Evaluating How Well Active Fault Mapping Predicts earthquake surface-rupture locations

Earthquake surface-fault rupture location uncertainty is a key factor in fault displacement hazard analysis and informs hazard and risk mitigation strategies. Geologists often predict future rupture locations from fault mapping based on the geomorphology interpreted from remote-sensing data sets. However, surface processes can obscure fault location, fault traces may be mapped in error, and a future rupture may not break every fault trace. We assessed how well geomorphology-based fault mapping predicted surface ruptures for seven earthquakes: 1983 M 6.9 Borah Peak, 2004 M 6.0 Parkfield, 2010 M 7.2 El Mayor–Cucapah, 2011 M 6.7 Fukushima-Hamadori, 2014 M 6.0 South Napa, 2016 M 7.8 Kaikoura, and 2016 M 7 Kumamoto. We trained geoscience students to produce active fault maps using topography and imagery acquired before the earthquakes. A geologic professional completed a “control” map. Mappers used a new “geomorphic indicator ranking” approach to rank fault confidence based on geomorphologic landforms. We determined the accuracy of the mapped faults by comparing the fault maps to published rupture maps. We defined predicted ruptures as ruptures near a fault (50–200 m, depending on the fault confidence) that interacted with the landscape in a similar way to the fault. The mapped faults predicted between 12% to 68% of the principal rupture length for the studied earthquakes. The median separation distances between predicted ruptures and strong, distinct, or weak faults were 15–30 m. Our work highlights that mapping future fault ruptures is an underappreciated challenge of fault displacement hazard analysis—even for experts—with implications for risk management, engineering site assessments, and fault exclusion zones

Virtual Shake Robot: Simulating Dynamics of Precariously Balanced Rocks for Overturning and Large-displacement Processes

 Understanding the dynamics of precariously balanced rocks (PBRs) is important for seismic hazard analysis and rockfall prediction. Utilizing a physics engine and robotic tools, we develop a virtual shake robot (VSR) to simulate the dynamics of PBRs during overturning and large-displacement processes. We present the background of physics engines and technical details of the VSR, including software architecture, mechanical structure, control system, and implementation procedures. Validation experiments show the median fragility contour from VSR simulation is within the 95% prediction intervals from previous physical experiments, when PGV/PGA is greater than 0.08 s. Using a physical mini shake robot, we validate the qualitative consistency of fragility anisotropy between the VSR and physical experiments. By overturning cuboids on flat terrain, the VSR reveals the relationship between fragility and geometric dimensions (e.g., aspect and scaling ratios). The ground motion orientation and lateral pedestal support affect PBR fragility. Large-displacement experiments estimate rock trajectories for different ground motions, which is useful for understanding the fate of toppled PBRs. Ground motions positively correlate with large displacement statistics such as mean trajectory length, mean largest velocity, and mean terminal distance. The overturning and large displacement processes of PBRs provide complementary methods of ground motion estimation

Geotechnical Field Reconnaissance: Gorkha (Nepal) Earthquake of April 25, 2015 and Related Shaking Sequence

The April 25, 2015 Gorkha (Nepal) Earthquake and its related aftershocks had a devastating impact on Nepal. The earthquake sequence resulted in nearly 9,000 deaths, tens of thousands of injuries, and has left hundreds of thousands of inhabitants homeless. With economic losses estimated at several billion US dollars, the financial impact to Nepal is severe and the rebuilding phase will likely span many years. The Geotechnical Extreme Events Reconnaissance (GEER) Association assembled a reconnaissance team under the leadership of D. Scott Kieffer, Binod Tiwari and Youssef M.A. Hashash to evaluate geotechnical impacts of the April 25, 2015 Gorkha Earthquake and its related aftershocks. The focus of the reconnaissance was on time-sensitive (perishable) data, and the GEER team included a large group of experts in the areas of Geology, Engineering Geology, Seismology, Tectonics, Geotechnical Engineering, Geotechnical Earthquake Engineering, and Civil and Environmental Engineering. The GEER team worked in close collaboration with local and international organizations to document earthquake damage and identify targets for detailed follow up investigations. The overall distribution of damage relative to the April 25, 2015 epicenter indicates significant ground motion directivity, with pronounced damage to the east and comparatively little damage to the west. In the Kathmandu Basin, characteristics of recorded strong ground motion data suggest that a combination of directivity and deep basin effects resulted in significant amplification at a period of approximately five seconds. Along the margins of Kathmandu Basin structural damage and ground failures are more pronounced than in the basin interior, indicating possible basin edge motion amplification. Although modern buildings constructed within the basin generally performed well, local occurrences of heavy damage and collapse of reinforced concrete structures were observed. Ground failures in the basin included cyclic failure of silty clay, lateral spreading and liquefaction. Significant landsliding was triggered over a broad area, with concentrated activity east of the April 25, 2015 epicenter and between Kathmandu and the Nepal-China border. The distribution of concentrated landsliding partially reflects directivity in the ground motion. Several landslides have dammed rivers and many of these features have already been breached. Hydropower is a primary source of electric power in Nepal, and several facilities were damaged due to earthquake-induced landsliding. Powerhouses and penstocks experienced significant damage, and an intake structure currently under construction experienced significant dynamic settlement during the earthquake. Damage to roadways, bridges and retaining structures was also primarily related to landsliding. The greater concentration of infrastructure damage along steep hillsides, ridges and mountain peaks offers a proxy for the occurrence of topographic amplification. The lack of available strong motion records has severely limited the GEER team’s ability to understand how strong motions were distributed and how they correlate to distributions of landsliding, ground failure and infrastructure damage. It is imperative that the engineering and scientific community continues to install strong motion stations so that such data is available for future earthquake events. Such information will benefit the people of Nepal through improved approaches to earthquake resilient design

Validation of meter-scale surface faulting offset measurements from high-resolution topographic data

Studies of active fault zones have flourished with the availability of high-resolution topographic data, particularly where airborne light detection and ranging (lidar) and structure from motion (SfM) data sets provide a means to remotely analyze submeter-scale fault geomorphology. To determine surface offset at a point along a strike-slip earthquake rupture, geomorphic features (e.g., stream channels) are measured days to centuries after the event. Analysis of these and cumulatively offset features produces offset distributions for successive earthquakes that are used to understand earthquake rupture behavior. As researchers expand studies to more varied terrain types, climates, and vegetation regimes, there is an increasing need to standardize and uniformly validate measurements of tectonically displaced geomorphic features. A recently compiled catalog of nearly 5000 earthquake offsets across a range of measurement and reporting styles provides insight into quality rating and uncertainty trends from which we formulate best-practice and reporting recommendations for remote studies. In addition, a series of public and beginner-level studies validate the remote methodology for a number of tools and emphasize considerations to enhance measurement accuracy and precision for beginners and professionals. Our investigation revealed that (1) standardizing remote measurement methods and reporting quality rating schemes is essential for the utility and repeatability of fault-offset measurements; (2) measurement discrepancies often involve misinterpretation of the offset geomorphic feature and are a function of the investigator’s experience; (3) comparison of measurements made by a single investigator in different climatic regions reveals systematic differences in measurement uncertainties attributable to variation in feature preservation; (4) measuring more components of a displaced geomorphic landform produces more consistently repeatable estimates of offset; and (5) inadequate understanding of pre-event morphology and post-event modifications represents a greater epistemic limitation than the aleatoric limitations of the measurement process

Geotechnical Effects of the 2015 Magnitude 7.8 Gorkha, Nepal, Earthquake and Aftershocks

This article summarizes the geotechnical effects of the 25 April 2015 M 7.8 Gorkha, Nepal, earthquake and aftershocks, as documented by a reconnaissance team that undertook a broad engineering and scientific assessment of the damage and collected perishable data for future analysis. Brief descriptions are provided of ground shaking, surface fault rupture, landsliding, soil failure, and infrastructure performance. The goal of this reconnaissance effort, led by Geotechnical Extreme Events Reconnaissance, is to learn from earthquakes and mitigate hazards in future earthquakes

Paleoseismology of the southern Panamint Valley fault: Implications for regional earthquake occurrence and seismic hazard in southern California.

[1] Paleoseismologic data from the southern Panamint Valley fault (PVF) reveal evidence of at least four surface ruptures during late Holocene time (0.33–0.48 ka, 0.9–3.0 ka, 3.3–3.6 ka, and >4.1 ka). These paleo‐earthquake ages indicate that the southern PVF has ruptured at least once and possibly twice during the ongoing (≤1.5 ka) seismic cluster in the Mojave section of the eastern California shear zone (ECSZ). The most recent event (MRE) on the PVF is also similar in age to the 1872 Owens Valley earthquake and the geomorphically youthful MRE on the Death Valley fault. The timing of the three oldest events at our site shows that the PVF ruptured at least once and possibly thrice during the well‐defined 2–5 ka seismic lull in the Mojave section of the ECSZ. Interestingly, the 3.3–3.6 ka age of Event 3 overlaps with the 3.3–3.8 ka age of the penultimate (i.e., pre‐1872) rupture on the central Owens Valley fault. These new PVF data support the notion that earthquake occurrence in the ECSZ may be spatially and temporally complex, with earthquake clusters occurring in different regions at different times. Coulomb failure function modeling of the Panamint Valley and Garlock faults reveals significant stress interactions between these two faults that may influence future earthquake occurrence. Specifically, our models suggest a possible rupture sequence whereby an event on the southern Panamint Valley fault can lead to the potential triggering of an event on the Garlock fault, which in turn could trigger the Mojave section of the San Andreas Fault

The late Holocene history of Lake Cahuilla : two thousand years of repeated fillings within the Salton Trough, Imperial Valley, California

To constrain the timing of the past seven lake highstands in the Salton Trough, we compiled 423 radiocarbon dates, of which 284 are reliable and have good stratigraphic control, from paleoseismic and archeological sites in the basin. We developed two OxCal models that assume most charcoal, wood, seeds, and twigs recovered from organic mats at or near the shoreline are derived from material that grew within the lake footprint, and therefore date a dry period between lakes. Charcoal samples collected from lacustrine clastic strata may have also been derived from fires burned during a dry period. As an initial constraint, we assume that samples older than those in earlier lake deposits have age inheritance. Assuming the dates are accurately described by their respective 2σ uncertainties, we ran all dates that would run in a preliminary OxCal model, and then removed those with a poor agreement index as defined in OxCal. From this, of the 423 total dates in the compilation, 151 dates are used in the base model and 149 dates are used in an alternative model, with the differences in the models resulting from choices of whether to include or exclude specific dates that may or may not be representative of a particular dry period between lakes. Where the two models agree, the results are robust, but where the models differ, any differences are taken as uncertainty in the lake ages. Historical accounts and a high-resolution paleohydrologic reconstruction allow us to refine some lake ages. The age windows for the past seven Lake Cahuilla highstands are 1731–1733 CE (Lake A), 1618–1636 CE (Lake B), 1486–1503 CE (Lake C), 1118–1165 or 1192–1241 CE (Lake D), 1007–1070 CE (Lake E), 930–966 CE (Lake F), and 612–5 BCE (Lake G). These ages represent the maximum allowable ranges during which a lake may have filled the basin up to the +13 m highstand elevation; the basin may have been dry for significant portions of each time window, though the lake filling and desiccation episodes may have extended beyond the stated highstand age range for each lake. If the paleohydrologic constraints are ignored, some of the lakes may have initiated earlier, by up to three decades. Additional dates would be needed to further bracket the ages of the earlier lakes. Notably, 120 of the original 284 reliable dates were rejected because they clearly violate stratigraphic ordering, implying that more than 40% of all radiocarbon dates in the Salton Basin exhibit statistically significant age inheritance.Ministry of Education (MOE)National Research Foundation (NRF)Published versionThis research was sup- ported by the Southern California Earthquake Center (SCEC Contribution # 10218). SCEC is funded by NSF Cooperative Agree- ment EAR-1600087 & USGS Cooperative Agreement G17AC00047. This research was also funded by the National Research Foundation Singapore and the Singapore Ministry of Education under the Research Centres of Excellence initiative. This is Earth Observatory of Singapore Contribution Number 318

Shortening Rate and Holocene Surface Rupture on the Riasi Fault System in the Kashmir Himalaya: Active Thrusting Within the Northwest Himalayan Orogenic Wedge

New mapping demonstrates that active emergent thrust faulting is occurring within the fold-and-thrust belt north of the deformation thrust front in the NW Himalaya. The \u3e60-km-long Riasi fault system is the southeasternmost segment of a seismically active regional fault system that extends more than 200 km stepwise to the southeast from the Balakot-Bagh fault in Pakistan into northwestern India. Two fault strands, the Main Riasi and Frontal Riasi thrusts, dominate the fault system in the study area. The Main Riasi thrust places Precambrian Sirban Formation over folded unconsolidated Quaternary sediments and fluvial terraces. New age data and crosscutting relationships between the Main Riasi thrust and the Quaternary units demonstrate that the Main Riasi thrust accommodated shortening between 100 and 40 ka at rates of 6–7 mm/yr. Deformation shifted to the southern Frontal Riasi thrust splay after ca. 39 ka. Differential uplift of a 14–7 ka terrace yields a range of shortening rates between 3 and 6 mm/yr. Together, shortening across the two strands indicates that a 6–7 mm/yr shortening rate has characterized the Riasi fault system since 100 ka. Geodetic data indicate that an 11–12 mm/yr arc-normal shortening rate characterizes the interseismic strain accumulation across the plate boundary due to India-Tibet convergence. These data combined with rates of other active faults in the Kashmir Himalaya indicate that the Suruin-Mastgarh anticline at the thrust front accounts for the remainder 40%–50% of the convergence not taken up by the Riasi fault system. Active deformation, and therefore earthquake sources, include both internal faults such the Riasi fault system, as well as rupture of the basal décollement (the Main Himalayan thrust) to the thrust front. Limited paleoseismic data from the Riasi fault system, the historical earthquake record of the past 1000 yr, the high strain rates, and partitioning of slip between the Riasi fault system and the thrust front demonstrate that a substantial slip deficit characterizes both structures and highlights the presence of a regionally important seismic gap in the Kashmir Himalaya. Slip deficit, scaling relationships, and a scenario of rupture and slip on the basal décollement (the Main Himalayan thrust) parsed onto either the Riasi fault system or the thrust front, or both, suggests that great earthquakes (Mw \u3e 8) pose an even greater seismic hazard than the Mw 7.6 2005 earthquake on the Balakot-Bagh fault in Pakistan Azad Kashmir

Earthquake hazard uncertainties improved using precariously balanced rocks

Probabilistic seismic hazard analysis (PSHA) is the state‐of‐the‐art method to estimate ground motions exceeded by large, infrequent, and potentially damaging earthquakes; however, a fundamental problem is the lack of an accepted method for both quantitatively validating and refining the hazard estimates using empirical geological data. In this study, to reduce uncertainties in such hazard estimates, we present a new method that uses empirical data from precariously balanced rocks (PBRs) in coastal Central California. We calculate the probability of toppling of each PBR at defined ground‐motion levels and determine the age at which the PBRs obtained their current fragile geometries using a novel implementation of cosmogenic 10Be exposure dating. By eliminating the PSHA estimates inconsistent with at least a 5% probability of PBR survival, the mean ground‐motion estimate corresponding to the hazard level of 10−4 yr−1 (10,000 yr mean return period) is significantly reduced by 27%, and the range of estimated 5th–95th fractile ground motions is reduced by 49%. Such significant reductions in uncertainties make it possible to more reliably assess the safety and security of critical infrastructure in earthquake‐prone regions worldwide

Coseismic Rupture and Preliminary Slip Estimates for the Papatea Fault and Its Role in the 2016 Mw 7.8 Kaikōura, New Zealand, Earthquake

International audienceCoseismic rupture of the 19‐km‐long north‐striking and west‐dipping sinistral reverse Papatea fault and nearby structures and uplift/translation of the Papatea block are two of the exceptional components of the 14 November 2016 Mw 7.8 Kaikōura earthquake. The dual‐stranded Papatea fault, comprising main (sinistral reverse) and western (dip‐slip) strands, ruptured onshore and offshore from south of Waipapa Bay to George Stream in the north, bounding the eastern side of the Papatea block. Fault rupture mapping was aided by the acquisition of multibeam bathymetry, light detection and ranging (lidar) topography and other imagery, as well as differential lidar (D‐lidar) from along the coast and Clarence River valley. On land, vertical throw and sinistral offset on the Papatea fault was assessed across an aperture of ±100  m using uncorrected D‐lidar and field data to develop preliminary slip distributions. The maximum up‐to‐the‐west throw on the main strand is ∼9.5±0.5  m⁠, and the mean throw across the Papatea fault is ∼4.5±0.3  m⁠. The maximum sinistral offset, measured near the coast on the main strand, is ∼6.1±0.5  m⁠. From these data, and considering fault dip, we calculate a maximum net slip of 11.5±2  m and an average net slip of 6.4±0.2  m for the Papatea fault surface rupture in 2016. Large sinistral reverse displacement on the Papatea fault is consistent with uplift and southward escape of the Papatea block as observed from Interferometric Synthetic Aperture Radar (InSAR) and optical image correlation datasets. The throw and net slip are exceedingly high for the length of the Papatea fault; such large movements likely only occur during multifault Kaikōura‐type earthquakes that conceivably have recurrence times of ≥5000–12,000  yrs⁠. The role of the Papatea fault in the Kaikōura earthquake has significant implications for characterizing complex fault sources in seismic hazard models