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Disentangling Slip-rate Variability in Time and Space at the Cucamonga Fault, Southern California, USA

Description: 

Fault-slip rates are often interpreted in terms of fault structure and earthquake processes. For example, temporal variability may reflect earthquake super-cycles, which are relevant to time-dependent seismic hazard analyses. However, measuring slip rates at multiple time intervals frequently requires measurements at multiple points in space because piercing points of multiple ages rarely occur in the same landform. As a result, it may be difficult to disentangle temporal variability (from earthquake to earthquake) from spatial variability (along strike). Here, we review recent efforts to characterize both temporal and spatial variability of slip rate on the Cucamonga fault, located in greater Los Angeles at an intersection of the Transverse Range reverse-fault system and the San Jacinto-San Andreas plate boundary fault system. Multi-fault ruptures on these systems potentially allow larger magnitude (M >7) earthquakes near the urban center and potentially occur with periodicity akin to super-cycles. First, we show that cumulative fault displacement is variable along strike at many length scales, using lidar mapping. Next, we measure slip rate at different time intervals using cosmogenic radionuclide-dated alluvial surface offsets. We then apply calibrated geomorphic models of scarp diffusion and stream-channel incision to characterize the sources of along-strike variability and measure long-term (10⁵ yr) slip rates. We identify fault geometry and erosive scarp modification as two important sources of real and apparent spatial variability, respectively. Slip rates are constant at ~1 mm/yr at intervals of 20, 30, and 40 ka. Real, long-wavelength spatial variability also appears stationary in time. If multi-fault ruptures include the Cucamonga fault, these ruptures likely span the entire fault and produce similar, long-wavelength spatial patterns of displacement. On other fault systems, where spatial variability is not necessarily stationary in time, efforts to measure temporal variability to improve hazard analysis should consider ruling out contributions from real and apparent spatial slip-rate variability.

Session: Temporally Variable Records of Earthquake Behavior and Considerations for Seismic Hazard Analyse [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Devin

Student Presenter: No

Invited Presentation: Yes

Poster Number: 30

Authors