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 INTRODUCTION histories and earthquake potential for this fault style, as well as to provide an improved regional Normal faults in basalt have distinctive Normal faults are prevalent in basalt environ- seismic hazard assessment related to this fault. surface-trace morphologies and earthquake ments in response to the common association These techniques provide a viable alternative to evidence that provide information about between basaltic volcanism and rifting. Such traditional paleoseismologic analyses, such as the slip behavior and earthquake potential. faults have distinctive surface morphologies trenching, which are ill-suited for the analysis The 47-km-long Hat Creek fault in northern where they cut through near-surface lavas (Pea- of faulted lavas. California (USA), a useful case example of cock and Parfi tt, 2002; Grant and Kattenhorn, The Hat Creek fault is located within a vol- this fault style, is a segmented fault system 2004; White and Crider, 2006; Rowland et al., canic corridor between Mount Shasta and Las- located along the western margin of the 2007; Ferrill et al., 2011) and remain active dur- sen Peak, near the southern end of the Cascade Modoc Plateau that is a regional earthquake ing volcanic periods such that variably aged lava Range and its associated underlying subduction hazard. In response to interaction with spo- fl ows cut by the fault can be used as temporal system (Fig. 1) (Wills, 1991; Muffl er et al., radically active volcanic systems, surface markers of slip rates and slip history. We use the 1994; Blakely et al. 1997; Walker, 2008). Nor- ruptures have progressively migrated west- case example of the Hat Creek fault in north- mal faulting and recurring volcanic activity ward since the late Pleistocene, with older eastern California (USA) to illustrate the effi - from more than 500 vents over the past 7 m.y. scarps being successively abandoned. The cacy of using offset lava fl ows to constrain slip created a pervasively faulted volcanic region 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- Figure 1. Terrain map of north- tional segments and is at least 6.5 km longer ern California with the location McCloud than previously mapped, thus increasing the of the Hat Creek fault (box) knowledge of the regional seismic hazard. relative to populated areas and Our work details scarp geomorphic styles Fall River Cascades volcanoes Mount Mills and slip-analysis techniques that can be Shasta and Lassen Peak. CA— applied to any normal-faulted basalt envi- California; NV—Nevada; ID— Burney Modoc ronment. Applied to the Hat Creek fault, Idaho; OR—Oregon. Plateau we estimate that a surface-breaking rup- ture could produce an earthquake of ~Mw Old Station (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. *Corresponding author Geosphere; October 2013; v. 9; no. 5; p. 1397–1409; doi:10.1130/GES00910.1; 9 fi gures. Received 11 February 2013 ♦ Revision received 10 July 2013 ♦ Accepted 13 August 2013 ♦ Published online 13 September 2013 For permission to copy, contact [email protected] 1397 © 2013 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/5/1397/3345815/1397.pdf by guest on 24 September 2021 Blakeslee and Kattenhorn in the vicinity of Lassen Peak (Muffl er et al., fault in the region. It is highly segmented and is talus piles that refl ect gradual geomorphic mod- 1994). The fault is located in the extending arc composed of three subparallel systems of scarps ifi cation of the scarps to slope angles of ~30°– and/or backarc transition of the Cascadia sub- of different ages (Fig. 2A) that accrued a cumu- 45°. There is no evidence of disruption of the duction zone, marking the approximate west- lative throw in excess of 600 m (Muffl er et al., talus slopes by recent surface rupture, consistent ern margin of a Miocene and younger volcanic 1994; Walker, 2008). The oldest and largest with these scarps refl ecting an older, abandoned highland called the Modoc Plateau (White and system of scarps, referred to as the Rim, has as portion of the fault system. Lava fl ows at the top Crider, 2006). The Modoc Plateau hosts numer- much as ~350 m of throw and defi nes the eastern- of the footwall east of Murken Bench have been ous Neogene and Quaternary normal faults, most extent of the fault system. The 47-km-long K-Ar dated as 924 ± 24 ka (Clynne and Muf- similar in style to the Basin and Range prov- Pleistocene Rim consists of seven right-step- fl er, 2010), constraining the maximum age of ince to the east (LaForge and Hawkins, 1986). ping, northwest-oriented segments ranging in the fault system as Calabrian (late Pleistocene). The north-northwest-trending, west-dipping length from ~1–16 km (Walker, 2008). These The intermediate-aged fault scarps west of Hat Creek fault is the most prominent normal scarps are heavily vegetated and have prominent the Rim are colloquially referred to here as the W ′ W W W ′ 0 ′ A ′ B °25 1°3 °25 °30 40°55′ N 12 121 121 121 40°55′ N Cinder Butte 1 Murken Bench 40°50′ N 40°50′ N 2 3 Rim Previously 4 mapped Pali 40°45′ N Active Scarp 40°45′ N Active Scarp Newly 5 mapped Active Scarp 6 40°40′ N Elevation (m) Elevation (m) High : 2104 High : 2104 Low : 870 7 Low : 870 40°40′ N 0 4 8 km 0 2 4 km 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. 1398 Geosphere, October 2013 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/5/1397/3345815/1397.pdf by guest on 24 September 2021 Earthquake hazard of the Hat Creek fault Pali (a Hawaiian term for eroded basaltic cliffs) Although this region of California is seis- slip characteristics of the Hat Creek fault were and have accrued as much as ~175 m of throw mically active (Fig. 3), including small events documented in the USGS Quaternary fault data- (Walker, 2008). The Pali, also Pleistocene in (M<3.5) in the vicinity of the Hat Creek fault, base by Sawyer (1995) (http://geohazards.usgs age, extends for ~24 km and is made up of fi ve the Active Scarp has not experienced a surface- .gov/cfusion/qfault/); however, that analysis also left-stepping segments with generally north ori- breaking earthquake event in recorded history combines multiple faults into one system (59 km entations in the southern part of the fault system (~200 yr for northern California). Nonetheless, cumulative length), and the details of the fault where the Pali intersects the Rim, but chang- in the southern portion of the fault system, the slip history are poorly constrained, with a sug- ing to north-northwest orientations in the north Active Scarp offsets glacial deposits (younger gested recurrence interval in the range 1000– where the Pali segments approach the volcanic than 15 ka) by 20 m (Muffl er et al., 1994; U.S. 3000 yr and a slip rate of 1–5 mm/yr. edifi ce at Cinder Butte (Fig. 2A). Many of the Geological Survey, 1996), indicating that 35% Our investigation of the Hat Creek fault segments are overlapping and exhibit physically of the total maximum throw in the northern part tightly constrains offset and timing history that connected (i.e., breached) relay ramps, creating of the Active Scarp and perhaps as much as can be used to refi ne and advance seismic haz- a mechanically continuous system of interacting 80% of the average throw along the entire fault ard assessment. In conjunction with a revised segments. length has accrued since these deposits formed, cumulative length of the Active Scarp, the his- The youngest system of scarp segments, and suggesting that motion along the fault likely tory provides a more accurate estimate of the referred to here as the Active Scarp, has a continued from the late Pleistocene into the earthquake potential of that portion of the fault maximum displacement of 56 m just north of Holocene. system that is likely to rupture in a single event. Murken Bench (Fig. 2B) and exhibits evidence The U.S. Geological Survey (USGS) Quater- At risk are numerous local towns (Burney, Fall of repeated earthquake activity since the late nary fault database (http://earthquake.usgs.gov River Mills, Susanville, Red Bluff, and Redding Pleisto cene.