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.