Austin J. Elliott

Near-Field Deformation from the El Mayor–Cucapah Earthquake Revealed by Differential LIDAR

Earthquakes from Above Preparing for risks and hazards associated with large earthquakes requires detailed understanding of their mechanical properties. In addition to pinpointing the location and magnitude of earthquakes, postmortem analyses of the extent of rupture and amount of deformation are key quantities, but are not simply available from seismological data alone. Using a type of optical remote sensing, Light Detection and Ranging (LiDAR), Oskin et al. (p. 702) surveyed the surrounding area that ruptured during the 2010 Mw 7.2 El Mayor–Cucapah earthquake in Northern Mexico. Because this area had also been analyzed in 2006, a comparative analysis revealed slip rate and strain release on the shallow fault zone and a number of previously unknown faults. As remote imaging becomes cheaper and more common, differential analyses will continue to provide fault-related deformation data that complements modern seismological networks. Optical remote sensing before and after a large earthquake reveals its rupture dynamics. Large [moment magnitude (Mw) ≥ 7] continental earthquakes often generate complex, multifault ruptures linked by enigmatic zones of distributed deformation. Here, we report the collection and results of a high-resolution (≥nine returns per square meter) airborne light detection and ranging (LIDAR) topographic survey of the 2010 Mw 7.2 El Mayor–Cucapah earthquake that produced a 120-kilometer-long multifault rupture through northernmost Baja California, Mexico. This differential LIDAR survey completely captures an earthquake surface rupture in a sparsely vegetated region with pre-earthquake lower-resolution (5-meter–pixel) LIDAR data. The postevent survey reveals numerous surface ruptures, including previously undocumented blind faults within thick sediments of the Colorado River delta. Differential elevation changes show distributed, kilometer-scale bending strains as large as ~103 microstrains in response to slip along discontinuous faults cutting crystalline bedrock of the Sierra Cucapah.

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.

Evidence from coseismic slip gradients for dynamic control on rupture propagation and arrest through stepovers

Received 30 July 2008; revised 14 November 2008; accepted 18 December 2008; published 27 February 2009. [1] Analysis of historic slip distributions from large-magnitude continental strike-slip earthquake ruptures reveals a pronounced correlation between the distance over which slip decreases as a rupture approaches a structural ‘‘step’’ in the fault and the ability of the rupture to propagate through the step. Our analysis of coseismic slip gradients near these stepovers indicates that in earthquakes in which slip decreases gradually toward a step, rupture will not continue on the next segment. Conversely, in earthquakes in which slip decreases abruptly, rupture commonly renucleates on the next segment. The fact that ruptures that stopped had low slip gradients near stepovers, relative to those that continued, indicates that rupture dynamics control the propagation of rupture through stepovers. These results corroborate dynamic rupture models that show that ruptures in which slip decreases abruptly at a step generate strong seismic waves that serve to renucleate rupture on the opposite side of the structural step. There are several potential causes for gradual decrease of displacement as rupture approaches a stepover, including transference of deformation onto subsidiary structures, rheological contrasts that could dictate favored rupture propagation directions, the existence of stress shadows from previous earthquakes within the system, or other material or frictional heterogeneities. All of these are potentially observable, and, if mapped systematically, could provide the basis for a strong constraint on the likely end points of future earthquakes. Inasmuch as earthquake magnitude is strongly dependent on the size of the rupture, such predictions would be of great utility as a basic component of scenario-based earthquake rupture forecasts.

Earthquake monitoring gets boost from a new satellite

Europes Sentinel-1A spacecraft and its extraordinary images of slip from the South Napa earthquake herald a new era of space-based surveillance of faults.

Rupture termination at restraining bends: The last great earthquake on the Altyn Tagh Fault

Strike-slip rupture propagation falters where changes in fault strike increase Coulomb failure stress. Numerical models of this phenomenon offer predictions of rupture extent based on bend geometry, but have not been verified with field data. To test model predictions of rupture barriers, we examine rupture extent along a section of the sinistral Altyn Tagh Fault punctuated by three major double bends. We measure 3–8 m offsets and map >95 km of continuous scarps that define the most recent surface rupture. We document the eastern terminus of this rupture within the Aksay bend, where an undeformed Pleistocene alluvial fan we mapped and dated overlaps the fault. We conclude, based on this geomorphologic evidence, that multiple Holocene ruptures have stopped in the Aksay bend. Our field data validate model predictions of rupture termination at a >18° restraining bend and support use of geometric parameters to define expected earthquake sizes in seismic hazard models.

Multisegment rupture in the 11 July 1889 Chilik earthquake (Mw 8.0–8.3), Kazakh Tien Shan, interpreted from remote sensing, field survey, and paleoseismic trenching

The 11 July 1889 Chilik earthquake (M-w 8.0-8.3) forms part of a remarkable sequence of large earthquakes in the late nineteenth and early twentieth centuries in the northern Tien Shan. Despite its importance, the source of the 1889 earthquake remains unknown, though the macroseismic epicenter is sited in the Chilik valley, similar to 100 km southeast of Almaty, Kazakhstan (similar to 2 million population). Several short fault segments that have been inferred to have ruptured in 1889 are too short on their own to account for the estimated magnitude. In this paper we perform detailed surveying and trenching of the similar to 30 km long Saty fault, one of the previously inferred sources, and find that it was formed in a single earthquake within the last 700 years, involving surface slip of up to 10 m. The scarp-forming event, likely to be the 1889 earthquake, was the only surface-rupturing event for at least 5000 years and potentially for much longer. From satellite imagery we extend the mapped length of fresh scarps within the 1889 epicentral zone to a total of similar to 175 km, which we also suggest as candidate ruptures from the 1889 earthquake. The 175 km of rupture involves conjugate oblique left-lateral and right-lateral slip on three separate faults, with step overs of several kilometers between them. All three faults were essentially invisible in the Holocene geomorphology prior to the last slip. The recurrence interval between large earthquakes on any of these faults, and presumably on other faults of the Tien Shan, may be longer than the timescale over which the landscape is reset, providing a challenge for delineating sources of future hazard.

Interactive terrain visualization enables virtual field work during rapid scientific response to the 2010 Haiti earthquake

The moment magnitude (Mw) 7.0 12 January 2010 Haiti earthquake is the fi rst major earthquake for which a large-footprint LiDAR (light detection and ranging) survey was acquired within several weeks of the event. Here, we describe the use of virtual reality data visualization to analyze massive amounts (67 GB on disk) of multiresolution terrain data during the rapid scientifi c response to a major natural disaster. In particular, we describe a method for conducting virtual fi eld work using both desktop computers and a 4-sided, 22 m 3 CAVE immersive virtual reality environment, along with KeckCAVES (Keck Center for Active Visualization in the Earth Sciences) software tools LiDAR Viewer, to analyze LiDAR pointcloud data, and Crusta, for 2.5 dimensional surfi cial geologic mapping on a bare-earth digital elevation model. This system enabled virtual fi eld work that yielded remote observations of the topographic expression of active faulting within an ~75-km-long section of the eastern Enriquillo‐Plantain Garden fault spanning the 2010 epicenter. Virtual fi eld observations indicated that the geomorphic evidence of active faulting and ancient surface rupture varies along strike. Landform offsets of 6‐50 m along the Enriquillo‐ Plantain Garden fault east of the 2010 epicenter and closest to Port-au-Prince attest to repeated recent surface-rupturing earthquakes there. In the west, the fault trace is well defi ned by displaced landforms, but it is not as clear as in the east. The 2010 epi center is within a transition zone between these sections that extends from Grand Goâve in the west to Fayette in the east. Within this transition, between L’Acul (lat 72°40′W) and the Rouillone River (lat 72°35′W), the Enriquillo‐Plantain Garden fault is un defi ned along an embayed low-relief range front, with little evidence of recent surface rupture. Based on the geometry of the eastern and western faults that show evidence of recent surface rupture, we propose that the 2010 event occurred within a stepover that appears to have served as a long-lived boundary between rupture segments, explaining the lack of 2010 surface rupture. This study demonstrates how virtual reality‐based data visualization has the potential to transform rapid scientifi c response by enabling virtual fi eld studies and real-time interactive analysis of massive terrain data sets.

Point-based computing on scanned terrain with LidarViewer

As an alternative to grid-based approaches, point-based computing offers access to the full information stored in unstructured point clouds derived from lidar scans of terrain. By employing appropriate hierarchical data structures and algorithms for out-of-core processing and view-dependent rendering, it is feasible to visualize and analyze three-dimensional (3D) lidar point-cloud data sets of arbitrary sizes in real time. Here, we describe LidarViewer, an implementation of point-based computing developed at the University of California (UC), Davis, W.M. Keck Center for Active Visualization in the Earth Sciences (KeckCAVES). Specifically, we show how point-based techniques can be used to simulate hillshading of a continuous terrain surface by computing local, point-centered tangent plane directions in a pre-processing step. Lidar scans can be analyzed interactively by extracting features using a selection brush. We present examples including measurement of bedding and fault surfaces and manual extraction of 3D features such as vegetation. Point-based computing approaches can offer significant advantages over grids, including analysis of arbitrarily large data sets, scale- and direction-independent analysis and feature extraction, point-based feature- and time-series comparison, and opportunities to develop semi-automated point filtering algorithms. Because LidarViewer is open-source, and its key computational framework is exposed via a Python interface, it provides ample opportunities to develop novel point-based computation methods for lidar data.