a broad deformation zone at depth will localize
in generally weak unconsolidated surface materials. Moreover, analytical estimates and numerical simulations [Rice et
al., 2005; Ben-Zion and Shi, 2005] indicate that the width of
the actively deforming zone generally decreases with depth
due to the increasing confining stress. A narrow localization
at seismogenic depth is supported by the collapse of
relocated seismicity along large faults to tabular zones with
the smallest resolvable dimension [e.g., Nadeau et al., 1994;
Schaff et al., 2002; McGuire and Ben-Zion, 2005] and by
available observations from deeply exhumed faults [Sibson,
2003]. Based on the strong similarity between the observed
localization of shallow slip and those observed at shallow
boreholes and exhumed faults, combined with the discussed
seismological and theoretical studies, we infer that high slip
localization may be a general characteristic of the entire
brittle crust, at least for some major faults. This integrated
view of high localization of coseismic slip, if correct, places
important constraints for many issues, ranging from identification of the appropriate theoretical framework for studying earthquake and fault mechanics [Ben-Zion and Sammis,
2003] to the effective constitutive laws that are operating
during rupture [Ben-Zion, 2001; Rice, 2006].
[22] The width and straightness of the primary slip zones
during large earthquake ruptures are key geometrical quantities that control the detailed dynamical aspects and overall
form of deformation. For example, a highly localized slip
zone with straight geometry implies that the failure process
may be analyzed within a continuum-Euclidean framework,
while a broad deformation region with multiple slip zones
associated with rough geometry require granular or fractal
frameworks [Ben-Zion and Sammis, 2003]. In continuumEuclidean mechanical models, frictional sliding under shear
stress close to the static friction and narrow slip zones
require the production of substantial frictional heat [e.g.,
Bowden and Thomas, 1954; McKenzie and Brune, 1972;
Sibson, 1973; Lachenbruch, 1980; Mase and Smith, 1987;
Kanamori and Heaton, 2000; Fialko, 2004] that should be
observable in the form of melting products in exhumed
faults and localized anomalies in heat flow studies. Indeed,
some exhumed faults display pseudotachylites along or
within deeply exposed portions of a fault zone [Sibson,
1975; Wallace, 1976; Swanson, 1992; Wenk et al., 2000; Di
Toro and Pennacchioni, 2004], but these observations are
generally quite sparse and do not support widespread
melting. Observations of pseudotachylites tend to be associated with relatively small faults (or fault branches). However, whether they are present on large fault zone structures
(like the San Andreas fault) that accommodate very large
motion must await study of such faults exhumed to depths
of 5 –10 km. Similarly, there is no significant localized heat
flow along large faults such as the San Andreas as expected
from traditional models [Brune et al., 1969; Lachenbruch
and Sass, 1992].
[23] If localization and lack of melting products are
characteristic of faults at seismogenic depths, strong
dynamic weakening must occur during earthquake rupture.
There are many proposed mechanisms for dynamic reduction
of stress during earthquake rupture. These include acoustic
fluidization [e.g., Melosh, 1979; Mora and Place, 1994],
collision of rough surfaces [Lomnitz-Adler, 1991], hydrodynamic fluidization [Brodsky and Kanamori, 2001], silica
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gel lubrication [Goldsby and Tullis, 2002; Di Toro et al.,
2004], various thermal and fluid effects [e.g., Sibson, 1973;
Hirose and Shimamoto, 2005; Noda and Shimamoto, 2005;
Rice, 2006], and wrinkle-like rupture along a bimaterial
interface [e.g., Weertman, 1980; Ben-Zion and Andrews,
1998; Shi and Ben-Zion, 2006]. Finding which mechanisms
operate in natural faults requires a list of observable signatures that are expected from each and can be tested with
field observations. One example is the production of asymmetric rock damage across a bimaterial interface, especially
in the top few km, with more damage on the side with faster
seismic velocities [Ben-Zion and Shi, 2005]. Detailed geological mapping [Dor et al., 2006a, 2006b] and seismic
imaging using fault zone trapped and head waves [Lewis et
al., 2005, 2007] show strong damage asymmetry in sections
of the San Andreas and San Jacinto faults, which correlate
with the velocity structure as predicted for wrinkle-like
ruptures. Similar observations appear to characterize sections
of the North Anatolian fault [Dor et al., 2005; Yildirim et al.,
2005].
[24] In summary, observations of high localization of
co-seismic slip in shallow paleoseismic exposures, combined with similar observations from exhumed faults and
drill cores, seismological data and theoretical considerations, lead us to infer that slip may be localized throughout
the upper crust. If correct, this places strong constraints on
many mechanical models associated with fault rupture
during large earthquakes. Additional multisignal predictions
of theoretical models and detailed field observations of the
width, symmetry properties, straightness and other aspects
of primary slip zones will be required to resolve the proper
mechanical framework, or combination of frameworks,
appropriate for analyzing the behavior of natural faults.
[25] Acknowledgments. We thank Fred Chester, Terry Tullis, an
anonymous reviewer and an anonymous associated editor for useful comments. This work was partially funded by the Southern California Earthquake Center.
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Y. Ben-Zion, Department of Earth Sciences, University of Southern
California, Los Angeles, CA 90089-0740, USA.
T. K. Rockwell, Department of Geological Sciences, San Diego State
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