Geotechnical Data Synthesis for GIS-Based Analysis of Fault Zone Geometry and Hazard in an Urban Environment

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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, Paci c 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

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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

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 performed 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. Advances in these areas would greatly improve our understanding California (Fig. 1) (Fisher and Mills, 1991; Rockwell, 2010; Sahakian et al., 2017). of faulting and earthquakes and improve our ability to assess seismic hazard From La Jolla, the RCFZ extends south along the Interstate Highway 5 corri- to populated regions. As a case study, the compiled geotechnical data were dor and then diverges near Old Town San Diego (Kern and Rockwell, 1992;

117°12’0”W 117°11’0”W 117°10’0”W 117°9’0”W 117°8’0”W

0 0.5 1 Rose Canyon fault zone km Balboa Park

32°44’0”N San Diego Int’l Airport

Florida Canyon fault San Diego Bay Downtown 32°43’0”N Figure 3. Street map of greater downtown San Diego “Downtown San Diego region showing Alquist-Priolo San Diego graben” (AP) zones and faults from the U.S. Geo- fault logical Survey (USGS) fault database (USGS-CGS, 2006). Black box shows the extent of Figures 6, 7, and 8. Background imagery: ESRI, HERE (https://www.here .com/strategic-partners/esri), Garmin, OpenStreetMap contributors, and the GIS 32°42’0”N community. Spanish Bight fault

Coronado Island

32°41’0”N Coronado fault AP zones USGS faults

Silver Strand fault

Singleton et al., 2019). One strand trends toward the San Diego International of Mount Soledad revealed six Holocene events on the RCFZ yielding a recur- Airport and the other toward downtown San Diego (Fig. 2). Both faults splay rence interval of ~1800 yr (Lindvall and Rockwell, 1995; Rockwell, 2010), but offshore into San Diego Bay, where they continue as the Spanish Bight fault recent trenching near Old Town suggests a recurrence interval of 700–800 yr (from the airport) and the Coronado and Silver Strand faults (from downtown). during the late Holocene (Singleton et al., 2019). Data from several geotech- The offshore faults accommodate transtension across San Diego Bay created nical studies were used to further constrain the RCFZ most recent event to an by a releasing stepover between the onshore Rose Canyon fault and offshore age of A.D. 1650 ± 120 yr (Rockwell, 2010). Descanso fault (Fig. 2) (Moore and Kennedy, 1975; Legg, 1985; Rockwell, 2010; Maloney, 2013). The Descanso fault continues south along and adjacent to the coast of Baja California, Mexico. Stratigraphy The stepover across San Diego Bay has resulted in localized transtension and subsidence and the evolution of multiple north‐trending en echelon faults San Diego is a coastal city that has been experiencing regional uplift (0.13– within the bay (Moore and Kennedy, 1975; Rockwell, 2010; Maloney, 2013). To 0.14 m/k.y.) with localized transpression and transtension during the eustatic the east, the north‐south–trending La Nacion fault zone has been interpreted sea-level cycles of the Quaternary, which resulted in stratigraphy that was as the eastern boundary of the pull‐apart basin (Anderson et al., 1989) (Fig. 2). deposited in both marine and nonmarine environments (Kern and Rockwell, The La Nacion fault zone is composed of west-dipping, anastomosing normal 1992; Haaker et al., 2016). Formations found in the stratigraphy of the downtown faults with >60 m of vertical offset observed in the Pliocene San Diego For- San Diego area include the San Diego Formation (3–1.5 Ma), the Lindavista mation (Hart, 1974). Formation (1.5–0.7 Ma), and the Bay Point Formation (0.13–0.08 Ma) (Kennedy, Downtown San Diego contains several Holocene‐active splays of the RCFZ. 1975; Kennedy and Peterson, 1975; Kennedy and Tan, 1977; Tan and Kennedy, Since the late 1970s, geotechnical investigations have identified two zones 1996; Abbott, 1999). There is a regional unconformity between the Lindavista and of faulting that comprise the AP zones downtown (Fig. 3). The western zone Bay Point Formations corresponding to a gap in deposition of ~0.57 m.y. Both encompasses the San Diego fault, which trends N5°–6°W and dips 60°–80°E “Lindavista Formation” and “Bay Point Formation” are older terms that have with measured vertical offsets, east‐side down, of between 3 and 10 m and a been recently discontinued, and the associated deposits now are commonly minimum lateral offset of ~3 m (Treiman, 1993; 2002). The eastern zone extends referred to as paralic deposits. However, because the terms are used in many south into San Diego Bay and includes the “downtown graben,” which is a of the earlier reports included in the project to describe subsurface stratigraphy, transtensional basin inferred via three north‐south–trending faults (Treiman, the terms will be used here. Also present in most sites downtown are paleosols 1993, 2002). The western graben fault is described as striking from north-south and Holocene overburden deposits such as alluvium, colluvium, stream or river to N15°W with eastward dip angles that range from 63° to 86°. The normal terrace deposits, and undocumented fill placed by humans for development. sense of displacement is east-side-down, and evidence for lateral displace- Identification of these deposits in the subsurface in geotechnical studies allows ment has been identified. The central graben fault includes multiple changes for relative dating of fault offset to determine the recency of fault activity. in strike, but dip angles are relatively consistent, ranging from vertical to 68°E. Both vertical and horizontal displacement have been observed on the fault. The central graben fault appears to continue offshore across San Diego Bay as ■■ METHODS the Silver Strand fault. The eastern graben fault trends N10°–20°W and shows evidence for both vertical and lateral offset. Dip angles vary from vertical to Data for this project came from geotechnical reports that were completed westward dipping at ~60° with apparent displacement down to the west. The by several different private geotechnical consulting firms (Table 1). Within the surface expression of faulting in the “downtown graben” was also observed reports, data were primarily found in the form of trench logs, boring logs, CPT and illustrated in an 1876 bird’s-eye-view drawing of downtown San Diego, logs, and geologic cross sections, with lesser contributions from seismic lines prior to most development (Glover, 1876). The fault is drawn laterally offset- and test pits. The information contained in the logs was originally obtained ting topographic features. on site by a licensed engineering geologist. Most of the reports collected for Historic seismicity has been observed in the downtown San Diego and this project were bound paper reports, which were unbound and scanned into San Diego Bay vicinity. Astiz and Shearer (2000) located several earthquakes PDF format, and then cataloged (Table 1). occurring between A.D. 1981 and 1997 of

and a parcel map of San Diego from the San Diego Association of Govern- maps were georeferenced, they were moved into group layers in the project ments (SANDAG) were all downloaded from ESRI’s online database and used table of contents, organized by the decade in which they were created (Fig. 4). as georeferencing base maps. Within each geotechnical report was a site map To estimate the total uncertainty in the process of georeferencing site maps, showing locations of collected data. The site map from each report was georef- 10 georeferenced maps were randomly selected from the downtown area. For erenced to the base layer individually using the ArcMap georeferencing toolbar. each site, the ESRI World Imagery accuracy was determined with the ArcMap None of the site maps contained markings for geographic coordinates, so Identify tool. All sites reported an accuracy of 4.06 m. The other two base maps all georeferencing was done using other mapped features tied from the site do not include reported accuracy. Therefore, the center of an intersection map to the base map layers. Most site maps showed parcel boundaries or adjacent to the site was used to measure offset distance between the World property lines for the study area. For those maps, control points were placed Imagery and World Street Map layers. Additionally, the offset between the along the parcel boundaries or property lines and tied to the SANDAG parcel World Street Map and parcel map layers was determined by measuring the map. For site maps where these boundaries were unavailable, control points distance between an identifiable corner of a building, lot, or park on the World were placed on street corners and around buildings and tied to the ESRI World Street Map layer and the same corner on the parcel map. Finally, the total root Imagery and World Street Map base layers. There was a small number of site mean square (RMS) error reported in the ArcMap georeferencing control point maps that showed neither parcel boundaries nor building locations. For these table was recorded for each site map. All of these measurements are reported sites, other landmarks, such as vegetation, parking lots, and driveways, were in Table 2. The average uncertainty from each source of error was calculated, tied to the ESRI World Imagery and World Street Map base layers. Once the site and then the total uncertainty (U) was calculated to be the square root of the

TABLE 1. COMPANIES CONTRIBTING GEOTECHNICAL REPORTS Company name No. of reports Allied Earth Technology, Inc. 1 Christian Wheeler Engineering 2 Converse Consultants 1 Construction Testing Engineering, Inc. 8 Davis Earth Materials, Inc. 1 Engineering Geology Consultants 1 Geocon, Inc. 9 GeoLogic Associates 1 Geotechnics Incorporated 5 Group Delta Consultants, Inc. 2 Kleinfelder, Inc. 21 LawCrandall, A Division of Law Engineering and 1 Environmental Services, Inc. Figure 4. Table of contents for the ESRI Leighton and Associates, Inc. 25 ArcMap project showing group layers Medall, Aragon Geotechnical, Inc. 2 for organization of data. Michael W. Hart Ninyo Moore Geotechnical and Environmental Sciences 6 Consultants NOA Services, Inc. 1 Owen Consultants 1 Petra Geotechnical, Inc. 1 Professional Services, Inc. 1 Robert Prater Associates 1 RORE, Inc. 1 Rugg Geosciences, Inc. 1 Southern California Soil Testing, Inc. 5 CCA Southland, A Division of Southland Geotechnical, Inc. 3 Starboard Development Company 1 TerraCosta Consulting Group, Inc. 2 RS Corporation 18 Woodward-Clyde Consultants 6

TABLE 2. MEASREMENTS OR ESTIMATED TOTAL NCERTAINTY IN THE GEOREERENCING PROCESS Site no. ESRI World Offset: Imagery Offset: Street map Total RMS Site map file name Intersection measured Parcel measured Map accuracy minus street map minus parcel map error m m m m 1 .06 2.033 1.152 1.11 E St. - G St. - Kettner - Columbia - geocon 80.PNG E Street and Kettner Boulevard NW corner of Pantoja Park 2 .06 2.501 0.26 0.603 11th-15th-C St- St owen 89.PNG B Street and 11th Avenue NW corner of northwesternmost building to NW corner of northwesternmost parcel 3 .06 3.226 0.852 0.06 Beech-Cedar-India-Kettner cte 97.PNG Cedar Street and Kettner Boulevard SE corner of northwesternmost building to SE corner of northwesternmost parcel .06 2.865 0.275 0.125 Dwntwn Sub Marina Area site A kleinfelder 98.PNG ront Street and G Street NW corner of northwesternmost building to NW corner of northwesternmost parcel 5 .06 2.201 5.35 0.202 India St Hawthorn St rugg 00.PNG Hawthorn Street and Kettner Boulevard NE corner of northwesternmost building to NE corner of northwesternmost parcel 6 .06 0.952 0.882 0.03 820 Ash St, 105 131 Pacific Hwy kleinfelder 0.PNG Pacific Highway and Beech Street SW corner of westernmost building to SW corner of northwesternmost parcel 7 .06 .27 1.116 0.178 0 17th St geocon 08.PNG 16th Street and Island Avenue NW corner of northwesternmost building to NW corner of northwesternmost parcel 8 .06 .26 0.981 0.079 B St-C St-State-nion RS 11.PNG State Street and C Street NE corner of northwesternmost building to NE corner of northwesternmost parcel 9 .06 1.183 1.815 2.21 189 Park Blvd cte 15.PNG Imperial Avenue and Park Boulevard SE corner of southwesternmost parking lot to SW corner of southeasternmost parcel 10 .06 5.5 0.888 0.099 625 Broadway cte 12.PNG E Street and 6th Avenue NW corner of northwesternmost building to NW corner of northwesternmost parcel

Mean .06 2.8995 1.3732 0.5077 As reported in ESRI ArcMap georeferencing tools RMSroot mean suare. Center of intersection used to measure offset between ESRI World Imagery and ESRI World Street Map layers. Always the northwest corner of the site, unless noted. Southwest corner of the site, because the northwest corner was obscured by buildings. Location used to measure offset between World Street Map and parcel map layers. Mean of all 10 sites measured across all columns. Notes: ESRI World Imagery WGS: accessed through ArcGIS Online. Sources: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, SDA, SGS, AE, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS ser Community. ESRI World Street Map: accessed through ArcGIS Online. Sources: Esri, HERE, DeLorme, SGS, Intermap, increment P Corp., NRCAN, Esri Japan, METI, Esri China Hong Kong, Esri Thailand, MapmyIndia, © OpenStreetMap contributors, and the GIS ser Community. Parcel Map: PARCELSSOTH shapefile downloaded from the San Diego Association of Governments SANDAG; GIS using legal recorded data provided by the County Recorders and Assessors Office. See the County ARCC website at https://arcc.sdcounty.ca.gov/Pages/default.asp for more information about ta parcels.

sum of squares for all sources of error (World Imagery [wu], World Street Map locations. The drawn symbols were then converted to features and added to

[su], parcel map [pu], and georeferencing control points [gu]), calculated as: the ArcMap project. A feature was created for each individual report and then grouped by data type and organized by year within each group (e.g., a group 2 2 2 2 U = ( w u + su + pu + gu ). (1) containing all boring data from all reports was broken into sublayers, organized by year, of boring locations from each site) (Fig. 4). This allows the user This resulted in an average estimated total uncertainty of 5.20 m for the to turn on and off all of one type of data at a time. 10 sites. While this is an average estimate, it offers a method of quantifying Once data locations were digitized and divided into sublayers, additional uncertainty that can be applied across the entire project. Where knowing site information for each sublayer was added to its attribute table (Fig. 5). Col- uncertainty is crucial, we suggest calculating total uncertainty by this method umns were created that included: Name, the name of the feature (e.g., B‐2, for individual sites of interest. As this project is based on the work of others, Trench Log, etc.); ReportName, the name of the associated report; Company, it is difficult to quantify other sources of uncertainty introduced during field the geotechnical firm that did the work;Date , the report date; and ProjectNo, investigations and generation of original site maps. a reference number used by the geotechnical firm for record keeping (Fig. 5). The georeferenced site maps were used to digitize locations of the various Additionally, the columns Latitude and Longitude were created to give coor- data recorded on each map. Using the drawing toolbar in ArcMap, data loca- dinate data to boring, CPT, hand auger, and test pit sublayers. tions were traced using point symbols for boring, CPT, hand auger, and test pit Next, the image(s) of the data log(s) from the original report were linked to locations, and lines or polygons for trench, cross section, and seismic profile each corresponding feature in ArcMap. This was accomplished by creating a

Figure 5. Example attribute table for a set of borings from one geotechnical report.

folder within the project catalog where all images of data logs were stored as by the engineering geologists that performed the original work. To distinguish individual files. To link the image of a specific data log with the corresponding potentially active from less potentially active, the trench logs and geologic feature on the map, a column was created in every data attribute table called cross sections were used to view and interpret the extent of faulting through Image. Here, the image file name for the given data log was placed within the the stratigraphic sections. The CGS updated their fault classifications in 2018 row for that feature, and the Hyperlink property of the sublayer was enabled. to simply “Holocene-active” if younger than 11,700 yr, or “pre-Holocene” if The user could then left‐click on a feature using the hyperlink tool to display older than 11,700 yr. Nevertheless, the reports used in this study were based the corresponding data log. on the older definitions, and so those definitions were adopted for hazard classification in the geodatabase. Finally, an earthquake hazard group layer was made based on the mapped Hazard Classification faults and their classification. For every site included in this project, its boundary was outlined atop the base map in ArcMap and grouped to represent the In ArcMap, faults identified and recorded in geotechnical reports were level of seismic hazard risk at that site. Sites where no faults were encoun- traced using the georeferenced site map to create line features for each fault tered were grouped as hazard level 0 sites. Sites where faults classified as classification. The faults were classified and grouped asactive , potentially less potentially active were encountered were grouped as hazard level 1 sites. active, or less potentially active. These definitions are modified from the pre- Sites where potentially active faults were encountered were grouped as hazard vious California Geological Survey (CGS) fault classifications, which were level 2 sites. And, sites with active faulting were grouped as hazard level 3 sites. adopted by the geotechnical community during the fault investigations synthe- These hazard levels, too, were given a group layer with sublayers based on sized in the geodatabase. Herein, an active fault is defined as one that displays classification. It should be emphasized that ahazard level 0 does not indicate evidence of movement within the Holocene (the last 11,500 yr). By the CGS that a site would suffer no impact from an earthquake on the RCFZ, but rather definition, a potentially active fault displays evidence for movement prior that the site is not located directly above any known fault traces. to 11,500 yr ago, but within the Quaternary period (the last 1.8 m.y.) (Bryant, 2010). This project further splits the CGS-defined potentially active faults into two groups; potentially active and less potentially active. This was done to Fault Geometry accommodate the moving of the Pliocene‐Pleistocene boundary from 1.8 Ma to 2.6 Ma in 2009. Herein, a fault defined asless potentially active shows evidence Strike orientation data were taken from all faults plotted in the project of movement within the San Diego Formation and/or the Lindavista Formation from geotechnical reports. This was done by downloading and installing the (3.0–0.7 Ma). A fault defined aspotentially active shows evidence of movement EasyCalculate add‐on for ArcMap (https://www.ian-ko.com/free/EC10/EC10 within the Bay Point Formation (0.13–0.08 Ma). Due to the gap in deposition _main.htm), which measures an azimuth direction for line features (e.g., fault within the region, activity between 0.7 and 0.13 Ma could not be identified. traces) within ArcMap. Because some fault traces change direction, the mapped Fault classification in this project is directly based on the classifications made faults were first split at all vertices along the polyline using theSplit Line

at Vertices ArcMap tool. Then strike was measured from the middle of each the ESRI layers that make up the base of the map (World Imagery, World Street fault segment. Measured strike orientations for each fault classification were Map). To create the geodatabase, >400 reports were collected and assessed placed in that layer’s attribute table. Dip angles and directions for each fault, from various geotechnical firms. Many of the reports were decades old, and taken from associated geotechnical reports, were also recorded in attribute some reports were found to be inadequate for inclusion due to lack of data, tables when available. A histogram of dip angles for each group of faults was uninterpretable data (e.g., faded pages, illegible hand‐drawn logs), missing also generated. Strike orientation data were exported to MATLAB to generate pages, or incomplete data logs. From the reports collected, 268 were included rose diagrams that plot strike orientations and mean direction for each fault in the project. Still, 42 of these reports have missing data. These are reports classification group. For the rose diagrams, and subsequent statistical tests, that are only missing one or two data logs but still contain enough data to be the strike data were normalized by fault segment length. Each 10 m length useful. The 268 reports included in the study yielded a total of 2020 georef- of a segment was counted as one data point (e.g., a 10 m segment produced erenced data points. These data points are made up of 922 boring locations, one data point of the same strike while a 100 m segment produced 10 data 290 CPT locations, six hand auger locations, 54 test pit locations, 554 trench points of the same strike). locations, 189 cross section locations, and five seismic lines (Figs. 6 and 7). Three basic statistical analyses were run in MATLAB, following Trauth (2007), The geodatabase was made publicly available through Weidman et al. (2019). on the strike orientation data for active faults, potentially active faults, and less potentially active faults. The tests were performed on a 95% confidence level and include a Pearson chi-squared test (Pearson, 1900) for randomness Hazard Classification of directional data, a Rayleigh test (Mardia, 1972) for the significance of a mean direction, and an F-test (Snedecor and Cochran, 1989) for the difference Reports were assessed to assign a hazard classification to each site and plot between two sets of directions. The chi-squared test compares the empirical faults that were encountered by the engineering geologist during field work. frequency distribution of fault directional data (strike orientations) with a uni- A total of 93 faults were mapped and classified (Fig. 8). Nine were classified form distribution to determine randomness. The Rayleigh test uses the mean as less potentially active faults, 35 as potentially active faults, and 49 as active resultant length, a measurement based on the computed sine and cosine of faults. Faults were traced from each original report’s site map. Fault traces on each strike direction, which increases proportionally to the significance of the site maps are generally confined only to the study area, which resulted in many mean direction. If the measured mean resultant length is greater than the crit- short and discontinuous fault segments (each about a block in length). These ical mean resultant length (taken from table 10.1 in Trauth [2007] from Mardia segments were not connected within the ArcMap project to best represent [1972], using significance level of 0.05), then the null hypothesis is rejected the original data, but mapped faults may represent strands or segments of and the data are claimed to have a preferred direction. The F-test compares a larger, more continuous fault zone, which could be interpreted by the user. resultant lengths of each of two data sets with the combined resultant length Hazard levels for each site were assessed based on the presence (or absence) of both data sets to give an F‐value. If the measured F‐value is greater than the of faults within the different classifications. In total, 223 sites within the down- critical F‐value (taken from standard F‐value tables), then the null hypothesis is town area were given a seismic hazard classification (Fig. 8). Of those sites, 172 rejected and it is concluded that the data sets are not from the same population. sites were classified ashazard level 0 and outlined in green; eight sites were classified ashazard level 1 and outlined in yellow; 22 sites were classified as hazard level 2 and outlined in orange; and, 21 sites were classified ashazard ■■ RESULTS level 3 and outlined in red. There are three sites in the study that do not have a hazard classification Geodatabase and are outlined in black on the map (Fig. 8). Each site contains some geologic data (e.g., boring data, CPT data, etc.), but lacks any information on faulting Within ArcMap, the finished project contains 23 group layers (Fig. 4), which because a fault evaluation was not performed. Therefore, they could not be include group layers for the three fault classifications active( , potentially active, given a hazard risk classification. There are also three sites that have a hazard and less potentially active), four hazard classifications hazard( levels 0–3), classification, but no fault classification. In all three cases, the reports indi- seven types of data logs (borings, CPTs, augers, test pits, trenches, cross cated the existence of faults classified by this study asless potentially active, sections, and seismic lines), and five layers for site maps grouped by decade but did not plot the faults on the site map. Because this study is meant to (1970s, 1980s, 1990s, 2000s, 2010s). The total size of the ArcMap project is ~5.2 show results based on geotechnical data, the sites were classified ashazard gigabytes, which includes digitized images of site maps and data logs from level 1. However, there was too little information to accurately plot faults on geotechnical reports. Also included are the background layers for the San the map at these sites. Diego parcel map (from SANDAG), the U.S. Geological Survey and California The faults plotted from geotechnical reports generally follow the pattern Geological Survey (USGS-CGS) Quaternary fault map (USGS-CGS, 2006), and of faults from the USGS-CGS fault database (Fig. 9). However, many faults

117°10’30”W 117°9’30”W 117°8’30”W

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Trench logs 5 Balboa Park Seismic logs

5 Interstate 5

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ESRI, HERE, Garmin, © OpenStreetMap contributors, and the GIS community.

Figure 6. Street map of downtown San Diego, California (USA), study area showing digitized line features of cross section, trench, and seismic logs. Within the ESRI ArcMap project, each log contains metadata in the feature attribute table and a link to the digitized copy of the original report’s log. Background imagery: World Light Gray Canvas Base; ESRI, HERE, Garmin, OpenStreetMap contributors, and the GIS community.

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Boring logs 5 CPT logs 5 Balboa Park Hand auger logs

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ESRI, HERE, Garmin, © OpenStreetMap contributors, and the GIS community.

Figure 7. Street map of downtown San Diego, California (USA), study area showing digitized point features of boring, cone penetration test (CPT), hand auger, and test pit logs. Within the ESRI ArcMap project, each log contains metadata in the feature attribute table and a link to the digitized copy of the original report’s log. Background imagery: World Light Gray Canvas Base; ESRI, HERE, Garmin, OpenStreetMap contributors, and the GIS community.

117°10’30”W 117°9’30”W 117°8’30”W

0 0.25 0.5 N km

Less potentially active faults Potentially active faults 5 Balboa Park Active faults

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Little Hazard level 1 32°43’30”N Italy 5 Hazard level 2 Hazard level 3 Hazard unclassified

5 Interstate 5

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ESRI, HERE, Garmin, © OpenStreetMap contributors, and the GIS community.

Figure 8. Street map of downtown San Diego, California (USA), study area with faults mapped and grouped by classification and hazard classification shown for each site included in the study. Background imagery: World Light Gray Canvas Base; ESRI, HERE, Garmin, OpenStreetMap contributors, and the GIS community.

117°10’30”W 117°9’30”W 117°8’30”W Fig. 10B RCFZ-SD segment Balboa 0 0.35 0.7 km Little Park Italy 32°43’30”N N

San Diego 5 Bay Figure 9. Street map of downtown San Di- ego, California (USA), study area zoomed to the extent of mapped faults. Also shown are the Alquist-Priolo (AP) zone boundar- “Downtown Fig. 10A ies and the U.S. Geological Survey (USGS)

Florida Canyon fault 32°43’0”N graben” faults (USGS-CGS, 2006) for comparison 5 Less potentially active RCFZ-SD segment with results from this study. RCFZ-SD faults segment—Rose Canyon fault zone–San Di- ego segment; CFZ—Coronado fault zone; Potentially active faults Active faults outside AP SSFZ—Silver Strand fault zone. Background Active faults zone imagery: World Light Gray Canvas Base; ESRI, HERE, Garmin, OpenStreetMap con- USGS faults tributors, and the GIS community. AP zones 32°42’30”N 5 Interstate 5

SSFZ

CFZ

are mapped differently for at least part of their length. Eight of the nine less east‐side-down sense of displacement, with estimates of vertical offset per potentially active faults, all 35 potentially active faults, and 40 of the 49 active event to be on the order of 0.3–0.6 m (Geocon, Inc., 2013). Additionally, at a faults differ from the USGS-CGS faults by >5.2 m (estimated uncertainty) bend in a fault segment located south of the “downtown graben,” the maxi- for at least part of their extent. The biggest differences were observed in the mum distance between an active fault mapped in geotechnical investigations potentially active faults, where many of the faults do not appear to align with and that mapped in the USGS-CGS database is ~28.3 m (Fig. 10A). The total any part of a USGS-CGS fault (Fig. 9). Most active faults track more closely georeferencing uncertainty for this site was calculated to be 7.31 m by the with the USGS-CGS faults, but mismatch for part of their extent. The active same calculations outlined in the Methods section, which is smaller than the faults also fall within the AP zones, with the exception of three faults classified measured difference. Furthermore, the fault was identified by the investigating as active that were identified one block east of the current western AP zone geotechnical firm in a trench that extended across the entire southern length boundary and were not included in the USGS-CGS fault map (Figs. 9 and 10A). of the site, including across the location of the USGS-CGS mapped fault (Geo- The site map associated with the faults shows two fault zones, fault zone A con, Inc., 2004) (Fig. 10A). The investigation did not show evidence of a fault and fault zone B. Fault zone A comprises two faults that trend to the northwest at the location of the USGS-CGS mapped fault, justifying a mismatch in the at approximately N17°W to N20°W and dip to the west at 65° (Geocon, Inc., two databases at this location. 2013). Displacement on these faults was not resolved, but a west‐side-down There are also sites in the downtown area, located across faults in the sense of motion is suggested (Geocon, Inc., 2013). Minor faulting also discov- USGS-CGS database, where reports from geotechnical investigations uncov- ered during trenching operations shows vertical displacement of 0.3–0.6 m ered no evidence for active or potentially active faults. Because this study (Geocon, Inc., 2013). Fault zone B comprises one fault that trends northwest directly reflects the geotechnical data, these sites were assigned tohazard from the southeastern corner of the site, parallel with faults in fault zone A, level 0. In some locations, the USGS-CGS faults just barely cross the corner or but turns toward the northeast. Fault zone B dips to the east and shows an edge of the site, and therefore the inconsistency could be a result of location

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Less potentially active faults Potentially active faults USGS-Geotech fault 32˚42’40”N di erence Active faults Geocon, Inc. (2004) USGS faults AP zones

Hazard level 0

Hazard level 1 B 0 0.05 0.1 km Geocon, Inc. (2008) Hazard level 2

5 Hazard level 3 32˚43’40”N RCFZ-SD segment Trench logs Boring logs

5 Interstate 5

32˚43’30”N CTE (2016)

117˚10’20”W 117˚10’10”W 117˚10’0”W

Figure 10. (A) Close-up on the “downtown graben” area of San Diego, California (USA) (see Fig. 9 for location) showing faults and hazard classification with some active faults outside the Alquist-Priolo (AP) zone. USGS-Geotech fault difference indicates the mismatch between faults mapped in this project based on geotechnical reports and faults mapped in the USGS-CGS (2006) database. The Geocon, Inc. (2004) trench shows location of the trench that was used to map the location of the fault zones across this location as discussed in the text. USGS—U.S. Geological Survey; FZA—fault zone A; FZB—fault zone B. (B) Close-up on Little Italy area (see Fig. 9 for location) showing geotechnical investigations across the Rose Canyon fault zone–San Diego (RCFZ-SD) segment where no evidence for faulting was observed. Background imagery: World Light Gray Canvas Base; ESRI, HERE, Garmin, OpenStreetMap contributors, and the GIS community.

uncertainty or lack of data at the site edges. However, there were also some in active fault strike moving from south to north (Fig. 9). To the south, active larger inconsistencies observed. For example, there are two sites located in faults trend to the north coming out of San Diego Bay, whereas further north, the Little Italy section of northern downtown where trenches were dug directly they take a more northeast trend and then turn back toward the northwest across a mapped segment of the RCFZ but a fault zone was not encountered near the “downtown graben” (Fig. 9). Approximately half of the active faults (Fig. 10B). This northern San Diego segment is not included in the AP zones, have near-vertical to vertical dip angles. The other half exhibit dips that vary but is mapped in the USGS-CGS database as a segment of the RCFZ extend- from east to west and have dip angles ranging from 50° to 79° (Fig. 12; Table 3). ing south from Old Town. At the northern site, where the investigation was Active faults that have shallower dips are concentrated around the “downtown conducted in 2008 by Geocon, Inc., the trench was dug to 2.4 m depth where graben,” with faults on the west side of the graben dipping to the east and unfaulted Bay Point Formation (Pleistocene age) was encountered (Geocon, faults on the east side of the graben dipping to the west. Inc., 2008). At the southern site, a 1.5-m-deep trench was excavated in 2016 by The strike data from each group of faults were tested against each other Construction Testing & Engineering, Inc. (CTE), and unfaulted Pleistocene-age using the F‐test to determine if the difference between the data sets is statisti- soils exhibiting very well-developed argillic horizons were encountered (CTE, cally significant. When comparing theactive faults with the potentially active 2016). The San Diego segment also crosses a third site in Little Italy that has faults, the F-test concluded that the two groups have statistically significant been classified ashazard level 0. However, at this site, located between the directional differences. The same conclusion resulted between the potentially other two trenching sites, only soil borings were collected, and the boring active faults and the less potentially active faults. However, the F-test concluded transect did not cross the trace of the USGS-CGS fault location (Fig. 10B). that a statistical difference cannot be determined between the strikes of the active faults and those of the less potentially active faults.

Fault Zone Geometry ■■ DISCUSSION To identify possible trends in fault activity that may be related to fault orientation or dip, the strikes and dips for all faults mapped in the study (if available) Geodatabase were analyzed (Table 3). All faults classified asless potentially active exhibit strike orientations that vary from approximately N40°W to N30°E, with a mean The compilation of individual geotechnical investigations provides a central- strike direction of N9°W (Fig. 11A). The chi-squared test results indicated that ized database for study of the RCFZ and associated hazards. Prior to creation of strike directions are not random. The mean direction was also tested using this database, the community would have needed to sift through hundreds of the Rayleigh test, which found the data to have a preferred direction. Based on ungeoreferenced paper reports to extract data. The ArcMap project allows the data from included reports, dip angles of the less potentially active faults vary user to view the location of various types of data and directly open images of from 50° to near vertical (Fig. 12; Table 3). Dip directions are both to the east the data logs through hyperlinks in the project. The subsurface data provide and west and exhibit normal separation in most cases, with beds separated detailed information on stratigraphy and faulting that would otherwise be from as little as 5 cm up to 2.7 m. difficult to obtain in the urban environment of downtown San Diego. These Faults classified aspotentially active exhibit strike orientations that vary data include geologic descriptions of sedimentary units from borings and from approximately N26°W to N50°E, with a mean strike direction of N16°E trenches, geotechnical properties of sedimentary units from CPT logs, and (Fig. 11B). The chi-squared test results showed that the data are not random, descriptions of structural observations including offset or folded strata, biotur- and the Rayleigh test indicated that there is a preferred direction. According to bation, fissures, and soft-sediment deformation structures. Future work based the geotechnical reports, most dip angles exhibit minor deviations from vertical on this geodatabase could include mapping of three-dimensional stratigraphic to the east and west. Only three of the 35 potentially active faults have a dip architecture and modeling of ground acceleration and liquefaction for different angle shallower than 70° (Fig. 12; Table 3). Offset beds along the potentially earthquake scenarios. Additional layers could also be added for future seis- active faults exhibit oblique offset in most cases that varies from centimeters mic hazard investigations, such as building zonation or population density. to meters. Spatial analysis of these trends shows that potentially active faults The geodatabase was made publicly available through Weidman et al. (2019). in the western half of the study area, near San Diego Bay, are dominantly north The work of compiling, scanning, and reviewing reports, the building of to NW trending, while the eastern half, near Interstate Highway 5, contains the geodatabase, and the initial fault geometry analysis were all conducted by potentially active faults that dominantly trend NE. a single student for a Master’s thesis project (Weidman, 2017). The onerous Strike orientations of the active faults vary from approximately N78°W to task of compiling reports was made easier by working directly with several N48°E with a mean direction of N9°W (Fig. 11C). The chi-squared test results consulting firms that provided copies of reports. This had the added benefit indicated that the data are not random, and the Rayleigh test showed that of helping to build relationships between the geotechnical and academic com- the data have a preferred direction. In map view, there is an obvious change munities. Similar geodatabase projects could be useful in other areas of the

TABLE 3. ALT STRIKE AND DIP DATA ROM DOWNTOWN SAN DIEGO AREA, CALIORNIA SA Less potentially active faults Potentially active faults Active faults Strike Dip Strike Dip Strike Dip 326.3 65SW 5.1 Near vertical 307.9 SW 336.1 N/A 12.8 To the west 329.2 63–79NE 332.0 65SW 38.0 58W 335.8 85NE 326.1 86NE 359.0 30W 35.3 50NE, 60SE .6 70–80E 25.0 72W to vertical 3.6 Near vertical 13.5 70–75E 1.3 72W to vertical 331.0 Near vertical 8.0 80E80W 13.8 72W to vertical 331.8 80NE 336.5 Near vertical to 50–8W 35.9 72W to vertical 31.6 Near vertical 33.6 65E 22.8 72W to vertical 30.8 Near vertical 35.9 Near vertical 282.3 70SW 3.3 N/A 312.6 N/A 29.3 E–W 319. 80SW 2.2 72W to vertical 339. 70–86NE 2.0 72W to vertical 36.2 63E to vertical 0.6 72W to vertical 35. 80NE 5.3 E–W 37. 53NE 31.1 E–W 353.7 West 11.2 E–W 350.7 65W 32.8 72W to vertical 356.7 55–75W 352.0 N/A 336.8 70W 355.8 80SE 338.1 65SW 352.9 N/A 3.9 87–90SE 353.8 N/A 37. Near vertical 25.3 77–88NW to vertical 333.0 Near vertical 1.0 N/A 339.5 75NE 23.1 70–90E 356.1 Near vertical to vertical 33.6 85W 38.9 Near vertical 29.8 ertical 0.0 Near vertical 2.6 82E 338.0 Near vertical 35.7 73–76N 35.3 Near vertical 38.7 50–60SE 32.1 Near vertical 18.0 Near vertical 331. Near vertical 333.9 Near vertical 339.7 Near vertical 5.0 80E 33.0 N/A 13.7 85NE 350.8 Near vertical 2.8 N/A 20. 80SE 32.5 65W 330. 65W 339.7 East 32.6 68E 22.0 ertical 33.9 N/A 17.8 80SE 17.3 80SE 22.6 ertical 3.8 N/A 37.9 75W 358.9 72NW

Strike is reported in degrees clockwise from north. Strike direction is always chosen in the NW or NE uadrant, regardless of dip direction. Notes: N/Ainformation on fault dip was not available in the original geotechnical report from which the fault was mapped. E–Wthe original geotechnical reports indicated fault strands dipping both to the east and west, without providing more detailed information on the amount of dip or dip direction for each fault observed.

Less potentially active faults 18 Less potentially active Mean = 350.6° 0 16 n = 51 Potentially active 330 30 14 Active s t 12 u l f a

300 60 10 o f

r 8

10 6 u m b e

5 N 270 90 4 2

Potentially active faults 0 50 55 60 65 70 75 80 85 90 Mean = 15.7° 0 Dip angle (degrees) n = 221 330 30 Figure 12. Dip angle histograms for each fault classification based on data from Table 3.

300 60 world where major fault zones trend through densely populated and developed regions. One such area is the San Francisco, California, region where multiple 40 strike-slip faults trend through cities surrounding San Francisco Bay, including 30 20 the San Andreas and Hayward faults. Fault zones can be highly complex along 10 270 90 strike, and the detailed fault investigations that are possible through GIS can improve understanding of fault zone geometry, mechanics, and evolution, which in turn can improve seismic hazard assessments. Active faults Mean = 350.6° 0 n = 341 330 30 Hazard Classification

The geodatabase was used for initial assessment of seismic hazard for the downtown San Diego area and for comparison with existing fault maps and 300 60 regulatory zones. Two active fault zones, fault zones A and B, were discovered outside the current AP zone in eastern downtown (Figs. 9 and 10A). These sites should be considered when the AP zone is updated, which may result in moving 80 the eastern AP zone boundary to include the active fault traces. The faults are 40 60 20 close to the “downtown graben”, and therefore could be considered part of the 270 90 deformation associated with localized transtension at the graben. Alternatively, Figure 11. Rose diagram plots of strike orientations for less potentially active the northeasterly trend of fault zone B along with its sense of slip suggest that it faults (A), potentially active faults (B), and active faults (C). Because the strike could represent the southern terminus of the Florida Canyon fault. If fault zone of a fault can be plotted as two opposing directions, 180° apart, the strike di- B is in fact the southern terminus of the Florida Canyon fault, then it implies a rection that was chosen for use in the analysis was always in the northeast or northwest quadrant (i.e., from 0°–90° and 270°–360°), regardless of dip direction. series of en echelon graben structures between the Rose Canyon and Florida Blue segments show direction of fault strike, with length of the segment repre- Canyon fault zones, and that the region of transtension and seismic hazard asso- senting a length-scaled fault strike where each 10 m fault length is equal to one ciated with the San Diego Bay stepover affects a much larger area of San Diego. unit. Concentric circles are labeled in italic text to indicate number of fault units. Red line shows the mean strike direction for each group of faults. n represents The data also revealed areas with a contradiction in fault zone location the number of fault units plotted in each rose diagram. between the geotechnical report data and the USGS-CGS fault map (Figs. 9

and 10). Some of these contradictions may be related to uncertainty between geometry could be related to spatial patterns associated with regional and fault maps, but many differences were greater than our total estimated uncer- local stress or relative location within the San Diego Bay pull-apart basin. tainty and therefore should be considered when these fault databases are used The maximum horizontal stress direction in this region is N20°E–N40°E for hazard assessment and zoning decisions, especially where differences are (Yang and Hauksson, 2013), consistent with strike-slip faulting along a principal large, as is the case south of the “downtown graben” (Fig. 10A). The USGS- displacement zone with roughly similar trend to the active and less potentially CGS San Diego fault segment through Old Town may also require a more active faults, and Riedel shears with roughly similar trend to the potentially significant adjustment, as two separate trenches across its trend did not reveal active faults (Cloos, 1928; Riedel, 1929; Sylvester, 1988). However, stress fields evidence for faulting (Fig. 10B). However, it is also possible that the fault zone are known to be highly variable on regional and local scales (Zoback, 1992; is correctly located, but that the age of the most recent event is older than Yale, 2003; Montone et al., 2012), and the subsampling of the fault zone shown stratigraphy exposed in both trenches (Pleistocene age). here is limited to a very small section of the RCFZ, making it difficult to fully The geodatabase also revealed faulting considered potentially active that is assess relationships between fault zone geometry and stress field. Changes in not included in the USGS-CGS database, or the AP zones. For example, there fault activity with fault strike in this area could also be related to the evolution is a roughly north-south–trending series of potentially active fault segments of the San Diego Bay pull‐apart basin, and could depend more on variability mapped near the waterfront, south of Little Italy, and some other segments in fault dip and sense of motion, which can be highly complex at fault ste- are mapped trending southwest between the “downtown graben” and San povers and bends (e.g., McClay and Dooley, 1995; Wakabayashi et al., 2004; Diego Bay (Fig. 9). Based on our classification, these faults have been active Wu et al., 2009). A preliminary assessment of fault dip indicates that a higher in the late Quaternary, but not during the Holocene. Although the earthquake percentage of active and potentially active faults are steep (>85°) compared risk for these faults is considered lower due to lack of Holocene activity, the to less potentially active faults, but there is a high degree of variability in the faults may still be important for models of hazard or fault zone evolution. data with few patterns that emerge (Fig. 12). It could also be interpreted that Furthermore, recent work has demonstrated that earthquakes may propagate all faults are part of the same active zone with the less potentially active faults across both active and inactive structures in the same event (Vallage et al., and potentially active faults representing evolutionary phases of the system 2016). The strike of the potentially active fault along the waterfront suggests that shut off as the main faults were established. Other recent examinations that it may be a northern continuation of the offshore Coronado fault zone, of fault zone geometry used high-resolution surface mapping technology which is included in AP zone regulations (Fig. 9). (e.g., lidar) across greater fault zone extents, and included more information on fault dip, sense of slip, and fault zone width (e.g., Barth et al., 2012; Teran et al., 2015; Vallage et al., 2016; Scott et al., 2018). These studies were also Fault Zone Geometry conducted in undeveloped areas, which again highlights the relative difficulty of assessing fault zones through urban areas where much of this information The analysis of fault strike and dip illustrates that there is much variability is not available. Expanding the geodatabase beyond downtown San Diego to in fault geometry along strike in the downtown San Diego section of the RCFZ. include information from both geotechnical and scientific investigations would This may locally be associated with the San Diego Bay stepover, but highly vari- improve interpretations of larger-scale fault zone geometry. able geometry has also been observed elsewhere along the RCFZ, including the The detailed fault mapping and assessment of fault dip compiled by this offshore extension north of La Jolla (Sahakian et al., 2017). Although the strike study illustrate that the RCFZ exhibits localized normal deformation in down- of active faults was found to be statistically different than that of potentially town San Diego as it approaches the releasing stepover at San Diego Bay, and active faults, it was found not to be statistically different from less potentially that the southernmost faults of the RCFZ appear to extend south across the active faults (Fig. 11). If we consider that the less potentially active faults are downtown area and into the stepover. Both observations suggest a highly com- oldest (longest time since rupture), followed by potentially active faults, and plex geometry of faulting that could influence earthquake rupture patterns and then active faults (most recently ruptured), this pattern could indicate a change should be considered for seismic hazard assessments. Further investigation in stress that first made rupture on approximately NW-trending faults more into the pull-apart basin evolution is warranted, especially with consideration likely, followed by a period where more NE-trending faults were active, and of the fault segments that step offshore into San Diego Bay. then a return to activity on more NW-trending faults. In this scenario, it appears that the less potentially active faults were not reactivated with the active faults following a similar trend, but this may be related to the lower dip angles of ■■ CONCLUSIONS the less potentially active faults compared to the more vertical active faults (Fig. 12). It has also been shown that in areas of slip partitioning, a temporal • The compilation of geotechnical data from hundreds of individual reports change in the stress field is not needed to explain differences in slip between into a geodatabase allows for more comprehensive study of a highly adjacent faults (Wesnousky and Jones, 1994). Alternatively, the observed fault complex urban fault zone.

• Geotechnical data from the subsurface beneath urban areas is an effec- Kennedy, M.P., 1975, Geology of the western San Diego metropolitan area, California, in Geology tive way to map stratigraphic architecture and fault zone geometry that of the San Diego Metropolitan Area, California: California Division of Mines and Geology Bulletin 200, p. 11–39. are otherwise obscured by development at the surface. Kennedy, M.P., and Peterson, G.L., 1975, Geology of the eastern San Diego metropolitan area, • The Rose Canyon fault zone in San Diego, California, exhibits transten- California, in Geology of the San Diego Metropolitan Area, California: California Division of sional deformation as the main fault strands approach a releasing stepover Mines and Geology Bulletin 200, p. 45–56. Kennedy, M.P., and Tan, S.S., 1977, Geology of National City, Imperial Beach, and Otay Mesa in the downtown area. quadrangles, southern San Diego metropolitan area, California: California Division of Mines and Geology Map Sheet 29, scale 1:24,000. Kern, J.P., and Rockwell, T.K., 1992, Chronology and deformation of Quaternary marine shorelines, ACKNOWLEDGMENTS San Diego County, California, in Fletcher, C.H., and Wehmiller, J.F., eds., Quaternary Coasts of the United States: Marine and Lacustrine Systems: SEPM (Society for Sedimentary Geology) We are indebted to the geotechnical firms that contributed reports to this project, especially Special Publication 48, p. 377–382, https://doi.org/10.2110/pec.92.48.0377. AECOM, Geocon, Kleinfelder, Leighton and Associates, and Construction Testing & Engineering. Legg, M.R., 1985, Geologic structure and tectonics of the Inner Continental Borderland offshore This research was funded by the California Geological Survey through student support. We are northern Baja California, Mexico [Ph.D. thesis]: Santa Barbara, University of California, 410 p. also grateful for thoughtful reviews by Nicolas Barth and two anonymous reviewers, which have Lindvall, S.C., and Rockwell, T.K., 1995, Holocene activity of the Rose Canyon fault zone in San greatly improved this manuscript. Diego, California: Journal of Geophysical Research, v. 100, p. 24,121–24,132, https://doi .org /10.1029/95JB02627. Lozos, J.C., Oglesby, D.D., Brune, J.N., and Olsen, K.B., 2015, Rupture propagation and ground REFERENCES CITED motion of strike‐slip stepovers with intermediate fault segments: Bulletin of the Seismological Society of America, v. 105, p. 387–399, https://doi.org/10.1785/0120140114. 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