Are Creep Events Big? Estimations of Along‐Strike Rupture Lengths

Abstract

Segments of many faults are observed to slip aseismically at the surface. On the central segment of the San Andreas Fault, aseismic slip accumulates largely in creep events: few mm bursts of slip which occur every few weeks to months. But even though we have observed creep events worldwide since the 1960s, we still do not know how big most events are or which forces drive them. To address this uncertainty, we systematically identify creep events along the central San Andreas Fault and determine their along‐strike rupture extents. We first use cross‐correlation and visual inspection to identify events at individual creepmeters. With data from 18 USGS creepmeters, we identify 2120 records of creep events between 1985 and 2020. We then search for slip that is closely timed across multiple creepmeters. We identify 306 instances of closely timed slip, which could indicate 306 creep events that rupture multiple creepmeter locations. Through visual inspection and statistical analysis of timing, we identify a variety of creep event types, including single‐creepmeter events, small (<2 km) events, medium‐sized (3–6 km) events, large (>10 km) events, and events that rupture multiple fault strands. The existence of many large (> $ > $few‐km) events suggests that creep events are not produced by small, rainfall‐associated perturbations; they are more likely driven by complex or heterogeneous frictional weakening and they may provide a window into the dynamics of a larger scale slip on the San Andreas Fault.

Space–Time Asymmetry in Earthquake Pairs along the Central San Andreas Fault: Evidence for Small Earthquake Links at Long Distances

“…However, we find no identifiable signals, suggesting that any slow-slip The Seismic Record events at the time of the pairs are too small to observe with the strainmeters. We have also examined the CSAF creep-event catalog of Gittins and Hawthorne (2022), and find no clear correlation between observed creep events and the times of our earthquake pairs. In addition, observed creep-event propagation velocities are all less than 500 km/day, with most velocities less than 100 km/day.…”

“…The creep rate varies along strike, reaching the maximum of about 25-35 mm/yr (e.g., Ryder and Bürgmann, 2008;Maurer and Johnson, 2014;Scott et al, 2020;Liu et al, 2022) south of Bitterwater, where it nearly equals the long-term fault slip rate and decreasing to the north toward San Juan Bautista. Although the long-term average creep rate has been nearly constant since 1970 (Ryder and Bürgmann, 2008), creepmeters have detected numerous individual creep events with durations of hours to days (e.g., Gittins and Hawthorne, 2022). InSAR analyses have also suggested temporal changes in creep rate at time scales of months to years (Khoshmanesh et al, 2015;Khoshmanesh and Shirzaei, 2018), some of which appear to have a seasonal component (Li et al, 2022).…”

Scattered M3–4 Slip Bursts Within Creep Events on the San Andreas Fault

“…We systematically search for strain offsets during and just before and after creep events recorded at creepmeter XHR, as identified by (Gittins & Hawthorne, 2022). For each event, we estimate a long‐term trend in the strain data from the 7 days before the creep event.…”

“…Sometimes, two or more strain offsets are associated with a common creep event, so the 71 strain offsets are associated with 44 surface creep events recorded at either the XHR or CWN creepmeter. Some of these 44 creep events are recorded at both creepmeters, with approximately 20%–25% of creep events rupturing both creepmeter locations (Gittins & Hawthorne, 2022).…”

“…Correlation between the timing of surface rupturing creep events, as recorded by creepmeters at different locations along the San Andreas Fault, show that creep events can span large distances, despite their small displacements (Gittins & Hawthorne, 2022). Similar observations of kilometer‐long creep event ruptures have been identified using tiltmeters (Mchugh & Johnston, 1976) and strainmeters (Mortensen et al., 1977).…”