Loading...
Please wait, while we are loading the content...
Similar Documents
Targeted High-Resolution Structure from Motion Observations over the Mw 6.4 and 7.1 Ruptures of the Ridgecrest Earthquake Sequence
| Content Provider | Semantic Scholar |
|---|---|
| Author | Donnellan, Andrea Lyzenga, Gregory Ansar, Adnan Goulet, Christine A. Wang, Jui-Pin Pierce, Marlon E. |
| Copyright Year | 2020 |
| Abstract | Cite this article as Donnellan, A., G. Lyzenga, A. Ansar, C. Goulet, J. Wang, and M. Pierce (2020). Targeted HighResolution Structure from Motion Observations over the Mw 6.4 and 7.1 Ruptures of the Ridgecrest Earthquake Sequence, Seismol. Res. Lett. XX, 1–9, doi: 10.1785/0220190274. We carried out six targeted structure frommotion surveys using small uninhabited aerial systems over the Mw 6.4 and 7.1 ruptures of the Ridgecrest earthquake sequence in the first three months after the events. The surveys cover approximately 500 × 500 m areas just south of Highway 178 with an average ground sample distance of 1.5 cm. The first survey took place five days after the Mw 6.4 foreshock on 9 July 2019. The final survey took place on 27 September 2019. The time between surveys increased over time, with the first five surveys taking place in the first month after the earthquake. Comparison of imagery frombefore and after theMw 7.1 earthquake shows variation in slip on themain rupture and a small amount of distributed slip across the scene. Cracks can be observed and mapped in the high-resolution imagery, which show en echelon cracking, fault splays, and a northeast-striking conjugate fault at the Mw 7.1 rupture south of Highway 178 and near the dirt road. Initial postseismic results show little fault afterslip, but possible subsidence in the first 7–10 days after the earthquake, followed by uplift. Introduction (Experiment Motivation) The Ridgecrest earthquake sequence began on Thursday, 4 July 2019 at 10:33 a.m. local Pacific Daylight time (PDT), following some smaller earthquakes, with an Mw 6.4 northeast-striking left-lateral earthquake northeast of the town of Ridgecrest and south of China Lake. The event was followed by an aftershock sequence, an Mw 5.4 event at 04:08 a.m. PDT on 5 July 2019, and the Mw 7.1 mainshock at 08:19 p.m. PDT on 5 July 2019. The Mw 7.1 event initiated north of the northeast end of the Mw 6.4 earthquake and ruptured bilaterally. The Mw 6.4 and 7.1 events ruptured the ground surface. The majority of both ruptures occurred within the Naval Air Weapons Station (NAWS) China Lake and were not easily accessible. The southern portions of both earthquakes ruptured south of China Lake, which is bounded to the south by Highway 178. To observe any postseismic afterslip on the ruptures, we targeted small areas of both ruptures using small uninhabited aerial systems (sUAS) or drones. We selected our survey areas based on detailed field mapping conducted immediately following the earthquake sequence (Brandenberg et al., 2019; C. A. Goulet et al. unpublished manuscript, 2020, see Data and Resources). Locations were just south of Highway 178 on the Mw 6.4 and 7.1 ruptures (Fig. 1). Both were easy to access, as close to the epicenters as possible, given the restricted Naval Air Weapons Station China Lake and exhibited clear surface ruptures. The goal of the observations was to collect repeated structure from motion (SfM) observations at targeted areas of each rupture to measure postseismic motion over time. Because the ruptures were fairly long and continuous, in relatively consistent geologic structure, we assumed that relative small area measurements within the ruptures would reflect similar behavior of the fault along much of the length of the rupture. The rupture length for theMw 6.4 rupture is on the order of 15 km, and the rupture of theMw 7.1 is on the order of 50 km, whereas the length of fault sections measured by our surveys is on the order of 0.5 km. Our focus was on understanding temporal evolution of the fault zone at small targeted locations. The goal was to compare afterslip for this earthquake to afterslip observed for the 1992 Mw 7.3 Landers earthquake sequence (Hauksson et al., 1993) and 1999 Hector Mine earthquake (Dreger and Kaverina, 2000) that also occurred in the Eastern 1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.; 2. Southern California Earthquake Center, University of Southern California, Los Angeles, California, U.S.A.; 3. Indiana University, Cyber Infrastructure Building, Bloomington, Indiana, U.S.A. *Corresponding author: andrea@jpl.caltech.edu © Seismological Society of America Volume XX • Number XX • – 2020 • www.srl-online.org Seismological Research Letters 1 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220190274/4967856/srl-2019274.1.pdf by The Claremont Colleges Library user on 23 March 2020 California Shear Zone. The Landers earthquake showed shallow fault afterslip in the centimeter range (Sylvester, 1993; Shen et al., 1994) and the Hector Mine earthquake experienced decimeters of slip at survey mode Global Positioning System (GPS) stations (Jacobs et al., 2002; Owen et al., 2002). These measurements can be used combined with Global Navigation Satellite System and Interferometric Synthetic Aperture Radar (InSAR) measurements to test whether afterslip extends to the surface or occurs deeper in the crust. Shallow slip would show up in the repeated local surveys and possibly in the other geodetic surveys, whereas deeper slip only would not be detectible within the local survey areas but might be with the broader coverage methods. The areas surveyed were also to cover a wide enough zone across each rupture to measure shallow off-fault deformation. Coseismic off-fault deformation can be assessed from the width of theobserved fracture zoneanddifferent fault strands in the local area. High-resolution observations that span a sufficient width across the fault zone can also be compared to determine the amount and extent of distributed deformation within the survey area. Identification of fractures and surface rupture can be compared to those identified from field mapping or other methods. Instrument Deployment and Details We carried out repeated measurements over theMw 6.4 and 7.1 ruptures south of Highway 178 (Table 1; Fig. 1). Data were collected using Parrot Anafi sUAS (Fig. 2). The vehicles each have a 21 megapixel integrated camera and gimbal and an onboard standard accuracy GPS, which geotags the images. We flew our surveys using an app called Pix4DCapture, a free flight planning app for mobile devices from Pix4D (see Data and Resources). We flew double grids with the camera facing forward, 20° up from nadir. The images had a front overlap of 80° and a side overlap of 70°. We flew the vehicles at 45 m above ground level, balancing coverage, flight time, and ground sample distance. Pix4DCapture saves the projects, so we were able to reproduce the flights for each survey. We made minor adjustments to the surveys between the early flights. We deployed cloth iron cross survey targets, which we have left at the study locations (Fig. 2). As of our last survey for this article, all of the targets were still in place. Because the targets serve as reference points within each survey in time, it does not matter if they move between surveys. We added one more target to the Mw 6.4 rupture to improve the geometry after the second survey and also slightly expanded our flight survey grid. By late September, we had staked down the center of each ground control point (GCP) with the goal of preserving their locations and presence. As discussed subsequently, locations of the targets moved up to 10 cm between surveys and are probably not useful for determining any postseismic motions within the survey area. We surveyed the GCPs using a Septentrio Real Time Kinematic (RTK) GPS system with base and rover stations. We logged data at the base station while we flew and post processed the data using the National Geodetic Survey’s mail in Online Positioning User Service (OPUS) processing system. The base station broadcasts RTK corrections to the rover, and the location of the rover relative to the base station was recorded for each GCP. The formal error for the base station solutions produced by OPUS is 1.1–1.7 cm. We corrected the location Figure 1. (a) Location map of Ridgecrest rupture sequence. Yellow lines show Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) (Field et al., 2015) faults for context. Red lines show rupture locations. Red arrow points from study location. (b)Mw 6.4 and 7.1 ruptures near Highway 178. Black lines show interpreted ruptures (Scharer, written comm., 2019). Red boxes outline structure from motion (SfM) target areas and orthomosaic results. Orange lines show not fully verified ruptures, and blue lines show cracks inferred from the SfM products. 2 Seismological Research Letters www.srl-online.org • Volume XX • Number XX • – 2020 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220190274/4967856/srl-2019274.1.pdf by The Claremont Colleges Library user on 23 March 2020 of the GCPs after we obtained the OPUS solution using software we wrote at Jet Propulsion Laboratory (JPL) called OpusCorrect.py. The root mean square (rms) error of the GCPs, based on Pix4d output, is about 2 cm. We reserved one target as a check point to validate the precision. We collected six sets of observations at each location with the time between observations increasing as time since the earthquakes increases (Table 1). Postseismic motions typically follow a log decay (Scholz, 1972), which is the rationale for increasing time between observations as time passes. We will continue to collect data infrequently over time. We downloaded the images from the vehicles and processed them using the commercial software package Pix4Dmapper, which is designed to produce survey grade results (see Data and Resources). Pix4D is a commercial photogrammetry software package tailored to sUAS. The software generates 3D reconstructions using a technique called SfM in which shape is determined from images collected from different perspectives (Wallach and O ‘Connell, 1953; Ullman, 1979; Marr, 1980). The initial step is to load geotagged images of the survey area into Pix4D and run an ini |
| File Format | PDF HTM / HTML |
| DOI | 10.1785/0220190274 |
| Alternate Webpage(s) | https://media.physics.hmc.edu/media/pubs/srl-2019274.1.pdf |
| Alternate Webpage(s) | https://doi.org/10.1785/0220190274 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
