4 Discussion
The recurrence estimates reported here range from ~220
years (Semidi section) to ~1000 years (Sanak section)
for Mw ≥ 8.5 events from geologic data (Table 1) and
from 50 years (Mw 8.1, Fox Islands section) to 4750
years (Mw 8.3, Sanak section) from geodetic data (Table
2). The two approaches provide different, but complementary, views of
rupture behavior. Along the energetic coasts of Alaska, geologic data
capture only the largest ruptures, generally with preserved evidence of
vertical deformation above detection limits of >0.2 m
(Hawkes et al., 2010; Shennan et al., 2016) or tsunami runup
> 5 m above the modern tidal range (Nelson et al., 2015;
Witter et al., 2016, 2019). The geologic data also represents events
that typically rupture multiple fault sections, which is demonstrated by
historical events and inferred for prehistoric earthquakes – in this
regard, the geologic recurrence rates should be viewed as participation
rates in ruptures >Mw 8.5 for each fault section for which
geologic data are available. The geodetic data are used to approximate
strain accumulation and release by single fault section, and so
recurrence rates of these events are necessarily shorter and the
inferred earthquake magnitudes are smaller than events recorded by
geology.
A primary advantage of the geodetic recurrence model is that it
estimates moment accumulation on the model subduction interface sections
in a general way. Because coupling magnitude and coupled area generally
trade off in geodetic models, the exact location of the coupled patches
is not critical. Instead, our goal is to estimate the moment
accumulation budget available for interface ruptures, rather than using
geodesy to strictly define rupture patches. By calculating the
recurrence on sections we have defined a priori , we provide
information that can be interpreted in the broader context of section
magnitude frequency distributions and multi-section ruptures. The
coupling polygons we present here are not meant to strictly correlate
with rupture patches. Complex non-unique coupling models are often
presented for subduction zone interfaces, including for some portions of
the AASZ (Li et al., 2016), and the relation between interseismic
coupling and interface ruptures can be modeled in intricate ways (Small
& Melgar, 2021). However, the relation between geodetic coupling models
(strain accumulation) and eventual rupture (strain release) is not
straightforward, and the assumption that complex depictions of
interseismic coupling uniquely predict future rupture patterns is not
clearly supported (Noda and Lapusta, 2013; Tsang et al., 2015; Witter et
al., 2019). For example, persistent asperity models such as most
recently presented by Zhao et al. (2022) for the Shumagin and Semidi
sections of the AASZ use recent ruptures to create non-unique backslip
scenarios that fit sparse GNSS measurements reasonably well. These
models offer limited utility for hazard forecasts because they
effectively predict characteristic earthquakes (Schwartz, 1999) unless
the persistent asperity assumption is relaxed and modified (Avouac,
2015), and a primary objective in modern seismic hazard modeling is to
move beyond the assumption that past earthquakes uniquely predict future
ruptures (Field et al., 2014).
Our effort to assign recurrence values along the AASZ points to several
potential future studies and opportunities. Geologic studies would be
beneficial to characterize rupture behavior in the western
~1,250 km of the subduction zone, for which no data are
currently available. Geodetic data are fundamentally important for
understanding subduction zone hazard, and a denser permanent GNSS
network throughout the AASZ would improve hazard estimates – and
especially on the seafloor, where recent GNSS-Acoustic studies (Brooks
et al., 2023) have demonstrated the importance of seafloor geodesy.
Instead of a single model, future coupling and slip-deficit models from
geodesy might be presented as a suite of models that encompass the
broadest possible range of uncertainty, such as multiple geodetic models
presented in Schmalzle et al. (2014) and Mariniere et al. (2021).
The AASZ experienced a series of major ruptures in the 20th century
along much of its length. Recurrence intervals for these largest
ruptures are many centuries long, and it was previously assumed that
apparently unruptured portions of the interface in the historical
period, or seismic gaps (Davies et al., 1981), would be most likely to
host future ruptures, and conversely, that regions that ruptured in the
20th century were likely no longer hazardous in the 21st century.
However, re-rupture of a historical great earthquake rupture patch by
subsequent large events has been documented or inferred along the AASZ
(Schwartz, 1999; Tanioka & Gonzalez, 1998; Brooks et al., 2023).
Complex rupture overlap is also supported by current observations and
models of subduction interface frictional behavior, which demonstrate
that subduction interfaces are a mosaic of slip patches along strike and
down dip (Lay et al., 2012). The regions of interface slip associated
with historical events are very poorly known (Nicolsky et al., 2016;
Witter et al., 2019) and the extremely generalized historical rupture
areas from aftershocks (Tape & Lomax, 2022) and coarse rupture models
(Johnson & Satake, 1993) are insufficient to accurately define
historical slip patches. Our recurrence results indicate that most
sections we define along the AASZ are capable of >
Mw 8 ruptures roughly every century, and that the
locations of historical ruptures and presumed spatial variations in
interseismic coupling are only loose constraints on future rupture
locations and magnitudes.