Fault‐Slip Distribution of the 1999 M_w 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

Structure and geochronological constraints on the ductile deformation observed along the GSSZ and CSSZ, Yunnan, China

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, September 2004.Some pages folded.Includes bibliographical references.The mechanism by which the Cenozoic post-collisional northward motion of India relative to Eurasia and South China was accommodated along its eastern boundary is still a poorly understood aspect of the tectonic evolution of SE Asia. This thesis focuses on a critically important area within the India-Eurasia collision zone, a region known as the Three Rivers area, and presents the first structural and geochronological data from several transects across two poorly documented linear metamorphic belts located in between the Sagaing fault and the Ailao Shan Shear Zone: the Gaoligong Shan Shear Zone (GSSZ) and the Chong Shan Shear Zone (CSSZ) in Yunnan, China. The presently inactive GSSZ is an important dextral Cenozoic (and possibly latest Mesozoic) ductile shear zone and appears to be the only right-lateral shear zone with the appropriate orientation to accommodate major pre- ca. 11 Ma northward movement of India relative to Eurasia. The previously unknown CSSZ is a long (>300 km) and wide (10- 20 km) metamorphic belt containing an assemblage low- to high-grade rocks from possibly several tectonic units which contain a prominent sub-vertical foliation and a sub-horizontal stretching lineation. Preliminary U/Pb results indicate that the ductile dextral shearing terminated by -18 Ma. Our regional map compilations indicate that the GSSZ and its proposed conjugate, the ASSZ, likely terminate within the Bangong Co-Nujiang suture zone.(cont.) This thesis documents the dextral GSSZ as the conjugate pair to the sinistral ASSZ, and thus constrains the limits of the crustal fragments that extruded to the southeast. Based on our reinterpretations of the geological maps of Tibet and the Three Rivers area, however, we propose that the extruded crustal material originated east of the easternmost corner of the Indian indentor, and did not create space for the penetration of India;. while at least limited extrusion did occur, there is no compelling geological evidence to indicate large-scale eastward movement of crustal material from directly north of the India indentor. Data presented in this thesis also indicate that extruded region was dismembered into at least two major crustal fragments, separated by the CSSZ.by Sinan Osman Akciz.Ph.D

Ground-based and UAV-based photogrammetry:a multi-scale, high-resolution mapping tool for structural geology and paleoseismology

This contribution reviews the use of modern 3D photo-based surface reconstruction techniques for high fidelity surveys of trenches, rock exposures and hand specimens to highlight their potential for paleoseismology and structural geology. We outline the general approach to data acquisition and processing using ground-based photographs acquired from standard DSLR cameras, and illustrate the use of similar processing approaches on imagery from Unmanned Aerial Vehicles (UAVs). It is shown that digital map and trench data can be acquired at ultra-high resolution and in much shorter time intervals than would be normally achievable through conventional grid mapping. The resulting point clouds and textured models are inherently multidimensional (x, y, z, point orientation, colour, texture), archival and easily transformed into orthorectified photomosaics or digital elevation models (DEMs). We provide some examples for the use of such techniques in structural geology and paleoseismology while pointing the interested reader to free and commercial software packages for data processing, visualization and 3D interpretation. Photogrammetric models serve to act as an ideal electronic repository for critical outcrops and observations, similar to the electronic lab book approach employed in the biosciences. This paper also highlights future possibilities for rapid semi-automatic to automatic interpretation of the data and advances in technology

Pre-Miocene birth of the Yangtze River

The development of fluvial systems in East Asia is closely linked to the evolving topography following India–Eurasia collision. Despite this, the age of the Yangtze River system has been strongly debated, with estimates ranging from 40 to 45 Ma, to a more recent initiation around 2 Ma. Here, we present 40Ar/39Ar ages from basalts interbedded with fluvial sediments from the lower reaches of the Yangtze together with detrital zircon U–Pb ages from sand grains within these sediments. We show that a river containing sediments indistinguishable from the modern river was established before ~23 Ma. We argue that the connection through the Three Gorges must postdate 36.5 Ma because of evaporite and lacustrine sedimentation in the Jianghan Basin before that time. We propose that the present Yangtze River system formed in response to regional extension throughout eastern China, synchronous with the start of strike–slip tectonism and surface uplift in eastern Tibet and fed by strengthened rains caused by the newly intensified summer monsoon

Space- and time- dependent probabilities for earthquake fault systems from numerical simulations: feasibility study and first results

In weather forecasting, current and past observational data are routinely assimilated into numerical simulations to produce ensemble forecasts of future events in a process termed “model steering”. Here we describe a similar approach that is motivated by analyses of previous forecasts of the Working Group on California Earthquake Probabilities (WGCEP). Our approach is adapted to the problem of earthquake forecasting using topologically realistic numerical simulations for the strike-slip fault system in California. By systematically comparing simulation data to observed paleoseismic data, a series of spatial probability density functions (PDFs) can be computed that describe the probable locations of future large earthquakes. We develop this approach and show examples of PDFs associated with magnitude M > 6.5 and M > 7.0 earthquakes in California.Peer reviewe

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