fault rupture and fault deformnation

Fault Rupture and Surface Deformation:

Defining the Hazard

JEROME A. TREIMAN

California Geological Survey, 888 South Figueroa Street, Suite 475, Los Angeles,CA 90017

Key Terms: Fault, Surface Rupture, Secondary Faults,Site Investigation, Setback

ABSTRACT

Surface fault rupture can be a complex phenomenoninvolving brittle fracture and closely associated defor-mation. In order to characterize the hazard tostructures from surface rupture it is the task of thegeologist to identify where rupture and related defor-mation have occurred as well as the style and magnitudeof past displacements and to interpret these data so asto anticipate future fault rupture. To extrapolate thisinformation from the data that are preserved ordiscovered in investigations requires an appreciationof the variety of surface rupture processes andexpression. This paper looks at the problem in termsof three questions one must ask: 1) Where should faultrupture and deformation be anticipated?; 2) How muchslip, and in what sense, should be anticipated?; and 3)How should the hazard be addressed when data areincomplete? It is concluded that there will remain anumber of uncertainties in the assessment of futurefault rupture and that mitigation strategies mustinclude a sufficient margin of safety to accommodatethese uncertainties.

INTRODUCTION

Simply put, a strict interpretation of California’sAlquist-Priolo Earthquake Fault Zoning Act (the‘‘Act’’) and related policies dictates that one may notbuild structures for human occupancy across the traceof any fault that has had surface rupture during theHolocene epoch, regardless of the amount orprobability of future displacement. This interpreta-tion is based on the introductory wording of the Act,which states that its purpose is to assist localgovernment in their ‘‘… responsibility to prohibitthe location of developments and structures forhuman occupancy across the trace of active faults.’’This purpose is further interpreted in Section 3603(a)of the California Code of Regulations (CCR), which

states the following: ‘‘No structure for humanoccupancy … shall be permitted to be placed acrossthe trace of an active fault. Furthermore, as the areawithin fifty (50) feet of such active faults shall bepresumed to be underlain by active branches of thatfault unless proven otherwise …, no such structuresshall be permitted in this area.’’ In Californiaregulatory language an ‘‘active fault’’ is one that‘‘has had surface displacement within Holocene time’’[CCR, Title14, 13601(a)].

The consulting and development community, andsome local government agencies, have recently askedfor flexibility in mitigating minor fault rupture thatwould not be a hazard to structures if properlymitigated by design (for example, Sexton [2008]). Onits face this would appear to be a reasonable request.However, addressing this request calls up the sagewarning to ‘‘Be careful what you ask for …,’’ for theanswer places a large burden of responsibility onthose who would seek to define, identify, and quantify‘‘minor fault rupture.’’ Fault-related deformation canspan an entire spectrum of surface features across acontinuum of scales and magnitudes. Somewherewithin this range of phenomena there are thresholdsthat differentiate deformation that would be ofsignificance to structures from a lesser or broader,non-hazardous degree of deformation. The actualvalues that might be assigned to these thresholds,which will vary with the state of engineering design,engineering practice, and societal concerns regardingacceptable risk, are not a part of this paper. I intendto show that quantifying potential future rupture onindividual fault traces and providing assurance thatsuch rupture will be ‘‘minor’’ are not simple tasks.Considering the cost, or even the likelihood of successof the necessary investigations, mitigation by designmay not be appropriate or practical for many, if notmost, faults.

This paper is derived from a talk given at the AEG-Shlemon Specialty Conference held February 19–20,2009, in Palm Desert, California. The conferenceexamined fault hazards with respect to site-specificengineering projects, and, hence, this paper focuses onissues that will face the consultant who is trying toassess the hazard for a proposed structure. In order

Environmental & Engineering Geoscience, Vol. XVI, No. 1, February 2010, pp. 19–30 19

for the engineering community to even begin todiscuss whether displacement can be mitigated ormust be avoided at any particular site, a geologist’scharacterization of the style, distribution, and mag-nitude of potential displacements is needed. Suchcharacterization requires an appreciation of thevariability (both spatial and temporal) of expressionof faults of different styles and activity. The issuesinvolved in the characterization of fault rupture andrelated surface deformation define a varied landscapeof readily observable and more subtle phenomena,hypothesis and fact, data and processes, all viewedthrough the eyes of experience. It is not the purposeof this brief paper to provide any answers, or even themethodologies, but rather to provide a reconnais-sance map of that terrain. Although there is a decidedCalifornia bias toward strike-slip faulting in thispaper, most of the observations are readily applicableto other faulting styles.

WHERE SHOULD FAULT RUPTURE ANDDEFORMATION BE ANTICIPATED?

To anticipate where fault rupture and relateddeformation may occur requires an understandingof both general faulting characteristics and thespecific geologic and tectonic characteristics of theproject site. Faults, in their simplest configurations(Figure 1), encompass some combination of threebasic styles of faulting: (a) strike slip—in response toshear; (b) normal—in response to extension; and (c)reverse or thrust—in response to compression. Faultrupture is commonly more complex than the singletraces and mechanisms shown in these block dia-grams as a result of the variety of stresses along afault zone, variability in fault geometry, displacementhistory, and varying earth materials. Anticipatingwhere fault rupture may develop requires a familiaritywith the variety of expression that may occur, whichitself is a product of the fault type, fault dip, amountof displacement, and the material being faulted. Somegood references related to various expressions of faultrupture include Gordon (1971), Johnson et al. (1997),Kelson et al. (2001), Lawson (1908), McCalpin(1996), Tchalenko (1970), and Yeats et al. (1997).

One might ask why the determination of faultlocation should be such a difficult task. Don’t wehave maps showing where the faults are? Consider amap of the Homestead Valley and Emerson Faultsprior to the 1992 Landers earthquake (Figure 2a). Itshows the main traces of the faults, but a look at thepost-earthquake rupture map (Figure 2b) shows thatthe actual pattern of historic fault rupture is muchmore complex. Although some earthquake rupturemay be simple, it may not be easy to predict that

simplicity beforehand. Because minor faults are notas well preserved at the surface, unless there has beena historic surface rupture on the fault we are notlikely to appreciate the full extent of related faultingand deformation. It is this full distribution of strainthat has to be characterized if we want to designstructures to either avoid or safely withstand distrib-uted fault displacement.

If prior mapping is not adequate, can we trench tofind all of the rupture traces? Again, consider anexample. In the following hypothetical trenches(Figure 3a) several significant and minor faults areexposed (short lines across trench outlines). Some

Figure 1. The three principal styles of faulting: (a) strike slip, (b)normal, and (c) reverse (block diagrams modified from figure 4 inClark and Hauge [1971]).

Treiman

20 Environmental & Engineering Geoscience, Vol. XVI, No. 1, February 2010, pp. 19–30

faults can only be seen on one side of the trench withno apparent continuity. A reasonable interpretationof the fault pattern is shown. However, the faultsselected for this example are part of an actual faultpattern from the Hector Mine earthquake (Fig-ure 3b). As can be seen, the actual breadth andcomplexity of faulting might not have been wellprojected or anticipated based solely on the trenchexposures. Furthermore, a step back to look at the

broader pattern of faulting (Figure 3c) shows thatthis is still an evolving en echelon fault system, andfuture rupture (indicated by the heavy dashed line)may well strike obliquely across the prior surfacerupture.

We may also misjudge the potential distribution ofsurface faulting if a trench was not deep enough orextensive enough to observe all active splays or ifsome faulting was not visible in the trench. Bonilla

Figure 2. Mapped fault traces of the Homestead Valley and Emerson Faults in the Melville Lake 7.5-minute quadrangle. (a) Showsmapped fault traces prior to the 1992 Landers earthquake. (b) Shows actual rupture pattern mapped after the earthquake [(a) modified fromCDMG, 1988; (b) mapping compiled by California Geological Survey (CGS)].

Figure 3. Fault rupture may only be incompletely exposed in trenches, as illustrated in this example. (a) Three hypothetical trenches exposemultiple small and larger shears (heavy and thin lines at trench outline) that might be used to construct two simple principal fault traces(dashed lines). The ‘‘observed’’ faults in (a) are modeled on historic surface rupture along the Lavic Lake Fault during the 1999 Hector Mineearthquake, plotted in (b), which shows a more complex fault pattern than might be constructed from minimal data exposed only intrenches. (c) Shows this fault segment in the context of the larger fault pattern (thin linework), revealing that it is within a still-developingright-lateral shear zone that will likely produce more aligned and continuous faults with future displacement (heavy dashed line).

Fault Rupture and Surface Deformation


and Lienkaemper (1991) identified three differentcategories of ‘‘non-visibility’’ of faults in trenchinvestigations. Obscure fault traces (traces not visiblein some geologic units) occurred in 14 percent of thetrenches that they reviewed. In 45 percent of thetrenches faults appeared to die out upward, whereother data showed that surface displacement hadoccurred. Upward die-out was found in as many as 70percent of trenches where strike-slip faulting wasinvolved. In 30 percent of the trenches there werefaults that appeared to die out downward. The studyof Bonilla and Lienkaemper (1991) pointed out thatjust because fault rupture is not visible in a trenchdoes not mean it did not occur. Their findingshighlighted the importance of having multiple faultexposures in order to properly characterize a fault.

Complex fault rupture commonly occurs wherethere are changes in fault geometry. Changes in strikeor stepovers along a strike-slip fault can yield both

compression and extension with resultant verticalmovements. Figure 4 shows examples of transten-sional and transpressional deformation along a strike-slip fault where the trace is slightly oblique to theprincipal stress. Figure 5 shows examples of com-pressional uplift at a fault stepover. Changes in thedip of normal or reverse faults can cause secondarynormal or thrust faults in the hanging wall of the fault(Figure 6). A fault that steepens toward the surfacewill cause compression above a reverse or thrust fault(Figure 6d) but extension in a normal fault. Incontrast, a reverse fault that shallows toward thesurface will cause extension in the hanging wall(Figure 6c).

Rupture complexity can involve splays, closelyspaced parallel faults, and warping. Tchalenko (1970)demonstrated how complexity within a strike-slipfault zone may develop from an initial pattern of en

Figure 4. Examples of surface rupture with a component ofdisplacement that is oblique to the fault. In (a) there is acomponent of extension, with a resultant down-dropped areawithin the shear zone (Lavic Lake Fault, 1999). The inset shows anegative flower structure. (b) Shows a fault with a component ofcompression and consequent uplift of a sliver within the fault zone(Emerson Fault, 1992). The inset shows a positive flower structure[(a) photo by J. Treiman, CGS; (b) photo by W. Bryant, CGS].

Figure 5. (a) Vertical aerial view of thrust-faulting and uplift at acompressional stepover along the Lavic Lake Fault, 1999 HectorMine earthquake (photo by I. K. Curtis, portion of frame 8–12);(b) uplifted playa sediments at a compressional stepover along theLavic Lake Fault (photo by K. Hudnut, U.S. Geological Survey).

Treiman


echelon faults (Figure 7). Although they have beendeveloped at laboratory scales, these fault patternsrepeat in nature at all scales, from the microscopic toexamples like the Lavic Lake Fault (Figure 3c) tofault zones that are several kilometers wide. If a faultstrand seems to be dying out in a series of trenches,consider looking for a stepover. Just as fault patterns

may evolve in strike-slip faulting, surface rupturealong evolving thrust or reverse faults may bepreceded by folding and then by shifts of locationfor the locus of most-active faulting (Figures 8 and 9).

Multiple parallel fault strands are also observedand may occur in all types of faulting. Examples areshown from strike-slip, normal, and reverse faultsituations (Figure 10a–c). Vertical fault displacementsoften involve multiple hanging wall faults accompa-nied by localized folding (Figure 6). Warping may bejust as damaging to a foundation as a discrete faultrupture (Figure 11).

Figure 12a–f illustrates some of the potentialvariation in surface expression of faults. Althoughthis figure shows vertical displacements and defor-mation, similar variations may occur in strike-slipdisplacement. Figure 12a shows a simple displace-ment of a presumed significant amount. Thresholdsof mitigable displacement, as mentioned in theintroduction to this paper, may be in the form of a

Figure 6. Variable expression (schematic) of thrust fault tips, asobserved in the 1999 Chi-Chi, Taiwan earthquake. Note the effectof changes in dip of the fault plane (modified from Kelson et al.,2001; their figure 26).

Figure 7. Results of Reidel shear experiment in laboratorydemonstrate integration of en echelon shears in a developingstrike-slip fault zone; D-value is displacement across the fault ateach stage of the experiment (from Tchalenko, 1970; their figure4).

Figure 8. Reconstruction of fault and scarp development dem-onstrating variable age and location of active fault traces. (a)Shows earlier trace and (c) shows latest evolution of the scarp.Note how the location of active slip can shift from one event to thenext (from Meghraoui et al., 1988; their figure 12).



certain maximum displacement, and avoidance maybe the best approach if the displacement is large. Butwhat if this maximum is distributed across severalfaults or a tightly constrained fold? Figure 12b showsthe same total displacement across two faults, andFigure 12c shows the same total displacement dis-tributed across a series of individually minor faults.Figure 13 shows an example of distributed faultingacross a zone of folding (Ventura Fault). If thisfaulting is confined to a narrow zone a structure’sfoundation may still respond as if it were a single

brittle rupture. The geologist needs to anticipate howmuch displacement might occur and over how broadof a zone. Only with this information can the engineerbegin to consider design recommendations.

Figure 9. Progression of a blind thrust fault into a surface fault.In (a) the fault has created an antiformal hill or ridge. In (b) thethrust fault is plowing underneath a shallow sediment layer withan active backthrust reaching the surface up the left slope of theridge. In (c) the primary fault has broken through to the surface,essentially deactivating the original backthrust. The example ismodeled from the Springville Fault in Ventura County,California (Whitney and Gath, 1991).

Figure 10. Multiple parallel faults: (a) parallel and sub-parallelstrike-slip faults in the Homestead Valley Fault Zone (Johnson et al.,1997; plate 4); (b) small parallel fractures in the hanging wall of anormal fault, Dixie Valley earthquake (Steinbrugge collection,courtesy of the National Information Service for EarthquakeEngineering, University of California, Berkeley); (c) northward viewof strands of the Cucamonga Fault zone—youngest strand (south-ernmost) marked by shafted arrows and oldest marked by solid whitearrowheads (photo by D. Morton, U.S. Geological Survey).

Treiman


Figure 12d illustrates the same fault-caused defor-mation as in Figure 12c, but without the brittlerupture extending to the surface. Figure 12e, with anarrower zone of folding, is similar in effect toFigure 12a. The comparability of these situations fordesign considerations should be evident and illus-trates the shortcomings of strictly interpreting regu-lations, such as the Alquist-Priolo Act, to apply onlyto surface rupture. As mentioned earlier, fault ruptureand related deformation occur within a continuum,extending to the more distributed deformation inFigure 12c or 12f, and raise the question ‘‘What is asurface fault?’’ This question can really be asked intwo ways: (1) How close can faults be to each otherbefore they are no longer considered as individualfaults?; and (2) How close to the surface must a faultbe to be considered a surface rupture hazard? Theformer question is raised by situations such as thatshown in Figure 10a or Figure 12c. The secondquestion arises in situations such as that depicted inFigure 12f or as illustrated in a trench exposure of abranch of the San Jacinto Fault in San Bernardino,California (Figure 14). Although this example in-cludes fairly shallow faulting, one can imagineincrementally increasing this depth until some geol-

ogists no longer will consider it a surface fault. Asdescribed earlier, faulting may also progressivelyapproach the surface with successive rupture events,and it may be up to the geologist to propose whetherthe next event will be the one to break the surface. Itwill be a challenge for geologists, engineers, reviewers,and regulators to apply rules and thresholds todifferentiate how these situations should be treated.

HOW MUCH SLIP SHOULD BE ANTICIPATED?

Once we have established where past rupture hasoccurred and inferred the likely distribution of futurefaulting we still need to evaluate how significant thatrupture might be to a structure. This will depend onboth the style and magnitude of the displacement.Just as with fault location, we often judge the natureof potential displacement based on evidence of pastdisplacements. That evidence, however, may beincomplete. Incomplete data result in part from gapsin the geologic record, but also from our limitedability to observe and interpret the data that remain.Even where slip data are preserved and recorded theymay be misleading because of variability in themagnitude of fault displacement. Variations in faultstyle also need to be considered to understand howslip may be distributed at the ground surface.

Incomplete Data

Problems with characterizing potential displace-ments and fault hazard first arise with the necessarilylimited nature of a site investigation. Geographically

Figure 11. Complex fold, backthrusts, and uplift at the leadingedge of the Chelungpu Fault (thrust) during the Chi-Chiearthquake (Lee et al., 2001).

Figure 12. Diagrams of relationships between fault displacementand surface displacement (scale intentionally is indeterminate). (a)Simple single fault offsets ground surface; (b) two close simplefaults create same total vertical separation as in (a); (c) multipleclosely spaced faults create same vertical separation within samedistance as in (b); (d) buried fault creates same surfacedeformation as multiple surface faults in (c); (e) buried fault withnarrower zone of folding more closely mimics single surface fault;(f) deeper buried fault causes broader warp of surface.



limited investigations, and particularly trenches, giveus a very narrow view of the past earthquake historyand, consequently, of the future potential for fault-related deformation. Each exposure is a snapshot, ifyou will, of the fault expression in one very limitedlocation and usually within a relatively restricted timeframe, which means there is a possibility that we arenot seeing the maximum displacement possible. Evenmultiple exposures allow for varying interpretationsand likely do not give us a complete picture of thebreadth and variability of faulting (as shown inFigure 3). Furthermore, most trenches only exposethe geometry of the fault in two dimensions. We may

see vertical separations (given favorable stratigraphy),but it is much more difficult to judge the lateralcomponent of slip, or the total slip, from thesevertical exposures. Additional trenches and/or three-dimensional trenches are needed if we even hope toelucidate the true slip potential at a site, unless theinvestigation site is undisturbed and retains geomor-phic indicators of past displacement. As discussedearlier, the study of Bonilla and Lienkaemper (1991)showed that trench exposures often provide only anincomplete record of past displacement, and evidenceof individual rupture events is not always visible inevery trench. Without the ability to see all of the pastfaults it will be extremely difficult to characterize thedisplacement potential.

Temporal Variation

At any point along a fault the amount of slip perevent may vary from one earthquake to the next,meaning that the slip observed from a prior eventmay not necessarily be characteristic of that fault. Arecent example of different slip distributions alongnearly identical rupture segments occurred along theParkfield segment of the San Andreas Fault in 1966and 2004 (Lienkaemper et al., 2006). The 2004 eventhad less slip at almost all locations than did the 1966event (Figure 15). The slip difference from oneearthquake to the next may result from a wide rangeof factors, including variation in accrued strain (dueto variable time intervals between ruptures), incom-plete release of strain in the last event (or the nextone), changing strain thresholds for rupture, irregulartransfer of slip across stepovers, strain accommoda-tion through folding, evolving fault geometry, posi-tion along the fault rupture, and total rupture length.

Figure 13. Multiple extensional faults with normal separation lie within the scarp of the Ventura Fault (modified from Sarna-Wojcicki etal., 1976).

Figure 14. Shallow faulting within the flank of uplift along theSan Jacinto Fault zone, San Bernardino, California (modifiedfrom Leighton & Associates, 1996).

Treiman


If a fault is being loaded (or relieved) by interactionwith other nearby or regional faults, then even thestrain rate on a fault may vary with time. If there havebeen physical changes that affect fault propagationsince the prior rupture, such as sedimentation, erosion(or grading), or changes in saturation or consolida-tion, the next surface rupture may also be moreconstrained or diffuse than a past event.

Differing length of rupture and overlap of rupturesegments is indicated for the southern San AndreasFault (Weldon et al., 2004; Biasi and Weldon, 2009).The observations of these authors led to the conclusionthat at some specific locations there have been differentamounts of slip during different events. This may beparticularly relevant where a study site is near the endof one rupture event but in the middle of another. Anextreme example of temporal variation is documentedby historic rupture in California’s Mojave Desert. In1979 there was a small earthquake swarm between theJohnson Valley Fault and the Homestead Valley Fault.The swarm was accompanied by very minor strike-slipshear (10 cm maximum, but commonly ,4 cm) alongthe two fault zones (Hill et al., 1980). A trench in 1980might have exposed minor fracturing to the surface,with perhaps a few centimeters of vertical separationon only the most prominent fractures. However, in1992 the same area experienced 1 m to 2 m of right-lateral displacement that might not have been char-acterizable prior to that event.

Spatial Variation

In any given earthquake slip is commonly variablealong the strike. Such variation, across a variety offault styles, is evident in the survey of a number ofhistoric ruptures by Wesnousky (2008). In this datawe see a considerable variation in surface displace-ment magnitude along strike-slip, normal, reverse,and thrust rupture events. If a fault study is onlylooking at paleoseismic data for a short section of a

fault, or over a short time frame, the variability, andparticularly the maximum potential displacement,may not be recognized. As just one example, considerthe slip distribution from the 1999 Duzce earthquake(Figure 16). A conservative approach (in hindsight)would be to construct the upper curve to modeldesign displacements for a similar rupture. However,without benefit of historic rupture or more completepaleoseismic data, a trench at the location indicatedby the arrow might have led to the lower slipdistribution curve for that fault segment. Slipmagnitude will also vary in a more general sensebased on where you are within a rupture segment. Asin the Duzce earthquake example, displacement mustdiminish to zero at the ends of the rupture, with oneor more slip maxima distributed along the rupture. Ifyour site is near the end of a segment, the slip fromthe last event may not be an accurate indication ofslip potential elsewhere along the segment (or at thatlocation in a different rupture event). Thus, it isimperative to have data from more than one site totry to characterize the displacement potential of anactive fault.

Variations in Style

There may also be variation in fault style within anevent that can affect slip geometry and magnitude.Inflections in the fault geometry may lead to variouslocal areas of compression or extension (as shown inFigures 4 and 6). Strain partitioning may yield boththrust faulting and strike-slip faulting on separate

Figure 15. Comparison of fault displacement associated with the1966 and 2004 Parkfield earthquakes. The 1966 event ruptured,37 km and the 2004 earthquake ruptured ,32 km of the fault;both were ,M6.0 (modified from Lienkaemper et al., 2006).

Figure 16. Plot of displacement values along the fault rupturefrom the 1999 Duzce earthquake. Upper curve added to plot isconservative envelope of rupture magnitudes for that earthquake.Lower dotted curve (also added) is what might be constructed forthe same segment in the absence of historical data and based on asingle trench at the arrow (modified from Wesnousky, 2008;electronic supplement, section 1).



faults within a single site (Figure 17). Carefulconsideration of overall fault style and geometry isrequired to interpret slip style and displacementmagnitudes within a complex zone of multiple traceswhen your only data comes from trenches. In order toreasonably assess the displacement potential of a faultone must have data from several past events and alsomust understand the style of faulting and interactionswith other faults. Where we don’t have a clear surfacerupture we need to anticipate the locus of maximumdeformation based on a reasonable model of the faultstyle and past displacements.

HOW DO WE ADDRESS THE HAZARD BASEDON INCOMPLETE DATA?

Having recognized that even a very competentinvestigation may not identify every potential faulttrace and may not be able to forecast the maximumdisplacement, a reasonable approach to the problemof such incomplete data is the judicious application ofbuffers around the uncertainty. The classic approachis the spatial buffer—a structural setback zone wideenough to contain the possible distribution of brittlerupture and associated deformation. A secondapproach, now being explored by the State Mining& Geology Board and addressed at the recent AEG-Shlemon Specialty Conference, is the design buffer—structural accommodation of minor displacementwith a sufficient factor of safety to encompass arealistic margin of error. Both of these approaches areessentially deterministic, and the latter approach is

predicated upon the ability of the geologist tounderstand and characterize the displacement poten-tial within a fault zone.

Probabilistic approaches to fault displacement andfault distribution are also currently being explored,but primarily with respect to lifelines, which may nothave the luxury of fault avoidance (PEER, 2009). Ifsociety or local communities can agree on what is anacceptable risk for occupied structures, then proba-bilistic analyses (when better developed) may providea third approach to the fault rupture hazard problemfor structures. Classic displacement-magnitude rela-tionships have been put forth by Wells and Copper-smith (1994) and may be useful for estimating themagnitude of displacement along the primary faultbased on rupture length or earthquake magnitude (ifknown). The distribution of slip across secondaryfaults is still being evaluated (Coppersmith andYoungs, 2000; Youngs et al., 2003; and Petersen etal., 2006). These studies have been primarily fornormal and strike-slip faulting, and circumstancesmay require a fallback to deterministic approaches ifone intends to design for such rupture, especiallywhere a reverse or thrust displacement component isinvolved. Without an adequate characterization ofthe distributed ruptures structural setbacks willremain the primary mitigation.

CLOSING

In this paper I have catalogued some of the reasonsfor the uncertainty associated with assessing the

Figure 17. Mapped traces of the Simi Fault within a regulatory zone (CDMG, 1999) include a high-angle left-lateral trace and lower-anglethrust faults. Strain on this left-oblique fault may be partitioned between these different traces in this area of the Simi Valley East 7.5-minutequadrangle (Treiman, 1998).

Treiman


potential location, style, and magnitude of futurerupture. The point of dwelling on these uncertaintiesis to provide the consultant with a sense of whatmight not be evident in a typical fault investigationand, in fact, what might not be discovered in even amore thorough study. Yet these very factors arecritical in the attempt to design a structure towithstand such deformation. One part of the solutionlies, of course, in careful investigation and analysis.Another part comes from experience and exposure tothe various possibilities. In that regard, consider thisan armchair adventure. The geologist who wouldcharacterize fault rupture for design purposes mustthink beyond the walls of his trenches to the biggerpicture that encompasses the full complexity of thefault zone.

REFERENCES

BIASI, G. P. AND WELDON, R. J., II, 2009, San Andreas fault

rupture scenarios from multiple paleoseismic records: String-

ing pearls: Bulletin Seismological Society America, Vol. 99,

pp. 471–498.

BONILLA, M. G. AND LIENKAEMPER, J. J., 1991, Factors Affecting

the Recognition of Faults Exposed in Exploratory Trenches:

U.S. Geological Survey Bulletin 1947, 54 p.

CALIFORNIA DIVISION OF MINES AND GEOLOGY (CDMG), 1988,

Special Studies Zones, Melville Lake Quadrangle, Official

Map: California Division of Mines and Geology, 1:24,000, 1

March 1988 [note: this is an archived obsolete version of the

Official Map].

CALIFORNIA DIVISION OF MINES AND GEOLOGY (CDMG), 1999,

Earthquake Fault Zones, Simi Valley East Quadrangle,

Official Map: California Division of Mines and Geology,

1:24,000, 1 May 1999.

CLARK, W. B. AND HAUGE, C. J., 1971, When … the earth quakes

… you can reduce the danger: California Geology, Vol. 24,

No. 11, pp. 203–216.

COPPERSMITH, K. J. AND YOUNGS, R. R., 2000, Data needs for

probabilistic fault displacement hazard analysis: Journal

Geodynamics, Vol. 29, pp. 329–343.

GORDON, F. R., 1971, Faulting during the earthquake at

Meckering, western Australia—14 October 1968: Bulletin

Royal Society New Zealand, Vol. 9, pp. 95–96.

HILL, R. L.; PECHMAN, J. C.; TREIMAN, J. A.; MCMILLAN, J. R.;

GIVEN, J. W.; AND EBEL, J. E., 1980, Geologic study of the

Homestead Valley earthquake swarm of March 15, 1979:

California Geology, Vol. 33, pp. 60–67.

JOHNSON, A. M.; FLEMING, R. W.; CRUIKSHANK, K. M.; MARTO-

SUDARMO, S. Y.; JOHNSON, N. A.; JOHNSON, K. M.; AND WEI,

W., 1997, Analecta of Structures Formed during the 28 June

1992 Landers–Big Bear, California, Earthquake Sequence:

U.S. Geological Survey Open-File Report 97-94, 59 p.

KELSON, K. I.; KANG, K.-H.; PAGE, W. D.; LEE, C.-T.; AND CLUFF,

L. S., 2001, Representative styles of deformation along the

Chelungpu Fault from the 1999 Chi-Chi (Taiwan) earth-

quake, geomorphic characteristics and responses of man-

made structures: Bulletin Seismological Society America,

Vol. 91, No. 5, pp. 930–952.

LAWSON, A. C., 1908, The California Earthquake of April 18, 1906,

Report of the State Earthquake Investigation Commission:

Carnegie Institution of Washington, Publication No. 87,Vol. 1, Part 1, 451 p.

LEE, J.-C.; CHEN, Y.-G.; SIEH, K.; MUELLER, K.; CHEN, W.-S.;CHU, H.-T.; CHAN, Y.-C.; RUBIN, C.; AND YEATS, R., 2001, Avertical exposure of the 1999 surface rupture of theChelungpu Fault at Wufeng, western Taiwan; Structuraland paleoseismic implications for an active thrust fault:Bulletin Seismological Society America, Vol. 91, No. 5,pp. 914–930.

LEIGHTON & ASSOCIATES, 1996, Seismic Hazard Assessment of theSan Bernardino Valley College Campus, San Bernardino,California: unpublished consultant report, Leighton &Associates, 30 August 1996.

LIENKAEMPER, J. J.; BAKER, B.; AND MCFARLAND, F. S., 2006,Surface slip associated with the 2004 Parkfield, California,earthquake measured on alignment arrays: Bulletin Seismo-logical Society America, Vol. 96, No. 4B, pp. S239–S249.

MCCALPIN, J., 1996, Paleoseismology: Academic Press, San Diego,CA, 588 p.

MEGHRAOUI, M.; PHILIP, H.; ALBAREDE, F.; AND CISTERNAS, A.,1988, Trench investigations through the trace of the 1980 ElAsnam thrust fault; Evidence for paleoseismicity: BulletinSeismological Society America, Vol. 78, No. 2, pp. 979–999.

PEER, 2009, Surface Fault Displacement Hazard Workshop:Pacific Earthquake Engineering Research Center, May 20–21, 2009: Electronic document, available at http://peer.berkeley.edu/events/2009/sfdc_workshop/

PETERSEN, M.; CAO, T.; DAWSON, T.; WILLS, C.; AND SCHWARTZ, D.,2006, Estimating fault displacement hazard for strike-slipfaults: Seismological Society of America, Abstracts of theAnnual Meeting: Seismological Research Letters, Vol. 77,No. 2, p. 277.

SARNA-WOJCICKI, A. M.; WILLIAMS, K. M.; AND YERKES, R. F.,1976, Geology of the Ventura Fault, Ventura County,California: U.S. Geological Survey Miscellaneous FieldStudies Map MF-781, 3 sheets, 1:6000.

SEXTON, C. J., 2008, Implementing the California EarthquakeFault Zoning Act, a proposal for change: EnvironmentalEngineering Geoscience, Vol. 14, No. 1, pp. 43–51.

TCHALENKO, J. S., 1970, Similarities between shear zones ofdifferent magnitudes: Geological Society America Bulletin,Vol. 81, pp. 1625–1640.

TREIMAN, J. A., 1998, Simi–Santa Rosa Fault Zone in theMoorpark, Newbury Park, Simi Valley East, Simi ValleyWest, and Thousand Oaks Quadrangles, Ventura County,California: California Division of Mines and Geology, FaultEvaluation Report FER-244, 5 October 1998, 21 p., tables,maps (1:24,000).

WELDON, R.; SCHARER, K.; FUMAL, T.; AND BIASI, G., 2004,Wrightwood and the earthquake cycle; What a longrecurrence record tells us about how faults work: GSAToday, Vol. 14, No. 9, pp. 4–10.

WELLS, D. L. AND COPPERSMITH, K. J., 1994, New empiricalrelationships among magnitude, rupture length, rupturewidth, rupture area, and surface displacement: BulletinSeismological Society America, Vol. 84, pp. 974–1002.

WESNOUSKY, S. G., 2008, Displacement and geometrical charac-teristics of earthquake surface ruptures: Issues and implica-tions for seismic-hazard analysis and the process of earth-quake rupture: Bulletin Seismological Society America,Vol. 98, No. 4, pp. 1609–1632 & online electronic supple-ments.

WHITNEY, R. A. AND GATH, E. M., 1991, Structure, tectonics, andsurface rupture hazard at the Las Posas anticline, VenturaCounty, California. In Blake, T. F. and Larson, R. A.(Editors), Engineering Geology along the Simi–Santa Rosa



Fault System and Adjacent Areas, Simi Valley to Camarillo,Ventura County, California: Association of EngineeringGeologists, Southern California Section Field Trip Guide-book, Vol. 2, pp. 164–182.

YEATS, R. S.; SIEH, K.; AND ALLEN, C. R., 1997, The Geology ofEarthquakes: Oxford University Press, New York, 568 p.

YOUNGS, R. R.; ARABASZ, W. J.; ANDERSON, R. E.; RAMELLI, A. R.;AKE, J. P.; SLEMMONS, D. B.; MCCALPIN, J. P.; DOSER, D. I.;

FRIDRICH, C. J.; SWAN, F. H., III; ROGERS, A. M.; YOUNT, J.C.; ANDERSON, L. W.; SMITH, K. D.; BRUHN, R. L.; KNUEPFER,L. K.; SMITH, R. B.; DEPOLO, C. M.; O’LEARY, K. W.;COPPERSMITH, K. J.; PEZZOPANE, S. K.; SCHWARTZ, D. P.;WHITNEY, J. W.; OLIG, S. S.; AND TORO, G. R., 2003, Amethodology for probabilistic fault displacement hazardanalysis (PFDHA): Earthquake Spectra, Vol. 19, No. 1,pp. 191–219.

Treiman


fault rupture and fault deformnation

Documents