Figure 13. (a) Oblique view looking to the northwest where the offshore
Queen Charlotte fault steps 3 km east to the Fairweather fault. Data
sources for onshore and offshore elevation models described in Figure 2
caption. The location of the southern termination of the offshore Icy
Point-Lituya Bay reverse fault is inferred from seismic profiles shown
in Figure 5. (b) Conceptual block model depicting transpression, or
oblique contraction, in a right-lateral transform plate boundary. Arrows
indicate relative fault slip rates (after Wesnousky, 2005). (c) Our
tectonic model for the restraining double bend along the southern
Fairweather fault consists of a one-sided, positive flower structure
that includes the steeply dipping Fairweather fault to the east of Icy
Point and two reverse faults, the Icy Point-Lituya Bay and Finger
Glacier faults. The Icy Point-Lituya Bay fault splays off the
Fairweather fault at seismogenic depths; the Finger Glacier thrust fault
is a shallow secondary structure. The Icy Point-Lituya Bay fault and the
Fairweather fault accommodate oblique contraction where the Yakutat
block obliquely collides into North America.
Uplift and contraction at Icy Point are accommodated by faults with
reverse slip, strike slip, and oblique-slip behaviors. Because of its
near-vertical dip (Tocher, 1960; Doser and Lomas, 2010), the Fairweather
fault does not accommodate contraction. Shortening along the restraining
double bend is accommodated by reverse faults, most notably the offshore
Icy Point-Lituya Bay fault. Because the Icy Point peninsula is uniformly
uplifted (Figure 9b and c), there must be vertical displacement on the
Fairweather fault during some earthquakes. Kinematically, this requires
that some earthquakes on the Fairweather fault involve oblique slip.
Alternatively (or, in addition), the Fairweather fault on the east side
of the Icy Point peninsula may slip in a bimodal fashion during
independent strike-slip and vertical-slip fault ruptures (e.g., Barnhart
et al., 2015).
Several observations guide our conceptual model of fault-driven
deformation west of the restraining bend at Icy Point. First, the
obliquity between the strike of the Fairweather fault and Yakutat-North
America plate motion (22° ± 8°; Brothers et al., 2020), provides the
setting for the restraining double bend and accounts for the topographic
relief of the contractional foothills (Fairweather and Yakutat
foothills) to the west of the Fairweather fault. Second, at Icy Point,
uplifted Holocene shorelines on Terrace B imply an earthquake cadence
that averages 3.4 m of coseismic uplift every 460–1040 yrs. Third, the
Fairweather fault on the east margin of Icy Point has accommodated at
least 25 m of vertical, west-side-up displacement in the past 10 ky
based on uplifted periglacial and outwash deposits at the Kaknau Cliff
section. Finally, because the 1958 Mw7.8 Fairweather
earthquake was predominantly strike slip and caused no detectable
vertical displacement at Icy Point, there must be a set of earthquakes
with different rupture modes than the 1958-type earthquake because,
every 460–1040 years, at least one plate boundary earthquake causes
measurable, ~3.4 m, coseismic uplift of the Icy Point
peninsula. By comparison, the average recurrence interval for
predominantly strike-slip earthquakes on the Fairweather fault like the
1958 event (3.5 m per event) is <100 years (Witter et al.,
2021).
5.4 A one-sided, positive flower structure fault model
A conceptual fault model of deformation at the restraining double bend
that accommodates a fast-slipping transform plate boundary with
extraordinary rock uplift rates at Icy Point is one in which the
principal strand of the Fairweather fault forms the eastern margin of a
“one-sided,” positive flower structure (Figure 13) (Woodcock and
Fischer, 1986; Bruhn et al., 2004; Pavlis et al., 2004). The structure
initiates 1–2 km south of Icy Point, where the offshore Queen Charlotte
fault steps 3 km east to the onshore Fairweather fault, undergoes a
~20° bend, and ends in the north near Lituya Bay (Witter
et al., 2021). The Queen Charlotte fault may merge with the east-dipping
Finger Glacier fault as implied by offshore seismic profiles (Figure 5).
Transpression imposed by the >20° obliquity between the
strike of the Fairweather fault and Yakutat-North America plate motion
(Brothers et al.., 2020) drives the asymmetrical flower structure at Icy
Point; contraction is accommodated primarily on the Icy Point-Lituya Bay
reverse fault that splays off the Fairweather fault at seismogenic
depths (<20 km depth, Table 5). The simple fault model
depicted in Figure 13 does not explain the complex dynamics required by
the continual lateral advection of crust through the corner of a
stationary restraining bend.
The asymmetrical flower structure model accommodates shortening between
crustal slivers along the northeastern edge of the Yakutat block (Bruhn
et al., 2004; Pavlis et al., 2004). The model we propose is consistent
with geodetic block models that place convergence on reverse faults west
of the Fairweather fault (Elliott et al., 2010; Elliott and Freymueller,
2020) and differs from models of slip partitioning that infer
substantial convergence on unidentified structures east of the fault
under the Fairweather Range (McAleer et al., 2009). We present an
alternative model that infers that the strain-weakened edge of the
Yakutat block abuts the strong crystalline core of the Fairweather Range
and that high rates of horizontal and vertical deformation are localized
west of the Fairweather fault. Our contention that the near vertical
Fairweather fault accommodates substantial vertical slip is similar to
models of vertical extrusion attributed to oblique convergence along the
sub-vertical Denali fault (Benowitz et al., 2022).
5.5 Fault rupture scenarios accommodating oblique contraction
Several fault rupture scenarios could account for the uplift rates
recorded at Icy Point in the Holocene. A ‘joint rupture’ scenario
involves the simultaneous ruptures of the Fairweather fault and the
offshore Icy Point-Lituya Bay fault. In this scenario, coseismic slip on
the Fairweather fault at depth propagates upward and intersects the Icy
Point-Lituya Bay fault where a component of slip splays off the primary
strand of the Fairweather fault. Slip is partitioned into reverse slip
on the blind Icy Point-Lituya Bay fault and vertical- or oblique-slip on
the Fairweather fault. This scenario consists of joint rupture of both
faults and causes uplift of the Icy Point peninsula. There may be
seaward (westward) tilting during uplift because a blind reverse fault
like the Icy Point-Lituya Bay fault can exhibit diminishing slip towards
the tip, promoting more uplift above the location where the reverse
fault splays off the Fairweather fault. However, the measured westward
slopes of terraces B and C (Figure 9) do not exceed seaward gradients
typically cut by shore platform formation along the coast.
An alternative to joint rupture of both faults is a scenario that
invokes independent ruptures of the Fairweather fault and the Icy
Point-Lituya Bay fault. This ‘independent rupture’ scenario entails
vertical- or oblique-slip on the Fairweather fault that relieves a
substantial component of vertical strain, and subsequent reverse slip on
the offshore, blind Icy Point-Lituya Bay fault that relieves strain
oriented perpendicular to the plate boundary. The oblique-slip events on
the Fairweather fault must be balanced by events with sufficient
horizontal slip that eventually sum to the long-term fault slip rate.
The oblique-convergent ruptures that caused the 2010 and 2021 Haiti
earthquakes offer comparative analogs of complex, serial ruptures along
an oblique contractional fault system involving strike-slip and reverse
faults (Hayes et al., 2010; Okuwaki and Fan, 2022).
Both earthquake scenarios require that the Fairweather fault is an
oblique-slip fault at seismogenic depths and accommodates both vertical
and horizontal slip. The vertical component of slip is recorded in the
northern two seismic profiles (panels A and B) in Figure 5, which mark
the commencement of growth of the “Fairweather foothills.” The
Fairweather foothills, and the Yakutat foothills to the north, record
contraction of crustal slivers along the northeastern edge of the
Yakutat block (Bruhn et al., 2004; Pavlis et al., 2004; Elliott and
Freymueller, 2020). Horizontal shortening occurs through slip on the Icy
Point-Lituya Bay and Yakutat faults; rock uplift is the result of the
vertical component of slip on the Fairweather fault and reverse faults
that define the crustal slivers to the west.
Multiple earthquake scenarios are supported by observations of coseismic
slip in other transpressional fault systems that change the Coulomb
stress on adjacent strike-slip and reverse faults and either promote or
inhibit failure (Lin and Stein, 2004). Along the Fairweather fault,
rupture of adjacent reverse faults can promote failure along strike-slip
faults. For example, the 1899 Mw8.1 Yakutat Bay
earthquake promoted failure on the Fairweather fault at the northwestern
section of the 1958 rupture (Rollins et al., 2020) and blind thrust
faults promote failure over broad areas of the overlying crust (Lin and
Stein, 2004). Rupture of the Fairweather fault also likely promoted
failure on reverse faults northwest of Yakutat Bay (Rollins et al.,
2020). For comparison, the Mw 7.9 San Andreas fault
earthquake of 1857 promoted failure on nearby thrust systems including
the Coalinga and White Wolf reverse faults (Lin and Stein, 2004). The
Mw 7.9, 2002 Denali fault earthquake and its foreshock,
the Mw 6.7 Nenana Mountain earthquake, provide an
example of stress transfer from the Nenana Mountain strike-slip
foreshock to the hypocentral area of the Denali earthquake mainshock
(Anderson and Ji, 2003). Stress transfer from the Nenana Mountain
earthquake promoted complex reverse-oblique and strike-slip ruptures on
the Susitna Glacier thrust and Denali faults, respectively
(Eberhart-Phillips et al., 2003; and Aagaard et al., 2004). In this
context, Coulomb stress changes resulting from complex oblique-slip,
reverse, and strike-slip fault ruptures along the restraining double
bend north of Icy Point may periodically promote vertical slip on the
Fairweather fault.
5.6 Earthquake source parameters for the offshore Icy Point-Lituya Bay
thrust fault
Slip on the offshore Icy Point-Lituya Bay thrust fault represents a
source of earthquakes and tsunamis along the Gulf of Alaska coast in
addition to strike-slip ruptures along the Fairweather fault. The 1958
Mw7.8 Fairweather earthquake primarily relieved shear
strain parallel to the plate boundary. Published slip rates for the
Fairweather fault (46–58 mm/yr) imply average recurrence intervals of
60–140 years for Mw>7 strike-slip
earthquakes (Plafker et al., 1978; Witter et al., 2021). This
investigation derives fault source parameters for repeated Holocene
ruptures on the Icy Point-Lituya Bay thrust fault that relieved
fault-normal strain, caused coseismic uplift at Icy Point, and included
substantial vertical displacement on the Fairweather fault. Moreover,
rupture of offshore thrust faults may explain large tsunamis in Lituya
Bay in 1853–1854, ca. 1874, and ca. 1899 (Miller 1960).
Reverse slip on the Icy Point-Lituya Bay thrust fault evidenced by
uplifted Holocene terraces at Icy Point occur no more than every
460–1040 years. Assuming a simple fault geometry for a thrust or
reverse fault striking subparallel to the plate boundary and dipping
between 45°–75° and coseismic uplift of 3–5 m per earthquake based on
terrace riser heights, we estimate that the slip during past events on
the Icy Point-Lituya Bay fault ranged between 3.1 and 10 m (Table 5).
Because the reverse fault is blind, its dip is unknown. Shallower fault
dips (30°–45°) are most consistent with geodetic estimates of
fault-normal rates of motion (5–14 mm/yr of shortening) perpendicular
to the plate boundary (Table 5) (Elliott and Freymueller, 2020); steeper
dips (60°–75°) are required if the reverse fault splays off the
Fairweather fault at seismogenic depths (10–16 km). Hypothetical
ruptures of the Icy Point-Lituya Bay fault with rupture lengths equal to
or exceeding the distance between Icy Point and Lituya Bay (40–70 km)
could potentially generate Mw7–7.5 earthquakes
(Wesnousky, 2008; Stirling et al., 2013); larger events are suggested by
the 3.4 m average vertical separation of shorelines on Terrace B (Moss
et al., 2022). Complex events that include simultaneous rupture of the
Icy Point-Lituya Bay fault (e.g., Mw7–7.5) and the
Fairweather fault (e.g., Mw7.8) are implied by our
results, and could potentially generate Mw7.9
earthquakes (derived by summing the moments of multiple events
(Kanamori, 1983)).