Through-going Rupture Research Articles

Effect of Asymmetric Topography on Rupture Propagation Along Fault Stepovers

AbstractComplex fault systems are often located in regions with asymmetric topography on one side of a fault, and these systems are very common in Southern California. Along these fault systems, geometrical complexities such as stepovers can impact fault rupture. Previous rupture dynamic studies have investigated the effect of stepover widths on throughgoing rupture, but these studies didn't examine the influence of topography on the rupture behavior. To investigate the effect of asymmetric topography on rupture dynamics at stepovers, I consider three cases: (a) a flat topography, (b) a positive (mountain), and (c) a negative (basin) topography on only one side of the fault system outside of the stepover. In each case, I use the 3D finite element method to compute the rupture dynamics of these fault systems. The results show a significant time dependent variation of the normal stress for the topography cases as opposed to the flat surface case, which can have an important impact on rupture propagation at the stepover. For a positive topography on the right of the rupture propagation, there is a clamping effect behind the rupture front that prevents the rupture to jump a wider extensional stepover. The opposite is observed for a negative topography or for a positive topography on the left side of the rupture propagation, where the rupture can jump over a wider compressional stepover. These results suggest that topography should be considered in dynamic studies with geometric complexities such as stepovers, and perhaps bends and branched fault systems.

Geologic and geomorphic evidence for multi-phase history of strands of the San Andreas fault through the San Gorgonio Pass structural knot, southern California

Abstract The San Gorgonio Pass region of southern California is a locus of extensive Quaternary deformation within a multi-strand section of the San Andreas fault zone. The geomorphology of the San Gorgonio Pass region reflects the complicated history of geologic events in the formation of this structurally complex region. We define fault-bounded blocks in San Gorgonio Pass and focus on two that are characterized by extensive crystalline bedrock outcrops with similar bedrock lithologies. These two blocks are separated by the San Bernardino strand of the San Andreas fault. Morphometric variables, including local relief, slope, slope distribution, and surface roughness, consistently demonstrate distinctions between the bedrock upland regions of the two blocks. Geologic observations of the region highlight differences in Quaternary units within the two blocks, reflective of the differing surficial processes active in each block. Within the Kitching Peak block, the morphology highlights a lineament that we informally name the Lion Canyon lineament. This boundary more clearly differentiates the two regions, as compared to the mapped San Bernardino strand, and may represent the previously active strand or bounding structure in this section. The distinction in morphology and surficial processes leads to our interpretation that the Kitching Peak and Pisgah Peak blocks have experienced different uplift histories. This further leads to the conclusion that the San Bernardino strand, broadly defined, has been integrated, at some point in the past, with the Banning strand, allowing for through-going rupture along the fault system. This connectivity may have occurred along the Burro Flats section of the San Bernardino strand or the Lion Canyon lineament. The fault connection along the mapped trace of the San Bernardino strand is not currently evident at the surface, however, suggesting that the integration has been disrupted. We propose this is due to intervals of N-S compression in the region, manifest as slip along the San Gorgonio Pass fault zone and other regional faults. We present evidence for lateral displacement along the San Bernardino and Banning strands of the San Andreas fault, discuss the implications of these displacements, and propose a sequence of fault activity, including multiple phases of activity along the San Bernardino and Banning strand pathway to account for the structural complexity and lack of surficial fault continuity.

Comment on “Active Crustal Deformation in the Trans‐Mexican Volcanic Belt as Evidenced by Historical Earthquakes During the Last 450 Years” by G. Suárez et al.

Comment on “Active Crustal Deformation in the Trans‐Mexican Volcanic Belt as Evidenced by Historical Earthquakes During the Last 450 Years” by G. Suárez et al.

What Can Surface-Slip Distributions Tell Us about Fault Connectivity at Depth?

ABSTRACT Fault systems with stepovers and gaps along strike are ubiquitous in nature, and many modern earthquakes (e.g., 1992 Landers, 1999 Hector Mine, 2016 Kaikōura, and 2019 Ridgecrest) have shown that ruptures can readily propagate across some disconnections, while being halted by others. It is quite possible, however, that many faults that appear discontinuous at the surface are in fact connected at depth, facilitating throughgoing rupture, and potentially increasing earthquake size. The present work explores whether the mapped surface slip in an earthquake is indicative of the connectivity of the fault system at depth. If there is a signal of subsurface connectivity in the surface-slip pattern, then the connectivity of the system could potentially be inferred. Through 3D dynamic rupture modeling of faults with along-strike gaps of various depths, I explore whether the amplitude or the spatial distribution of slip after an earthquake could be used to diagnose fault connectivity at depth. I find that, in general, fault segments that are connected up to shallow depths of 1–2 km and are relatively long along strike compared to the seismogenic depth tend to have higher slip gradients at their edges than faults that are connected at greater depth, or that are disconnected to the bottom of the seismogenic zone. Systematic slip gradient differences at fault segment edges have been recorded in past earthquakes, giving hope that the modeled effect can be detected in many cases, even though mapped surface slip is affected by a number of different sources of heterogeneity. The results provide an alternative explanation for observations that stepovers that allow throughgoing rupture tend to have higher slip gradients than those at which rupture terminates: perhaps many such stepovers are connected at depth, which could persistently favor throughgoing rupture. There may be implications for interpretation of apparent fault discontinuities worldwide.

Dynamic models of earthquake rupture along branch faults of the eastern San Gorgonio Pass region in California using complex fault structure

AbstractGeologic data suggest that the Coachella Valley segment of the southern San Andreas fault (southern California, USA) is past its average recurrence time period. At its northern edge, this right-lateral fault segment branches into the Mission Creek and Banning strands of the San Andreas fault. Depending on how rupture propagates through this region, there is the possibility of a throughgoing rupture that could lead to the channeling of damaging seismic energy into the Los Angeles Basin. The fault structures and potential rupture scenarios on these two strands differ significantly, which highlights the need to determine which strand provides a more likely rupture path and the circumstances that control this rupture path. In this study, we examine the effect of different assumptions about fault geometry and initial stress pattern on the dynamic rupture process to test multiple rupture scenarios and thus investigate the most likely path(s) of a rupture that starts on the Coachella Valley segment. We consider three types of fault geometry based on the Southern California Earthquake Center Community Fault Model, and we create a three-dimensional finite-element mesh for each of them. These three meshes are then incorporated into the finite-element method code FaultMod to compute a physical model for the rupture dynamics. We use a slip-weakening friction law, and consider different assumptions of background stress, such as constant tractions and regional stress regimes with different orientations. Both the constant and regional stress distributions show that rupture from the Coachella Valley segment is more likely to branch to the Mission Creek than to the Banning fault strand. The fault connectivity at this branch system seems to have a significant impact on the likelihood of a throughgoing rupture, with potentially significant impacts for ground motion and seismic hazard both locally and in the greater Los Angeles metropolitan area.

Evidence for Rupture Through a Double Benioff Zone During the 2017 Mw 8.2 Chiapas, Mexico Earthquake

AbstractWe combine multi‐array relative back‐projection of high‐frequency P waves and finite‐fault modeling of low‐frequency P waves from the 8 September 2017 Mw 8.2 Chiapas, Mexico earthquake to image unilateral rupture on a southeast‐northwest striking subvertical fault plane over depths of 10–70 km. Improved multi‐array relative back‐projection estimates of rupture speed and fault geometry provide key refinements to the slip model from finite‐fault modeling. Compilation of prior seismicity and aftershocks reveals the earthquake initiated in the lower plane of a double Benioff zone (DBZ) and ruptured with large slip across the entire DBZ. The previously aseismic interior does not appear to be due to lack of stress as both planes show downdip extension stresses. Instead, tomographic imaging of DBZs find high Poisson's ratio with fast velocities in the DBZ interior, so we propose that highly hydrated harzburgite generates velocity‐strengthening behavior that inhibits earthquake nucleation, yet it can slip seismically during dynamic weakening from a large throughgoing rupture.

Slip distribution of the 2014 Mw = 8.1 Pisagua, northern Chile, earthquake sequence estimated from coseismic fore‐arc surface cracks

AbstractThe 2014 Mw = 8.1 Iquique (Pisagua), Chile, earthquake sequence ruptured a segment of the Nazca‐South America subduction zone that last hosted a great earthquake in 1877. The sequence opened >3700 surface cracks in the fore arc of decameter‐scale length and millimeter‐to centimeter‐scale aperture. We use the strikes of measured cracks, inferred to be perpendicular to coseismically applied tension, to estimate the slip distribution of the main shock and largest aftershock. The slip estimates are compatible with those based on seismic, geodetic, and tsunami data, indicating that geologic observations can also place quantitative constraints on rupture properties. The earthquake sequence ruptured between two asperities inferred from a regional‐scale distribution of surface cracks, interpreted to represent a modal or most common rupture scenario for the northern Chile subduction zone. We suggest that past events, including the 1877 earthquake, broke the 2014 Pisagua source area together with adjacent sections in a throughgoing rupture.

Fault geometry, rupture dynamics and ground motion from potential earthquakes on the North Anatolian Fault under the Sea of Marmara

SUMMARY Using the 3-D finite-element method, we develop dynamic spontaneous rupture models of earthquakes on the North Anatolian Fault system in the Sea of Marmara, Turkey, considering the geometrical complexity of the fault system in this region. We find that the earthquake size, rupture propagation pattern and ground motion all strongly depend on the interplay between the initial (static) regional pre-stress field and the dynamic stress field radiated by the propagating rupture. By testing several nucleation locations, we observe that those far from an oblique normal fault stepover segment (near Istanbul) lead to large through-going rupture on the entire fault system, whereas nucleation locations closer to the stepover segment tend to produce ruptures that die out in the stepover. However, this pattern can change drastically with only a 10° rotation of the regional stress field. Our simulations also reveal that while dynamic unclamping near fault bends can produce a new mode of supershear rupture propagation, this unclamping has a much smaller effect on the speed of the peak in slip velocity along the fault. Finally, we find that the complex fault geometry leads to a very complex and asymmetric pattern of near-fault ground motion, including greatly amplified ground motion on the insides of fault bends. The ground-motion pattern can change significantly with different hypocentres, even beyond the typical effects of directivity. The results of this study may have implications for seismic hazard in this region, for the dynamics and ground motion of geometrically complex faults, and for the interpretation of kinematic inverse rupture models.

Structural and Geomorphic Characteristics of Landslides at Coyote Mountain, Anza-Borrego Desert State Park, California

Coyote Mountain is an 8-mi-long (13 km) elongate fault block made up of granitic and metamorphic rocks in northeastern San Diego County, California. A series of landslides, most of which have distinct morphology and failure mechanisms, occurs in the tonalite and gneiss underlying the steep southwest slope of the mountain. The southernmost landslide area is the Peg Leg Smith landslide complex, which is composed of several translational slides and a unique remnant of a long-runout rock avalanche. In the central portion of the mountain, two distinct landslide types underlie the slopes near Coyote Peak. The first is represented by a pair of rock-block landslides, the Coyote Peak landslides, which failed along foliation planes in metamorphic rock. The second is the Coyote Ridge landslide, a 2-mi-wide (3.2 km) area of incipient landsliding in highly fractured tonalite. The Alcoholic Pass landslides, located at the northwestern end of the mountain block, are situated in tonalite. This complex consists of two juxtaposed landslides that failed at nearly right angles to each other. The base of the northernmost landslide is not exposed, and the failure mechanism is postulated to have been block sliding along a well-developed fracture system. The basal rupture zone of the southern landslide is composed of coarse, matrix-rich breccia. The southern flank of the slide grades into linear scarps which define the head of the Coyote Ridge landslide. The Alcoholic Pass landslides are concluded to be rare examples of fully developed translational failure resulting from formation of a through-going rupture surface created by incremental movement along interconnecting fractures.

Relatively simple through-going fault planes at large-earthquake depth may be concealed by the surface complexity of strike-slip faults

Abstract At the surface, strike-slip fault stepovers, including abrupt fault bends, are typically regions of complex, often disconnected faults. This complexity has traditionally led geologists studying the hazard of active faults to consider such stepovers as important fault segment boundaries, and to give lower weight to earthquake scenarios that involve rupture through the stepover zone. However, recent geological and geophysical studies of several stepover zones along the San Andreas fault system in California have revealed that the complex nature of the fault zone at the surface masks a much simpler and direct connection at depths associated with large earthquakes (greater than 5 km). In turn, the simplicity of the connection suggests that a stepover zone would provide less of an impediment to through-going rupture in a large earthquake, so that the role of stepovers as segment boundaries has probably been overemphasized. However, counter-examples of fault complexity at depth associated with surface stepovers are known, so the role of stepovers in fault rupture behaviour must be carefully established in each case.

The Dynamics of Strike-Slip Step-Overs with Linking Dip-Slip Faults

Fault step-overs with linking dip-slip faults are common features on long strike-slip fault systems worldwide. It has been noted by various researchers that under some circumstances, earthquakes can jump across fault step-overs to cascade into large events, while under other circumstances rupture is arrested at step-overs. There is also evidence that fault step-overs may be preferential locations for earthquake nucleation. The present work uses the 3D finite element method to model the dynamics of strike-slip fault systems with step-overs and linking dip-slip faults. I find that the presence of a linking normal or thrust fault greatly increases the ability of earthquake rupture to propagate across the step-over, leading to a larger event. Additionally, dilational step-overs with linking normal faults are more prone to through-going rupture than compressional step-overs with linking thrust faults. This difference is due to the sign of the normal stress increment on the dip-slip fault caused by slip on the strike-slip segments: Slip on the strike-slip segments causes a negative (unclamping) normal stress increment on the linking normal fault in a dilational step-over, whereas the opposite effect occurs on the linking thrust fault in a compressional step-over. Even in cases for which both dilational and compressional step-overs can experience through-going rupture, dilational step-overs typically experience higher slip, particularly on the linking normal fault. In the compressional case, rupture nucleation on the linking thrust fault may increase the likelihood of through-going rupture compared to nucleation on one of the strike-slip segments. Near the intersections between the fault segments, the stress interaction between the fault segments also causes a significant rotation of rake away from that which would be inferred from the regional stress field. The results help to emphasize the importance of two-way interactions between nearby fault segments during the earthquake rupture process. The results also may have implications for the probability of large earthquakes along geometrically complex strike-slip fault systems, and may help explain why step-overs sometimes act as barriers and other times as nucleation locations for large earthquakes.

Lack of continuity of the San Andreas Fault in southern California: Three‐dimensional fault models and earthquake scenarios

The 1200‐km‐long San Andreas Fault loses its apparent continuity in southern California near San Gorgonio Pass [Allen, 1957], which raises significant questions given the dominant role of this fault in active California tectonics. What is the fundamental three‐dimensional (3‐D) geometry and kinematic behavior of the San Andreas fault system in this complex region? Is a throughgoing, if complex, San Andreas rupture from the Mojave Desert to the Coachella Valley possible? We have explored the issue of 3‐D continuity by mapping over 60 faults in this region to depths of 15–20 km from hypocenter locations and focal mechanisms. We were able to constrain the 3‐D geometry of the San Andreas fault zone (SAF) near San Gorgonio Pass from the 3‐D geometry of the fault network surrounding it. The most likely configuration is for the San Andreas Fault to merge into the shallow‐dipping San Gorgonio Pass thrust northwest of Indio. We concluded that there is no direct continuity at present but rather a network of faults, and the only kind of rupture possible for the SAF in this region is a complex rupture, involving both strike‐slip and reverse faulting. GPS measurements also suggest that despite the fact that large motions must have occurred in the past based on offset geologic markers, only minor motion is occurring today in this area. Applying our findings about the fault geometry, we explored several simple earthquake scenarios to determine the most favorable conditions for a throughgoing rupture of the San Andreas fault system from the Mojave Desert to the Coachella Valley.

Influence of Releasing Step-Overs on Surface Fault Rupture and Fault Segmentation: Examples from the 17 August 1999 Izmit Earthquake on the North Anatolian Fault, Turkey

The 17 August 1999 Izmit earthquake broke four distinct structural segments of the North Anatolian fault, from east to west: the Karadere, Sakarya, Sapanca, and Golcuk fault segments. These segments are separated from one another and the adjoining Yalova and Duzce fault segments to the west and east, respectively, by distinct right-releasing step-over and/or gaps in the fault trace. These include, from east to west, the Eften Lake step-over (2-4 km wide), Akyazi gap (6-13 km long), Sapanca step-over (1-2 km wide), Golcuk step-over (1-2 km wide), and Karamursel step-over (4-5 km wide). The earthquake nucleated within or near the Golcuk step-over with bilateral rupture propagation to the west and east. To the west, about 4-5.5 m of surface rupture (up to 7 m of subsurface rupture) on the Golcuk segment was mostly, if not entirely, arrested by the 4- to 5-km-wide Karamursel step-over. To the east, about 3-4 m of surface rupture (about 4 m of subsurface rupture) propagated through the 1- to 2-km-wide Sapanca step-over, triggering up to 4.5-5.5 m of surface rupture (up to 5-6 m of subsurface rupture) on the Sakarya segment. Rupture on the Sakarya segment stepped across the Akyazi gap triggering 1-1.5 m of rupture on the Karadere segment. Rupture on the Karadere segment was arrested by the 2- to 4-km-wide Eften Lake step-over. Modeling of the transient stress field associated with the rupture propagation agrees well with the field observations. The modeled fault rupture of 5 m on the Golcuk segment appears to decrease the transient stress on the eastern tip of the adjoining Yalova segment across the 4- to 5-km-wide Karamursel step-over, even if we allow rupture on the Golcuk segment to extend along the entire southern margin of the step-over. Our model shows that regardless of the amount of slip, in a generic sense, it is difficult to increase stress on the tip of an adjoining fault segment across a 5-km-wide step-over. On the other hand, fault rupture of 3 m on the western tip of the Sapanca fault segment significantly increases the transient stress on the adjoining Sakarya segment across the 1- to 2-km-wide Sapanca step-over, allowing rupture propagation across the basin. An evaluation of 30 historical strike-slip earthquakes involving 59 step-over basins supports our field observations and stress modeling of the Izmit fault rupture. Historical strike-slip events with small to large displacements usually propagate through step-overs less than 1-2 km wide. With increasing displacement, larger and larger step-overs can be ruptured through. The empirical data generally show a ratio ranging from 2:1 to 1:1 between step-over width (in kilometers) and strike-slip displacement (in meters) for a through-going rupture. However, step-overs of 4- to 5-km width appear to always arrest fault rupture, regardless of the amount of displacement. These results, therefore, provide important constraints for evaluating the probability of multisegment ruptures on segmented strike-slip faults. Manuscript received 20 August 2000.

The large aftershocks of the Northridge earthquake and their relationship to mainshock slip and fault-zone complexity

AbstractThe distribution of fault slip for the Northridge mainshock and two MW ≧ 6 aftershocks is estimated using an approach that inverts the far-field momentrate functions derived by empirical Green's function deconvolution. The slip for these events was found to have a strong negative correlation with the locations of small aftershocks, and there is a notable clustering of aftershocks near the edges of slip, suggesting that these events are triggered by a redistribution of stress following the mainshock rupture. The two largest aftershocks ruptured the updip regions adjacent to the leading edge of the mainshock rupture. One aftershock, 11 hr after the main event, initiated 8 km west of the mainshock slip terminus and ruptured east to the western edge of mainshock slip. This event was found to have ruptured the same south-dipping fault as the mainshock. The two events, however, are separated by a lateral offset in the depth of seismicity that serves as a geometrical barrier to through-going rupture. An MW 6.2 aftershock approximately 1 min after the main event is inferred to have ruptured an updip region of the mainshock fault that is relatively devoid of small aftershock seismicity. This event was located at the eastern edge of the sequence that overlaps with the north-dipping fault responsible for the 1971 San Fernando earthquake. Although the south-dipping plane is favored for this aftershock, it is not possible to rule out that it occurred on the north-dipping fault structure, or within the hanging block of the mainshock.