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Location, Structure, and Seismicity of the Seattle Fault Zone, Washington: Evidence from Aeromagnetic Anomalies, Geologic Mapping, and Seismic-Reflection Data

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Location, Structure, and Seismicity of the Seattle Fault Zone, Washington: Evidence from Aeromagnetic Anomalies, Geologic Mapping, and Seismic-Reflection Data Location, structure, and seismicity of the Seattle fault zone, Washington: Evidence from aeromagnetic anomalies, geologic mapping, and seismic-reflection data Richard J. Blakely* Ray E. Wells U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA Craig S. Weaver U.S. Geological Survey, University of Washington, Seattle, Washington 98195, USA Samuel Y. Johnson U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225, USA ABSTRACT crustal seismicity and the region of uplift olution seismic data to delimit the near-surface produced by the M 7 Seattle earthquake of locations of fault strands beneath the major A high-resolution aeromagnetic survey of A.D. 900–930. waterways, the aeromagnetic data detail the the Puget Lowland shows details of the Se- location, length, and subsurface geometry of attle fault zone, an active but largely con- Keywords: aeromagnetic maps, Cascadia the Seattle fault zone along the entire southern cealed east-trending zone of reverse fault- subduction zone, earthquakes, fault zones, margin of the Seattle basin. This result is a ing at the southern margin of the Seattle Puget Sound, seismic reflection profiles. critical step in improving models of crustal basin. Three elongate, east-trending mag- deformation used to estimate earthquake haz- netic anomalies are associated with north- ards in this densely populated urban area. Two INTRODUCTION dipping Tertiary strata exposed in the rupture models for the fault zone are formu- hanging wall; the magnetic anomalies in- lated on the basis of aeromagnetic and various dicate where these strata continue beneath Earthquake hazards from shallow crustal geologic and geophysical data. glacial deposits. The northernmost anoma- faults are poorly understood in most of the Pacific Northwest (Yelin et al., 1994). Crustal ly, a narrow, elongate magnetic high, pre- GEOLOGIC SETTING cisely correlates with magnetic Miocene earthquakes occur relatively infrequently and volcanic conglomerate. The middle anom- are difficult to relate to poorly mapped faults, aly, a broad magnetic low, correlates with yet geophysical surveys indicate that faults Geologic mapping (Yount and Gower, thick, nonmagnetic Eocene and Oligocene exist in the shallow subsurface beneath many 1991), seismic-reflection data (Johnson et al., marine and fluvial strata. The southern of the densely populated regions of western 1994, 1999), and geophysical models (Pratt et anomaly, a broad, complex magnetic high, Oregon and Washington (e.g., Johnson et al., al., 1997) are consistent with an interpretation correlates with Eocene volcanic and sedi- 1994, 1996, 1999; Blakely et al., 1995, 2000). that the Seattle fault zone consists of multiple mentary rocks. This tripartite package of Much of the Puget Lowland is covered by east-trending, north-verging thrust faults. Mo- anomalies is especially clear over Bain- surficial deposits, water, and dense vegetation, tion on the fault zone has displaced Eocene bridge Island west of Seattle and over the and information about crustal faults (Fig. 1) volcanic and sedimentary bedrock northward region east of Lake Washington. Although has come largely from marine seismic-reflec- relative to the deep, sediment-filled Seattle ba- attenuated in the intervening region, the tion profiling (Pratt et al., 1997; Johnson et sin to the north (Fig. 1) (Johnson et al., 1994). Ͼ pattern can be correlated with the mapped al., 1994, 1996, 1999; Brocher et al., 2001; Seismic-reflection data indicate 7kmof strike of beds following a northwest-strik- Calvert et al., 2001; T.M. Van Wagoner, R.S. post-Eocene throw across the fault zone (Pratt ing anticline beneath Seattle. The aeromag- Crosson, K.C. Creager, G. Medema, and L. et al., 1997; ten Brink et al., 1999). Johnson netic and geologic data define three main Preston, 2001, personal commun.) and poten- et al. (1994) inferred that the Seattle fault zone strands of the Seattle fault zone identified tial-field surveys (Yount and Gower, 1991; has been active from 40 Ma to the present and in marine seismic-reflection profiles to be Gower et al., 1985). To improve our under- represents an east-trending transpressive zone subparallel to mapped bedrock trends over standing of the crustal framework of this re- transferring strain from right-lateral faults lo- a distance of Ͼ50 km. The locus of faulting gion, the U.S. Geological Survey conducted a cated southeast and northwest of the Seattle coincides with a diffuse zone of shallow high-resolution aeromagnetic survey of the fault zone (Fig. 1). Despite these large offsets entire Puget Lowland region (see Appendix). and a long history of deformation, the loca- *E-mail: [email protected]. By using published interpretations of high-res- tions of the Seattle fault zone and its various GSA Bulletin; January 2002; v. 114; no. 1; p. 169–177; 6 figures. For permission to copy, contact [email protected] ᭧ 2002 Geological Society of America 169 BLAKELY et al. to the north, and sparse outcrops can be traced eastward along strike for Ͼ50 km. However, the cover of young glacial deposits, water, and vegetation makes it difficult to map the pre- cise location and configuration of the Seattle fault zone between the widely spaced seismic- reflection crossings, particularly beneath the highly developed regions of Seattle, Bremer- ton, and Bellevue. For these reasons, the Se- attle area is an excellent candidate for high- resolution potential-field studies. AEROMAGNETIC INTERPRETATION The Seattle uplift (Fig. 1) is underlain at shallow depth by a complex package of Eo- cene and younger volcanic and sedimentary rocks. The contrasting magnetic properties of these rocks are ideal for aeromagnetic map- ping of structures in the middle and upper crust. Along the Seattle fault zone, a distinc- Figure 1. Regional setting. Large-scale map shows isostatic residual gravity over the Se- tive pattern of magnetic anomalies follows the attle basin and surrounding areas, where gravity lows reflect thick sections of basin-filling eastward trend of bedrock in the upthrown deposits. Faults generalized from Yount and Gower (1991) and Johnson et al. (1996). block and reliably reflects the underlying, Crosshatch pattern indicates urbanized areas. S—Seattle, T—Tacoma, E—Everett, B— steeply dipping stratigraphy. Bremerton. Inset: Dotted rectangle shows area of large-scale map of Figure 1; dashed rectangle indicates area of Figures 2, 3, 4, and 6. Strands of the Seattle Fault Zone Aeromagnetic data over the southern mar- strands have remained uncertain along most of This relatively simple thrust-fault model is gin of the Seattle basin (Fig. 2) display a pack- its length. complicated by the recent discovery of an age of three east-trending magnetic anomalies. The frontal fault of the Seattle fault zone east-striking scarp on Bainbridge Island From north to south, they consist of an elon- was the likely source of a M 7 earthquake that (Bucknam et al., 1999), referred to as the Toe gate, narrow magnetic high, a broad magnetic occurred ϳ1100 yr ago (A.D. 900–930), caus- Jam Hill scarp. Contrary to the long-term his- low, and a complex magnetic high; their east- ing tectonic uplift (Bucknam et al., 1992), tory on the Seattle fault zone, the topographic west extent is Ͼ50 km. The northern anomaly landslides (Jacoby et al., 1992), and a local expression along the Toe Jam Hill scarp is (anomaly A in Fig. 2) is remarkably linear and tsunami (Atwater and Moore, 1992). Uplift consistent with a north-side-up fault, and re- narrow; it trends east from Dyes Inlet to Puget patterns from that earthquake are consistent cent geologic field evidence confirms this in- Sound and from Lake Washington to 10 km with the south-side-up model for the fault. terpretation (Nelson et al., 1999). Moreover, a east of Lake Sammamish. On Bainbridge Is- Field evidence from Bainbridge Island (Buck- M 4.9 earthquake that occurred near Bremer- land, this anomaly directly overlies a basalt nam et al., 1992) indicates that the pre-uplift ton in 1997 had a focal mechanism also con- conglomerate within the Miocene fluvial de- shoreline at Restoration Point, ϳ1.5 km south sistent with north-side-up movement (Weaver posits of the Blakely Harbor Formation (Ful- of Eagle Harbor, was 7 m lower than it is to- et al., 1999; T.M. Van Wagoner, R.S. Crosson, mer, 1975), which strikes east and dips 72Њ– day, whereas the pre-uplift shoreline at a K.C. Creager, G. Medema, and L. Preston, 80ЊN. East of Lake Washington, a similar marsh near Winslow, on the north side of Ea- 2001, personal commun.), and other relocated anomaly also is caused by a steeply dipping gle Harbor, was 1.5 m higher (R.C. Bucknam, earthquake hypocenters throughout the area of Miocene volcanic conglomerate correlative 2001, written communication). Thus, near Ea- the Seattle fault zone have components of with the Blakely Harbor Formation. These gle Harbor, an active strand of the Seattle fault north-side-up motion (T.M. Van Wagoner, R.S. rocks were presumably deposited in the Se- zone must lie within narrow spatial limits near Crosson, K.C. Creager, G. Medema, and L. the topographic surface. A similar pattern is Preston, 2001, personal commun.). The impli- seen on the east side of Puget Sound: At Alki cations of the Toe Jam Hill fault and recent Figure 2. (A) Aeromagnetic anomalies over Point, south of the frontal fault, the pre-uplift earthquakes are discussed subsequently. the Seattle fault zone and Seattle uplift. shoreline was6mlower than it is today (R.C. The location of the deformation front and Letters A, B, and C indicate anomalies dis- Bucknam, 2001, written commun.), whereas several thrusts in the Seattle fault zone are rea- cussed in text. Dotted line shows location of at West Point north of the frontal fault, the sonably well determined in Puget Sound and magnetic profile (Fig.
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