ALBERT

Description: Large [moment magnitude (M(w)) 〉/= 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 M(w) 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 ~10(3) microstrains in response to slip along discontinuous faults cutting crystalline bedrock of the Sierra Cucapah.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Oskin, Michael E -- Arrowsmith, J Ramon -- Hinojosa Corona, Alejandro -- Elliott, Austin J -- Fletcher, John M -- Fielding, Eric J -- Gold, Peter O -- Gonzalez Garcia, J Javier -- Hudnut, Ken W -- Liu-Zeng, Jing -- Teran, Orlando J -- New York, N.Y. -- Science. 2012 Feb 10;335(6069):702-5. doi: 10.1126/science.1213778.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Geology, University of California, Davis, 1 Shields Avenue, Davis, CA 95618, USA. meoskin@ucdavis.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22323817" target="_blank"〉PubMed〈/a〉

Description: Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse ~20 kilometers in width, ~6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at ~3.3 kilometers per second over ~140 kilometers. The smooth slip onset, indicating a large (~5-meter) slip-weakening distance, caused moderate ground shaking at high frequencies (〉1 hertz; peak ground acceleration, ~16% of Earth's gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of 4 to 5 seconds caused the collapse of tall structures, including cultural artifacts.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Galetzka, J -- Melgar, D -- Genrich, J F -- Geng, J -- Owen, S -- Lindsey, E O -- Xu, X -- Bock, Y -- Avouac, J-P -- Adhikari, L B -- Upreti, B N -- Pratt-Sitaula, B -- Bhattarai, T N -- Sitaula, B P -- Moore, A -- Hudnut, K W -- Szeliga, W -- Normandeau, J -- Fend, M -- Flouzat, M -- Bollinger, L -- Shrestha, P -- Koirala, B -- Gautam, U -- Bhatterai, M -- Gupta, R -- Kandel, T -- Timsina, C -- Sapkota, S N -- Rajaure, S -- Maharjan, N -- New York, N.Y. -- Science. 2015 Sep 4;349(6252):1091-5. doi: 10.1126/science.aac6383. Epub 2015 Aug 6.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Geology and Planetary Sciences, California Institute of Technology (Caltech), Pasadena, CA 91125, USA. ; BerkeleySeismological Laboratory, University of California (UC)-Berkeley, Berkeley, CA 94720, USA. ; Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, UC-San Diego, La Jolla, CA 92037, USA. ; Jet Propulsion Laboratory (JPL), Caltech, Pasadena, CA 91109, USA. ; Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. Department of Geology and Planetary Sciences, California Institute of Technology (Caltech), Pasadena, CA 91125, USA. ; Department of Mines and Geology, Lainchour, Kathmandu, Nepal. ; Nepal Academy of Science and Technology, Khumaltar, Lalitpur, Nepal. ; Department of Geological Sciences, Central Washington University (CWU), Ellensberg, WA 98926, USA. ; Tri-Chandra Campus, Tribhuvan University, Ghantaghar, Kathmandu, Nepal. ; U.S. Geological Survey (USGS), Pasadena, CA 91106, USA. ; Pacific Northwest Geodetic Array and Department of Geological Sciences, CWU, Ellensberg, WA 98926, USA. ; UNAVCO, Boulder, CO 80301, USA. ; Departement Analyse et Sureveillance de l'Environnement (DASE), Commissariat a l'Energie Atomique (CEA), 91297 Bruyeres-le-Chatel, Arpajon, France.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/26249228" target="_blank"〉PubMed〈/a〉

Description: 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 (〈25 m). As rupture zone thickness increases, the number of scarps in a given transect increases, and the scarp with the greatest relative amount of coseismic slip decreases. Rupture zones in previously undeformed alluvium become wider and have more complex arrangements of secondary fractures with oblique slip compared to those with pure normal dip-slip or pure strike-slip. Field relations and lidar (light detection and ranging) difference models show that as magnitude of coseismic slip increases from 0 to 60 cm, the links between kinematically distinct fracture sets increase systematically to the point of forming a throughgoing principal scarp. Our data indicate that secondary faults and penetrative off-fault strain continue to accommodate the oblique kinematics of coseismic slip after the formation of a thoroughgoing principal scarp. Among the widest rupture zones in the Sierra Cucapah are those developed above buried low angle faults due to the transfer of slip to widely distributed steeper faults, which are mechanically more favorably oriented. The results from this study show that the measureable parameters that define rupture zone fabric allow for testing hypotheses concerning the mechanics and propagation of earthquake ruptures, as well as for siting and designing facilities to be constructed in regions near active faults.

Description: We measured offsets on tectonically displaced geomorphic features along 80 km of the Clark strand of the San Jacinto fault (SJF) to estimate slip-per-event for the past several surface ruptures. We identify 168 offset features from which we make over 490 measurements using B4 light detection and ranging (LiDAR) imagery and field observations. Our results suggest that LiDAR technology is an exemplary supplement to traditional field methods in slip-per-event studies. Displacement estimates indicate that the most recent surface-rupturing event (MRE) produced an average of 2.5–2.9 m of right-lateral slip with maximum slip of nearly 4 m at Anza, a Mw 7.2–7.5 earthquake. Average multiple-event offsets for the same 80 kms are ~5.5??m, with maximum values of 3 m at Anza for the penultimate event. Cumulative displacements of 9–10 m through Anza suggest the third event was also similar in size. Paleoseismic work at Hog Lake dates the most recent surface rupture event at ca. 1790. A poorly located, large earthquake occurred in southern California on 22 November 1800; we relocate this event to the Clark fault based on the MRE at Hog Lake. We also recognize the occurrence of a younger rupture along ~15–20??km of the fault in Blackburn Canyon with ~1.25??m of average displacement. We attribute these offsets to the 21 April 1918 Mw 6.9 event. These data argue that much or all of the Clark fault, and possibly also the Casa Loma fault, fail together in large earthquakes, but that shorter sections may fail in smaller events.Online Material: Topographic contour maps and hillshades generated from B4 LiDAR data, corresponding field photographs, and data tables comparing LiDAR-based and field-based slip measurements for individual geomorphic features along the Clark fault.

Description: 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. Online Material: Tables of LiDAR displacement measurements; slip distributions plots; Google Earth index files and locations where displacements were made; LiDAR data files, and access information to view screen captures of the displacement measurements.

Description: Toppling analysis of a precariously balanced rock (PBR) can provide insight into the nature of ground motion that has not occurred at that location in the past and, by extension, can constrain peak ground motions for use in engineering design. Earlier approaches have targeted 2D models of the rock or modeled the rock–pedestal contact using spring-damper assemblies that require recalibration for each rock. Here, a method to model PBRs in 3D is presented through a case study of the Echo Cliffs PBR. The 3D model is created from a point cloud of the rock, the pedestal, and their interface, obtained using terrestrial laser scanning. The dynamic response of the model under earthquake excitation is simulated using a rigid-body dynamics algorithm. The veracity of this approach is demonstrated through comparisons against data from shake-table experiments. Fragility maps for toppling probability of the Echo Cliffs PBR as a function of various ground-motion parameters, rock–pedestal interface friction coefficient, and excitation direction are presented. These fragility maps indicate that the toppling probability of this rock is low (less than 0.2) for peak ground acceleration (PGA) and peak ground velocity (PGV) lower than 3 m/s 2 and 0.75 m/s, respectively, suggesting that the ground-motion intensities at this location from earthquakes on nearby faults have most probably not exceeded the above-mentioned PGA and PGV during the age of the PBR. Additionally, the fragility maps generated from this methodology can also be directly coupled with existing probabilistic frameworks to obtain direct constraints on unexceeded ground motion at a PBR’s location. Electronic Supplement: Text and figures describing the steps involved in the modeling of precariously balanced rock (PBR)–pedestal geometry, including dense point cloud representation and final 3D model, hazard deaggregation, and toppling probability.

Description: 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.

Description: We present the results of a survey of mud pots in the Wister Unit of the Imperial Wildlife Area. Thirty-three mud pots, pot clusters, or related geothermal vents (hundreds of pots in all) were identified, and most were found to cluster along a northwest-trending line that is more or less coincident with the postulated Sand Hills fault. An extrapolation of the trace of the San Andreas fault southeastward from its accepted terminus north of Bombay Beach very nearly coincides with the mud pot lineament and may represent a surface manifestation of the San Andreas fault southeast of the Salton Sea. Additionally, a recent survey of vents near Mullet Island in the Salton Sea revealed eight areas along a northwest-striking line where gas was bubbling up through the water and in two cases hot mud and water were being violently ejected.