Diverse rupture modes for surface-deforming upper plate earthquakes in the southern Puget Lowland of Washington State Alan R. Nelson1,*, Stephen F. Personius1, Brian L. Sherrod2, Harvey M. Kelsey3, Samuel Y. Johnson4, Lee-Ann Bradley1, and Ray E. Wells5 1Geologic Hazards Science Center, U.S. Geological Survey, MS 966, PO Box 25046, Denver, Colorado 80225, USA 2U.S. Geological Survey at Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA 3Department of Geology, Humboldt State University, Arcata, California 95521, USA 4Western Coastal and Marine Geology Science Center, U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA 5Geology, Minerals, Energy, and Geophysics Science Center, U.S. Geological Survey, 345 Middlefi eld Road, MS 973, Menlo Park, California 94025, USA ABSTRACT earthquakes. In the northeast-striking Saddle migrating forearc has deformed the Seto Inland Mountain deformation zone, along the west- Sea into a series of basins and uplifts bounded Earthquake prehistory of the southern ern limit of the Seattle and Tacoma fault by faults. One of these, the Nojima fault, pro- Puget Lowland, in the north-south com- zones, analysis of previous ages limits earth- duced the 1995 Mw6.9 Hyogoken Nanbu (Kobe) pressive regime of the migrating Cascadia quakes to 1200–310 cal yr B.P. The prehistory earthquake, which killed more than 6400 peo- forearc, refl ects diverse earthquake rupture clarifi es earthquake clustering in the central ple, destroyed the port of Kobe, and caused modes with variable recurrence. Stratigraphy Puget Lowland, but cannot resolve potential $100 billion in damage (Chang, 2010). Unlike and Bayesian analyses of previously reported structural links among the three Holocene the Inland Sea of Japan, the historical record of and new 14C ages in trenches and cores along fault zones. seismicity in the Puget Lowland is short, and backthrust scarps in the Seattle fault zone information about past large, surface-deforming restrict a large earthquake to 1040–910 cal yr INTRODUCTION earthquakes must come from paleoseismol- B.P. (2σ), an interval that includes the time of ogy. In this densely forested, heavily glaciated the M 7–7.5 Restoration Point earthquake. The seismic hazard of active convergent mar- region, the most successful paleoseismology A newly identifi ed surface-rupturing earth- gins derives not only from great earthquakes on studies begin with aeromagnetic mapping and quake along the Waterman Point backthrust the megathrust, but also from shallow faulting in seismic refl ection surveys to identify potentially dates to 940–380 cal yr B.P., bringing the num- the actively deforming forearc. Along the Cas- active fault zones, move to searches for fault or ber of earthquakes in the Seattle fault zone cadia margin, the cities of Seattle and Tacoma fold scarps and deformed shorelines on lidar in the past 3500 yr to 4 or 5. Whether scarps occupy the Puget Lowland, a structurally com- (light detection and ranging; i.e., airborne laser record earthquakes of moderate (M 5.5–6.0) plex, seismically active forearc trough between swath mapping) imagery, and follow up with or large (M 6.5–7.0) magnitude, backthrusts the Coast Range and the Cascade volcanic arc. studies of Holocene stratigraphy beneath scarps of the Seattle fault zone may slip during mod- Deformation of the lowland is caused by the and shorelines to date prehistoric earthquake erate to large earthquakes every few hundred clockwise motion of western Oregon, pushing deformation (Barnett et al., 2010; e.g., Blakely years for periods of 1000–2000 yr, and then northward and compressing western Washing- et al., 2002; Johnson et al., 2004a; Sherrod et al., not slip for periods of at least several thou- ton against slower moving Canada (Fig. 1A; 2004a, 2008; Kelsey et al., 2012). sands of years. Four new fault scarp trenches Wells and Simpson, 2001; McCaffrey et al., Paleoseismology has uncovered several in the Tacoma fault zone show evidence of late 2007). North-south shortening of the Puget aspects of fault and earthquake behavior in the Holocene folding and faulting about the time Lowland at ~4.4 mm/yr (McCaffrey et al., 2007) Seattle-Tacoma-Saddle Mountain fault systems. of a large earthquake or earthquakes inferred has produced thrust fault–bounded uplifts that 1. A very large earthquake is recorded by an from widespread coseismic subsidence ca. subdivide the lowland into four basins (Fig. 1B; uplifted marine terrace fi rst identifi ed at Res- 1000 cal yr B.P.; 12 ages from 8 sites in the Pratt et al., 1997; Brocher et al., 2001; Blakely toration Point on Bainbridge Island (Fig. 2A). Tacoma fault zone limit the earthquakes to et al., 2002). Much of the north- to northeast- The terrace, uplifted during an earthquake on 1050–980 cal yr B.P. Evidence is too sparse directed strain in the southern Puget Lowland the master-ramp thrust of the Seattle fault, can to determine whether a large earthquake was is accommodated by deformation in the Seattle be traced over tens of square kilometers on the closely predated or postdated by other earth- fault zone, the Tacoma fault zone, the Saddle fault hanging-wall block (Bucknam et al., 1992; quakes in the Tacoma basin, but the scarp Mountain deformation zone, and along the Sherrod et al., 2000; Kelsey et al., 2008; Figs. of the Tacoma fault was formed by multiple Olympia fault (Figs. 1B, 1C). 2A and 3). The Puget Lowland tectonic setting is similar 2. Surface-rupturing reverse faults are well *Email: [email protected] to that of southwest Japan, where the westward- documented in the hanging-wall blocks of the Geosphere; August 2014; v. 10; no. 4; p. 769–796; doi:10.1130/GES00967.1; 14 fi gures; 1 table; 1 supplemental fi le. Received 22 July 2013 ♦ Revision received 24 February 2014 ♦ Accepted 3 June 2014 ♦ Published online 24 June 2014 For permission to copy, contact [email protected] 769 © 2014 Geological Society of America Nelson et al. Seattle and Tacoma faults, as well as along Figure 1 (on following page). (A) Kinematic model of the Cascadia forearc (simplifi ed from faults of the Saddle Mountain deformation zone Wells et al., 1998; Wells and Simpson, 2001). Northward migration of the Oregon Coast (Fig. 1C; Witter et al., 2008; Blakely et al., 2009; Range squeezes western Washington against the North American plate, producing faults and this paper). In the central Seattle fault zone, the earthquakes in the Puget Lowland. Red lines are faults; black arrows show motions of tec- surface ruptures are on short, en echelon back- tonic blocks. (B) Tectonic setting of the Seattle and Tacoma fault zones and the Saddle Moun- thrusts (Nelson et al., 2003a), and their relation tain deformation zone in the southern Puget Lowland region (modifi ed from Blakely et al., to slip on the master thrust at depth is problem- 2009). The Puget Lowland occupies the area within the orange dashed line. Red and green atic (Kelsey et al., 2008). stipples indicate regions of uplift and sedimentary basins, respectively, as defi ned by gravity 3. Widespread evidence for surface-deform- anomalies. Holocene or suspected Holocene faults are shown in black. Faults in the Seattle ing earthquakes within ~100–200 yr of 1000 and Tacoma fault zones and the Saddle Mountain deformation zone are shown in red. Basins cal yr B.P. extends over at least 800 km2 (Buck- and regions of uplift: BB—Bellingham basin; EB—Everett basin; KA—Kingston arch; SB— nam et al., 1992; Sherrod, 2001) and probably Seattle basin; SU—Seattle uplift; DB—Dewatto basin; TB—Tacoma basin; OU—Olympia >4000 km2 (Blakely et al., 2009; Martin and uplift. Faults: BCF—Boulder Creek fault; LRF—Leech River fault; DMF—Devil’s Mountain Bourgeois, 2012) of the southern Puget Low- fault; SWIF—southern Whidbey Island fault; RMF—Rattlesnake Mountain fault; SMF— land. This includes the Seattle fault zone, the Saddle Mountain faults; CRF—Canyon River fault; FCF—Frigid Creek fault; WRF—White Tacoma fault zone and Tacoma basin, and the River fault; OF—Olympia fault. (C) Southern Puget Lowland showing the Seattle fault zone, Saddle Mountain deformation zone (Fig. 1C; Tacoma fault zone, Saddle Mountain deformation zone, Olympia fault, and related late Holo- Atwater and Moore, 1992; Bucknam et al., 1992; cene fault scarps (base from Finlayson, 2005). Faults (red lines) are inferred from evidence Bucknam and Biasi, 1994; Atwater, 1999; Sher- summarized by Brocher et al. (2004), Johnson et al. (2004b), Karel and Liberty (2008), Liberty rod et al., 2000; Hughes, 2005; Kelsey et al., and Pratt (2008), Witter et al. (2008), Blakely et al. (2009), Clement et al. (2010), and Tabor 2008; Blakely et al., 2009; Arcos, 2012). et al. (2011). Fault scarps (black lines) are as identifi ed on lidar (light detection and ranging) 4. Apparent quiescent intervals of 6000– imagery by many investigators (e.g., Harding and Berghoff, 2000; Haugerud and Harding, 8000 yr have been inferred for 2 faults marked 2001; Nelson et al., 2002; Haugerud et al., 2003; Sherrod et al., 2004a; Nelson et al., 2008; by short scarps of the central Seattle fault zone Witter et al., 2008; Blakely et al., 2009). Colored symbols mark sites with evidence of surface (Nelson et al., 2003a; this paper). The signifi - deformation (from Bucknam et al., 1992; Sherrod, 2001; Sherrod et al., 2004a; Tabor et al., cance of the apparent clustering of earthquakes 2011; Arcos, 2012): green (inverted triangles)—coseismic subsidence, purple (triangles)— on these faults in the late Holocene is unclear, coseismic uplift, blue (circle)—no movement. Faults marked by the Price Lake scarps include partly because the structural relation of the faults the Saddle Mountain East fault (SME) and Saddle Mountain West fault (SMW).