Probabilistic Estimates of Surface Coseismic Slip and Afterslip for Hayward Fault Earthquakes

Abstract

We examine the partition of long-term geologic slip on the Hayward fault into interseismic creep, coseismic slip, and afterslip. Using Monte Carlo simulations, we compute expected coseismic slip and afterslip at three alinement array sites for Hayward fault earthquakes with nominal moment magnitudes ranging from about 6.5 to 7.1. We consider how interseismic creep might affect the coseismic slip distribution as well as the variability in locations of large and small slip patches and the magnitude of an earthquake for a given rupture area. We calibrate the estimates to be consistent with the ratio of interseismic creep rate at the alinement array sites to the geologic slip rate for the Hayward fault. We find that the coseismic slip at the surface is expected to comprise only a small fraction of the long-term geologic slip. The median values of coseismic slip are less than 0.2 m in nearly all cases as a result of the influence of interseismic creep and afterslip. However, afterslip makes a substantial contribution to the long-term geologic slip and may be responsible for up to 0.5-1.5 m (median plus one standard deviation [S.D.]) of additional slip following an earthquake rupture. Thus, utility and transportation infrastructure could be severely impacted by afterslip in the hours and days following a large earthquake on the Hayward fault that generated little coseismic slip. Inherent spatial variability in earthquake slip combined with the uncertainty in how interseismic creep affects coseismic slip results in large uncertainties in these slip estimates.

Long‐Term Afterslip of the 2004M 6.0 Parkfield, California, Earthquake—Implications for Forecasting Amount and Duration of Afterslip on Other Major Creeping Faults

“…For continental strike-slip faults, the surface afterslip record of the 2004 Parkfield earthquake remains the only record longer than 2 yrs that fully spans the fault-zone slip using a uniform methodology (Aagaard et al, 2012). Therefore, in most cases we have been unable to confirm how quickly afterslip was completed for other historical events.…”

“…For purposes of forecasting final slip, we used the AFTER program Budding et al, 1989) to invert postseismic slip observations (i.e., differential displacement between two sides of the fault), obtaining best-fitting coefficients for an empirical afterslip relation (1) that fits afterslip observations for continental strike-slip earthquakes well: E Q -T A R G E T ; t e m p : i n t r a l i n k -; d f 1 ; 3 1 3 ; 4 9 7 ut u f = 1 T t c 1 (Aagaard et al, 2012). Displacement, u accumulates with time t, in days following the earthquake, toward a final value u f (in cm), which includes coseismic and postseismic afterslip; in which T may be described as the duration, or the time for which this relation behaves as a power law with exponent c. After time T, the afterslip continues but at a lower rate while approaching u f asymptotically.…”

“…A recent moderate-magnitude suburban earthquake in the San Francisco Bay region, the 2014 M 6.0 South Napa, California, earthquake exhibited sizeable (∼0:5 m) surface afterslip , seriously damaging fault-crossing natural gas and water pipelines, and residential structures , delaying their permanent repair for many months. We expect that most of these larger anticipated (M ≥ 6:7) regional earthquakes would be accompanied by much larger and more damaging afterslip continuing for years (Aagaard et al, 2012).…”

“…The apparent lack of interseismic creep may explain its relatively brief (∼1 yr) afterslip duration, which allowed scientists to provide reasonably prompt and accurate forecasts of final afterslip to the public Lienkaemper et al, 2016). However, long-term afterslip forecasting for larger future earthquakes in this region may be more challenging, based on our limited knowledge of surface afterslip duration from just a few historical continental strike-slip ruptures on creeping faults, such as the 2004 Parkfield event and others (Lienkaemper et al, 2006;Aagaard et al, 2012). Unfortunately, previous studies have used afterslip records limited to at most about 2 yrs, most less than 1 yr, so the natural variation in afterslip magnitude and duration remains poorly understood.…”

Mobile Laser Scanning Applied to the Earth Sciences

“…In all likelihood, the increased ability to make repeat observations, leverage preexisting data sets (see http:// www . opentopography .org/), and change detection techniques [Aryal et al, 2012;Nissen et al, 2012] will significantly affect the study of previously unobservable or transient processes such as earthquake afterslip [Aagaard et al, 2012] or catastrophic debris flow initiation [Iverson et al, 1997]. Balloon-mounted observations (assuming airspace free of obstacles) would give an overview of the area in question, and backpack-or vehicle-mounted observations would allow researchers to focus on specific features of interest.…”