USING AFTERSHOCKS AND 3D MODELS OF THE M7.3 LANDERS, CA EARTHQUAKE TO CONSTRAIN SUBSURFACE STRUCTURE OF A COMPLEX FAULT SYSTEM Betsy Madden & David D. Pollard Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 e-mail: [email protected]

Abstract fault development and thus for hydrocarbon production as well as seismic hazard. The M7.3 June 28 1992 Landers earthquake The nature of multi-fault earthquakes raises ruptured N and NW across segments of five sub- interesting and perplexing questions about the nature of parallel faults. The rupture geometry in plan-view is three-dimensional fault structure and the character of complex and raises the question as to whether the fault seismic events such as: structure at depth is equally complex. This project aims to constrain the subsurface fault geometry by comparing aftershock focal mechanisms with modeled How do mapped rupture traces and seismicity at failure planes determined at aftershock locations. depth relate to subsurface fault geometry? Observed aftershocks are taken as evidence of the local How does fault structure influence fault interaction stress state at depth and models are evaluated by their and fault development? ability to reproduce that local stress. Structural models How does spatial variation in geology at depth with variable levels of fault segmentation, remote stress affect fault geometry and earthquake rupture? orientation, and fault dip are compared. The local stress state at depth likely is influenced by the Much scientific literature is devoted to the topic of subsurface geology, which controls both in the internal fault linkage and segmentation. Non-planar fault angle of friction and the elastic moduli. The effect of geometries, slip distributions and 3D imaging of faults lateral and vertical changes in geology are not have been utilized to support different kinematic and modeled, but are addressed in the final section. geometric models of fault growth over multiple seismic events (Walsh et al 2003). For example, in the “isolated Keywords: strike-slip faults, 3D fault structure, aftershocks, Poly3D, Landers

Introduction Multi-fault, strike-slip earthquakes such as the Landers event are not unique. Such behavior has been documented for several other earthquakes including the 1989 Loma Prieta earthquake, the 1999 Izmit earthquake (Barka 1999, Reilinger et al 2000) and the 2002 Denali earthquake (Eberhart-Phillips et al 2003). This has led to efforts to characterize statistically when ruptures jump from one fault to another (e.g. Black and Jackson 2008, Wesnousky 2008), and to model the mechanics of such behavior (e.g. Fliss et al 2005, Templeton et al 2009). These events are of great concern because they can be larger than earthquakes predicted for any one fault involved (Jackson 1996, Oglesby et al 2003, Black and Jackson 2008). Multi- fault ruptures also suggest that the relationship between the behavior of faults in complex, active tectonic Figure 1. a) Map showing the Landers event and regimes and the fault traces mapped at the surface or other recent earthquakes in the Landers area. interpreted along depth slices of seismic reflection data Inset shows location in California (from Langenheim and Jachens 2002, Figure 1). also is complex. The intricacy of this relationship between fault traces in 2-dimensions and fault structure in 3-dimensions has implications for fault behavior,

Stanford Rock Fracture Project Vol. 21, 2010 K-1 fault model”, faults initiate independently, begin to a) interact, and then link to form larger faults over several earthquake cycles (Peacock and Sanderson 1991, Cartwright et al 1995, Childs et al 2009). Fault linkage has been shown to occur through secondary structures or as through-going linkage of the main structures (e.g. Segall and Pollard 1980, Segall and Pollard 1983). In one study, Lohr et al (2008) use the out-of-plane surface geometry and slip distribution along a 15km- long fault imaged from seismic reflection data to identify four “generations” of previously segmented faults. Walsh et al (2003) suggest that the isolated fault model overlooks the spatial organization of fault segments, the out-of-plane fault propagation, and the likelihood of fault bifurcation during propagation in heterogeneous media. These support the initiation and evolution of faults as one connected system, termed the “coherent fault model” (Walsh et al 2003) and account for the influence of subsurface geology on fault behavior. Aggregate fault slip distributions of coherent fault arrays appear smooth, similar to the slip distribution for a single, isolated fault (Walsh et al 2003). While one cannot rule out that both linkage of isolated faults and coherent development of fault segments occur as a fault system develops, it may be that these processes are distinct from one another and that factors controlling such behavior, such as the local state of stress around faults, can be explored with b) mechanical models. While it is difficult to determine and model the stress state that has surrounded a fault over its entire development, aftershocks from individual seismic events provide insight into the local stress state at discrete locations following an earthquake. c) Individual large aftershocks are used here to constrain structural models of the Landers faults and address the relationship between surface fault traces and subsurface geometry. Studying fault behavior below the surface in a multi-fault earthquake provides insight into fault development during a single seismic event.

Data The Landers earthquake occurred in the Mojave Desert area of southeastern California, in the Eastern California Shear Zone, an area marked by multiple right-lateral strike-slip faults and an area of high seismic activity, as shown in Figures 1 and 2. The Figure 2. a) Faults in the Eastern California Shear earthquake ruptured parts of five right-stepping faults Zone. Colors indicate recency of faulting, prior to named, from south to north, the Johnson Valley, 1986. Box shows location of 2b). b) Fault map Landers-Kickapoo, Homestead Valley, Emerson and with Landers surface rupture overlaid. Star the Camp Rock faults (Figure 2b). The rupture traveled shows earthquake epicenter. Small box outlines almost unilaterally north from its epicenter along the location of Figure 3a. c) Key showing timing of southern portion of the Johnson Valley fault. Following fault rupture pr16 ior to 98 (from Bortugno, 1986). the Landers mainshock, geologists undertook extensive

Stanford Rock Fracture Project Vol. 21, 2010 K-2 a) mapping of the surface deformation that was well exposed in the desert environment (e.g. Hart et al 1993, Johnson et al 1993, Aydin and Du 1995, McGill and Rubin 1995, Spotila and Sieh 1995, Zachariasen and Sieh 1995, Johnson et al 1997, Fleming et al 1998). This work utilizes a digital database of the rupture compiled from these surface maps by geologists at the California Geological Survey (CGS 2002, Bryant 2004). The overlay in Figure 2b is a simplification of the rupture extending along all five faults. Figure 3a shows the detailed rupture trace along the Johnson Valley, Landers-Kickapoo and southern Homestead Valley faults, while Figure 3b shows an example of a detailed rupture map from which this digitized surface trace was compiled. The work presented here focuses on the Johnson Valley fault. Focal mechanisms for aftershocks relocated by Zanzerkia (2003) were determined by Jean Hardebeck at the U.S. Geological Survey in March, 2010. Aftershocks along relatively straight, mature faults such as the Calaveras fault in Northern California (Figure 4), appear confined to the fault traces in map view and the extent of the fault at depth in cross-section, especially after relocation (Waldhauser and Ellsworth 2000, Simpson et al 2006, Schaff et al 2002).

b)

Figure 4. a) Map-view of aftershocks along the Calaveras fault prior to relocation. Plot is 40km wide Figure 3. a) Digitized rupture trace of Landers and 12 km high. b) Aftershocks along strike of fault earthquake along Johnson Valley fault for location at depth prior to relocation. Boxes are 6km across outlined in Figure 2b. Box outlines location of 3b and 12 km deep. c) Map-view of relocated (CGS 2002, Bryant 2004). b) Detailed map of aftershocks. Plot is 40km wide and 12 km high. d) surface deformation in Flamingo Heights area Relocated aftershocks along strike of fault at depth. along Johnson Valley fault from which digitized Boxes are 6km across and 12 km deep. rupture traces were made (CGS 2002, Bryant 2004). (Schaff et al 2002, Figures 2, 3, 6).

Stanford Rock Fracture Project Vol. 21, 2010 K-3 Yet aftershocks along the Landers rupture are divergent solutions. Along the central part of the fault (Figure 7), from the main fault trend and tightly clustered, as 379 aftershocks have focal mechanism solutions and shown in the aftershock density plot compiled by Liu et 107 of these have uncertainties under 35, the upper al (2003) (Figure 5). These authors find that, of the 40% uncertainty limit for aftershocks analyzed here. of aftershocks that occurred within 500m from the Distinct clustering is apparent in map-view and at mainshock rupture plane (approximated as a series of depth (Figure 6) and several of these clusters have been straight lines along the rupture path), less than 30% identified as the “focus areas” (Figure 7). While several exhibit focal mechanisms similar to that of the focus areas have been explored, only Focus Area 1b mainshock. They conclude that the fault damage zone, (FA1b), FA2d and FA2e are presented here because not the mainshock fault planes, dictates aftershock they include the largest numbers of aftershocks with locations and orientations. focal mechanism solutions and large events with focal This analysis takes a mechanical perspective by mechanisms solutions of low uncertainty. While attributing the aftershocks to the local stress state and hypothesizes that the events are not random occurrences in a damage zone, but clearly dictated by the stress changes resulting from the mainshock. Thus, aftershock data is used to constrain structural models to gain insight into fault segmentation, remote stress orientation and fault dip as well as the relationship between fault geometry expressed at the surface and structure at depth.

Methods 9414 aftershocks occurred along the Johnson Valley fault from June 28, 1992 through the end of 1998. 1593 events, approximately 17%, have focal mechanism

Figure 5. a) Plot of aftershock density along Landers Figure 6. a) Aftershocks along the Johnson Valley rupture. Faults are labeled with initials: EP (Eureka fault in a) map-view and b) perpendicular to the Peak), JV (Johnson Valley), HV (Homestead Valley), fault trace from the East. EM (Emerson) and CR (Camp Rock). b) Histogram of aftershock depths (Liu et al 2003, Figure 2).

Stanford Rock Fracture Project Vol. 21, 2010 K-4 aftershocks along the central Johnson Valley fault PARAMETER VALUE predominantly exhibit strike-slip and normal motion, Poisson’s ratio 0.25 Shear modulus strikes are heterogeneous (Figure 8a). Yet within 30 GPa individual focus areas, dominant strike orientations are (Liu et al 2003) identified for certain types of motion. For example, Young’s modulus 75 GPa Figures 8b-d show that right-lateral, strike-slip motion Bulk modulus 50 GPa is oriented in different directions in the three different Porosity 0 focus areas. This illustrates that the focus areas are Density 3 distinct not only in aftershock spatial distribution, but (Langenheim and Jachens, 2700kg/m also by aftershock orientation and thus relatively 2002) uniform the local state of stress. Half-space yes Aftershock focal mechanism solutions include two Remote stress magnitude max horizontal stress gradient: (Lund and Zoback, 1999) 30e3 Pa/m possible fault planes, both of which are included in the min horizontal stress gradient: plots shown in Figure 8. This fault plane ambiguity 15e3 Pa/m vertical stress gradient: introduces an additional challenge in working with aftershocks. Assumptions are often made in order to 25e3 Pa/m select one of the two fault plane solutions, such as an Remote stress orientation N07E (Hauksson 1994, Aochi and N15E agreement between aftershock and mainshock Fukuyama 2002) N22E 1-segment Fault model 4-segment 9-segment Vertical Fault dip 80E 80W 0.6 Internal coefficient of friction (for failure planes only)

Table 1: Model parameters.

orientation and sense of motion (e.g. Hauksson 1994). In this project, modeling the local stress state ideally will allow the correct aftershock fault plane solution to be selected from the two options without making a priori assumptions about the stress state and fault behavior at that location. The choice to display right-lateral events in Figure 8 is arbitrary, but informed by the presence of possible right-lateral events in all focus areas, allowing for comparison of variations in fault plane orientations between areas. Events with left-lateral and normal motion also exhibit dominant orientations, though these are not shown. As mentioned, aftershocks are used in this work to provide additional constraint on three-dimensional structural models of the Johnson Valley fault. The modeling tool utilized in this work is Poly3D, a quasi- static, boundary element program that relates either displacement discontinuity or traction on triangular dislocation elements, and remote stress or strain conditions to stress, strain and displacement fields in the surrounding isotropic, linear-elastic half-space (Comninou and Dunders 1975, Thomas 1993). Poly3D accommodates non-planar 3D fault geometries by using triangular rather than rectangular elements (e.g. Okada Figure 7. a) Focus areas (FAs) along Johnson 1985). Valley fault: FA1b (green), FA2d (blue) and FA2e (pink). Larger black box outlines the central part of the fault referred to in the text. Units are meters.

Stanford Rock Fracture Project Vol. 21, 2010 K-5 Poly3D has been used extensively to study fault mechanics (e.g. Willemse 1997, Maerten et al 2005, Dair and Cooke 2009) and the correspondence between local stress perturbations caused by primary faults and the formation of secondary faults (e.g. Maerten et al 2002) and is well suited for the analysis presented here. Results are displayed along observation grids created by the user. A non-planar grid was constructed from the locations of all the aftershocks with focal mechanism solutions along the central Johnson Valley fault, so the stress tensor has been determined at each aftershock location. Structural models of faults with 10cm or more slip were constructed from maps of surface rupture traces (CGS 2002) at three spatial scales: 1:1,000,000, 1:500,000 and 1:33,333, as shown in Figure 9. These models increase in their level of segmentation over this range of scales from a continuous, 1-segment model, to 4- and 9-segment models. In addition to variations in segmentation, model variables also include fault dip (90, 80E, 80W) and remote stress orientations (N07E, N15E, N22E), for a total of 27 model configurations. Table 1 summarizes these variables and the additional parameters specified in Poly3D. Models are evaluated by their ability to reproduce the stress state reflected by the focal mechanisms of the first and second aftershocks of M2.8 or greater within each focus area. The orientations of potential failure planes modeled at the aftershock locations are calculated with the Coulomb criterion illustrated in Figure 10, where = ½*atan(1/) and is the angle between the greatest compressive stress (1) and the failure planes. is equal to 30 for the internal coefficient of friction () of 0.6 utilized here. The effects of the three variables (segmentation, remote stress orientation and fault dip) on the modeled failure planes are demonstrated in Figure 11, where observed aftershocks for all events located along the central Johnson Valley fault are plotted with potential failure planes for several model configurations. The orientations of modeled events shift in orientation based RAKE MOTION on the different local stress states at the aftershock locations resulting from the different model scenarios. 0 Leftlateral Figure 11a shows variation due to the model 90 Reverse segmentation, Figure 11b shows the effect of the remote stress orientation, and Figure 11c shows 180 Rightlateral changes due to fault dip with all other variables held 270 Normal constant. For the model configurations shown in Figure 11, it appears that the remote stress orientation has the Figure 8. a) Strike and rake of observed aftershocks smallest effect on potential failure plane orientation, along the central Johnson Valley fault. b) FA1b: Rake while fault dip has the most effect. The 1- and 9- of all events, strikes for aftershocks with rakes 160- segment models both produce distinct clusters of 200(right-lateral). c) FA2d: same as b). d) FA2e: same as b). All aftershocks occur 6/28/1992- potential failure planes, while the 4-segment model 12/31/1998 and have uncertainties below 35. produces more variable orientations. In general,

Stanford Rock Fracture Project Vol. 21, 2010 K-6 aftershocks appear to be more heterogeneous than Results modeled failure planes. This may be due to uncertainty associated with the aftershock focal mechanisms, which Focus Area 1b range up to 35, and to fault plane ambiguity (see Methods), which means that two solutions are plotted FA1b (Figure 7, green box) occurs on the for each event. Despite this, it seems apparent that more compressional side of a slight bend along the south- low-dip aftershocks (likely to be normal events) are central Johnson Valley fault. 66 events in this area have observed than are captured in model results. focal mechanism solutions and 17 of these have an uncertainty less than 35. Most are strike-slip events with rakes clustering around 0 and 180 (Figure 8b). Right-lateral potential solutions for these events cluster at strikes of 150 and 315. The first large aftershock in this area (# 3034783) was a M3.6 event that occurred at 550m depth on 7/4/1992, six days following the mainshock. The two focal mechanism solutions are:

(1a) strike: 155, dip: 76, rake: -163 (right-lateral) (1b) strike: 61, dip: 74, rake: -15 (left-lateral).

The second large aftershock (#3069069) to occur in this area was a M3.75 event at 1480m depth on 10/6/1992. The two focal mechanism solutions are:

(2a) strike: 347, dip: 77, rake: -165 (right-lateral) (2b) strike: 254, dip: 75, rake: -13 (left-lateral).

These aftershocks and the modeled results at their locations for each of the 27 model combinations are shown in Figure 17. The first aftershock is not well matched by any of the results for the dipping models, suggesting that the fault is vertical through this area. a) b) Results are slightly better for the 1- and 4-segment models relative to the 9-segment models. The closest fit is given by the fault plane solution (1a) and a vertical, 1-segment fault with a remote stress of N07E,

Figure 9. Structural models for 20km Johnson Valley fault as shown in Figure 3a shown with aftershocks. a) 1-segment, b) 4-segment, c) 9-segment models.

Figure 10. Coulomb criterion for the orientation () of potential failure planes. 1 is the greatest compressive stress, 3 is the least compressive c) stress.

Stanford Rock Fracture Project Vol. 21, 2010 K-7 suggesting that the fault also is through going in this location. The best fit for both models is the 1-segment, a) vertical fault with a remote stress of N15E or N22E, consistent with the first event except in remote stress orientation. This model is in agreement with fault plane solutions (1a) and (2a), both right-lateral events. The second aftershock was a larger event and may b) have occurred on pre-existing plane that was oriented more optimally for failure than the first event and may provide a better constraint on the structural model. These aftershocks occurred approximately 1km apart and the second event may have been influenced by the first event. This is addressed in the discussion section and will be tested in future work by including c) aftershocks planes in fault models.

Focus Area 2d

The aftershocks in FA2d form a lineation in map- Figure 11. Stereonets of normals to aftershock fault view that splays southeast off the main trace of the planes (red) and modeled failure planes (blue) for Johnson Valley fault just north and on the opposite side several model configurations. Each row has one of the fault from the area of extensive surface variable parameter with all other parameters held deformation shown in Figure 3b. 131 events have focal constant. a) Variable is fault segmentation: 1- mechanism solutions in this area and 50 of these have segment (left) to 4-segment (middle) and 9-segment an uncertainty under 35 and are analyzed here. Rakes (right), remote stress: N07E, dip: 90. b) Variable is are predominantly strike-slip, with many normal events remote stress orientation: N07E (left), N15E occurring as well. Right-lateral potential solutions for (middle), N22E (right), 4-segment model, dip: 90. c) Variable is fault dip: 80W (left), 90 (middle), 80E these events strike at approximately 125 or 315. (right), 4-segment model, remote stress at N15E. M3.51 (#3053187) and M3.59 (#3053192) aftershocks occurred 100m and twenty minutes apart on Many of the models show results consistent with 8/11/1992 at 3520m and 3550m depth. The first, at both observed aftershock orientations, but the models 11:08pm, has the fault plane solutions: that produce the best results for the first event do not produce the best results for the second event. This (1a) strike: 336, dip: 78, rake: -174 (right-lateral) suggests that stress state of the second event may well (1b) strike: 245, dip: 84, rake: -12 (left-lateral). have been affected by the first event, due to their proximity in time and space. The best-fit model for both The second, at 11:27pm, has the fault plane solutions: events is a 9-segment fault dipping 80E with a remote (2a) strike: 146, dip: 81, rake: -159 (right-lateral) stress orientation of N15E, consistent with the model (2b) strike: 53, dip: 69, rake: -10 (left-lateral). matching fault plane solution (1a) except for in the orientation of the remote stress. This model is in These events and the modeled failure planes at these agreement with fault plane solutions (1a), a right-lateral locations for the models are shown in Figure 18. event, and (2b), a left-lateral event. The first event is best fit by many of the model It is interesting to note that a M4.01 also occurs in configurations that include a fault dipping 80E as well this location one year later on 7/8/1993. This event as the 9-segment faults with any dip angle and a remote (#3112773) occurs at 2950m depth with fault plane solutions as follows: stress orientation of N22E. This suggests that the fault is segmented and/or dips through this region, in contrast (3a) strike: 131, dip: 77, rake: 164 (right-lateral) to its relatively straight surface trace. The two best (3b) strike: 225, dip: 74, rake: 14 (left-lateral). results are for right-lateral fault plane solution (1a) and a 9-segment fault dipping 80E with a remote stress Fault plane solution (3a) is consistent with (2a), while orientation of N07E and for fault plane solution (1b) (3b) is consistent with (1b), but neither (2a) nor (1b) are and a 9-segment, vertical fault with a remote stress the fault plane solutions matched by the best-fit models. orientation of N22E. This suggests that the stress state in this area may be changing over the one year between the second and

Stanford Rock Fracture Project Vol. 21, 2010 K-8 third largest events, in addition to between the first and a remote stress orientation of N15E or N22E. As the second large events. fault progresses northward into FA2d, it dips to the East and/or is segmented similarly to the 9-segment model. Focus Area 2e Many model configurations produce results that fit both large events that occurred in FA2e, just north of FA2d, The 20 events in FA2e with low-uncertainty, out of shortly after the mainshock. The best-fit results support the 79 events with focal mechanisms that occurred in a segmented fault that changes here to dip West. this area, have a similar distribution of rakes to those in Several general observations about the aftershocks FA2d, but with even more normal events. Right-lateral can be highlighted from these preliminary results. First, potential fault planes strike predominantly at 150, with it appears that both right-lateral and left-lateral events some events also oriented at 0. The spatial distribution are occurring in aftershock clusters FA2d and FA2e. of events is less tightly clustered than in FA2d. Thus the assumption that aftershocks mirror the strike The first large aftershock (#3041872) was a M3.59 and motion of the mainshock does not necessarily hold that occurred on 7/12/1992, two weeks after the for all events, in all locations along a fault. mainshock, at 5710m depth. The fault plane solutions Second, the influence of aftershocks on one another for this event are as follows: is likely important, especially for close, large events. This will be addressed in future models by including (1a) strike: 338, dip: 82, rake: -154 (right-lateral) aftershock fault planes and studying the resulting stress (1b) strike: 244, dip: 64, rake: -9 (left-lateral). orientations and nearby failure planes. Third, it seems that the state of stress is changing The next largest event that year (#3075134) was a over time in both FA1b and FA2d, where the 2nd large M2.81 aftershock that occurred on 11/7/1992 at 1180m aftershock fits better with higher remote stress depth with orientations than the first large event. This stress change may be due to the influence of the first large (2a) strike: 145, dip: 64, rake: -167 (right-lateral) aftershocks. It also appears that the stress state in FA2d (2b) strike: 49, dip: 78, rake: -27 (left-lateral). has changed prior to the summer of 1993, when a third large event occurred with different fault planes than These events occurred some distance from one either of those for the first two aftershocks taking place another, so it is not likely that the first influenced the in the summer of 1992. stress state of the second. Therefore, it may be that the This raises the question as to when the dominant stress state is constant between these events and that orientations in aftershock clusters shown in Figure 8 both may be used to constrain the structural model. become persistent: after the mainshock, after the first There are several configurations shown in Figure 19 large aftershock, or after the first two large events? Or that produce failure plane orientations consistent with does the stress state continue to change for some time? both focal mechanisms, but the best fit is for a 9- One way to address this question is by looking at the segment fault dipping 80W with a remote stress correspondence between the general pattern of failure orientation of N15E. This model fits fault plane plane orientations for a certain fault model with the solutions (1a), a right-lateral event, and (2b), a left- lateral event. For a remote stress orientation of N07E, any level of segmentation produces good results. For a higher angle of remote stress, the 4- and 9-segment a) b) models produce the best results. For a fault dipping 80E, the 1-segment model with a N15E remote stress orientation and the 4-segment model with a higher angle of remote stress fit both aftershocks. This area will have to be studied more closely to decipher between these results.

Summary and Discussion of Results The first, large aftershocks in FA1b and FA2d occur close together and the second event is likely influenced Figure 12: Normals to observed aftershock fault by the first event, a factor that will be studied more planess (red) in FA1b plotted with normals to closely in later models. Preliminary results taking both potential failure planes (blue) from a) the model events into consideration suggest a vertical, through- that best fits the first large aftershock occurring in FA1b and b) the model that best fits both the going southern Johnson Valley fault through FA1b with first and second aftershocks in FA1b.

Stanford Rock Fracture Project Vol. 21, 2010 K-9 general pattern of aftershock orientations for a planes are included, which increases the scatter of particular area. Figure 12a shows the aftershocks observed and modeled events. This is enhanced because plotted with the failure plane orientations of the best-fit the potential failure planes are determined based on an model for the first large aftershock in FA1b. Figure 12b internal coefficient of friction of 0.6. Therefore, vertical shows the aftershocks plotted with the failure plane failure planes strike 60 apart. Orientations for vertical orientations for the best-fit model for the both the first aftershocks, however, strike 90 apart. Nonetheless, the and second large aftershocks in FA1b. Figure 13 does patterns appear to be consistent with different the same for FA2d, though includes 3 stereonets observations of stress change within the different focus because the first aftershock was well matched by two areas. different model configurations. Figure 14 shows the results for the best-fit model in FA2e, which is the same Influence of Local Geology for the first aftershock alone and for the first and second aftershocks together. The local geology likely influences the geometry of The general pattern of aftershocks in FA1b are major faults and the characteristics of associated better matched by the model configuration that best fit seismicity. Figure 15 shows the Landers trace and both of the first large aftershocks, as this model results aftershocks overlaid on a surface geologic map. In this in a greater scatter of aftershock orientations that more area of the Mojave Desert, Mesozoic quartz monzonite closely resembles the observed events. It is difficult to and diorite as well as older granite and gneiss outcrop draw conclusions for the plots of events in FA2d, but in from Pleistocene and younger surficial sediments in general it seems that aftershock orientations are not various stages of consolidation (Dibblee 2008). Tertiary well matched by any of the three models. This is basalt flows cap some mountains in the area, but do not consistent with the observation that the stress here likely extend to depth, though associated dikes do cut continues to change following the second large through many of the older rocks in certain locations. aftershock. The plot of observed events within FA2e The extent of the igneous and metamorphic rocks at appears to be relatively well matched by the modeled depth is not well known, but in a recent study, failure planes, which is consistent with the theory that Langenheim and Jachens (2002) identify a large diorite stresses here are not changing by the time of the second inclusion nearby, referred to as the Emerson Lake Body large event. In general, it is difficult to draw (ELB, Figure 16), using magnetic and gravity data. The conclusions from these plots because both failure ELB is located east of the Landers rupture trace and west of the faults involved in the Hector Mine earthquake that occurred near Landers in 1999 (Figures 1, 16). The authors suggest that this high-density body a) b) (D=2750kg/m3) extends to at least 15km depth and that few aftershocks associated with either earthquake occur in this area. While there are right-lateral faults that trend NW-SE above the ELB, Langenheim and Jachens (2002) note that these have much lower records of slip than do the faults outside of this area and including those involved in the Landers rupture. The authors’ cross-section through the southern part of the Landers

c)

Figure 13: Normals to observed aftershock fault planes (red) in FA2d plotted with normals to potential failure planes (blue) from a) the first Figure 14: Normals to observed aftershock fault model that best fits the first large aftershock planes (red) in FA2e plotted with normals to occurring in FA2d, b) the second model the fits the potential failure planes (blue) from the model that first large aftershock and c) the model that best fits best fits the first and both the first and second both the first and second aftershocks in FA2d. Stanford Rock Fracture Project Vol. 21, 2010 K-10 area (Figures 16c and 16d, line A-A’) shows another quartz-diorite crust (Whitney et al 2004) and may be density variation at the Landers rupture, with a slightly appropriate given the presence of the ELB higher density to the east (2700kg/m3), closer to the (Langenheim and Jachens 2002). While model ELB, and a slightly lower density west of the rupture parameters will not change the orientation of the stress (2670kg/m3). tensor, and therefore will not change the potential Changes in rock type affect rock behavior by failure planes, they will affect the magnitudes of the changing the elastic moduli and the internal coefficient stresses in the system and therefore the locations most of friction. This can impact models such as those prone to failure. utilized in this work. Laboratory tests demonstrate the Rock types also vary by their internal coefficient of large variability of Young’s moduli within one rock friction, however, and this will affect the orientation of type, as well as variation between rock types. For rock potential failure planes being modeled. Internal types present at Landers, average values range from coefficients of friction range from 0.51-0.68 for 68GPa for gneiss, to 45GPa for granite and 22GPa for sandstone to 0.60-0.64 for granite (Jaegar et al 1979). sandstone (Pollard and Fletcher 2005, Table 8.2). The choice of 0.6 used here fits within these ranges, but Poisson’s ratios range from 0.22 to 0.25 for the same may actually vary from location to location, thus three rock types (Pollard and Fletcher 2005, Table 8.2). varying the angle between the two potential failure The choice of 75GPa for the Young’s modulus used in planes. This could affect the agreement between the the models for this project agrees with values for a orientations of modeled failure planes and observed events. Determining the orientation of the potential failure planes can be done with different values for the internal coefficient of friction in areas where different rock types are suspected. The choice of elastic moduli, however, must remain constant for the model using Poly3D and so poses a challenge. Different choices for these values can be used when studying different portions of the rupture, as would perhaps be important for the rupture along the Homestead Valley fault, north of the Johnson Valley fault focused on in the work present here. The Homestead Valley fault not only more closely approaches the ELB, but also cuts through multiple outcrops of quartz monzonite and older granitic rocks, as shown in Figures 15 and 16. This may have a great influence on the multi-fault nature of the Landers rupture.

Conclusions The approach presented here and the preliminary results suggest that aftershocks and an understanding of local geology, in conjunction with mechanical models, can be used to constrain models of complex, non-planar faults and improve interpretation of those faults at depth. In addition, this approach provides a way to use the local stress state in which aftershocks occur to determine their fault plane from the two provided in the focal mechanism solution. Thus, integrating geological and geophysical data provides new constraint on fault geometry at depth, remote stress orientation, and possible changes in stress over time. This approach will next be applied at the step-over between the Johnson Valley and Homestead Valley faults to better understanding the fault geometry at depth and gain insight into fault development over a single seismic event and multi-fault ruptures in general. Figure 15. Geologic quadrangle maps by Dibblee (2008) with aftershocks and surface rupture overlaid. Key adaptoed fr m Dibble e (2008).

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Figure 16. a) Pseudogravity and b) basement gravity plots showing the Emerson Lake Body, a high density body of Jurassic diorite between the Landers and Hector Mine earthquake surface ruptures Seismicity is overlaid. c) Same map as b) showing location of cross-section A-A’. d) Cross-section A-A’. No vertical exaggeration. Densities are in kg/m3, pseudodensity (pD) and magnetic susceptibility (S) are in 10-3SI. Stars show epicenters. (from Langenheim and Jachens 2002, Figure 6 and 7). Stanford Rock Fracture Project Vol. 21, 2010 K-12 Figure 17: FA1b Figure 18: FA2d

Stanford Rock Fracture Project Vol. 21, 2010 K-13 Figures 17-19. Read plots in landscape view. First largest (red dot) and second largest (blue dot) normals to aftershock fault planes in the focus area plotted with normals to potential failure plane solutions (red and blue stars) at each aftershock location for all 27 model configurations. Rows differ by fault dip: top row shows results for vertical fault models, middle row for faults dipping 80W and the last row for faults dipping 80E. Fault segmentation varies with each stereonet from 1-, to 4- to 9-segments, so that the 1st, 4th and 8th columns are all 1- segment models. Remote stress orientation is N07E for the first 3 columns, N15E for the middle 3 columns, and N22E for the last 3 columns.

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