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Deep Slip Occurs Prior to Surface Creep Events on the San Andreas Fault

Scientists have used creepmeters to observe the surface expression of creep events along the San Andreas Fault since the 1960s. These creep events represent episodic slip bursts that have a duration of 1–2 days with a small amount of steady background creep between events (e.g., Gittins & Hawthorne, 2022; Gladwin et al., 1994; Nason et al., 1974; Schulz, 1989). During a creep event, the fault typically slips a few millimeters over several hours to days, with events separated by a few weeks to months. At the surface, creep events accommodate >50% of slip along the San Andreas Fault (Gittins & Hawthorne, 2022), but the evolution of slip at depth is less well constrained because of a lack of depth resolution provided by the current creepmeter design.

To observe deeper – down to 10 km – we use strainmeters. This study investigates the deep slip associated with surface recordings of creep events. We analyze strain observations in the 10 hrs before and during creep events at the northern end of the Central San Andreas Fault. We identify 71 offsets in the strain time series created by short bursts of slip within longer creep events. Then, we perform a grid search to determine the location, depth, and magnitude of these slip bursts by modeling rectangular slip patches in a homogeneous elastic half-space. We find that these slip bursts occur up to 7 km from the recorded location of surface rupture associated with the longer creep event, as observed by creepmeters. 42–55% of the slip bursts are likely to occur below 4 km depth and have moments equivalent to Mw 3.2–4.1 earthquakes. Our observations suggest that creep events are not always simply shallow, triggered phenomena. They originate spontaneously from a range of locations, including depths that are comparable to the depths of nearby earthquakes. Creep events also show complex propagation histories, triggering several slip locations as they propagate horizontally. These strain offsets within creep events show a nucleation and propagation history that needs further exploration, perhaps with a more optimal instrument configuration and design.

Session: Characteristics and Mechanics of Fault Zone Rupture Processes, from Micro to Macro Scales - II

Type: Oral

Room: Tikahtnu Ballroom C

Date: 5/2/2024

Presentation Time: 11:15 AM (local time)

Presenting Author: Daniel Gittins

Student Presenter: No

Additional Authors