Research Paper GEOSPHERE Geotechnical data synthesis for GIS-based analysis of fault zone geometry and hazard in an urban environment GEOSPHERE, v. 15, no. 6 Luke Weidman, Jillian M. Maloney, and Thomas K. Rockwell Department of Geological Sciences, San Diego State University, San Diego, California 92182, USA https://doi.org/10.1130/GES02098.1 12 figures; 3 tables ABSTRACT 119°0’0”W 118°0’0”W 117°0’0”W 116°0’0”W 115°0’0”W OR CORRESPONDENCE: [email protected] ID Many fault zones trend through developed urban areas where their geo- Los Angeles SAF morphic expression is unclear, making it difficult to study fault zone details 34°0’0”N CITATION: Weidman, L., Maloney, J.M., and Rockwell, NV T.K., 2019, Geotechnical data synthesis for GIS-based and assess seismic hazard. One example is the Holocene‐active Rose Canyon CA analysis of fault zone geometry and hazard in an urban fault zone, a strike‐slip fault with potential to produce a M6.9 earthquake, environment: Geosphere, v. 15, no. 6, p. 1999– 2017, SJF which traverses the city of San Diego, California (USA). Several strands trend PV-CBF Pacific https:// doi.org /10.1130 /GES02098.1. EF MX through densely populated areas, including downtown. Much of the devel- NI-RCFZ Ocean oped environment in San Diego predates aerial imagery, making assessment Science Editor: Shanaka de Silva 33°0’0”N Associate Editor: Jose M. Hurtado of the natural landscape difficult. To comply with regulations on development in a seismically active area, geotechnical firms have conducted many private, San Diego United States Received 30 November 2018 small‐scale fault studies in downtown San Diego since the 1980s. However, SCF Mexico SDTF Revision received 7 June 2019 each report is site specific with minimal integration between neighboring sites, Pacic DF Accepted 2 August 2019 and there exists no resource where all data can be viewed simultaneously on a Ocean regional scale. Here, geotechnical data were mined from 268 individual reports SMVF Published online 16 October 2019 32°0’0”N Baja, and synthesized into an interactive geodatabase to elucidate fault geome- N Mexico try through downtown San Diego. In the geodatabase, fault segments were 50 km ABF assigned a hazard classification, and their strike and dip characterized. Results show an active zone of discontinuous fault segments trending north-south Figure 1. Regional map of southern California (USA) with generalized traces of major fault zones in eastern downtown, including active faults outside the mapped regulatory in red. Black square over San Diego shows the outline of Figure 2. Inset in the upper right corner Earthquake Fault Zone. Analysis of fault geometry shows high variability along indicates the location of the map within western North America. SAF—San Andreas fault; SJF— strike that may be associated with a stepover into San Diego Bay. This type San Jacinto fault; EF—Elsinore fault; NI-RCFZ—Newport-Inglewood–Rose Canyon fault zone; PV-CBF—Palos Verdes–Coronado Bank fault; DF—Descanso fault; SDTF—San Diego Trough fault; of geodatabase offers a method for compiling and analyzing a high volume SCF—San Clemente fault; SMVF—San Miguel Vallecitos fault; ABF—Agua Blanca fault. Inset map: of small-scale fault investigations for a more comprehensive understanding OR—Oregon; ID—Idaho; NV—Nevada; CA—California; MX—Mexico. of fault zones located in developed regions. RCFZ poses a major seismic hazard for the San Diego region, as it strikes ■ INTRODUCTION through densely populated areas, including downtown. Downtown San Diego has been well developed since the early 1900s, prior The city of San Diego is the 8th most populous city in the United States to aerial imagery or high-resolution topographic maps, making geomorpho- and is located on the southernmost coast of California (USA), within the logical recognition of faulting difficult. Furthermore, the dense development border zone between the Pacific and North American plates. At the latitude of of the downtown area precludes traditional fault zone studies, in which fault San Diego, the plate boundary includes a wide zone of faulting from the San exposure is required. This is similar to several other major cities in California Andreas fault in the east to the coastal and offshore faults of the California (e.g., Los Angeles, San Francisco Bay area) and worldwide (e.g., Izmit, Turkey; Borderlands (Fig. 1). The Rose Canyon fault zone (RCFZ) is a coastal fault zone, Wellington, New Zealand; Kumamoto, Japan), where fault zones are obscured characterized by right‐lateral motion and a long-term slip rate of ~1–2 mm/yr, by development. Additionally, the RCFZ is a complex fault zone, and downtown This paper is published under the terms of the which is capable of producing a M6.9 earthquake (Anderson et al., 1989; Lind- San Diego sits at the edge of a major releasing stepover where the RCFZ steps CC-BY-NC license. vall and Rockwell, 1995; Rockwell and Murbach, 1996; Rockwell, 2010). The offshore across San Diego Bay, a pull-apart basin (Fig. 2). The total step from © 2019 The Authors GEOSPHERE | Volume 15 | Number 6 Weidman et al. | Characterization of fault zone geometry and hazard in an urban environment Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1999/4880349/1999.pdf 1999 by guest on 25 September 2021 Research Paper 117°18’0”W 117°15’0”W 117°12’0”W 117°9’0”W 117°6’0”W 117°3’0”W Faults 5 Interstate La Jolla Mt. 5 32°50’0”N Soledad Pacific Ocean RCFZ Mission 8 Population per cell Bay Figure 2. Map of the Rose Canyon fault zone (RCFZ) 0 through San Diego (SD), California (USA) and across 0–1 Old Town the San Diego Bay pull-apart basin. Black box shows 32°45’0”N 1–5 the extent of Figure 3. Grid shows population count 5 5–10 per grid cell (~1 km2) (source: LandScan 2017, Oak 10–25 SD Airport Ridge National Laboratory, UT-Battelle, LLC, https:// 25–50 landscan .ornl .gov/). DF—Descanso fault; SBF— 50–100 Spanish Bight fault; CF—Coronado fault; SSF—Silver 100–250 Downtown LNFZ Strand fault; LNFZ—La Nacion fault zone. 250–500 500–1000 1000–2500 2500–5000 5000–10,000 10,000–168,386 San 32°40’0”N Diego CF Bay SSF SBF DF 5 N 2 km the RCFZ to the offshore Descanso fault is >10 km, so a throughgoing rupture better the geology and seismic hazard of the area. The RCFZ’s location through is not predicted (Wesnousky, 2006). Nevertheless, rupture models show that the populated city and classification as Holocene active (Lindvall and Rockwell, the presence of smaller faults within the stepover can have a complicated effect 1995) place restrictions on development. These restrictions are in place through on rupture propagation across a step, with some scenarios suggesting that the Alquist‐Priolo Earthquake Fault Zoning Act, which prohibits the location of rupture could propagate onto intermediate faults within the step (Lozos et al., most structures for human occupancy across the traces of active faults, and 2015). Stepover geometry also evolves over time and could result in temporally through the City of San Diego Downtown Special Fault Zone, which requires complex rupture patterns with changes to the amount of slip accommodated a fault evaluation for any new or additional development near active fault on various fault segments (e.g., Wakabayashi et al., 2004; Wu et al., 2009). traces. These regulations define a zone around known active fault traces where Therefore, a detailed understanding of fault geometry near and across the the restrictions are in effect, herein referred to as the Alquist-Priolo zone (AP San Diego Bay stepover is important for accurate hazard assessments for the zone) (Fig. 3). Within the AP zone, fault investigations are routinely conducted region and for improving our understanding of stepover evolution. for proposed development projects by various geotechnical firms. The fault This project represents the first attempt to synthesize geotechnical data from investigations can include trenching, sediment borings, cone penetration tests downtown San Diego, gathered by the geotechnical community, to understand (CPTs), and geophysical subsurface imaging, which are then used to define GEOSPHERE | Volume 15 | Number 6 Weidman et al. | Characterization of fault zone geometry and hazard in an urban environment Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/6/1999/4880349/1999.pdf 2000 by guest on 25 September 2021 Research Paper the stratigraphy and fault geometry across the proposed development site, used to assess RCFZ geometry as it relates to evolution of the San Diego Bay typically about the size of a city block. These types of data are included in site stepover. Specifically, the orientations of fault segments were compared with reports submitted to the firm’s clients and city officials, but the data from the the timing of the most recent activity on those segments to assess whether reports have never been compiled before into a single geodatabase for a more the evolution of the pull-apart basin may have resulted in a change in active complete view of the RCFZ through the entire downtown area. fault orientation through time. For this study, data were pulled from 268 geotechnical investigations per- formed by various consulting firms between 1979 and 2016, and were compiled into a comprehensive GIS fault and seismic hazard map of downtown San ■ GEOLOGIC BACKGROUND Diego. The resulting geodatabase could contribute to an active fault database for use in updating the city’s seismic safety element, and aid the science com- Rose Canyon Fault Zone munity by helping to establish fault characteristics and complexities along strike, map subsurface stratigraphy beneath downtown for use in ground The RCFZ is the southern continuation of the Newport‐Inglewood fault acceleration and liquefaction models, and potentially illuminate recurrence zone that strikes south from Los Angeles, continues along the continental intervals, patterns of multi‐segment ruptures, and evidence for long-term shelf edge, and then trends onshore just north of Mount Soledad in La Jolla, slip rate.