ABSTRACT The post–Thomas Fire debris flows of 9 January 2018 killed 23 people, damaged 558 structures, and caused severe damage to infrastructure in Montecito and Carpinteria, CA. U.S. Highway 101 was closed for 13 days, significantly impacting transportation and commerce in the region. A narrow cold frontal rain band generated extreme rainfall rates within the western burn area, triggering runoff-driven debris flows that inundated 5.6 km2 of coastal land in eastern Santa Barbara County. Collectively, this series of debris flows is comparable in magnitude to the largest documented post-fire debris flows in the state and cost over a billion dollars in debris removal and damages to homes and infrastructure. This study summarizes observations and analyses on the extent and magnitude of inundation areas, debris-flow velocity and volume, and sources of debris-flow material on the south flank of the Santa Ynez Mountains. Additionally, we describe the atmospheric conditions that generated intense rainfall and use precipitation data to compare debris-flow source areas with spatially associated peak 15 minute rainfall amounts. We then couple the physical characterization of the event with a compilation of debris-flow damages to summarize economic impacts.

The 2020 M-w 5.8 Lone Pine earthquake, the largest earthquake on the Owens Valley fault zone, eastern California, since the nineteenth century, ruptured an extensional stepover in that fault. Owens Valley separates two normal-faulting regimes, the western margin of the Great basin and the eastern margin of the Sierra Nevada, forming a complex seismotectonic zone, and a possible nascent plate boundary. Foreshocks began on 22 June 2020; the largest M-w 4.7 foreshock occurred at similar to 6 km depth, with primarily normal faulting, followed similar to 40 hr later on 24 June 2020 by an M-w 5.8 main-shock at -7 km depth. The sequence caused overlapping ruptures across a similar to 0.25 km(2) area, extended to similar to 4 km(2), and culminated in an similar to 25 km(2) aftershock area. The main-shock was predominantly normal faulting, with a strike of 330 degrees (north-northwest), dip-ping 60 degrees-65 degrees to the east-northeast. Comparison of background seismicity and 2020 Ridgecrest aftershock rates showed that this earthquake was not an aftershock of the Ridgecrest mainshock. The M-w-m(B) relationship and distribution of ground motions suggest typical rupture speeds. The aftershocks form a north-northwest-trending, north-northeast-dipping, 5 km long distribution, consistent with the rupture length esti-mated from analysis of regional waveform data. No surface rupture was reported along the 1872 scarps from the 2020 M-w 5.8 mainshock, although, the dipping rupture zone of the M-w 5.8 mainshock projects to the surface in the general area. The mainshock seismic energy triggered rockfalls at high elevations (> 3.0 km) in the Sierra Nevada, at distances of 8-20 km, and liquefaction along the western edge of Owens Lake. Because there were similar to 30% fewer aftershocks than for an average southern California sequence, the aftershock forecast probabilities were lower than expected. ShakeAlert, the earthquake early warning system, provided first warning within 9.9 s, as well as subsequent updates.

Cite this article as Brooks, B. A., J. Murray, J. Svarc, E. Phillips, R. Turner, M. Murray, T. Ericksen, K. Wang, S. Minson, R. Burgmann, et al. (2020). Rapid Geodetic Observations of Spatiotemporally Varying Postseismic Deformation Following the Ridgecrest Earthquake Sequence: The U.S. Geological Survey Response, Seismol. Res. Lett. XX, 1–16, doi: 10.1785/0220200007. Supplemental Material The U.S. Geological Survey’s geodetic response to the 4–5 July 2019 (Pacific time) Ridgecrest earthquake sequence comprised primarily the installation and/or reoccupation of Global Navigation Satellite System (GNSS) monumentation. Our response focused primarily on theUnited States’Navy’s China Lake Naval AirWeapons Station base (NAWSCL). This focuswas becausemuch of the surface rupture occurred on theNAWSCL and because of NAWSCL access restrictions only permitting Federal and State of California personnel. In total, we measured or are still measuring at 24 sites, 14 of which were on the NAWSCL and, as of this writing, operational. The majority of sites were set up as continuous stations logging at either 1 sample per second or 1 sample per 15 s. Two stationswere recording a 200 m cross-rupture aperture starting ∼10 hr after the M 6.4 event, and they recorded the coseismic displacements of the M 7.1. Approximately, 1 hr after the M 7.1 event, two new stations were recording a ∼200 m cross-rupture aperture of the surface rupture. In the days following, we established the rest of the stations ranging to a distance of ∼ 15 km from the M 7.1 principal rupture trace. The lack of differential displacement across the M 6.4 rupture during the M 7.1 event suggests that it did not reactivate theM 6.4 plane. The lack of differential cross-fault displacement for both events suggests that rapid shallow afterslip did not occur at those two locations. The postseismic time series from these stations shows centimeters of horizontal displacement over periods of a few months. They record a mixture of fault-parallel and fault-normal displacements that, in conjunction with analysis of more spatially complete Interferometric Synthetic Aperture Radar displacement fields, suggest that both poroelastic and afterslip phenomena occur along the M 6.4 and 7.1 rupture planes. Using preliminary data from these and other regional stations, we also explore the Ridgecrest sequence’s effect on regional GNSS time series and the differentiation of long-term postseismic motions and secular deformation rates. We find that redefining a common-mode noise filter using different GNSS stations that are assumed to be unaffected by the earthquakes results in small but systematic differences in the regional velocity field estimate. Introduction The southern California 2019 Ridgecrest earthquake sequence from 4 to 5 July (Pacific time) was a multifault rupture comprising a left-lateral M 6.4 foreshock (∼11:7 km depth) on a northeast-trending fault plane followed ∼34 hr later by a right-lateral M 7.1 (∼8 km depth) mainshock on a 1. Earthquake Science Center, U.S. Geological Survey, Moffett Field, California, U.S.A.; 2. Earthquake Science Center, U.S. Geological Survey, Menlo Park, California, U.S.A.; 3. Department of Earth and Planetary Science, University of California, Berkeley, Berkeley, California, U.S.A.; 4. Earthquake Science Center, U.S. Geological Survey, Pasadena, California, U.S.A.; 5. Earthquake Science Center, U.S. Geological Survey, Vancouver, Washington, U.S.A.; 6. California Geological Survey, Los Angeles, California, U.S.A. *Corresponding author: bbrooks@usgs.gov © 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/10.1785/0220200007/5049208/srl-2020007.1.pdf by University of California Berkeley Library user on 01 June 2020 northwest-trending fault plane (Ross et al., 2019; Stewart et al., 2019) (Fig. 1a). Both events produced substantial surface rupture (∼1–5 m) measured over distances of 10s of kilometers (Stewart et al., 2019). Although previously unmapped, the causative faults reside in the Little Lake fault zone, a northwest-trending series of faults that occur in a complicated portion of the Eastern California Shear Zone (ECSZ) that is ∼25 km east of the southern terminus of the southern Sierra Nevada fault zone (itself the western limit of Basin and Range extension) and ∼15 km north of the east-northeast–westsouthwest-trending left-lateral Garlock fault. As a result of this setting, the region experiences a complicated background combination of right-lateral ECSZ-related stress, extensional stress from Basin and Range deformation, and left-lateral, Garlock-related stress (Becker et al., 2005). Given that it was the first major seismic event in southern California for two decades, the Ridgecrest sequence generated a substantial amount of scientific interest and research. Early reports have focused on the surface disruption (Stewart et al., 2019) and multifault nature of the sequence (Ross et al., 2019). Here, we report on the rapid geodetic response to the Ridgecrest sequence led by the U.S. Geological Survey (USGS). Much of the surface rupture for both events occurred within the limits of the United States’ Navy’s China Lake Naval Air Weapons Station base (NAWSCL). Because of NAWSCL access restrictions, USGS and California Geological Survey personnel were the only scientists permitted onsite and to deploy equipment. Immediately following the M 6.4 event, our team maintained close contact and collaboration with the large group of academic geodetic researchers from the Southern California Earthquake Center (SCEC) (Floyd et al., 2020). Our principal scientific objective is to better understand postseismic deformation processes—both the surface deformation they produce and, through inference, the physical processes controlling slip on the fault planes. The spatiotemporal signatures of different postseismic processes such as poroelastic rebound and dilation (Peltzer et al., 1996), fault afterslip (Marone et al., 1991), and viscoelastic relaxation (Pollitz et al., 2001; Bürgmann and Dresen, 2008) are fundamental characteristics of continental lithosphere that can only be studied following infrequent seismic events. The postseismic response to crustal faulting still poses first-order questions: what is the relative contribution of fault afterslip versus viscoelastic mantle relaxation? When and why do poroelastic postseismic responses occur? Why do some faults have shallow afterslip and others not? For instance, the large amount of shallow, rapid afterslip for the Mw 6.0 South Napa earthquake was a surprise (Floyd et al., 2016; Brooks et al., 2017). The shallowest portion of seismogenic faults (<1 km depth) has gained much recent attention, in particular, because of the significant increase in sensing capability for near-field surface 0 75 150 km −122° −120° −118° −116° −114° 34° 36° 38° 40° (a) Figure 1. (a) Regional location map showing western U.S. state boundaries andmapped Quaternary fault traces (red lines). The blue box indicates the study area and Ridgecrest earthquakes surface rupture (black lines) shown in (b). (b) Study area showing the city of Ridgecrest, location of the China Lake Naval Air Weapons Station base (NAWSCL) (cyan lines), mapped Quaternary faults (pink lines), the Ridgecrest earthquake sequence surface rupture trace (red lines, C. B. DuRoss et al., unpublished manuscript, see Data and Resources), Plate Boundary Observatory continuous stations (yellow squares), and stations installed or occupied as part of this study (green squares). Red vectors are coseismic displacements estimated for the M 7.1 event. Note that displacements from stations RCRW and RCRE are so similar, they plot on top of one another. GF, Garlock fault; LLFZ, Little Lake fault zone; SNFZ, Sierra Nevada fault zone. Stations RCRW and RCRE as well as 71RW and 71RE are along Highway 178. Spray-paint markers discussed in the Instrument Deployment and Details section were installed between each of these two locations.(Continued) 2 Seismological Research Letters www.srl-online.org • Volume XX • Number XX • – 2020 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220200007/5049208/srl-2020007.1.pdf by University of California Berkeley Library user on 01 June 2020 deformation (Nissen et al., 2014; Zinke et al., 2014; Milliner et al., 2015; Brooks et al., 2017). Given that historical surface ruptures predominate in the databases from which empirical relations for seismic hazard analyses are derived (Wesnousky, 2008), it is important to document the prevalence of rapid shallow afterslip. In addition, the presence or absence of rapid shallow afterslip (occurring minutes to days after the mainshock) is critical to the current debate about the amount of coseismic slip that reaches the surface in continental strike-slip faulting events (Simons et al., 2002; Fialko et al., 2005; Dolan and Haravitch, 2014; Xu et al., 2016) and whether a deficit in shallow coseismic slip could be rapidly recuperated by afterslip, as it was, for instance in the 2004 Mw 6.0 Parkfield earthquake (Bilham, 2005; Langbein et al., 2006) and the South Napa event (Lienkaemper et al., 2016; Brooks et al., 2017). Robust quantification of the postseismic deformation field both in the nearand far field of faults also potentially permits placing rheological constraints on the shallowest portion of slipping faults for which, recently, there has been much interest, especially in the field of fault displacement hazard analysis (Petersen et al., 2011). To address these questions, it is critical to rapidly collect geodetic data on surface deformation, especially in the near field. Although space-based instruments have the capability of widespread imaging, their response time (days) is not necessarily adequate to capture the afterslip decay that occurs

ABSTRACTSurface rupture in the 2019 Ridgecrest, California, earthquake sequence occurred along two orthogonal cross faults and includes dominantly left-lateral and northeast-striking rupture in the Mw 6.4 foreshock and dominantly right-lateral and northwest-striking rupture in the Mw 7.1 mainshock. We present >650 field-based, surface-displacement observations for these ruptures and synthesize our results into cumulative along-strike displacement distributions. Using these data, we calculate displacement gradients and compare our results with historical strike-slip ruptures in the eastern California shear zone. For the Mw 6.4 rupture, we report 96 displacements measured along 18 km of northeast-striking rupture. Cumulative displacement curves for the rupture yield a mean left-lateral displacement of 0.3–0.5 m and maximum of 0.7–1.6 m. Net mean vertical displacement based on the difference of down-to-the-west (DTW) and down-to-the-east (DTE) displacement curves is close to zero (0.02 m DTW). The Mw 6.4 displacement distribution shows that the majority of displacement occurred southwest of the intersection with the Mw 7.1 rupture. The Mw 7.1 rupture is northwest-striking and 50 km long based on 576 field measurements. Displacement curves indicate a mean right-lateral displacement of 1.2–1.7 m and a maximum of 4.3–7.0 m. Net vertical displacement in the rupture averages 0.3 m DTW. The Mw 7.1 displacement distributions demonstrate that maximum displacement occurred along a 12-km-long portion of the fault near the Mw 7.1 epicenter, releasing 66% of the geologically based seismic moment along 24% of the total rupture length. Using our displacement distributions, we calculate kilometer-scale displacement gradients for the Mw 7.1 rupture. The steepest gradients (∼1–3 m/km) flank the 12-km-long region of maximum displacement. In contrast, gradients for the 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes are <0.6 m/km. Our displacement distributions are important for understanding the influence of cross-fault rupture on Mw 6.4 and 7.1 rupture length and displacement and will facilitate comparisons with distributions generated remotely and at broader scales.

The M-w 6.4 and M-w 7.1 Ridgecrest earthquake sequence occurred on 4 and 5 July 2019 within the eastern California shear zone of southern California. Both events produced extensive surface faulting and ground deformation within Indian Wells Valley and Searles Valley. In the weeks following the earthquakes, more than six dozen scientists from government, academia, and the private sector carefully documented the surface faulting and ground-deformation features. As of December 2019, we have compiled a total of more than 6000 ground observations; approximately 1500 of these simply note the presence or absence of fault rupture or ground failure, but the remainder include detailed descriptions and other documentation, including tens of thousands of photographs. More than 1100 of these observations also include quantitative field measurements of displacement sense and magnitude. These field observations were supplemented by mapping of fault rupture and ground-deformation features directly in the field as well as by interpreting the location and extent of surface faulting and ground deformation from optical imagery and geodetic image products. We identified greater than 68 km of fault rupture produced by both earthquakes as well as numerous sites of ground deformation resulting from liquefaction or slope failure. These observations comprise a dataset that is fundamental to understanding the processes that controlled this earthquake sequence and for improving earthquake hazard estimates in the region. This article documents the types of data collected during postearthquake field investigations, the compilation effort, and the digital data products resulting from these efforts.

First posted May 8, 2020 For additional information, contact: Contact Information, Menlo Park, Calif.Office—Earthquake Science CenterU.S. Geological Survey345 Middlefield Road, MS 977Menlo Park, CA 94025 We acquired multiple types of seismic data across the Hollywood Fault in Hollywood, Calif., and the Santa Monica Fault in Beverly Hills, Calif., in May and June 2018. On the basis of our data, we infer near-surface locations of various traces of these faults.From two separate profiles across the Hollywood Fault, we evaluated multiple seismic datasets and models, including guided-wave data, tomographic VP data, tomographic VS data, VP/VS and Poisson's ratio models derived from tomographic VP and VS data, Rayleigh-wave–based VS models, Love-wave–based VS models, VP/VS and Poisson's ratio models (derived from combinations of tomographic-based VP and surface-wave–based VS models), P-wave reflection images, and S-wave reflection images. All of these data and models can be used to delineate near-surface faulting, and the data consistently infer near-surface fault traces of the Hollywood Fault in the same locations. Importantly, the combined data indicate more than one near-surface fault trace of the Hollywood Fault. Between North Bronson and North Gower Avenues, evidence exists for a near-surface trace of the Hollywood Fault slightly south of Carlos Avenue. Farther west, along Argyle Avenue, our data contain high levels of cultural noise, but we interpret near-surface faulting slightly south of the intersection of Carlos and Argyle Avenues and between Carlos Avenue and Yucca Street.For the Santa Monica Fault in Beverly Hills, we acquired guided-wave data only along Lasky Drive between Moreno Drive and South Santa Monica Boulevard, owing to limited access permissions. However, we used two separate source locations to generate the guided-wave data (SP1 and SP2). The data from more distant source location (relative to the recording array, SP1) were noisy, but on the basis of those data, we infer near-surface faulting at several locations along Lasky Drive, with concentrated near-surface faulting slightly south of the intersection of Lasky Drive and Charleville Boulevard. Guided-wave data generated at the closer source location (relative to recording array, SP2) more clearly show evidence for distributed near-surface faulting at several locations along Lasky Drive, with concentrated faulting near the intersection of Lasky Drive and Charleville Boulevard.Although the seismic surveys across both faults provide strong evidence for the locations of near-surface fault traces, the seismic data provide little or no information about the rupture history of the fault traces.

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