SCEC Award Number 18048 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Testing for Holocene clustered earthquakes on the Ash Hill Fault, northern ECSZ
Name Organization
Christine Regalla Boston University
Other Participants Emerson Lynch, PhD student
Johanna Fischi, undergraduate student
SCEC Priorities 5b, 5d, 1a SCEC Groups Geology, SDOT
Report Due Date 04/30/2020 Date Report Submitted 10/27/2020
Project Abstract
This goal of this project is to determine the timing and magnitude of late Holocene rupture on the Ash Hill Fault, Eastern California Shear Zone, to test whether three earthquake temporally clustered with known events on the Panamint fault. We collected two primary data sets to test this hypothesis. First, we measured fault displacement from offset geomorphic markers using new high-resolution DEMS constructed using structure from motion photogrammetry. Second, we collected four samples for Feldspar luminescence to determine the ages of deposits offset by late Holocene earthquakes. These samples were collected in locations where soil chonosequence ages have been previously determined. We have found evidence for three late Holocene ruptures each with ~0.75-1.35 m of right lateral displacement and ~0.1-0.3m of vertical displacement, during the past ~4ka. New Feldspar IRSL ages and soils consequence age estimates on offset surfaces and ponded aeolian deposits bracket the three most recent ~0.3-0.8 ka, ~1.5-2.3 ka, and < 4 ka. These three ruptures overlap in time with the three most recent ruptures on the Panamint fault. These results suggest that it is possible that the Ash Hill and Panamint faults may rupture in the same, or very closely temporally related events.
Intellectual Merit Our displacement and rupture timing results have allowed us to determine that that the three most recent ruptures on the Ash Hill fault overlapped in time with the three most recent events on the adjacent Panamint Valley fault. These results suggest that the two faults may rupture synchronously, such that slip in a single event can be transferred from the Panamint fault to the Ash Hill fault, or vice versa, in a multi-fault rupture. Alternately, it may suggest that ruptures along one fault can lead to stress field perturbations that trigger slip the adjacent fault within days to decades of the initial event.
If the two faults are directly linked at depth, as would be necessary if the Panamint fault is a gently west-dipping fault, and the Ash Hill fault is near vertical, this slip transfer may occur directly, likely in a single event. If true, this geometric model requires active slip to be accommodated on a low angle Panamint fault. Alternately, if the two fault systems are not connected at depth, as could occur if both the Panamint and Ash hill faults have high-angle dips, these rupture timing data suggest that the transfer zone between the Panamint and Ash Hill faults may serve as a strain transfer “gate” between the two fault systems. In this model, the transfer zone may either allow for seismic slip to be transferred between the systems in a single event, or the transfer zone can act as a capacitor, storing strain and helping trigger slip on the adjacent fault.
In addition, the three late Holocene events we bracket on the Ash Hill fault fall within the time period of recent seismicity in the Owens and Garlock faults. This overlap suggests that the Ash Hill fault may be part of a system of faults in the northern ECSZ that have experienced a spatio-temporal clustering of earthquakes during the past ~1.5 ka.
Collectively, these results are relevant to SCEC5 science objectives 1 and 5 on: a) refining geologic slip rates on faults in southern California, b) the temporal variation of stress transfer on faults and c) placing bounds on the character and frequency of multi-fault ruptures and the potential for “seismic supercycles”.
Broader Impacts Results of this study help constrain the timing, recurrence, kinematics, and magnitude of Holocene earthquakes along the Ash Hill fault. These parameters that are currently incomplete in the USGS Quaternary Faults and Fold Database. This project supported the research of an early-career, female PI, Christine Regalla, and training on field mapping, data collection, and drone based SFM data collection and processing for 5 students at Regalla’s academic institutions. In particular, it provided training in drone-based SFM data collection and processing, and field sampling methods for luminescence, for two PhD students, Emerson Lynch and Emily Schottenfels. In addition, this project has provided training in SFM-based ortho-image and DEM generation for two female MS students (one BIPOC) and one female, disabled, undergraduate student as part of a research work-study.
Exemplary Figure Figure 4: Timing of the last three late Holocene earthquakes along the Ash Hill and Panamint faults (black bars/arrows), constrained by soil age estimates (from E. McDonald, DRI; Regalla et al 2016), feldspar luminescence (from S. Mahan, USGS; This Study), and 14C of surface and deposits that bracket rupture events (Kirby 2016). Grey bars indicate time interval in which the rupture histories for both faults overlap.