DOI: 10.25349/d9vw3d
title: Holocene earthquake rupture of the XEOLXELEK–Elk Lake fault in the Greater Victoria area, British Columbia, Canada

description: Subduction forearcs are subject to seismic hazard from upper plate faults
 that are often invisible to instrumental monitoring networks. Identifying
 active faults in forearcs therefore requires integration of geomorphic,
 geologic, and paleoseismic data. We demonstrate the utility of these
 approaches in a highly populated region of Vancouver Island, Canada, by
 combining lidar remote sensing, historical imagery, field investigations,
 and shallow geophysical surveys to identify a previously unrecognized
 active fault, the XEOLXELEK - Elk Lake fault, in the northern Cascadia
 forearc, ~ 10 km north of the city of Victoria. Lidar-derived bare-earth
 digital terrain models and historical air photos show a ~ 2.5 m-high scarp
 along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching
 and electrical resistivity tomography surveys across the scarp reveal a
 single reverse-slip earthquake produced a fault-propagation fold above a
 blind southwest-dipping fault. Five geologically plausible OxCal models of
 radiocarbon-dated charcoal from deformed deposits and one colluvial wedge
 constrain the likely earthquake age to between 4.7 and 2.3 ka (68%
 confidence interval). Fault-propagation fold modeling indicates ~ 3.2 m of
 reverse slip on a blind, 50° southwest-dipping fault can reproduce the
 observed deformation. Fault scaling relations suggest a M 6.1–7.6
 earthquake with a 13 to 73 km long surface rupture and 2.3 to 3.2 m of
 dip-slip may be responsible for the deformation observed in the
 paleoseismic trench. An earthquake near this magnitude in Greater Victoria
 could result in major damage, and the results of this study highlight the
 importance of using both remote sensing and field studies to identify and
 characterize active faults in forearcs and regions with recent glaciation.
description: This README.txt file was updated on 21/09/2023 by N. Harrichhausen GENERAL
 INFORMATION 1. Title of Dataset: Holocene earthquake rupture of the
 XEOLXELEKElk Lake fault in the Greater Victoria area, British Columbia,
 Canada. Dryad, Dataset. 2. Author Information A. Principal Investigator
 Contact Information Name: Nicolas Harrichhausen Institution: ISTerre,
 Universit Grenoble Alpes Address: Grenoble, France Email:
 n.harrichhausen@univ-grenoble-alpes.fr; nharrichhausen@gmail.com 3. Date
 of data collection (06/2021 - 10/2021) 4. Geographic location of data
 collection: Saanich, British Columbia, Canada 5. Information about funding
 sources that supported the collection of the data: This research was
 supported by National Science Foundation Earth Sciences (NSF EAR) grants
 #1756943 and #2046278 to Dr. Morell, NSF EAR grant #1756834 to Dr.
 Regalla, funding from the USGS National Cooperative Geologic Mapping
 Program to Dr. Bennett, an NSERC Discovery grant #2017-04029 and Canada
 Research Chair to Dr. Nissen, an NSERC Alexander Graham Bell Canada
 Graduate scholarship to T. Finley, an NSERC Post Graduate Scholarship
 (PGGSD - 504197 - 2017) and Centre National d'tudes Spatiale (CNES)
 postdoctoral fellowship to N. Harrichhausen, and a University of Victoria
 Jamie Cassels Undergraduate Research Award to E. McLeod. SHARING/ACCESS
 INFORMATION 1. Licenses/restrictions placed on the data: Data under
 copyright of: Harrichhausen, et al., (2023). Holocene earthquake rupture
 of the XEOLXELEKElk Lake fault in the Greater Victoria area, Canada 2.
 Links to publications or works that cite or use the data: * Harrichhausen,
 N., Finley, T., Morell, K. D., Regalla, C, Bennett, S.E.K., Leonard, L.J.,
 Nissen, E., McLeod, E., Lynch, E. M., Salomon, G., Sethanant, I., (2023).
 Paleoseismic study of the XEOLXELEK-Elk Lake fault: A newly identified
 Holocene fault in the northern Cascadia forearc near Victoria, British
 Columbia, Canada. Proceedings of the 11th International INQUA Meeting on
 Paleoseismology, Active Tectonics and Archeoseismology. * Harrichhausen,
 N., Finley, T., Morell, K. D., Regalla, C., Bennett, S.E.K., Leonard,
 L.J., Nissen, E., McLeod, E., Lynch, E. M., Salomon, G., Sethanant, G.
 (Submitted to JGR: Solid Earth). Discovery of an active forearc fault in
 an urban region: Holocene rupture on the XEOLXELEKElk Lake fault,
 Victoria, British Columbia, Canada 3. Recommended citation for this
 dataset: Harrichhausen, N., Finley, T., Morell, K. D., Regalla, C.,
 Bennett, S.E.K., Leonard, L.J., Nissen, E., McLeod, E., Lynch, E. M.,
 Salomon, G., Sethanant, G. (2023). Holocene earthquake rupture of the
 XEOLXELEKElk Lake fault in the Greater Victoria area, British Columbia,
 Canada. Dryad, Dataset. DATA & FILE OVERVIEW 1. File List (with
 descriptions): Harrichhausen\_et\_al\_AugerHoles.docx: Basic sediment
 descriptions of sediment encountered in auger holes with depths in
 centimeters. Harrichhausen\_et\_al\_trench\_structure.xlsx: Bedding and
 fault plane measurements in degrees.
 Harrichhausen\_et\_al\_unit\_descriptions.xlsx: Detailed unit descriptions
 of units shown in trench logs (Fig. 5; Fig. S2, Harrichhausen et al., (JGR
 Solid Earth)). Harrichhausen\_et\_al\_ERT\_3m\_Apr22\_2021.stg
 Harrichhausen\_et\_al\_XELF\_ERT\_3m\_west\_terrain.txt
 Harrichhausen\_et\_al\_ERT\_1m\_Apr22\_2021.stg
 Harrichhausen\_et\_al\_XELF\_ERT\_1m\_west\_terrain.txt
 Harrichhausen\_et\_al\_Waterski\_Beach\_ERT\_3m\_Oct21\_2022.stg
 Harrichhausen\_et\_al\_Waterski\_ERT\_terrain.txt .stg files: resistivity
 data collected using the AGI Ministing. Following 3 lines of header
 information, the next set of lines (marked RESIST) contain the data
 collected during contact resistance testing, prior to the electrical
 resistivity tomography (ERT) survey. For these data, the format is as
 follows: Record number, RESIST, date, time, V/I, error in tenths of
 percent, output current in mA, address of the first electrode used for the
 test, address of the second electrode, gain range for the receiver, high
 voltage output for the current transmitter, measurement time, number of
 measurement cycles. The remaining lines (marked DIP-DIP) contain the data
 collected during the dipole-dipole ERT survey, with the following data
 format: Record number, DIP-DIP, date, time, V/I, error in tenths of
 percent, output current in mA, apparent resistivity in Ohm-m, dipole size
 a (m), distance between dipoles na (m), X-coordinate (m) for the midpoint
 between the two dipoles, gain range for the receiver, high voltage output
 for the current transmitter, measurement time, number of measurement
 cycles. Note: the calendar dates are incorrect in these files the true
 dates are in the filenames. terrain.txt files: relative distance and
 elevation data collected using a total station. Following 3 lines of
 header information, the first column is horizontal distance (m) along the
 profile, and the second column is relative elevation (m). README.md
 METHODOLOGICAL INFORMATION 1. Description of methods used for
 collection/generation of data: The structural data and unit descriptions
 were collected during the excavation of a \~31 m-long, 1-2 m-wide,
 &lt;3 m-deep paleoseismic trench perpendicularly across the the scarp
 of the XEOLXELEKElk Lake fault (XELF) on the eastern shore of XEOLXELEK
 (Elk Lake) in August 2021. Structural measurements were taken using a
 Brunton compass were compiled and analyzed using the programs Stereonet
 10.1.6, available at https://www.rickallmendinger.net/. Detailed
 sedimentary descriptions were made for each geologic unit portrayed in the
 trench logs shown in the supporting information of the manuscript. The
 auger data was collected from up to 3.4 m-deep auger holes from the floor
 of, and alongside the trench. The electrical resistivity data were
 collected April 22, 2021 at Eagle Beach and October 21, 2021 at Waterski
 Beach. The resistivity files (*.stg) are correlated with levelling data
 that are also provided (*.txt). We used an Advanced Geosciences Inc. (AGI)
 MiniSting R1 resistivity meter with a 28-electrode cable in dipole-dipole
 electrode configuration. At Eagle Beach, data were collected along a
 profile co-located with the eventual western wall of the trench and
 extending beyond the trench footprint to the northeast and southwest.
 First, an 81 m-long profile was surveyed, centered on the topographic
 scarp, with an electrode spacing of 3 m and resolvable depth of \~16 m.
 This was followed by a 27 m-long profile nested within the longer line,
 with an electrode spacing of 1 m, to collect higher-resolution data of the
 shallow subsurface (to \~5.5 m depth). At Waterski Beach, we used the same
 methodology to conduct a 3-m spaced survey along an 80 m long profile
 centered on a slight \~60 cm high elevation rise coinciding with the along
 strike extension of the XELF east of XEOLXELEK. The survey was conducted
 at an angle of 63 relative to the fault scarp as a result of the nearby
 lakeshore. For all surveys, levelling data at each electrode position were
 surveyed with a Spectra Precision Focus 6 5 Total Station. 2. Methods for
 processing the data: Bedding and foliation planes were plotted and
 analyzed using Stereonet 10.1.6 software (Allmendinger et al., 2011;
 Cardozo & Allmendinger, 2013). Resistivity data were processed
 using AGIs EarthImager 2D software, the apparent resistivity data were
 inverted using a damped least-squares method, for 5 iterations, to produce
 resistivity models of the subsurface that minimized the root-mean-square
 misfit between the model-predicted and measured values. 3. Instrument- or
 software-specific information needed to interpret the data: Stereonet
 10.1.6 used for structural analyses. Publicly available at
 https://www.rickallmendinger.net/ Advanced Geosciences Inc. EarthImager 2D
 software used for resistivity data processing is available at
 https://www.agiusa.com/agi-earthimager-2d. 4. Standards and calibration
 information, if appropriate: N/A 5. People involved with sample
 collection, processing, analysis and/or submission: Harrichhausen, N.,
 Finley, T., Morell, K. D., Regalla, C.A., Bennett, S.E.K., Leonard, L.J.,
 Nissen., E., McLeod, E., Lynch, E. M., Salomon, G., Sethanant, G. all
 assisted in data collection. DATA-SPECIFIC INFORMATION: Information on
 variables are provided in excel spreadsheets. REFERENCES: Allmendinger, R.
 W., Cardozo, N., & Fisher, D. M. (2011). Structural geology
 algorithms: Vectors and tensors. Cambridge University Press. Cardozo, N.,
 & Allmendinger, R. W. (2013). Spherical projections with
 OSXStereonet. Computers & Geosciences , 51 , P. 193205