Technical Report- Cajon Pass Earthquake Gate Area: Active fault mapping and evaluation of potential sites to measure the Holocene slip rate along the Glen Helen and San Jacinto faults in the Cajon Pass area. Project Objectives The purpose of this study is to investigate the relationship between displacement along the northern San Jacinto fault zone and the San Andreas fault in the Cajon Pass area (Fig. 1) as part of the larger SCEC effort to understand the Cajon Pass Earthquake Gate. Previously published slip-rate and paleoseismic data from the two faults indicate that slip is transferred between the two faults in the Cajon Pass area (e.g., Onderdonk et al., 2018; McGill et al., 2013; Morton and Matti, 1993). We are working to determine where slip transfer between the two faults occurs, and if the two faults occasionally rupture together. To answers these questions, we first need measurements of the late Quaternary slip-rate and slip distribution along the three strands of the San Jacinto fault zone in the Cajon Pass area. This will allow us to document the distribution and pattern of Holocene deformation within the Earthquake Gate that has resulted from repeated ruptures through the area. Our specific objectives for 2018 included: 1. Produce a digital map of the late Quaternary fault structure, surface deformation, and recency of movement along the northern extent of the San Jacinto fault zone. 2. Identify new sites where slip-rate, slip-per-event, or rupture history can be evaluated on each fault. Figure 1. Faults in the Cajon Pass area mapped with key locations mentioned in the text: A= Sycamore Canyon, B= Scotland, C= Applewhite Campground, D= Glen Helen Regional Park, E= Texas Hill. Inset map shows location of Cajon Pass with slip-rate data (in mm/yr) from nearby sites on the San Jacinto and San Andreas faults. Methodology We have been using field investigations, LiDAR data, historic air photos, Google Earth, and previously published geologic maps to 1) create a new map of the northern San Jacinto fault zone and 2) identify and explore new slip-rate sites and paleoseismic sites. The map includes the late Quaternary fault structure, surface deformation, and recency of movement along the major fault strands. LiDAR data from San Bernardino County is used as the base for this map. We have also obtained historic air photos from the Map and Imagery Library at UCSB, and all consultant reports in the area from the California Geological Survey. We are using the LiDAR data, Google Earth, and historic air photos to identify and map active faults and associated deformation visible in the geomorphology. Trench logs and data from the consultant reports are being used to narrow down fault locations in the subsurface, gain information about the fault structure that was observed, and catalog any dating of offset units that was observed. Field investigations have been focused on 5 specific areas where our mapping has suggested there is potential for measuring fault slip-rates or gaining paleoseismic information. Field mapping will continue at these focus areas, but will expand over the coming summer to include the entire lengths of the fault strands. Results 1. Mapping late Quaternary deformation A) The Glen Helen strand shows the best geomorphic evidence for recent surface deformation. Near the Glen Helen Regional Park (location D on Fig. 1) there are well-preserved, 5 m tall scarps in late Pleistocene and Holocene fan deposits. 2 km to the north in Cajon Canyon we have mapped vertical and lateral offset in small fans that we infer to be Holocene. We expect that this location can be used to calculate a slip-rate and possibly date the last few earthquakes. This site is described below (Kenwood Site). Much of the northern part of the fault in Cajon Creek and upper Lytle Canyon parallels the canyon walls. There are visible scarps in bedrock and younger alluvium, but more work is needed to determine if these are fault scarps or erosional scarps from higher flow in Cajon and Lytle Creeks. We are also exploring the idea that the Glen Helen fault, or a strand of this fault, curves to the northeast into Lone Pine Canyon near Lost Lake (white dashed fault in Fig. 1), as is suggested by aligned stream deflections and benches in a north-facing hillside near Lost Lake. This is a critical hypothesis to evaluate since this possible fault strand would be a more direct link to the San Andreas fault. B) There are numerous deflected streams along the middle San Jacinto strand, and an apparent offset of the south canyon wall of Lytle Creek near the community of Scotland, but we have not yet identified clear geomorphic offsets in younger deposits that can be dated. Most of the geomorphic features we have mapped so far are developed in old crystalline rock, with a few developed in old fan deposits interpreted to be early Pleistocene by Morton and Matti (2001). However, consultant reports show evidence of Holocene offset in the subsurface in two spots along the fault; Sycamore Canyon, and at Scotland in Lytle Creek (Fig. 1). Field investigations in Sycamore Canyon did not reveal any obvious offset of Holocene deposits at the surface, but more detailed work is needed in this area. We have not yet done a detailed field investigation at the Scotland site. C) Deflected streams and scarps along the Lytle Creek strand that were used by Mezger and Weldon (1983) to calculate a slip rate are being re-evaluated. We have also been mapping deflected streams and slope breaks that may be associated with surface deformation, but so far all of these are developed in crystalline rock or older Pleistocene terrace deposits. These deposits are being dated with OSL by Figuieredo, but we expect ages of at least 10’s of thousands of years, so we have yet to identify any evidence of late Pleistocene or Holocene surface deformation along this strand. There are also no consultant reports along the Lytle Creek fault to provide subsurface information. However, our field investigations in the Texas Hill area (location E on Fig. 1) revealed that the main fault is exposed in a road cut along strike of a prominent scarp in older fan deposits. This road cut exposes approximately 3 m of apparent vertical separation of a soil horizon, with a colluvial wedge draping across the scarp. OSL samples were collected from this outcrop and are being dated at Univ. of Cincinnatti by Figuieredo. 2. Investigating possible slip-rate and slip history sites We have also been conducting field investigations of potential slip-rate sites we identified during mapping. We need more time to fully evaluate some sites (described below), but we have identified a very promising site on the Glen Helen fault in lower Cajon Canyon that we proposed to develop in 2019 to calculate a Holocene slip-rate. The site is located in the middle of the 10 km-long stepover in lower Cajon Creek where the San Andreas and northern San Jacinto fault zones overlap, and where ruptures are most likely to “jump” between the faults. We are calling this site the “Kenwood site”, based on its location near Kenwood road. Preliminary surficial mapping of the site shows Quaternary fan deposits that have been offset by the Glen Helen fault strand (Fig. 2). Two fault strands are visible in the surface topography at the site. One fault marks the steep linear mountain front and appears to be buried at the canyon mouths by small alluvial fans (Qf1) that we hypothesize to be Holocene based on preliminary field observations of the soils on these deposits. The Qf1 fans, and a younger deposit (Qf2), are offset by a second fault strand that is located 30 m to 50 m northeast of the mountain front. Approximately 30 m of right-lateral offset of one fan edge is visible in the topography (Fig. 2). Other sites that we have investigated in 2018 include: A. Sycamore Canyon- A geologic consulting report from the Sycamore Canyon and Sycamore Flat area (LOR Geotechnical, 1994) concluded that there is Holocene rupture on the San Jacinto strand through this area. A large fault zone was exposed in several trenches and deposits were dated to bracket the last movement on the fault between AD 1380 and AD 1810. Our preliminary field investigations of the canyon where these trenches were excavated did not reveal any obvious scarps or offset features at the surface. However, we will continue to evaluate the site to determine if 3D trenches at the mouth of one of the smaller tributary canyons would enable us to use buried channels to measure slip rate, slip-per-event, and more closely constrain the last event on this part of the fault. This area also has potential for future paleoseismic work based on the sequences of offset and overlapping deposits shown in the LOR Geotechnical trench logs. Figure 2. LiDAR DEM of the Kenwood site with faults (black lines), Quaternary deposits (colored shading), and proposed trench locations (blue rectangles). B. Scotland Site in upper Lytle Creek- Three strands of the middle San Jacinto fault cross Lytle Creek near the town of Scotland. The canyon wall of Lytle Creek appears to be offset along the eastern strand, and an apparent offset of a fluvial terrace riser can be seen in the LiDAR data along the central strand. A trench excavated across the central strand on the fluvial terrace (Clopine, 1987) exposed a fault zone that offset “floodplain deposits”. The units exposed in the trench were not dated by the consulting company, but the deposits were assumed to be late Holocene and multiple events are depicted in the trench logs.