Trona Post-Earthquake Geologic Fault Investigation

Abstract

San Bernardino County Fire, Office of Emergency Services (OES) staff, in partnership with Land Use Services, is conducting continuing surveys and monitoring of fault dis-placement and lateral spread caused by the July Earthquake Sequence in the Searles Valley – Indian Wells Valley area. We recognized the need for detailed mapping of the Trona area fault ruptures. Creeping motions and widely distributed fracture zones were observed late Friday night July 5th throughout the northern Searles Valley area in addition to sand boils and lateral ground spreading occurring from ground liquefaction. These patterns were roughly aligned with damaged and destroyed properties, damaged road surfaces, and broken underground utilities. Using a combination of known fault lines, inferred possible fault locations, information provided by aerial and satellite reconnaissance, and field reports, an area survey was conducted. Locations for the first survey were chosen after studying available data. The first faulting observed was on Trona Road in the West End area on the Saturday after the 7.1 event. A noticeable vertical tenting raised the road several inches, creating a definite traffic hazard. This fault is directly north of the County Public Works Yard with mapping showing the fault displacement continues under the Yards parking lot to the southeast and to the west. This fault was not shown on any USGS or CGS preliminary map. Additional area surveys were conducted of the Argus, Trona, and Pioneer Point communities in the Searles Valley area. Ground fractures were observed in all communities. Many buildings were damaged or destroyed, roads, water transmission and distribution systems damaged, electrical infrastructure was heavily damaged, natural gas transmission and distribution pipes were damaged, and waste water systems were broken and destroyed. Windshield survey included video recordings of all streets in the community, marking of damages and fractures with handheld GPS units and extensive still photography. August 3, 2019 Survey The first survey was in the western Searles Valley east of Trona Road on August 3, 2019. Shortly after starting east on Pinnacle Road the first fracture zone was discovered. This zone was several miles west of any expected ground motion. After mapping this area was tentatively named Searles Valley Fracture Zone 1. The zone is approximately 250 feet wide, 500 feet long, trends N 20 E, and has vertical displacement up to 3cm. Continuing east two additional unknown, unexpected fracture zones were discovered. These zones were named Searles Valley Fracture Zones 2 and 3, respectively. Similar fault displacement that was seen in Zone 1 was observed in these zones. The two zones were also mapped and photographed. The zone is approximately 250 feet wide, 600 feet long, trends N 20 E, and has vertical displacement up to 3cm. East of the Randsburg Wash Road, still on Pinnacle Road, the survey located the southern end of the Paxton Ranch Fault Zone (resulting from the 7.1 M EQ) crossing the road. Field surveys of this area revealed three areas of faulting not previously mapped or walked. The Survey Team also walked the known fracture zones of the Paxton Ranch Fault Zone. We mapped and photographed observed faulting movement in this zone. The mapped zone is approximately 300 feet wide, 3,500 feet long, trends N 20 E, and has vertical displacement ~ 3cm. North of Pinnacle Point on the southern reaches of Searles Lake a fifth zone of faulting and possible lateral spread (low angle landslide) was found. This feature was mapped and photographed. The day’s survey was over with only 1/3 of the planned mapping completed. This zone is approximately 700 feet wide, 1,200 feet long, trends N 50 E, and has vertical displacement up to 15 cm. August 30, 2019 Survey Another survey of the western Searles Valley was conducted on August 30. Searles Valley Fracture Zones 1, 2, and 3 were re-checked with additional fractures/faulting found on the north and south ends of Fracture Zone 1 and 2. No additional fracturing was found at Fracture Zone 3. The zone areas checked were greatly expanded from the August 3 survey. An older fault scarp, possibly connected to the Garlock Fault, was located and mapped along the south end of SVFZ 1. Fracture Zone 1 was extended on both ends. The zone is mapped to approximately 300 feet wide, 4,000 feet long, trends N 20 E, and has vertical displacement up to 3cm. The Paxton Ranch Fault Zone at Pinnacles Road had three areas of extensive new fracturing not observed August 3rd. Two of the trends were generally parallel to the original observations of August 3 but were in different locations. The third zone of fracturing was on an existing zone but had opened up considerably. The new and additional fractures/fault displacement were extensively mapped and photographed. The mapped zone is now approximately 1,500 feet wide, 5,000 feet long, and has vertical displacement up to 10 cm. The final stop of the day was at the “Miles from Anywhere Fault” crossing Trona Road at West End, first measured July 6, 2019. County Roads had removed the tented pavement, regraded the roadway and repaved the road during the week of July 8-12, 2019. The road was smooth upon completion. On Au-gust 30 a notable tenting of the pavement was noted although the asphalt road surface had not yet cracked. A survey of the area on east side of the known fracture zone showed additional cracking, with the displacement trending N45W. On the west side of Trona Rd, the fault line trends N85W to the west edge of the borrow pit. Additional movement was not found west of the area . The zone is approximately 150 feet wide, 1,200 feet long, trends N 50 W, and has vertical displacement up to 3cm. October 5, 2019 Survey A third survey was conducted on October 3, 2019. This survey covered areas south of the Garlock Fault northeast of Red Mountain; the area north of Searles Valley on Trona Rd to SR 178; the SR 178 area where both the Salt Wells Valley Fault and the Paxton Ranch Fault cross; and the Pioneer Point area of Trona. Active faulting was found in the Pioneer Point Area , in the Salt Wells Valley fault zone and the Paxton Ranch Fault Zone where the faults cross SR 187. The Salt Wells Valley Fault Zone rupture was indicated on the CGS maps as detected by instruments but no field trothing was conducted immediately after the earthquake. Our field investigation showed a fairly extensive rupture zone about one mile east of the observed faulting that crosses both SR 178 and Trona Rd. The fault trend roughly parallels the observed fault. Field work in the Pioneer Point area found three areas of fracturing. The first area was located south of Pioneer Point that crossed Trona Road and was observed trending N 45 E for about 0.25 miles. An extensive area of faulting was found on both sides of the roadway. There were no structures in this zone. This area was tentatively named the Cemetery Fault as its southern terminus is north of Trona Cemetery. The fault may extend into portions of Trona and the business district to the south. A second zone of fracturing was found just south of the Cemetery Fault, trending N30E. Damage to portions of the sewer lateral from Pioneer Point to the septic system may have been caused by movement of this fracture zone. This zone was named the School Fracture Zone as the south end is trending toward Trona High School. This zone needs further investigation as it may be faulting, lateral spreading/low angle landslide or a combination of the two types of movement. The third and most extensive area of faulting begins at the Pile of Rocks area of Pioneer Point and extends west southwest at the base of the Argus Mountains. This zone was walked for about ¾ mile by two teams. This series of faults has many en-echelon fractures, all with about 1 to 5 mm of vertical offset and an unmeasured amount of horizontal uplift. October 19, 2019 This survey focused on three areas. The team surveyed the West End area to determine the extent of the Miles Fault, the Trona area west of Point of Rocks – Pioneer Point, and the Argus area. The Miles Fault survey shows both low angle landslide (Lateral Spread) and faulting. The area surveyed now extends from the lake west 3/4 mile up canyon to the end of traversable terrain. Areas of lurch shattering was found along with faulting. Up canyon large boulders had rolled down into the canyon, bouncing while moving down. Some impressive craters were left by these boulders. The segment crossing Trona Road continues to develop an upward tenting feature typical of vertical uplift. To the east of Trona Road within the housing tract, areas of low angle land sliding was observed. This landslide also affects the County Public Works Yard for Trona. The Argus area survey was inconclusive as much of the observable faulting has been eliminated through reconstruction and grading. Additional fieldwork needs to be done. The Pioneer Point Fault System along the toe of the Argus Mountains now extends about 1.5 miles to the west southwest of Pile of Rocks at Trona Road. Most of the observed movement along this fault system is vertical and occurs over several parallel faults within the fault zone. The zone is about 250 to 400 feet in width for most of the observed fault. Near the west end of the system, a branch veers off trending to the south southwest towards the Searles Valley Minerals coal pile. Another field trip on December 14th covered the Trona, Argus, and Pioneer Point areas. Additional faulting was mapped with similar offset found. Mapping in the rest area of Trona confirmed possible faulting north of the Rest Area in addition to lateral spreading around the rest area. Additional field studies need to be completed in order to accurately develop a mitigation strategy for the Trona and Windy Acres areas. This may include development of Alquist-Priolo Fault zones/County zones; designating areas as liquefaction zones; or areas in need of a geotechnical report prior to issuance of a building permit. Current building codes need to be reviewed to determine if any modifications or changes to the codes are warranted. Earthquake Clusters and Direction Of Slip The Ridgecrest-Trona Fault and Earthquake Complex started on July 4th, 2019 with the generation of the M 6.4 earthquake. This quake initiated at a depth of 10.7 kilometers (6.7 miles) at 10:33 AM (PDT) along a short, previously unmapped, northeast-striking fault. This unmapped fault lies adjacent to, and approximately 250 meters (800 feet) southeast of a “section” of the Little Lake Fault [California Geological Survey (CGS), 2005; U.S. Geological Survey (USGS), 2006] located within the Salt Wells Valley. The movement jumped a 1 kilometer (2/3 mile) gap onto a longer, previously unmapped, northeast-striking fault. Together, these faults exhibited left-lateral, strike slip movement (Xu, personal communication). Surface mapping by CGS, the USGS, and other researchers described complex surface ruptures approximately 14.8 kilometers (9.3 miles) in length within the Salt Wells Valley. We informally refer to this unmapped section of the rupture as the “Salt Wells Valley Fault.” We also choose to include the initial rupture along the 1.9 kilometer (1.2 mile) section of the Little Lake Fault as part of the redefined “Salt Wells Valley” Fault Zone. This earthquake ended up being a foreshock of the July 5th M 7.1 Ridgecrest Earthquake. The main shock occurred along a northwest-striking, previously unmapped fault. Portions of this fault that didn’t rupture during this event were also previously mapped by CGS (2005) and the USGS (2006) as part of the Little Lake Fault. The main shock initiated at a depth of 8.0 kilometers (5.0 miles) at 8:20 PM (PDT). The USGS initially determined a bilateral subsurface rupture length of approximately 40 km (25 miles). This fault exhibited right-lateral, strike slip movement (Xu, personal communication). Additional InSAR mapping noted local areas of vertical uplift and down drop, particularly within the Argus Range (Xu, personal communication). Later surface mapping by CGS, the USGS, and other researchers described complex surface ruptures approximately 48 kilometers (30 miles) in length between the Coso Valley and the eastern portion of West Searles Valley. Both earthquakes and much of the surface rupture occurred within San Bernardino County and China Lake Naval Weapons Station (NAWS). As of the date of this work, more than 50,000 aftershocks have been recorded by the California Institute of Technology. Analyses of the aftershock sequence has noted the relatively shallow source depths of these quakes. Depth evaluations have also recognized the existence of several planar alignments of microquakes. The shallowest of these alignments are relatively parallel to the existing ground surface, while deeper alignments are inclined at shallow angles to the east. These planar alignments may represent tectonic movement along flatter faults, perhaps pre-existing detachment faults underlying the southern Basin and Range Province. Railroad track disfigurement occurred at several locations on the Trona Railroad tracks. This damage took several weeks to fully repair. Damage was a combination of lateral spread and horizontal offset. No rolling stock was derailed but the system was not operational for several weeks.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)