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Revised earthquake hazard of the Hat Creek fault, northern California:
A case example of a normal fault dissecting variable-age basaltic lavas
Matthew W. Blakeslee and Simon A. Kattenhorn*
Department of Geological Sciences, University of Idaho, 875 Perimeter Drive, MS 3022, Moscow, Idaho 83844-3022, USA
ABSTRACT
Normal faults in basalt have distinctive
surface-trace morphologies and earthquake
evidence that provide information about
the slip behavior and earthquake potential.
The 47-km-long Hat Creek fault in northern
California (USA), a useful case example of
this fault style, is a segmented fault system
located along the western margin of the
Modoc Plateau that is a regional earthquake
hazard. In response to interaction with spo-
radically active volcanic systems, surface
ruptures have progressively migrated west-
ward since the late Pleistocene, with older
scarps being successively abandoned. The
most recent earthquake activity broke the
surface through predominantly ca. 24 ka
basaltic lavas, forming a scarp with a maxi-
mum throw of 56 m. Past work by others
identifi ed 7–8 left-stepping scarp segments
with a combined length of 23.5 km, but did
not explicitly address the throw character-
istics, fault evolution, slip history, or earth-
quake potential. We address these defi ciencies
in our understanding of the fault system
with new fi eld observations and mapping
that suggest the active scarp contains 2 addi-
tional segments and is at least 6.5 km longer
than previously mapped, thus increasing the
knowledge of the regional seismic hazard.
Our work details scarp geomorphic styles
and slip-analysis techniques that can be
applied to any normal-faulted basalt envi-
ronment. Applied to the Hat Creek fault,
we estimate that a surface-breaking rup-
ture could produce an earthquake of ~Mw
(moment magnitude) 6.7 and a recurrence
interval of 667 ± 167 yr in response to a rapid
slip rate in the range 2.2–3.6 mm/yr, creat-
ing a moderate risk given a lack of historical
earthquake events.
INTRODUCTION
Normal faults are prevalent in basalt environ-
ments in response to the common association
between basaltic volcanism and rifting. Such
faults have distinctive surface morphologies
where they cut through near-surface lavas (Pea-
cock and Parfi tt, 2002; Grant and Kattenhorn,
2004; White and Crider, 2006; Rowland et al.,
2007; Ferrill et al., 2011) and remain active dur-
ing volcanic periods such that variably aged lava
fl ows cut by the fault can be used as temporal
markers of slip rates and slip history. We use the
case example of the Hat Creek fault in north-
eastern California (USA) to illustrate the effi -
cacy of using offset lava fl ows to constrain slip
histories and earthquake potential for this fault
style, as well as to provide an improved regional
seismic hazard assessment related to this fault.
These techniques provide a viable alternative to
traditional paleoseismologic analyses, such as
trenching, which are ill-suited for the analysis
of faulted lavas.
The Hat Creek fault is located within a vol-
canic corridor between Mount Shasta and Las-
sen Peak, near the southern end of the Cascade
Range and its associated underlying subduction
system (Fig. 1) (Wills, 1991; Muffl er et al.,
1994; Blakely et al. 1997; Walker, 2008). Nor-
mal faulting and recurring volcanic activity
from more than 500 vents over the past 7 m.y.
created a pervasively faulted volcanic region
For permission to copy, contact editing@geosociety.org
© 2013 Geological Society of America
1
Geosphere; October 2013; v. 9; no. 5; p. 1–13; doi:10.1130/GES00910.1; 9 fi gures.
Received 11 February 2013 ♦ Revision received 10 July 2013 ♦ Accepted 13 August 2013 ♦ Published online 12 September 2013
*Corresponding author
Burney
McCloud
Old Station
Fall River
Mills
Modoc
Plateau
Figure 1. Terrain map of north-
ern California with the location
of the Hat Creek fault (box)
relative to populated areas and
Cascades volcanoes Mount
Shasta and Lassen Peak. CA—
California; NV—Nevada; ID—
Idaho; OR—Oregon.
Blakeslee and Kattenhorn
2 Geosphere, October 2013
in the vicinity of Lassen Peak (Muffl er et al.,
1994). The fault is located in the extending arc
and/or backarc transition of the Cascadia sub-
duction zone, marking the approximate west-
ern margin of a Miocene and younger volcanic
highland called the Modoc Plateau (White and
Crider, 2006). The Modoc Plateau hosts numer-
ous Neogene and Quaternary normal faults,
similar in style to the Basin and Range prov-
ince to the east (LaForge and Hawkins, 1986).
The north-northwest-trending, west-dipping
Hat Creek fault is the most prominent normal
fault in the region. It is highly segmented and is
composed of three subparallel systems of scarps
of different ages (Fig. 2A) that accrued a cumu-
lative throw in excess of 600 m (Muffl er et al.,
1994; Walker, 2008). The oldest and largest
system of scarps, referred to as the Rim, has as
much as ~350 m of throw and defi nes the eastern-
most extent of the fault system. The 47-km-long
Pleistocene Rim consists of seven right-step-
ping, northwest-oriented segments ranging in
length from ~1–16 km (Walker, 2008). These
scarps are heavily vegetated and have prominent
talus piles that refl ect gradual geomorphic mod-
ifi cation of the scarps to slope angles of ~30°–
45°. There is no evidence of disruption of the
talus slopes by recent surface rupture, consistent
with these scarps refl ecting an older, abandoned
portion of the fault system. Lava fl ows at the top
of the footwall east of Murken Bench have been
K-Ar dated as 924 ± 24 ka (Clynne and Muf-
fl er, 2010), constraining the maximum age of
the fault system as Calabrian (late Pleistocene).
The intermediate-aged fault scarps west of
the Rim are colloquially referred to here as the
Elevation (m)
High : 2104
Low : 870
Rim
Pali
Active
Scarp
Cinder
Butte
1
2
3
4
5
6
7
Newly
mapped
Active Scarp
Previously
mapped
Active Scarp
Murken
Bench
48 km 24 km
00
AB
Elevation (m)
High : 2104
Low : 870
40°55′N
40°50′N
40°45′N
40°40′N
121°30′W
121°30′W
40°55′N
40°50′N
40°45′N
40°40′N
121°25′W
121°25′W
Figure 2. Digital elevation model of the Hat Creek fault (elevation range in meters) derived from the U.S. Geological Survey 30 m national
elevation data set. (A) Traces of west-dipping fault scarps in the Hat Creek fault system. The three scarp system components are informally
named (from oldest to youngest) the Rim, the Pali, and the Active Scarp. (B) Enlargement of the Active Scarp fault trace. Seven identifi ed
segments are numbered and shown in black. Newly mapped additions to the Active Scarp in the north (red) follow the base of one of the
Rim segment scarps.
Earthquake hazard of the Hat Creek fault
Geosphere, October 2013 3
Pali (a Hawaiian term for eroded basaltic cliffs)
and have accrued as much as ~175 m of throw
(Walker, 2008). The Pali, also Pleistocene in
age, extends for ~24 km and is made up of fi ve
left-stepping segments with generally north ori-
entations in the southern part of the fault system
where the Pali intersects the Rim, but chang-
ing to north-northwest orientations in the north
where the Pali segments approach the volcanic
edifi ce at Cinder Butte (Fig. 2A). Many of the
segments are overlapping and exhibit physically
connected (i.e., breached) relay ramps, creating
a mechanically continuous system of interacting
segments.
The youngest system of scarp segments,
referred to here as the Active Scarp, has a
maximum displacement of 56 m just north of
Murken Bench (Fig. 2B) and exhibits evidence
of repeated earthquake activity since the late
Pleisto cene. Surface-breaking ruptures of the
Active Scarp follow the older Pali scarps within a
few tens of meters of the base of the Pali, except
the southernmost Active Scarp segment, which
occurs a few tens of meters from the base of a
southern Rim segment (Fig. 2A). The majority
of the Active Scarp offsets the Hat Creek Basalt,
a low-potassium olivine tholeiite that covers
much of the hanging wall valley fl oor west of the
fault scarp. The basalt originated from a cluster
of vents 30 km to the south of the throw maxi-
mum and has been 40Ar/39Ar dated as 24 ± 6 ka
(Turrin et al., 2007). These lavas fl owed north
down the Hat Creek valley, which is bounded
on its western side by a ca. 500–800 ka volcanic
escarpment (there is no western bounding fault
antithetic to the Hat Creek fault).
Although this region of California is seis-
mically active (Fig. 3), including small events
(M<3.5) in the vicinity of the Hat Creek fault,
the Active Scarp has not experienced a surface-
breaking earthquake event in recorded history
(~200 yr for northern California). Nonetheless,
in the southern portion of the fault system, the
Active Scarp offsets glacial deposits (younger
than 15 ka) by 20 m (Muffl er et al., 1994; U.S.
Geological Survey, 1996), indicating that 35%
of the total maximum throw in the northern part
of the Active Scarp and perhaps as much as
80% of the average throw along the entire fault
length has accrued since these deposits formed,
and suggesting that motion along the fault likely
continued from the late Pleistocene into the
Holocene.
The U.S. Geological Survey (USGS) Quater-
nary fault database (http://earthquake.usgs.gov
/hazards/qfaults) currently gives an incomplete
picture of the fault and its recent activity. The
Hat Creek fault is listed as having been active
in the past 15 k.y.; however, the database does
not discriminate between different parts of the
fault system that have been active at different
times (i.e., Rim, Pali, and Active Scarp). The
USGS also provides various earthquake hazard
estimates for the Hat Creek fault system in the
2008 National Seismic Hazards Maps database
(http://geohazards.usgs.gov/cfusion/hazfaults
_search/disp_hf_info.cfm?cfault_id=8,%209);
however, the earthquake magnitude estimates
(which range from M6.5–7.2) combine three
different faults (Hat Creek, McArthur, and May-
fi eld faults; 97 km cumulative length) that are
unlikely to rupture in tandem. Details about the
slip characteristics of the Hat Creek fault were
documented in the USGS Quaternary fault data-
base by Sawyer (1995) (http://geohazards.usgs
.gov/cfusion/qfault/); however, that analysis also
combines multiple faults into one system (59 km
cumulative length), and the details of the fault
slip history are poorly constrained, with a sug-
gested recurrence interval in the range 1000–
3000 yr and a slip rate of 1–5 mm/yr.
Our investigation of the Hat Creek fault
tightly constrains offset and timing history that
can be used to refi ne and advance seismic haz-
ard assessment. In conjunction with a revised
cumulative length of the Active Scarp, the his-
tory provides a more accurate estimate of the
earthquake potential of that portion of the fault
system that is likely to rupture in a single event.
At risk are numerous local towns (Burney, Fall
River Mills, Susanville, Red Bluff, and Redding
are all within 90 km of the Hat Creek fault), a
hydroelectric infrastructure within 20 km of
the fault system, the Allen Telescope Array at
the Hat Creek Radio Observatory, only 1.5 km
west of the Active Scarp, and Lassen Volcanic
National Park, 25 km to the south.
The long-term geometric and kinematic
evolution of the fault system in the context of
regional tectonics was described in Walker
(2008) and Walker and Kattenhorn (2008), out-
lining temporally variable tectonic and mag-
matic infl uences that affected fault trace geom-
etries. For example, whereas the Rim is made
up of seven right-stepping segments, the later
scarps of the Pali and the Active Scarp all have
a consistent left-stepping en echelon pattern
(Fig. 2). This change in geometry likely refl ects
0
Hat Creek fault
Fault Trace
30 60 km
Legend
M 2.0–2.49
M 2.5–2.99
M 3.0–3.49
M 3.5–3.99
> M 4.0
Scale
M5.7 (5/23/13)
Figure 3. Seismic events from
1968 to 2011 in the general
area of the Hat Creek fault.
Data obtained from the North-
ern California Earthquake
Data Center (http://quake.geo
.berkeley.edu/). Despite being a
seismically active area, the Hat
Creek fault itself has produced
very few events and no his-
toric large earthquakes. White
star shows a moderate (M5.7)
earthquake on 23 May 2013.
Blakeslee and Kattenhorn
4 Geosphere, October 2013
the infl uence of regional tectonic patterns, par-
ticularly the Walker Lane belt to the southeast,
which transferred dextral shear into the Lassen
region (Blakely et al., 1997; Muffl er et al.,
2008), creating the left-stepping segment pat-
terns and potentially driving the ongoing activity
of the later scarps (Walker, 2008).
ACTIVE SCARP OBSERVATIONS
Field and Analytical Methods
Our observations of the morphology of the
surface trace of the Hat Creek fault are based
on seven separate fi eld visits to all segments
of the fault (Rim, Pali, and Active Scarp; Fig.
2A) between 2003 and 2012. The scarp can be
reached via numerous access roads and hik-
ing trails that lead off of State Route 89, which
comes within 1 km of the fault at segment 6
(Fig. 2B). A gravel road that is along much of
the top of the Rim provides numerous vantage
points along the Rim footwall. These visits
allowed us to collect extensive fi eld descriptions
of the scarp morphology along the entire length
of the fault, with emphasis on the most recent
surface ruptures along the Active Scarp.
In all locations visited along the Active Scarp,
we made detailed observations of the manner in
which the fault had interacted with the youngest
lava fl ows. The two main elements of the fault
trace, hanging wall monoclines and vertical
scarp faces (described in the following), were
examined to unravel the progressive sequence
of disruption of the lavas by repeated fault
motions. We noted the dimensions of columnar
blocks of lava that had been disrupted or moved
out of place along both the vertical scarp and the
upper surface of the hanging wall monocline.
Relative exposure ages of different portions of
the vertical scarp were qualitatively linked to the
amount of lichen coverage of exposed columns
in the scarp face.
We did not date samples of lava fl ows cut by
the Active Scarp, relying instead on robust ages
provided by previously published works. Gen-
eral fl ow directions of lavas were determined
based on fl ow extents relative to known source
vents for fl ows of different ages, also docu-
mented previously (Muffl er et al., 1994; Turrin
et al., 2007). These fl ow directions were used
to identify where lavas had fl owed toward, and
draped across, existing portions of fault scarps
located at lower elevations than the source vents.
Where lavas of different ages abut or overlap,
we attempted to discern the locations of the lava
contacts in the fi eld; however, the vegetation
density commonly makes such distinctions dif-
fi cult, in which case aerial photos and Google
Earth were used to approximate the contacts.
We recorded all observation locations along
the fault using a handheld Trimble GeoExplorer
6000 GPS device, which was also used to
record throw along the Active Scarp with a
vertical precision of as little as ~10 cm. Throw
profi les for the majority of the Active Scarp
are based on data derived from stereo imagery
(Walker, 2008); however, GPS elevations were
also obtained to derive throw profi les along por-
tions of the Active Scarp by physically walking
out both the hanging wall and footwall cutoffs
and collecting GPS data points at 5 s intervals.
All data were imported into an ArcGIS environ-
ment and superposed on a 30 m digital elevation
model basemap (Fig. 2).
GPS fi eld data record locations and elevations
and thus only account for the vertical compo-
nent of fault motion, or throw (T). Our analysis
of the fault motion history also considers the
component of motion in the plane of the fault
itself, referred to as the slip or displacement
(D). These parameters are related using the fault
dip in the subsurface (θ) such that T = D sin θ
(Fig. 4).
Scarp Geometry and Surface Morphology
The Active Scarp was previously mapped
as consisting of 7 (Walker, 2008) or 8 (Muffl er
et al., 1994) left-stepping segments spaced from
0.79 to 1.83 km apart and with a cumulative
length of 23.5 km. The discrepancy in number
of segments documented in past work simply
refl ects the interpretative nature of separating
out connected segments. We adopt the inter-
pretation of 7 segments along the previously
identifi ed extent of the Active Scarp (Fig. 2B).
The strike of the Active Scarp is subparallel to
the Rim and Pali segments in the southern third
of the fault system (segments 5–7 in Fig. 2B)
where the Rim and Pali converge; however, in
the northern part of the fault system, both the
Pali and the Active Scarp diverge from the domi-
nant trend of the older scarp system, follow-
ing a northwest trend toward the ~335-m-tall,
8.5-km-wide shield volcano Cinder Butte
(Fig. 2). The westward migration of successive
scarp system segments of the Hat Creek fault
suggests that Cinder Butte and its underlying
magmatic system may have focused the devel-
opment of the active portion of the fault in the
proximity of Cinder Butte. Such a model is con-
sistent with documented examples of magmatic
systems affecting fault growth and orientation in
response to local stress perturbations related to
magma pressure (Clifton and Schlische, 2003;
Clifton and Kattenhorn, 2006; Rowland et al.,
2007; Gudmundsson et al., 2009) or the topo-
graphic and mechanical attributes of volcanic
constructs (Friese, 2008; Jenness and Clifton,
2009). Lava fl ows from Cinder Butte covered
the northern end of the Pali scarp; however, the
Active Scarp subsequently dissected these lavas
and curved toward the center of Cinder Butte
(Fig. 5), indicating more recent motion on the
fault relative to Cinder Butte volcanism. The
fi nal stage of Cinder Butte volcanism ended at
38 ± 7 ka with the eruption of the basaltic ande-
site of Cinder Butte (Turrin et al., 2007).
The majority of the Active Scarp ruptures
through the 24 ± 6 ka Hat Creek Basalt, which
covers much of the Hat Creek valley and accu-
mulated at the base of preexisting scarps of the
Rim (in the south) and the Pali (further north).
Rupture of the Active Scarp through the Hat
Creek Basalt resulted in surface features that
are characteristic of active normal faults cut-
ting basaltic lava fl ows (Gudmundsson, 1987a,
1987b, 1992; Grant and Kattenhorn, 2004;
Martel and Langley, 2006), including verti-
cal scarps where the fault breached the surface
along columnar joints and subsequently accrued
throw, dilational cracks and fi ssures along the
base of the scarp, and as much as ~40 m wide
zones of basalt rubble at the base of the scarp
created by repeated episodes of fault rupture
(Fig. 6A).
Although geomorphic processes have affected
scarps of all ages along the Hat Creek fault,
evident in the rubble piles that collected along
Rim and Pali scarps in response to mass wast-
ing, earthquake-related ground shaking effects
are the dominant geomorphic modifi er along the
relatively youngest Active Scarp. For example,
signifi cant disaggregation of the network of
small columns (generally <20 cm across and
<30 cm tall) within the rapidly cooled upper
surface of the Hat Creek lavas is common even
where lava surfaces are subhorizontal, imply-
ing strong ground shaking (Fig. 6C). In some
DT
θ
Figure 4. Relationship between throw (T)
and displacement (D) along a normal fault
with dip, θ. GPS field measurements of
footwall and hanging wall cutoff elevations
indicate T. Slip rates along faults are based
on the cumulative amount of displacement,
determined using D = T/sin θ.
Earthquake hazard of the Hat Creek fault
Geosphere, October 2013 5
maximum throw on
Active Scarp (56 m)
38 ± 7 ka
24 ± 6 ka
basalt west of
Six Mile Hill
53.5 ± 2 ka
maximum throw on
Pali scarp (175 m)
maximum throw on
Rim scarp (350 m)
maximum throw on northern
segments of Active Scarp (30 m)
limit of Cinder Butte lavas
limit of Hat Creek Basalt lava
limit of Hat Creek lava
limit of Hat
Creek lava
CA 89
Cassel/Fall River Mills road
basaltic andesite of
Cinder Butte
Hat Creek Basalt
Legend
Previously mapped
Active Scarp
Newly mapped Active
Scarp
Pali
Rim
Figure 5. Perspective view down and to the northeast of the northern portion of the Hat Creek fault system, showing the Hat Creek and
the Cinder Butte lavas in relation to the Active Scarp (red), the Pali (green), and the Rim (blue). The newly mapped portion of the Active
Scarp (yellow) traces the Rim scarp northeast of Cinder Butte. Locations and amounts of maximum throw points on the various scarps are
as shown. Image courtesy of Google Earth Pro.
Blakeslee and Kattenhorn
6 Geosphere, October 2013
instances, individual columns were lifted up out
of place and onto the surface of the fl ow, leaving
cavities within the fl ow top that can be matched
to individual column blocks like puzzle pieces,
implying ground accelerations suffi cient to
overcome the weight of the blocks (i.e., >1 gn).
Large basalt columns (1–2 m wide and several
meters tall) within the hanging wall have tilted,
fallen over, or broken apart adjacent to the
scarp, suggesting these areas have undergone
signifi cant ground shaking. This shaking also
caused similar-sized columns in the footwall to
fall from the vertical scarp and infi ll the fi ssure
below, exposing less-weathered and relatively
lichen-free surfaces along the scarp wall. Older
surfaces are coated by lichen of species Rhizo-
carpon with maximum diameters of ~24 mm
that imply an exposure time of at least 250 yr
(e.g., Bull, 2000), although lichenometric dating
techniques are not robust.
The Active Scarp is also characterized by a
hanging wall fault-trace monocline (Fig. 6B),
representing the near-surface fl exing of the Hat
Creek Basalt above an upward-propagating
fault tip prior to initial surface breaching by
the fault. Such monoclines are common along
normal faults that rupture to the surface through
young basalt fl ows (Grant and Kattenhorn,
2004; Martel and Langley, 2006; White and
Crider, 2006; Rowland et al., 2007). Along the
Active Scarp, the monocline accounts for as
much as 33 m of throw (Walker, 2008), imply-
ing numerous fault slip events prior to breaching
of the surface along the upper hinge line of the
fold. Once the surface was breached by the fault,
monocline growth ceased, and all subsequent
surface throw accumulation was directed along
the vertical scarp.
Progressive disaggregation of the monocline
occurred along the Active Scarp in response to
the local effects of repeated fault rupture, with
the fi nal stage of monocline history being its
complete collapse, rendering the monocline
to a pile of rubble (Fig. 6A). The rubble pile
is commonly composed of intact columns of
basalt with individual dimensions of several
meters and with large open cavities between
the blocks. Monocline disaggregation is most
advanced along fault segments with the largest
cumulative throws (segments 2–6 in Fig. 2), and
is least advanced where throw decreases toward
the segment tips or at relay zones, implying a
relationship between cumulative fault activity
and monocline breakdown.
Taken together, these characteristics of
monoclines in various stages of breakdown sug-
gest repeated earthquake-induced ground shak-
ing events. Intact or only partially disaggregated
breached monoclines can be seen along portions
of the fault where the scarp height is at least
Avertical scarp
rubble from
collapsed
monocline
B
C
vertical scarp
rubble from
collapsed
monocline
Figure 6. (A) View to the southeast of the northern end of segment 6 (Fig.
2B) of the Active Scarp. The 22-m-high vertical scarp represents surface
breaching of a monocline along its upper hinge line. Progressive destruc-
tion, interpreted to be the result of earthquake shaking, has reduced the
monocline to a pile of rubble in this location. (B) Intact monocline just
north of Murken Bench, along segment 2 of the Active Scarp (Fig. 2B).
Monocline height here is ~21 m (Walker, 2008). (C) Disruption of basalt
blocks on the upper surface of the Hat Creek lavas at the top of the mono-
cline in B. Inferred earthquake-induced ground accelerations in excess
of 1 gn have resulted in ejected blocks (arrow) on top of the lava fl ow
surface. Scale at center of image is 15 cm long.
Earthquake hazard of the Hat Creek fault
Geosphere, October 2013 7
6 m, implying that multiple surface-breaching
earthquake events are needed to induce mono-
cline disaggregation or destruction after initial
breaching of the monocline upper hinge line.
Maximum Fault Throw
The northern portion of the Active Scarp
(segments 1 and 2; Fig. 2B), north of Murken
Bench, crosses the northern extent of the Hat
Creek Basalt along the fault (Fig. 5), beyond
which the fault ruptures through basaltic ande-
site (38 ± 7 ka) originating from several vents
concentrated at Cinder Butte (Turrin et al.,
2007). The maximum observed throw of 56 m
(determined from GPS data with 10 cm verti-
cal precision) offsets the northern extent of the
Hat Creek lavas at the transition to older Cinder
Butte lavas. Larger offsets are apparent for the
upper surface of the Cinder Butte lavas (at least
70 m but potentially as much as 83 m), indicat-
ing 14–27 m of throw accrual along the fault
during the ~14 k.y. period between the erup-
tion of the Cinder Butte lavas and the Hat Creek
lavas near the end of the Pleistocene.
Southward-fl owing lavas from Cinder Butte
were deformed and offset by the northern por-
tions of the Pali (and later the Active Scarp),
which propagated northwest toward Cinder
Butte as the fault system seemingly responded
to the Cinder Butte magmatic system (Figs.
7A, 7B) (Walker, 2008). The uncertainty in the
amount of Active Scarp offset of the Cinder
Butte lavas derives from the fact that later lavas
of this eruptive period fl owed south along the
axis of the Pali scarps, resulting in bifurcation
of lava fl ows into two lobes that fl owed onto the
fault footwall and down onto the hanging wall
(Fig. 7C). The resultant vertical offset of the
two lobes of the lava fl ow was thus not tectonic,
but geomorphic. Subsequent activity along the
fault after the eruption of the Cinder Butte lavas
requires some portion of the total offset to be
fault-related throw. In contrast, Hat Creek lavas
fl owed from the south and so accumulated only
in the hanging wall of the Pali scarp (Fig. 7D).
These lavas were subsequently deformed as the
Active Scarp broke to the surface through them,
such that the entire 56 m of throw within these
lavas is decidedly fault-related (Fig. 7E).
Ordinarily, it may be impossible to distin-
guish between the geomorphic and tectonic
components of offset where lavas have fl owed
along both the footwall and hanging wall sides
of an active fault because the height of the scarp
would need to be known at the time of lava
emplacement across the fault (which is unlikely
to be determinable). In such cases, the scarp
height during lava emplacement would be sub-
tracted from the cumulative offset at the time
of measurement to ascertain the total tectonic
offset. In the case of the Hat Creek fault, how-
ever, the Active Scarp postdates both the Cinder
Butte and Hat Creek lava fl ows and formed on
the hanging wall side of the older Pali scarps.
Hence, any offset of the Cinder Butte lavas
across the Active Scarp must be purely tectonic.
The southern margin of the south-fl owing Cin-
der Butte lavas on both the footwall and hang-
ing wall sides of the Active Scarp, where it is
onlapped by the north-fl owing Hat Creek lavas,
exhibits at least 70 m of cumulative fault throw.
A second measurement of 83 m of throw was
taken ~150 m north of the fi rst measurement.
In this location, the Active Scarp and Pali scarp
appear to merge (Fig. 7E); therefore, a portion
of this 83 m offset may be geomorphic, indicat-
ing that 70 m is the more reliable measurement
of minimum total tectonic throw for the Cinder
Butte lavas. The combination of geomorphic
and tectonic offset of young lavas highlights the
importance of incorporating the relative tim-
ing and propagation directions of fault-growth
events and lava-fl ow episodes and advancement
directions into fault-offset analyses in tectoni-
cally active volcanic settings in order to obtain
appropriate slip rates.
Revised Active Scarp Length
The previously identifi ed northern extent
of the Active Scarp terminates within Cinder
Butte (Muffl er et al., 1994). Our investigation
of the fault system and fi eld mapping northeast
of Cinder Butte suggests the continuation of
young rupture activity, implying the length
of the Active Scarp has been underestimated. In
this region, the base of the Rim scarp created
a buttress for lavas fl owing across the hanging
wall of the Rim (Fig. 5). Some of these lavas
fl owed eastward away from eruptive center at
Cinder Butte, covering an older fl ow surface of
the basalt west of Six Mile Hill, dated as 53.5 ±
2 ka (Muffl er et al., 2012). The lavas of the
basalt west of Six Mile Hill fl owed southward
along the base of the Rim, following a local
slope toward the throw maximum along the
Rim 8.5 km south of the source vents immedi-
ately south of the Cassel–Fall River Mills road
(Fig. 5). The surface morphologies of these
lavas are distinct from older Pleistocene fl ows
exposed in the Rim, defi ning a youthful but
rugged lava surface with numerous tumuli and
defl ation pits. The roughness of this lava sur-
face complicates the identifi cation of the east-
ern extent of the Cinder Butte lavas. The Cinder
Butte lavas appear to abut the base of the Rim
scarp due east of the high point of Cinder Butte
but likely did not reach the Rim to the north-
east of the high point (Muffl er et al., 2012) as
a result of emplacement across the southward
sloping surface of the older lavas of the basalt
west of Six Mile Hill (Fig. 5).
Young ruptures through the lavas along the
base of the northern Rim scarp show striking
similarities to the previously documented sur-
face ruptures along the fault segments south of
Cinder Butte. This portion of the fault was not
previously identifi ed as part of the Active Scarp,
with presumed late Pleistocene most recent
activity in the USGS Quaternary fault database.
The young ruptures along the base of the north-
ern Rim scarp motivate us to test if they are part
of the Active Scarp system (i.e., rupture in tan-
dem with previously identifi ed active segments),
resulting in a longer fault length that should be
considered in a seismic hazard analysis, or if
they represent independently rupturing parts of
the overall Hat Creek fault system.
To map the young scarps along the northern
Rim northeast of Cinder Butte, we walked the
entire length of the rupture (both footwall and
hanging wall), mapping it with differential GPS
to capture the segment geometries and throw
distribution. We identifi ed two new rupture seg-
ments (Fig. 2B) extending a total of 4 km, break-
ing the surface ~50 m west of the base of the
Rim scarp with a maximum throw of 30 m in
the southern segment. The surface rupture loca-
tion relative to the base of the Rim is identical
to the previously documented Active Scarp seg-
ments south of Cinder Butte, which break verti-
cally to the surface through the Hat Creek lavas
several tens of meters from the base of the Pali
scarps. The southern termination of the newly
identifi ed young rupture is located 4.5 km north-
east of the northern termination of the previously
identifi ed Active Scarp (segment 1) within Cin-
der Butte (Fig. 2B). The northern termination is
~0.8 km north of where the fault is crossed by
the Cassel–Fall River Mills road. Additional Rim
segments of the Hat Creek fault continue for at
least 6 km north-northwest beyond this point to
just north of the Pit River; however, no evidence
of recent activity was confi rmed by this study.
Similar to the geometry of cracks along the
Active Scarp segments south of Cinder Butte,
the newly discovered rupture zone contains
fracturing that has a left-stepping geometry,
indicating similar rupture kinematics (normal
with a small right-lateral component). Displace-
ment of the late Pleistocene lavas has created
features identical to those observed along the
Active Scarp within Hat Creek Basalt, such as
vertical scarps, dilational cracks and fi ssures,
the presence of a large fault trace monocline
in the hanging wall, and a large rubble zone
created by ground shaking and the breakdown
of the monocline (Fig. 8). The vertical fault
scarp exposes older Pleistocene basalt near its
Blakeslee and Kattenhorn
8 Geosphere, October 2013
southern end where late Pleistocene lavas did
not reach the base of the Rim; however, where
the youngest lavas abut the Rim, all of the throw
is taken up within those lava fl ows, partially
by the monocline and partially by throw along
the vertical scarp. Therefore, the accumulation
of the total throw must have occurred at least
within the past 53.5 ± 2 k.y. (i.e., since the erup-
tion of the basalt west of Six Mile Hill) and pos-
sibly within the past 38.5 ± 7 k.y., if the basaltic
andesite lavas of Cinder Butte reached the Rim
in the vicinity of the young ruptures.
The northernmost segment of the newly iden-
tifi ed scarp defi nitively cuts through the basalt
west of Six Mile Hill. Where exposed along the
fault scarp, this lava appears noticeably older
than basaltic andesite lavas from Cinder Butte,
with a signifi cantly greater amount of surface
lichen. Nonetheless, the fault scarp features in
the basalt west of Six Mile Hill are similar to
other portions of the Active Scarp, including a
variably disaggregated hanging wall monocline
and a vertical fault scarp. The throws within this
unit are as much as 23 m within the northern-
Pleistocene lavas
Cinder Butte
Pali scarp
Early Cinder
Butte lavas
Pleistocene lavas
Cinder Butte
Pali scarp
Early Cinder
Butte lavas
AB
>40 ka
(<53.5 ka)
~40 ka?
propagation
of fault
Pleistocene lavas
Cinder Butte
Pali scarp
C
~38 ka
Late Cinder
Butte lavas
lava partially
covers scarp
geomorphic oset of
Late Cinder Butte lavas
Pleistocene lavas
Cinder Butte
Pali scarp
D
~24 ka
Late Cinder
Butte lavas
Hat Creek lavas
Pleistocene lavas
Cinder Butte
Pali scarp
E
0 ka
Late Cinder
Butte lavas
Hat Creek lavas
56 m
vertical fault scarp
(dilational fault)
monocline
Active Scarp
Figure 7. Interpreted sequence of volcanism and fault activity where the Active Scarp cuts into Cinder Butte from the south. (A) Develop-
ment of Cinder Butte, the lavas of which overlie the 53.5 ± 2 ka basalt west of Six Mile Hill, resulted in a new portion of the Hat Creek fault
(the Pali) branching away from the older Rim scarps and propagating northwest toward Cinder Butte. (B) Fault offset of older Cinder
Butte lavas by continued growth of the Pali scarp to the northwest. (C) Eruption of later Cinder Butte lavas at 38 ± 7 ka, emplacing lava
fl ows onto both the footwall and hanging wall sides of the fault and creating a geomorphic offset of the lava. (D) Eruption of Hat Creek
lavas at 24 ± 6 ka. Northward transport of these lava fl ows along the hanging wall of the Pali scarp resulted in onlap of Cinder Butte lavas.
(E) Ongoing fault activity was manifested by a vertical fault (the Active Scarp) cutting up through Hat Creek lavas, forming a fault-trace
monocline and ultimately a vertical scarp, producing a total 56 m of vertical offset of Hat Creek lavas.
Earthquake hazard of the Hat Creek fault
Geosphere, October 2013 9
most recently active segment of the fault. The
combination of evidence based on surface mor-
phologies suggests that the Hat Creek fault rup-
tured through at least three late Pleistocene lava
fl ows of different ages, with young surface rup-
ture both south of Cinder Butte (the previously
identifi ed Active Scarp) and northeast of Cinder
Butte, along the base of old Rim segments.
DISCUSSION
If the newly mapped rupture trace is the
continuation of the Active Scarp to the north-
east of Cinder Butte, it indicates that the most
recent surface ruptures along the Hat Creek fault
extend further north than previously considered.
Therefore, the Hat Creek fault has remained
active along the Rim in the northern part of the
fault system, despite having abandoned the Rim
portion of the fault system further south. The
additional segments of the Active Scarp north-
east of Cinder Butte defi ne a 4.5 km right step
and a 2.5 km along-strike gap within the fault
system, increasing the total length of young
ruptures by 6.5 km compared to prior estimates.
Although predominantly a left-stepping fault
system in response to dextral-oblique extension,
this right step in the fault geometry is interpreted
to result simply from the effect of the Cinder
Butte magmatic system on the temporal evolu-
tion of the southern portion of the fault. Large
steps are not uncommon in segmented normal
fault systems and are not necessarily hindrances
to earthquake ruptures. For example, steps from
3 to 8 km wide were associated with both the
1954 Dixie Valley (moment magnitude, Mw 6.8)
and the 1915 Fairview Peak (Mw 7.2) earth-
quakes in Nevada (Zhang et al., 1991).
There are numerous lines of evidence to
suggest that the newly identifi ed segments are
part of the Active Scarp system and rupture in
tandem. For example, the various morphologic
features consistent with the Active Scarp, such
as vertical scarps fl anked by a fault-trace mono-
cline, the disaggregated appearance due to the
collapse of the monocline during earthquakes,
and offsets of relatively young lava fl ows, sug-
gest these fault segments have both undergone
recent rupture with signifi cant ground shaking
and should thus be considered together when
evaluating the credible earthquake magnitude
and seismic hazard potential of the region. To
test this assertion, we explore fault evolution
and earthquake scenarios in which the newly
mapped segment is fi rst treated as an indepen-
dent fault and then considered to be incorpo-
rated into the entire Active Scarp system.
Estimation Method for Earthquake Slip
and Recurrence
Empirical relationships between rupture
length and maximum surface displacement dur-
ing discrete earthquake events (e.g., Wells and
Coppersmith, 1994; Wesnousky, 2008) per-
mit the estimation of slip rates and recurrence
intervals between earthquakes if the ages of
offset layers are known. For example, Wells and
Copper smith (1994) presented empirical data
for normal fault lengths in the range 3.8 km to
75 km to generate a regression line for maxi-
mum displacement (MD, in meters) versus sur-
face rupture length (SRL, in kilometers). The
relationship is given by log (MD) = –1.98 +
1.51 × log (SRL) for normal faulting. Maximum
displacement per event can be used to deter-
mine the number of events required to accrue
the cumulative displacement along the fault. If
the age of the oldest offset unit is known, the
number of events in this time interval informs
us about the recurrence interval between events.
This method uses the simplifying assumption of
characteristic earthquake events (equal displace-
ment per event and constant rupture length).
Nonetheless, it provides a reasonable insight
into earthquake activity in the absence of addi-
tional information such as paleoseismologic
evidence from scarp trenching, which is typi-
cally limited to only the last several events and
is not well suited to paleoseismological analysis
in faulted basalt. Moreover, the Active Scarp
represents the reactivation of an existing fault
system at depth (the Pali and the Rim), which
controlled the rupture length of the Active Scarp
as it developed through young lavas. The surface
trace length thus likely remained approximately
constant through time, as did the maximum dis-
placement per event.
Regression relationships are based on maxi-
mum surface displacement, not throw (which
is the vertical component of the fault displace-
ment). Faults scarps at Hat Creek are vertical
ARim scarp
surface trace of newly mapped
Active Scarp segments
Bmonocline
surface trace of newly mapped
Active Scarp segments
Figure 8. Surface morphology of newly mapped segments of the
Active Scarp northeast of Cinder Butte. (A) The surface trace of
the fault within lavas of the basalt west of Six Mile Hill consists
of a variably intact, disaggregated, or collapsed monocline fl anking
a vertical scarp (view to the east). (B) View of an intact portion of
the monocline along the newly mapped segments, within the basalt
west of Six Mile Hill. Cinder Butte is visible on the skyline (view to
the southwest).
Blakeslee and Kattenhorn
10 Geosphere, October 2013
within ~50 m of the surface, where they break
through the Hat Creek lavas (Muffl er et al.,
1994; Walker, 2008); however, slip along the
fault below this depth occurs along a dipping
fault plane, resulting in a component of dila-
tion at the surface along the vertical scarp.
This phenomenon is typical of dilational faults
in basalt lavas (e.g., Grant and Kattenhorn,
2004; Ferrill et al., 2011). Throw is converted
to displacement assuming a typical subsurface
normal fault dip of 60° (Anderson, 1951). This
assumption is reasonable given that geomor-
phically modifi ed scarps of the Pali and Rim
have typical dips of ~45°, indicating originally
higher dips.
Earthquake Potential of Newly
Mapped Segments
The newly mapped, recent rupture segments
along the northern portion of the Rim have
a total rupture length of 4 km (i.e., within the
range used in the Wells and Coppersmith regres-
sion). If these segments rupture independently
of any other segments of the Hat Creek fault, the
slip rate and recurrence interval must accom-
modate the offset of the 53.5 ± 2 ka basalt west
of Six Mile Hill. Given the maximum throw of
30 m (corresponding to 34.6 m of displacement
along the fault plane; Fig. 4), the newly identi-
fi ed young fault scarps would have an associ-
ated slip rate of ~0.65 mm/yr (or in the range
0.6–0.7 mm/yr when accounting for lava age
uncertainty). Using the Wells and Coppersmith
(1994) regression, a normal fault with a length
of 4 km (which is somewhat low for a surface-
breaking fault rupture) should have an average
maximum displacement of only ~8.5 cm per
rupture event (equivalent to ~7.5 cm of throw),
implying a low recurrence interval of 134 ± 5 yr
to accrue 30 m of throw since the faulted lava
was erupted.
Rupture of these fault segments could pro-
duce a Mw 5.8 earthquake, using the regres-
sion relationship between moment magnitude
and maximum displacement per event (MD, in
meters), given as M = 6.61 + 0.71 × log (MD)
(Wells and Coppersmith, 1994). Such an event
would probably be felt regionally in northeast-
ern California. For example, a M5.7 earthquake
along a normal fault located ~65 km south-
southeast of the southern end of the Hat Creek
fault on 23 May 2013 was felt as far south as the
San Francisco Bay area, as well as north into
southern Oregon and east into central Nevada
(USGS earthquake database: http://earthquake
.usgs.gov). However, no seismic events have
been attributed to the fault in recorded human
history in the region (~200 yr), and there is
no fi eld evidence of a very recent rupture. It is
therefore more likely that the newly mapped
segments are part of a larger system that rup-
tures less frequently—specifi cally, the entire
Active Scarp portion of the Hat Creek fault.
Slip Rate Analysis
As additional evidence in support of this
assertion, we consider the throw distribution
along the entire length of the Active Scarp. It is
well documented that interacting normal fault
segments distribute throw throughout the length
of a fault system (i.e., kinematic coherence;
Figs. 9A, 9B), typically producing the maximum
throw at the center of the fault trace (e.g., Walsh
and Watterson, 1991; Dawers et al., 1993; Cart-
wright et al., 1995; Willemse, 1997). The throw
commonly exhibits an approximately elliptical
distribution, attenuating from the maximum at
the center of the surface trace to zero at the fault
tips, with local variability at segment bound aries
within the fault system related to either fault
growth history (Childs et al., 1995; Kattenhorn
and Pollard, 2001) or the partial accommodation
of fault throw by relay ramp deformation (Hug-
gins et al., 1995; Blakeslee, 2012).
Along the previously identifi ed seven seg-
ments of the 23.5 km Active Scarp, throw is
distributed among segments by mechanical
interaction, whereby the fault segments are
affected by the presence of one another despite
spacings or underlaps between segments of
hundreds of meters to several kilometers (Fig.
2B). However, the throw versus distance profi le
along these seven segments (Walker, 2008) does
not show a maximum throw at the center of the
total length (Fig. 9C). Instead, the throw pro-
fi le is greatly skewed toward its northern end,
where the 56 m maximum throw displaces the
Hat Creek Basalt within segment 2, just south
of Cinder Butte (Fig. 5). The throw profi le
shape suggests that the previously identifi ed
seven segments of the Active Scarp do not rep-
resent the full length of the active fault system,
requiring mechanical interaction with more
segments north of segment 1. The addition of
the newly mapped, young segments northeast
of Cinder Butte extends the active fault system
northward, producing a maximum throw more
centered along the rupture length and an over-
all throw distribution with a more symmetric,
somewhat elliptical shape (Fig. 9C). The throw
pattern implies that the newly mapped segments
are kinematically coherent with the previously
identifi ed Active Scarp, rather than being part
of an independent fault. Nonetheless, the throw
profi le is somewhat skewed toward the north
end, raising the possibility of additional active
segments farther north (Figs. 2 and 3), where the
Hat Creek fault approaches the Pit River; how-
ever, no clear evidence for young rupture (e.g.,
through alluvial deposits that cross portions of
the fault) was observed in that area during a
reconnaissance survey.
Mechanical interaction and resultant slip
partitioning in segmented normal fault systems
results in higher throws (both cumulative and
per earthquake event; e.g., Dawers and Anders,
1995; Willemse, 1997) and higher slip rates for
segments near the centers of fault systems. For
example, the 387 km Wasatch fault in Utah has
a slip rate of 1–2 mm/yr in the central segments,
decreasing to 0.5 mm/yr in the distal segments
(Machette et al., 1991). Analogously, the active
portion of the Hat Creek fault exhibits variable
slip rates in different segments along its length.
To characterize the earthquake potential, includ-
ing slip rates, slip-per-event, recurrence inter-
vals, and earthquake magnitude along a fault
system consisting of multiple segments that rup-
ture in tandem, we use the assumption that the
segment containing the maximum cumulative
throw should be used. In so doing, we account
for the maximum amount of throw that neces-
sarily accumulated during a determined time
interval: in this case, the age of the Hat Creek
lavas. Segment 2 has the maximum throw (56 m
in 24 ± 6 ka lavas) and hence the highest slip
rate averaged since the late Pleistocene: 2.7
mm/yr, or in the range 2.2–3.6 mm/yr given the
uncertainty of the Hat Creek lava age. This esti-
mate assumes a subsurface fault dip of 60° and
so a total displacement of 64.7 m in the plane
of the fault (Fig. 4). We conservatively assume
pure dip-slip motion along the fault, although
a slight dextral component of motion may be
present based on the presence of left-stepping
fractures along the surface rupture trace. Hence,
actual slip rates may be slightly higher than we
calculate.
The maximum throw of the Cinder Butte
lavas (at least 70 m) is less defi nitive; however,
this estimate would require a minimum slip rate
of 2.1 mm/yr (or in the range 1.8–2.6 mm/yr
taking into account lava age uncertainty) since
these slightly older lavas erupted, approximately
consistent with the slip rate deduced from the
offset of the Hat Creek lavas. Compared to other
normal fault slip rates, such as the Wasatch
fault, the Hat Creek fault slip rate is relatively
high. The 0.65 mm/yr slip rate computed for
the newly mapped segments northeast of Cinder
Butte is consistent with those segments being
at the distal end of the coseismic rupture seg-
ments of the Active Scarp. Analogously, at the
southern end of the Active Scarp, segment 6 has
an effective slip rate 1.0 mm/yr (or in the range
0.8–1.4 mm/yr taking into account lava age
uncertainty). The observed attenuation of the
slip rate to the distal segments is thus consistent
Earthquake hazard of the Hat Creek fault
Geosphere, October 2013 11
with our assertion that the newly mapped seg-
ment is part of the Active Scarp system.
Slip rate estimates provide some insights
into the timing of the long-term evolution of the
fault system. For example, a hypothesized stress
perturbation induced by the Cinder Butte mag-
matic system caused a new branch of the fault to
propagate away from the Rim and redirect fault
activity toward Cinder Butte, forming the Pali
scarp system west of the original Rim scarps.
Given the 175 m maximum throw along the Pali
system and the simplifying assumption of a con-
stant a slip rate of 2.7 mm/yr, the evolution of
the Pali may have commenced ca. 65 ka, indi-
cating ~30 k.y. of activity in the Cinder Butte
magmatic system prior to the eruption of the
youngest lavas at 38 ± 7 ka. In this time inter-
val, the Pali scarps could have acquired maxi-
mum throws of ~70 m by the time the basaltic
andesite lavas erupted. Although the highest Pali
scarps are ~5.4 km south of the southern extent
of the Cinder Butte lavas (Fig. 5), the geomor-
phic offset of bifurcating lava lobes fl owing
onto the footwall and hanging wall blocks of the
Pali scarp closer to Cinder Butte (Fig. 7C) may
nonetheless have been quite signifi cant. This
possibility strengthens the argument that only
the throw offset of Cinder Butte lavas across the
relatively younger Active Scarp can be reliably
used to estimate fault slip rates.
One fi nal consideration regarding slip rate
analysis relates to the seemingly anomalous
nature of the throw peak along segment 2 (Fig.
9C) where the maximum throw of 56 m was
measured by GPS. Although approximately
elliptical throw profi les have been noted along
other mechanically interacting segmented nor-
mal fault systems (Dawers and Anders, 1995;
Willemse, 1997), the throw peak along segment
2 appears to fall above any choice of ellipse to
approximate the overall throw profi le. There are
many reasons why this throw peak may have
occurred. One possibility is that throw has been
underestimated along the Active Scarp seg-
ments south of the throw peak as a result of the
throw being partitioned onto other fault seg-
ments during earthquakes. For example, in the
vicinity of Active Scarp segments 3 and 4, there
are two north-oriented fault scarps that appear
to link the Pali with the Rim (Fig. 2). Although
we found no clear evidence of recent rupture
along those segments, the Hat Creek lavas did
not reach these segments, which may make
recent rupture evidence diffi cult to identify.
Another possibility is that the overall pattern
of throw is highly skewed toward the north-
ern end of the Active Scarp, with a maximum
Throw (m)
Distance Along Fault Trace (km)
0
10
20
30
40
50
60
50101520253035
Active Scarp
New segments
1
2
3
4
5
6
7
data gap
newly mapped
segments
Throw or Slip
Distance Along Fault Trace
Throw or Slip
Distance Along Fault Trace
AB
C
Figure 9. (A) Idealized ellipti-
cal distribution of throw or slip
along the length of a continu-
ous normal fault composed of a
single segment. Thick line below
graph shows map view of fault
trace. (B) Distribution of throw
along a 650-m-long normal fault
composed of seven mechani-
cally interacting segments in the
Bishop Tuff in eastern Califor-
nia (after Willemse et al., 1997).
Dashed curve shows the approx-
imation of an elliptical profi le.
Distal segments underrepresent
this distribution. Actual fault
dip direction was not specifi ed.
(C) Cumulative throw profi le
of the Active Scarp (monocline
height plus fault scarp height).
Profi les in red are derived
from stereo imagery and indi-
cate seven previously identi-
fied segments of the Active
Scarp (numbered as in Fig.
2B). Roughness in the profi les
refl ects technique uncertainty
and surface topography of the
Hat Creek lavas. The profi le in
blue shows the newly mapped
segments at the northern end of
the fault system based on GPS
data collected in the fi eld with
10 cm vertical precision. The
new segments cumulatively cre-
ate a more elliptical throw pro-
fi le with the maxi mum throw
closer to the geo metric center
the fault trace.
Blakeslee and Kattenhorn
12 Geosphere, October 2013
at segment 2. Such skewed cumulative throw
profi les have been noted (e.g., Willemse et al.,
1996) where fault segments mechanically
interact with a nearby perturbing infl uence,
such as another fault (or in this case, perhaps
Cinder Butte).
Ultimately, throw profi les simply refl ect the
cumulative effects of interactions between dif-
ferent components of a constantly evolving seg-
mented fault system; therefore, no faults have
truly elliptical throw profi les, and local peaks
are not uncommon (Fig. 9B) (cf. Dawers and
Anders, 1995; Cartwright and Mansfi eld, 1998).
Our careful matching of Hat Creek lavas across
the Active Scarp at segment 2 using fi eld-based
GPS measurements leave us confi dent in the
accuracy of the 56 m throw accumulation (and
hence our calculated slip rate) since the Hat
Creek lavas erupted, pooled against the existing
Pali scarp, and were subsequently offset by the
development of the Active Scarp (Fig. 7).
Revised Credible Earthquake Magnitude
The additional active segments increase the
originally mapped rupture length by 6.5 km
to a total of 30 km. Applying the surface rup-
ture length to maximum displacement scaling
relationship for normal faults described earlier
(Wells and Coppersmith, 1994), a 30 km fault
should manifest a maximum displacement of
1.78 m per rupture event (or 1.54 m of maxi-
mum throw assuming a fault dip of 60° in the
subsurface), implying a recurrence interval of
667 ± 167 yr for the 56 m throw maximum in
24 ± 6 ka lavas. This recurrence interval neces-
sitates ~15 Holocene rupture events along the
Active Scarp in addition to the late Pleistocene
activity that postdated the eruptions of the basalt
west of Six Mile Hill, Cinder Butte basaltic
andesite, and the Hat Creek Basalt.
The concomitant potential earthquake mag-
nitude is Mw 6.7 based on maximum displace-
ment versus magnitude relationships for normal
faults, as previously described (Wells and
Copper smith, 1994). Magnitude versus rupture
area relationships (M = 3.93 + 1.02 × log (RA),
with RA in square kilometers) will provide the
same result for a seismogenic thickness of 15 km
(down-dip rupture width of 17.3 km); however,
greater seismogenic thicknesses will increase
slightly the maximum credible earthquake. For
example, an 18–20 km seismogenic thickness
would only increase the potential earthquake
magnitude to Mw 6.8. Given the relatively low
recurrence interval and the lack of historic earth-
quake events or knowledge of the timing of the
last event, the Hat Creek fault may thus provide
a greater probabilistic seismic hazard than has
been previously realized.
CONCLUSIONS
Field observations and mapping along the
Hat Creek fault lead us to propose the existence
of two previously overlooked northern segments
of the Active Scarp, providing new insights
about the geometry, evolution, and seismic
potential of the fault. The newly mapped seg-
ments are kinematically coherent with the pre-
viously identifi ed Active Scarp segments, with
a surface morphology in 53.5 ± 2 ka lava fl ows
identical to surface ruptures along the previ-
ously mapped Active Scarp within the 24 ±
6 ka Hat Creek Basalt to the south. Near the
center of the reinterpreted Active Scarp sys-
tem, a maximum of 56 m of offset of Hat Creek
Basalt implies a late Pleistocene–Holocene slip
rate of 2.2–3.6 mm/yr, implying a very active
extensional fault system with high strain rates,
possibly refl ecting the contribution of a local
magmatic extension component to the overall
strain budget.
Based on earthquake magnitude and slip-
per-event scaling relationships for the reinter-
preted 30 km rupture length, we estimate a
credible earthquake magnitude of Mw 6.7
and a recurrence interval of 667 ± 167 yr to
account for the cumulative throw. As one of
the most prominent faults in the area, but
one that lacks historical earthquake events
(the timing of the last event is unknown but
is likely in excess of 200 yr ago in order to
predate historical records of earthquakes in the
region), the active portion of the fault is thus
a potential signifi cant earthquake hazard for
northeastern California.
The Hat Creek fault study provides a con-
text for the evaluation of earthquake hazards,
slip rate analysis, and recurrence interval
determination in tectonically active volcanic
environments. Young basalt lavas can be rea-
sonably well dated using Ar-Ar analysis, and
thus provide useful temporal offset markers.
True tectonic offsets can be distinguished from
any geomorphic offset caused by lavas fl owing
across existing scarps through fi eld analysis of
fl ow directions and characteristics relative to
developing fault scarps. Using the simplify-
ing assumption of characteristic earthquake
events during the period of tectonic offset of
young lavas (i.e., constant slip-per-event and
earthquake recurrence), we provide a reason-
able methodology for evaluating seismic hazard
in faulted lavas. This technique is particularly
useful given the diffi culty of trenching analy-
sis in lavas and the limitations of other tech-
niques such as lichenometry and cosmogenic
nuclide analysis (which may be incapable of
distinguishing between events that are relatively
closely spaced in time).
ACKNOWLEDGMENTS
We thank Erin Walker for developing the fault
evolution history in our related study, Marie Jackson
for contributing the photogrammetry database of the
Active Scarp heights of the Hat Creek fault used in
our throw profi les, Nicole Bellino for lichen sam-
pling and analysis, Leslie Fernandes and Tom Sawyer
for fi eld assistance, and Patrick Muffl er and Robert
Krantz for helpful discussions. Portions of this work
were funded under National Science Foundation grant
EAR-1113677 and a Seed Grant from the University
of Idaho. Digital elevation models in Figure 2 were
derived from 30 m resolution data obtained from the
U.S. Geological Survey National Elevation Dataset
(http://nationalmap.gov/viewer.html). Earthquake data
used in the production of Figure 3 were obtained
from the Northern California Earthquake Data Cen-
ter (http://quake.geo.berkeley.edu/). We thank Juliet
Crider and Patrick Muffl er for their thoughtful reviews
of the original manuscript.
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