S. O. Akciz

Slip in the 1857 and Earlier Large Earthquakes Along the Carrizo Plain, San Andreas Fault

Slip, Tripped, and Faulted Earthquake risk assessment can be improved if we were able to quantify the recurrence and magnitude of slip events. Until recently though, a lack of sophisticated seismometers has forced us to rely on anecdotal evidence from those who survived major earthquakes or to look for clues in the landscape. Zielke et al. (p. 1119, published online 21 January; see the Perspective by Scharer) analyzed high-resolution images of the San Andreas Fault in southern California. The data showed that major surface ruptures, such as the 1857 Fort Tejon earthquake, resulted from slips of only about 5 meters; much less than previously thought. In a study that lends support to this discovery, Grant Ludwig et al. (p. 1117, published online 21 January; see the Perspective by Scharer) suggest from analysis of the geomorphic features of this region that several smaller earthquakes have occurred during recent centuries rather than infrequent but larger movements. The Perspective by Scharer (p. 1089) discusses how paleoseismological studies like these may be valuable for feeding data into earthquake prediction. The historical behavior of the San Andreas Fault may have been dominated by smaller, more frequent slip events. The moment magnitude (Mw) 7.9 Fort Tejon earthquake of 1857, with a ~350-kilometer-long surface rupture, was the most recent major earthquake along the south-central San Andreas Fault, California. Based on previous measurements of its surface slip distribution, rupture along the ~60-kilometer-long Carrizo segment was thought to control the recurrence of 1857-like earthquakes. New high-resolution topographic data show that the average slip along the Carrizo segment during the 1857 event was 5.3 ± 1.4 meters, eliminating the core assumption for a linkage between Carrizo segment rupture and recurrence of major earthquakes along the south-central San Andreas Fault. Earthquake slip along the Carrizo segment may recur in earthquake clusters with cumulative slip of ~5 meters.

Geometry, kinematics, and regional significance of the Chong Shan shear zone, Eastern Himalayan Syntaxis, Yunnan, China

The geology east of the Eastern Himalayan Syntaxis is poorly known, although it figures prominently in many models for the Cenozoic tectonics of the India-Eurasia collision and subsequent intracontinental deformation. The little known Chong Shan shear zone a ∼250-km–long and ∼10-km–wide metamorphic belt composed mainly of mylonitic augen gneisses and migmatites, forms a major shear zone developed during early Cenozoic extrusion of the Indochina crustal (lithospheric?) fragment. Foliation within the shear zone is moderately to steeply west dipping, and stretching lineations are subhorizontal, consistent with dominantly strike-slip transport. Kinematic indicators including rotated porphyroclasts, S-C fabrics, and asymmetric folds provide evidence for both dextral and sinistral movements. Our preliminary geo-chronological studies indicate that the Chong Shan shear zone has been active since at least ca. 34 Ma, and perhaps as early as 41 Ma. Strike-slip shearing continued at least until ca. 29 Ma, perhaps as late as ca. 24 Ma, and terminated by ca. 17 Ma. The Chong Shan shear zone, therefore, is not a belt of Precambrian metamorphic rocks as previously interpreted, but a Cenozoic shear zone of great significance, which was contemporaneous with movement on the left-lateral Ailao Shan shear zone and the right-lateral Gaoligong Shan shear zone two shear zones that bound the Indochina fragment on the east and west, respectively. Our data from the Chong Shan shear zone along with the data presented elsewhere from the Gaoligong Shan shear zone indicate that while the region between the Gaoligong Shan shear zone and Ailao Shan shear zone extruded to the southeast, it did not extrude as a single rigid block, but rather it was dismembered into at least two major fragments, the Baoshan to the west and the Lanping-Simao to the east separated by the Chong Shan shear zone. Our study of the Chong Shan shear zone suggests it and the other major early Cenozoic shear zones formed part of a broad major shear zone that passes into eastern Tibet between the Qiang-tang and Lhasa tectonic units.

Assembly of a large earthquake from a complex fault system: Surface rupture kinematics of the 4 April 2010 El Mayor–Cucapah (Mexico) Mw 7.2 earthquake

The 4 April 2010 moment magnitude (M_w) 7.2 El Mayor–Cucapah earthquake revealed the existence of a previously unidentified fault system in Mexico that extends ∼120 km from the northern tip of the Gulf of California to the U.S.–Mexico border. The system strikes northwest and is composed of at least seven major faults linked by numerous smaller faults, making this one of the most complex surface ruptures ever documented along the Pacific–North America plate boundary. Rupture propagated bilaterally through three distinct kinematic and geomorphic domains. Southeast of the epicenter, a broad region of distributed fracturing, liquefaction, and discontinuous fault rupture was controlled by a buried, southwest-dipping, dextral-normal fault system that extends ∼53 km across the southern Colorado River delta. Northwest of the epicenter, the sense of vertical slip reverses as rupture propagated through multiple strands of an imbricate stack of east-dipping dextral-normal faults that extend ∼55 km through the Sierra Cucapah. However, some coseismic slip (10–30 cm) was partitioned onto the west-dipping Laguna Salada fault, which extends parallel to the main rupture and defines the western margin of the Sierra Cucapah. In the northernmost domain, rupture terminates on a series of several north-northeast–striking cross-faults with minor offset (<8 cm) that cut uplifted and folded sediments of the northern Colorado River delta in the Yuha Desert. In the Sierra Cucapah, primary rupture occurred on four major faults separated by one fault branch and two accommodation zones. The accommodation zones are distributed in a left-stepping en echelon geometry, such that rupture passed systematically to structurally lower faults. The structurally lowest fault that ruptured in this event is inclined as shallowly as ∼20°. Net surface offsets in the Sierra Cucapah average ∼200 cm, with some reaching 300–400 cm, and rupture kinematics vary greatly along strike. Nonetheless, instantaneous extension directions are consistently oriented ∼085° and the dominant slip direction is ∼310°, which is slightly (∼10°) more westerly than the expected azimuth of relative plate motion, but considerably more oblique to other nearby historical ruptures such as the 1992 Landers earthquake. Complex multifault ruptures are common in the central portion of the Pacific North American plate margin, which is affected by restraining bend tectonics, gravitational potential energy gradients, and the inherently three-dimensional strain of the transtensional and transpressional shear regimes that operate in this region.

Century-long average time intervals between earthquake ruptures of the San Andreas fault in the Carrizo Plain, California

Paleoseismological data constrain the age, location, and associated magnitude of past surface-rupturing earthquakes; these are critical parameters for developing and testing fault behavior models and characterizing seismic hazard. We present new earthquake evidence and radiocarbon analyses that refi ne the chronology of the six most recent earthquakes that ruptured the south-central San Andreas fault in the Carrizo Plain (California, United States) at the Bidart Fan site. Modeled 95 percentile ranges of the earthquakes prior to the A.D. 1857 earthquake are A.D. 1631‐1823, 1580‐1640, 1510‐1612, 1450‐1475, and 1360‐1452. The average time interval between the last six earthquakes that ruptured the San Andreas fault in the Carrizo Plain is 88 ± 41 yr. This is less than the time since the most recent A.D. 1857 earthquake, less than all reported average intervals of prehistoric earthquakes along the entire San Andreas fault, and signifi cantly shorter than the 235 yr average used in recent seismic hazard evaluations. The new chronological data combined with recent slip studies imply that the magnitudes of the earthquakes that ruptured the southern San Andreas fault in the Carrizo Plain since ca. A.D. 1360 were variable, and suggest that the widely held view of rare but great surface rupturing earthquakes along this portion of the southern San Andreas fault should be reevaluated. 118°

Climate-modulated channel incision and rupture history of the San Andreas Fault in the Carrizo Plain

Slip, Tripped, and Faulted Earthquake risk assessment can be improved if we were able to quantify the recurrence and magnitude of slip events. Until recently though, a lack of sophisticated seismometers has forced us to rely on anecdotal evidence from those who survived major earthquakes or to look for clues in the landscape. Zielke et al. (p. 1119, published online 21 January; see the Perspective by Scharer) analyzed high-resolution images of the San Andreas Fault in southern California. The data showed that major surface ruptures, such as the 1857 Fort Tejon earthquake, resulted from slips of only about 5 meters; much less than previously thought. In a study that lends support to this discovery, Grant Ludwig et al. (p. 1117, published online 21 January; see the Perspective by Scharer) suggest from analysis of the geomorphic features of this region that several smaller earthquakes have occurred during recent centuries rather than infrequent but larger movements. The Perspective by Scharer (p. 1089) discusses how paleoseismological studies like these may be valuable for feeding data into earthquake prediction. The historical behavior of the San Andreas Fault may have been dominated by smaller, more frequent slip events. The spatial and temporal distribution of fault slip is a critical parameter in earthquake source models. Previous geomorphic and geologic studies of channel offset along the Carrizo section of the south central San Andreas Fault assumed that channels form more frequently than earthquakes occur and suggested that repeated large-slip earthquakes similar to the 1857 Fort Tejon earthquake illustrate typical fault behavior. We found that offset channels in the Carrizo Plain incised less frequently than they were offset by earthquakes. Channels have been offset by successive earthquakes with variable slip since ~1400. This nonuniform slip history reveals a more complex rupture history than previously assumed for the structurally simplest section of the San Andreas Fault.

Applications of airborne and terrestrial laser scanning to paleoseismology

Paleoseismic investigations aim to document past earthquake characteristics such as rupture location, frequency, distribution of slip, and ground shaking intensity—critical parameters for improved understanding of earthquake processes and refined earthquake forecasts. These investigations increasingly rely on high-resolution ( 2 /m. This situation refines interpretations of PBR exhumation rates and thus their effectiveness as paleoseismometers. Given that earthquakes disrupt Earth9s surface at centimeter to meter scales and that depositional and erosional responses typically operate on similar scales, ALS and TLS provide the absolute measurement capability sufficient to characterize these changes in challenging geometric arrangements, and thus demonstrate their value as effective analytical tools in paleoseismology.

Geologic and structural controls on rupture zone fabric: A field-based study of the 2010 Mw 7.2 El Mayor–Cucapah earthquake surface rupture

We systematically mapped (scales >1:500) the surface rupture of the 4 April 2010 Mw (moment magnitude) 7.2 El Mayor-Cucapah earthquake through the Sierra Cucapah (Baja California, northwestern Mexico) to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution, and internal configuration of shearing in a rupture zone. Fault zone thickness and master fault dip are strongly correlated with many parameters of rupture zone fabric. Wider fault zones produce progressively wider rupture zones and both of these parameters increase systematically with decreasing dip of master faults, which varies from 20° to 90° in our dataset. Principal scarps that accommodate more than 90% of the total coseismic slip in a given transect are only observed in fault sections with narrow rupture zones (

Fault‐Slip Distribution of the 1999 Mw 7.1 Hector Mine Earthquake, California, Estimated from Postearthquake Airborne LiDAR Data

The 16 October 1999 Hector Mine earthquake (M_w 7.1) was the first large earthquake for which postearthquake airborne Light Detection and Ranging (LiDAR) data were collected to image the fault surface rupture. In this work, we present measurements of both vertical and horizontal slip along the entire surface rupture of this earthquake based on airborne LiDAR data acquired in April 2000. We examine the details of the along‐fault slip distribution of this earthquake based on 255 horizontal and 85 vertical displacements using a 0.5 m digital elevation model derived from the LiDAR imagery. The slip measurements based on the LiDAR dataset are highest in the epicentral region, and taper in both directions, consistent with earlier findings by other works. The maximum dextral displacement measured from LiDAR imagery is 6.60±1.10  m, located about 700 m south of the highest field measurement (5.25±0.85  m). Our results also illustrate the difficulty in resolving displacements smaller than 1 m using LiDAR imagery alone. We analyze slip variation to see if it is affected by rock type and whether variations are statistically significant. This study demonstrates that a postearthquake airborne LiDAR survey can produce an along‐fault horizontal and vertical offset distribution plot of a quality comparable to a reconnaissance field survey. Although LiDAR data can provide a higher sampling density and enable rapid data analysis for documenting slip distributions, we find that, relative to field methods, it has a limited ability to resolve slip that is distributed over several fault strands across a zone. We recommend a combined approach that merges field observation with LiDAR analysis, so that the best attributes of both quantitative topographic and geological insight are utilized in concert to make best estimates of offsets and their uncertainties.

Geologic swath map of the lavic lake fault from airborne thermal hyperspectral imagery

The 1999 Hector Mine earthquake on the Lavic Lake fault produced a maximum right-lateral displacement of ∼5 m, but the long-term cumulative offset remains unresolved. To identify bedrock that has been offset by the fault, we produced a swath map from airborne hyperspectral imagery. High spatial and spectral resolution, along with a lack of significant vegetation cover helped us differentiate lithologic units and create a geologic map with supervised and unsupervised classifications. Supervised classifications over a small test site had an overall accuracy of 71 ± 1%, and some of the boundaries between units in our unsupervised classification correlate well with lithologic boundaries from a previously published geologic map that covers the same area. Our geologic fault swath map will help to resolve the total tectonic offset of bedrock along the Lavic Lake fault.

Three‐Dimensional Investigation of a 5 m Deflected Swale along the San Andreas Fault in the Carrizo Plain

Topographic maps produced from Light Detection and Ranging (LiDAR) data are useful for paleoseismic and neotectonic research because they pro- vide submeter representation of faulting-related surface features. Offset measurements of geomorphic features, made in the field or on a remotely sensed imagery, commonly assume a straight or smooth (i.e., undeflected) pre-earthquake geometry. Here, we present results from investigation of an ∼20 cm deep and >5 m wide swale with a sharp bend along the San Andreas fault (SAF) at the Bidart fan site in the Carrizo Plain, California. From analysis of LiDAR topography images and field measure- ments, the swale was initially interpreted as a channel tectonically offset ∼4:7 m. Our observations from exposures in four backhoe excavations and 25 hand-dug trenchettes show that even though a sharp bend in the swale coincides with the trace of the A.D. 1857 fault rupture, the swale formed after the 1857 earthquake and was not tectonically offset. Subtle fractures observed within a surficial gravel unit overlying the 1857 rupture trace are similar to fractures previously documented at the Phelan fan and LY4 paleoseismic sites 3 and 35 km northwest of Bidart fan, respectively. Collectively, the fractures suggest that a post-1857 moderate-magnitude earthquake caused ground cracking in the Carrizo and Cholame stretches of the SAF. Our obser- vations emphasize the importance of excavation at key locations to validate remote and ground-based measurements, and we advocate more geomorphic characterization for each site if excavation is not possible. Online Material: Figures of trench logs.