Liu
et al
.,
Sci. Adv.
8
, eabk1167 (2022) 23 February 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
1 of 6
GEOPHYSICS
A unified perspective of
seismicity and
fault coupling
along the
San Andreas Fault
Yuan-Kai Liu
1
*, Zachary E.
Ross
1
, Elizabeth S.
Cochran
2
, Nadia
Lapusta
1,3
The San Andreas Fault (SAF) showcases the breadth of possible earthquake sizes and occurrence behavior; in
particular, the central SAF is a microcosm of such diversity. This section also exhibits the spectrum of fault
coupling from locked to creeping. Here, we show that the observations of aseismic slip, temporal clustering of
seismicity, and spatial variations in earthquake size distributions are tightly connected. Specifically, the creep rate
along the central SAF is shown to be directly proportional to the fraction of nonclustered earthquakes for the
period 1984–2020. This relationship provides a unified perspective of earthquake phenomenology along the SAF,
where lower coupling manifests in weaker temporal clustering, with repeating earthquakes as an end-member.
This new paradigm provides additional justification for characterizing the northwest
∼
75
kilometers of the creeping
segment as a transition zone, with potential implications for seismic hazard.
INTRODUCTION
Active fault zones exhibit remarkable diversity in their seismic
activity over space and time. Some fault zones are silent, not pro-
ducing even a single detectable earthquake over decades, while others
produce incessant, steady activity on a daily basis (
1
). In some places,
the spatial distribution of seismicity is highly localized to small
zones that are hundreds of meters wide (
2
) but, in other places, is
distributed across tens of kilometers (
3
). The San Andreas Fault
(SAF) in California is perhaps the epitome of such varied behavior,
as it demonstrates essentially the entire spectrum of earthquake
behavior along its
∼
1100-km length. This variability can even occur
over relatively short distances, as seen, for example, on the central
section of the fault (Fig. 1), which produces large damaging
earthquakes, repeating earthquakes (
4
), tectonic tremor (
5
), and
occasional swarms.
Variability in the spatial and temporal distributions of earthquakes
may result from differences in the mechanical properties of the
fault. In particular, frictional properties can have a first-order effect
on most aspects of earthquake source processes (
6
), and variations
in these properties should have a major impact on the space-time
patterns of the seismicity (
7
–
9
). Perhaps the strongest observational
evidence of the link between frictional properties and earthquake
behavior comes from faults exhibiting assorted aseismic slip phe-
nomena, such as slow slip transients, steady creep, and postseismic
deformation (
10
). The presence (or lack thereof) of aseismic pro-
cesses is often explained with the concept of fault coupling (
11
–
13
),
whereby fault behaviors lie on a spectrum from fully locked to fully
creeping. Here, we use the term coupling to represent a purely kine-
matic notion defined by the ratio of the slip deficit rate to the total
slip rate from plate motion models or geologic records. Faults with
low coupling are often seen to have seismicity patterns distinct from
those with strong coupling, which include characteristically repeating
earthquakes (
4
,
14
), pronounced spatial streaks of seismicity (
15
),
and a lack of moderate-to-large earthquakes. In several subduction
zones, aftershock productivity and spatial density have been shown
to correlate with coupling estimates from fault slip across seismic
cycles (
16
,
17
). Together, these studies provide clues linking coupling
variations to aftershock productivity. Despite these seminal obser-
vations, we still lack a comprehensive and unified understanding of
how the dynamics of seismicity is related to the degree of coupling
on major faults, such as the SAF.
Geodetic investigations of faults have revealed that coupling
often varies strongly in space (
10
,
13
), with more strongly coupled
faults defining potential areas of coseismic moment release during
great earthquakes (
18
). Faults with low coupling accommodate
most of their slip as stable sliding (
19
). Factors controlling coupling
include a combination of effective stress and frictional properties
related to rheological and geometrical changes on the fault interface
(
20
–
22
). Earthquakes are associated with stick-slip failure of asper-
ities (i.e., regions that experience coseismic slip with negligible
interseismic creep), which fail recurrently as strain accumulates
(
23
,
24
). This leads to the expectation that seismicity rates should be
generally higher along faults with higher slip rates. For example,
subduction zones around the Pacific show a positive correlation
between the background rate of earthquakes and the plate conver-
gence rate (
25
).
Among major fault systems worldwide, the SAF stands out as
demonstrating the complete range of interseismic coupling along
strike (
26
–
28
). This variability can be seen within just the central
∼
225-km section of the fault (Fig. 1), despite a relatively uniform
long-term right-lateral slip rate of 34 ± 3 mm/year from both
geologic and geodetic observations (
29
). These aspects, together
with the wealth of available high-quality seismic and geodetic obser
-
vations, make the central SAF an ideal setting to study the relation-
ship between fault coupling and the dynamics of seismicity over
decadal time scales.
RESULTS
We estimate the fraction of nonclustered events along the 225-km-long
section of the central SAF, starting from the rupture of the 1989
M
6.9 Loma Prieta earthquake in the northwest to Cholame Valley
in the southeast (Figs. 1 and 2A). The fraction of nonclustered events
strongly varies along strike from less than 0.1 to
∼
0.8 (Fig. 2A).
1
Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125,
USA.
2
U.S.
Geological Survey, Earthquake Science Center, Pasadena, CA 91106, USA.
3
Department of Mechanical and Civil Engineering, California Institute of Technology,
Pasadena, CA 91125, USA.
*Corresponding author. Email: ykliu@caltech.edu
Copyright © 2022
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
NonCommercial
License 4.0 (CC BY-NC).
Downloaded from https://www.science.org at California Institute of Technology on February 23, 2022