location, structure, and seismicity of the seattle fault ...€¦ · tsunami (atwater and moore,...

For permission to copy, contact [email protected]� 2002 Geological Society of America 169

GSA Bulletin; January 2002; v. 114; no. 1; p. 169–177; 6 figures.

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. WellsU.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

Craig S. WeaverU.S. Geological Survey, University of Washington, Seattle, Washington 98195, USA

Samuel Y. JohnsonU.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225, USA

ABSTRACT

A high-resolution aeromagnetic survey ofthe Puget Lowland shows details of the Se-attle fault zone, an active but largely con-cealed east-trending zone of reverse fault-ing at the southern margin of the Seattlebasin. Three elongate, east-trending mag-netic anomalies are associated with north-dipping Tertiary strata exposed in thehanging wall; the magnetic anomalies in-dicate where these strata continue beneathglacial deposits. The northernmost anoma-ly, a narrow, elongate magnetic high, pre-cisely correlates with magnetic Miocenevolcanic conglomerate. The middle anom-aly, a broad magnetic low, correlates withthick, nonmagnetic Eocene and Oligocenemarine and fluvial strata. The southernanomaly, a broad, complex magnetic high,correlates with Eocene volcanic and sedi-mentary rocks. This tripartite package ofanomalies is especially clear over Bain-bridge Island west of Seattle and over theregion east of Lake Washington. Althoughattenuated in the intervening region, thepattern can be correlated with the mappedstrike of beds following a northwest-strik-ing anticline beneath Seattle. The aeromag-netic and geologic data define three mainstrands of the Seattle fault zone identifiedin marine seismic-reflection profiles to besubparallel to mapped bedrock trends overa distance of �50 km. The locus of faultingcoincides with a diffuse zone of shallow

*E-mail: [email protected].

crustal seismicity and the region of upliftproduced by the M 7 Seattle earthquake ofA.D. 900–930.

Keywords: aeromagnetic maps, Cascadiasubduction zone, earthquakes, fault zones,Puget Sound, seismic reflection profiles.

INTRODUCTION

Earthquake hazards from shallow crustalfaults are poorly understood in most of thePacific Northwest (Yelin et al., 1994). Crustalearthquakes occur relatively infrequently andare difficult to relate to poorly mapped faults,yet geophysical surveys indicate that faultsexist in the shallow subsurface beneath manyof the densely populated regions of westernOregon and Washington (e.g., Johnson et al.,1994, 1996, 1999; Blakely et al., 1995, 2000).

Much of the Puget Lowland is covered bysurficial deposits, water, and dense vegetation,and information about crustal faults (Fig. 1)has come largely from marine seismic-reflec-tion profiling (Pratt et al., 1997; Johnson etal., 1994, 1996, 1999; Brocher et al., 2001;Calvert et al., 2001; T.M. Van Wagoner, R.S.Crosson, K.C. Creager, G. Medema, and L.Preston, 2001, personal commun.) and poten-tial-field surveys (Yount and Gower, 1991;Gower et al., 1985). To improve our under-standing of the crustal framework of this re-gion, the U.S. Geological Survey conducted ahigh-resolution aeromagnetic survey of theentire Puget Lowland region (see Appendix).By using published interpretations of high-res-

olution seismic data to delimit the near-surfacelocations of fault strands beneath the majorwaterways, the aeromagnetic data detail thelocation, length, and subsurface geometry ofthe Seattle fault zone along the entire southernmargin of the Seattle basin. This result is acritical step in improving models of crustaldeformation used to estimate earthquake haz-ards in this densely populated urban area. Tworupture models for the fault zone are formu-lated on the basis of aeromagnetic and variousgeologic and geophysical data.

GEOLOGIC SETTING

Geologic mapping (Yount and Gower,1991), seismic-reflection data (Johnson et al.,1994, 1999), and geophysical models (Pratt etal., 1997) are consistent with an interpretationthat the Seattle fault zone consists of multipleeast-trending, north-verging thrust faults. Mo-tion on the fault zone has displaced Eocenevolcanic and sedimentary bedrock northwardrelative to the deep, sediment-filled Seattle ba-sin to the north (Fig. 1) (Johnson et al., 1994).Seismic-reflection data indicate�7 km ofpost-Eocene throw across the fault zone (Prattet al., 1997; ten Brink et al., 1999). Johnsonet al. (1994) inferred that the Seattle fault zonehas been active from 40 Ma to the present andrepresents an east-trending transpressive zonetransferring strain from right-lateral faults lo-cated southeast and northwest of the Seattlefault zone (Fig. 1). Despite these large offsetsand a long history of deformation, the loca-tions of the Seattle fault zone and its various

170 Geological Society of America Bulletin, January 2002

BLAKELY et al.

Figure 1. Regional setting. Large-scale map shows isostatic residual gravity over the Se-attle basin and surrounding areas, where gravity lows reflect thick sections of basin-fillingdeposits. Faults generalized from Yount and Gower (1991) and Johnson et al. (1996).Crosshatch pattern indicates urbanized areas. S—Seattle, T—Tacoma, E—Everett, B—Bremerton. Inset: Dotted rectangle shows area of large-scale map of Figure 1; dashedrectangle indicates area of Figures 2, 3, 4, and 6.

Figure 2. (A) Aeromagnetic anomalies overthe Seattle fault zone and Seattle uplift.Letters A, B, and C indicate anomalies dis-cussed in text. Dotted line shows location ofmagnetic profile (Fig. 5). (B) Aeromagneticanomalies filtered in order to emphasizeshallow magnetic sources. Data from Awere continued upward 50 m, then sub-tracted from the original data.

M

strands have remained uncertain along most ofits length.

The frontal fault of the Seattle fault zonewas the likely source of a M 7 earthquake thatoccurred�1100 yr ago (A.D. 900–930), caus-ing tectonic uplift (Bucknam et al., 1992),landslides (Jacoby et al., 1992), and a localtsunami (Atwater and Moore, 1992). Upliftpatterns from that earthquake are consistentwith the south-side-up model for the fault.Field evidence from Bainbridge Island (Buck-nam et al., 1992) indicates that the pre-upliftshoreline at Restoration Point,�1.5 km southof Eagle Harbor, was 7 m lower than it is to-day, whereas the pre-uplift shoreline at amarsh near Winslow, on the north side of Ea-gle Harbor, was 1.5 m higher (R.C. Bucknam,2001, written communication). Thus, near Ea-gle Harbor, an active strand of the Seattle faultzone must lie within narrow spatial limits nearthe topographic surface. A similar pattern isseen on the east side of Puget Sound: At AlkiPoint, south of the frontal fault, the pre-upliftshoreline was 6 m lower than it is today (R.C.Bucknam, 2001, written commun.), whereasat West Point north of the frontal fault, thepre-uplift shoreline was 3 m higher (Atwaterand Moore, 1992). Considering that sea levelhas risen�1 m since the uplift, net motionwas dominantly south-side up both west andeast of Puget Sound.

This relatively simple thrust-fault model iscomplicated by the recent discovery of aneast-striking scarp on Bainbridge Island(Bucknam et al., 1999), referred to as the ToeJam Hill scarp. Contrary to the long-term his-tory on the Seattle fault zone, the topographicexpression along the Toe Jam Hill scarp isconsistent with a north-side-up fault, and re-cent geologic field evidence confirms this in-terpretation (Nelson et al., 1999). Moreover, aM 4.9 earthquake that occurred near Bremer-ton in 1997 had a focal mechanism also con-sistent with north-side-up movement (Weaveret al., 1999; T.M. Van Wagoner, R.S. Crosson,K.C. Creager, G. Medema, and L. Preston,2001, personal commun.), and other relocatedearthquake hypocenters throughout the area ofthe Seattle fault zone have components ofnorth-side-up motion (T.M. Van Wagoner, R.S.Crosson, K.C. Creager, G. Medema, and L.Preston, 2001, personal commun.). The impli-cations of the Toe Jam Hill fault and recentearthquakes are discussed subsequently.

The location of the deformation front andseveral thrusts in the Seattle fault zone are rea-sonably well determined in Puget Sound andother waterways by marine seismic-reflectionstudies (Pratt et al., 1997; Johnson et al., 1994,1999). Geologic mapping (Yount and Gower,1991) shows that the Tertiary strata in thehanging wall of the fault are dipping steeply

to the north, and sparse outcrops can be tracedeastward along strike for�50 km. However,the cover of young glacial deposits, water, andvegetation makes it difficult to map the pre-cise location and configuration of the Seattlefault zone between the widely spaced seismic-reflection crossings, particularly beneath thehighly 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 atshallow depth by a complex package of Eo-cene and younger volcanic and sedimentaryrocks. The contrasting magnetic properties ofthese rocks are ideal for aeromagnetic map-ping of structures in the middle and uppercrust. Along the Seattle fault zone, a distinc-tive pattern of magnetic anomalies follows theeastward trend of bedrock in the upthrownblock and reliably reflects the underlying,steeply dipping stratigraphy.

Strands of the Seattle Fault Zone

Aeromagnetic data over the southern mar-gin of the Seattle basin (Fig. 2) display a pack-age of three east-trending magnetic anomalies.From north to south, they consist of an elon-gate, narrow magnetic high, a broad magneticlow, and a complex magnetic high; their east-west extent is�50 km. The northern anomaly(anomaly A in Fig. 2) is remarkably linear andnarrow; it trends east from Dyes Inlet to PugetSound and from Lake Washington to 10 kmeast of Lake Sammamish. On Bainbridge Is-land, this anomaly directly overlies a basaltconglomerate within the Miocene fluvial de-posits of the Blakely Harbor Formation (Ful-mer, 1975), which strikes east and dips 72�–80�N. East of Lake Washington, a similaranomaly also is caused by a steeply dippingMiocene volcanic conglomerate correlativewith the Blakely Harbor Formation. Theserocks were presumably deposited in the Se-

Geological Society of America Bulletin, January 2002 171

SEATTLE FAULT ZONE, WASHINGTON


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Figure 3. Geologic compilation of the area including the Seattle fault zone and Seattle uplift, simplified from Yount and Gower (1991),Frizzell et al. (1984), and Tabor et al. (1993). Seismic-reflection interpretation from Johnson et al. (1999). Uplift data from Atwater andMoore (1992) and R.C. Bucknam (2001, written commun.); values reported as elevation of pre-uplift (or pre-subsidence) shorelinerelative to present mean highest high water (MHHW). White dots indicate magnetic contacts interpreted from aeromagnetic data anddiscussed in text. A—Alki Point, D—Duwamish River, B—Beacon Hill, W—West Seattle, M—Mercer Island, E—Elliott Bay, R—Restoration Point, EH—Eagle Harbor, DI—Dyes Inlet.

attle basin and subsequently incorporated intothe hanging wall as the Seattle fault zonepropagated northward (Johnson et al., 1999).

The central magnetic low (anomaly B inFig. 2) west of Seattle correlates with upperEocene and Oligocene sedimentary strata ofthe Blakeley Formation (Fulmer, 1975). Eastof Seattle, the anomaly overlies undifferenti-ated marine and nonmarine sedimentaryrocks, also late Eocene and Oligocene in age(Yount and Gower, 1991). These strata havevery low magnetic susceptibility, as measuredin the field with a hand-held susceptibility me-ter, and are intermittently exposed betweenBremerton and Lake Sammamish.

The southern magnetic high (anomaly C inFig. 2) consists of high-amplitude, steep-gra-dient anomalies that overlie deformed Eocenevolcanic rocks in the hanging-wall block.Magnetic anomalies at the western end arecaused by basaltic basement of the CrescentFormation, whereas to the east, the high-am-plitude anomalies are caused by Cascade-de-rived andesitic rocks of the Tukwila Forma-tion (Yount and Gower, 1991).

This tripartite package is well expressedeast and west of Seattle. The intervening re-

gion, between Alki Point and Lake Washing-ton, coincides with folds that are transverse tothe Seattle fault zone (Yount and Gower,1991). Anomalies due to geologic sources aresubdued in this region, and high-amplitudeanomalies from cultural sources add complex-ity to the magnetic pattern over Seattle. Nev-ertheless, the aeromagnetic triplet, as a pack-age, can be traced beneath Puget Sound,downtown Seattle, and Lake Washington,where the sinuous shape of the aeromagneticanomalies follows the mapped strike of beds,and both anomalies and bedding sweep arounda mapped northwest-striking anticline east ofthe Duwamish River (Fig. 3). The southwardsweep of magnetic anomalies south of Seattleis mimicked by mid- and upper-crustal seismicvelocities (Brocher et al., 2001; T.M. VanWagoner, R.S. Crosson, K.C. Creager, G.Medema, and L. Preston, 2001, personal com-mun.), indicating that this sinuous shape is afundamental aspect of the hanging wall.

Computer-picked magnetic contacts (Blake-ly and Simpson, 1986) were used to divide thehanging wall into three magnetic lithologieslocated at shallow depth (Fig. 4): a thin stripof Miocene volcanic conglomerate extending

eastward from Bremerton to beyond LakeSammamish (anomaly A), Eocene volcanicrocks to the south (anomaly C), and nonmag-netic Eocene to Oligocene sedimentary rocksin the intervening gap (anomaly B). The entirepackage dips steeply northward, presumablyoffset at depth by the Seattle fault zone. Wenote that Miocene conglomerate does not cropout in the Seattle area on Beacon Hill whereanomaly A is curved and has lower amplitude.Anomalies may be subdued in this region be-cause the conglomerate has been forced todeeper depths or has been uplifted and erodedby the folding event related to the anticline.Despite the presence of the anticline, there isstill good agreement between the geologicstrikes and the strikes of the magnetic contactsover Beacon Hill (Fig. 3). Alternatively, theMiocene conglomerate deposited in this areamay still be in the Seattle basin and not yetincorporated into the hanging-wall block. Asdiscussed subsequently, this latter interpreta-tion would imply along-strike variation in thenorthward propagation of the fault zone sincethe conglomerate was deposited in the Mio-cene (Fulmer, 1975).

The location of the Seattle fault zone and



Figure 4. Interpretation of aeromagnetic, generalized geology, and seismic-reflection data of the area including the Seattle fault zoneand Seattle uplift. Stipple patterns show interpreted magnetic terranes. Faults are indicated by purple lines, and the deformation frontby a green line. Letters A, B, and C indicate anomalies discussed in text.

its various strands must be consistent with thelong-distance continuity of structure and post-Eocene stratigraphy as indicated by the geol-ogy and magnetic data. In Figure 4, we havetried to ‘‘connect the dots’’ provided by seis-mic-reflection crossings of the fault zone inthe waterways (Johnson et al., 1999). whileusing magnetic contacts and geologic map-ping to guide us in the intervening areas. Forexample, the shape and continuity of anomalyA between Dyes Inlet and Alki Point showsthat the Miocene conglomerate is steeply dip-ping and continuous in this area. Thus, east-trending faults observed in marine seismic-re-flection crossings immediately north and southof the conglomerate in Dyes Inlet, west ofBainbridge Island, and in Puget Sound, cannotsignificantly offset the conglomerate, and wepresume, therefore, that these faults have sim-ilar continuity between Dyes Inlet and AlkiPoint.

The deformation front, or hinge line, whereSeattle basin strata are dragged upward by thefrontal thrust fault, lies north of the steeplydipping magnetic conglomerate in the up-thrown block. The frontal fault, as located bythe reflection data, lies between the deforma-

tion front and the conglomerate unit, locallycoinciding with its upper contact. The frontalfault passes through Eagle Harbor, near thenorthern limit of uplift associated with the M7 earthquake�1100 yr ago (A.D. 900–930)(Bucknam et al., 1992). The Blakely Harborfault lies south of the conglomerate unit andlocally coincides with its southern contact.The Orchard Point fault follows the contactbetween Tertiary sediments and an upliftedblock of volcanic rocks to the south.

Is the Seattle Fault Segmented?

There is evidence from high-resolution seis-mic-reflection data (Johnson et al., 1999) fortwo north-trending tear faults in Puget Soundthat offset the Seattle fault zone by severalkilometers. According to Johnson et al.(1999), the tear faults cross the Miocene con-glomerate between Bainbridge Island and AlkiPoint. Significant lateral displacement on ei-ther fault should be evident as lateral displace-ments of anomaly A. We have noted twopoints along anomaly A where right-lateraloffsets are permissible (Fig. 4), but the strongcontinuity of anomaly A (Fig. 2) limits the

amount of offset on any crosscutting faults tono more than�1 km at this location.

The subdued nature of anomaly A beneathSeattle may relate directly to the easternmostof the proposed tear faults. The frontal faultwest of Seattle has propagated northwardsince deposition of the Miocene conglomerate,incorporating the conglomerate into the hang-ing wall in the process. The frontal fault be-neath Seattle, however, may not have propa-gated northward to the same degree, leavingthe conglomerate still deep within the Seattlebasin. The easternmost tear fault may mark adiscordance in fault propagation.

The possibility of north-south tear faults,the mapped anticline along the DuwamishRiver, the sinuous nature of the magneticanomalies between Alki Point and Lake Sam-mamish, and the subdued nature of anomalyA beneath Seattle all imply a degree of along-strike variation in the Seattle fault zone. Takenas a whole, we do not consider this along-strike variation to fundamentally segment thefrontal fault. The M 7 earthquake that oc-curred�1100 yr ago on the frontal fault up-lifted the hanging wall by 7 m both east andwest of the tear faults. This defining event in-


BLAKELY et al.

dicates in dramatic fashion that segmentationalong the Seattle fault zone does not precludethe possibility of large earthquakes in thefuture.

We also note a narrow anomaly extendingeastward from Elliot Bay to northern MercerIsland (Fig. 2). Part of this anomaly overliesthe Interstate-90 bridge over Lake Washingtonand may be caused by the bridge itself, al-though other bridges and freeways in the areado not produce similar anomalies. Harding etal. (1988) noted a change in the dip of seismicreflectors imaged in multichannel seismic-re-flection data along the west side of MercerIsland and interpreted the change in dip as in-dicating a possible Holocene fault located 200m south of the bridge. Recent examination ofthese data indicates that vertical offset of thesereflectors could not be greater than�10 m (T.Pratt, 2000, oral commun.). We show the aero-magnetic anomaly as a queried fault in Figure4, but additional field investigations are need-ed to evaluate this possibility.

SEISMICITY AND FAULT GEOMETRY

On June 23, 1997, a M 4.9 earthquake oc-curred�12 km west of downtown Seattle, justwest of Bainbridge Island (Weaver et al.,1999; T.M. Van Wagoner, R.S. Crosson, K.C.Creager, G. Medema, and L. Preston, 2001,personal commun.). The earthquake occurreddirectly beneath the inferred surface trace ofthe Blakely Harbor fault (Fig. 4). The depthof the main shock was initially estimated at 7km (Weaver et al., 1999), but recent hypocen-ter relocations by T.M. Van Wagoner, R.S.Crosson, K.C. Creager, G. Medema, and L.Preston (2001, personal commun.) indicate adepth of 11.5 km. The focal mechanism (Fig.4) indicated either north-side-up motion on anapproximately vertical east-trending surface,or northward displacement on a nearly hori-zontal surface. Aftershocks followed abovethe main-shock hypocenter, suggesting thatthe former solution is more likely, but neithersolution is consistent with the long-term his-tory of the Seattle fault zone, which wouldpredict south-side-up motion on the frontalfault.

However, the north-side-up solution for the1997 earthquake was consistent with the mor-phology of a Holocene fault scarp visible ona lidar image of Bainbridge Island (Bucknamet al., 1999; Nelson et al., 1999) (Figs. 3 and4), which shows up-to-the-north morphologyand lies approximately within the surface pro-jection of the preferred plane of the earth-quake focal mechanism. The Toe Jam Hillscarp, as it is referred to, is�2 km in length,

ranges from 1.5 to 10 m in height (Bucknamet al., 1999), and is parallel to the BlakelyHarbor fault. The north-side-up morphologyof the Toe Jam Hill fault scarp is consistentwith formation along a back thrust (Nelson etal., 1999), possibly the southern margin of a‘‘pop-up’’ formed by bending of the hanging-wall block as it rides up the frontal fault froma deeper detachment.

Taken together, the Bainbridge Island earth-quake sequence and the Toe Jam Hill scarpsuggest complexity in the spatial relationshipbetween the frontal fault, the Blakely Harborfault, and earthquakes responsible for the Ho-locene scarp. We have modeled magneticanomalies along the east shore of BainbridgeIsland to help determine the geometry of thesefaults (Fig. 5). Recent interpretations of seis-mic-reflection data (Pratt et al., 1997; tenBrink et al., 1999; Johnson et al., 1999;Brocher et al., 2001; Calvert et al., 2001) fromthis part of Puget Sound all agree that strandsof the Seattle fault zone dip southward, withdips ranging from 40� (ten Brink et al., 1999)to 60� (Johnson et al., 1999; Calvert et al.,2001). This range of southward dip was usedto set dip limits on all north-verging faults inFigure 5. The volcanic conglomerate was as-signed a dip of 74�N and an up-section thick-ness of 1 km on the basis of geologic fieldmapping (Yount and Gower, 1991). The con-glomerate was also assigned a magnetizationof 0.8 A/m, the maximum allowable accordingto 35 field measurements using a hand-heldsusceptibility meter. The Crescent Formationwas assigned a magnetization of 1.7 A/m,consistent with published values for CoastRange basalt elsewhere (0.1–3.5 A/m fromBromery and Snavely [1964], 2.75 A/m fromFinn [1990], and 3 A/m from Pratt et al.[1997]).

Figures 5A and 5B show the observed mag-netic anomaly and a permissible magneticmodel, respectively. Figures 5C and 5D showtwo interpretations based on the magneticmodel. Both include three thrust faults, in ac-cordance with Figure 4, and predict that thefrontal fault has offset the Miocene conglom-erate�1 km at a depth of�1.5 km. The Eo-cene Crescent Formation is multiply offset,with a total vertical throw of�7 km at thislocation, in agreement with seismic interpre-tations (ten Brink et al., 1999; Pratt et al.,1997). In both interpretations, the 1997 Bain-bridge Island earthquake occurred in the foot-wall of the frontal fault.

The length and height of the Holocenescarp suggest that the earthquake(s) thatcaused the scarp would have been larger thanM 6 (Hemphill-Haley and Weldon, 1999) and,

if the rupture surface were near vertical, itmay have ruptured into the footwall of thefrontal fault. The spatial relationship andshared sense of motion between the HoloceneToe Jam Hill scarp and the 1997 BainbridgeIsland earthquakes support an argument that asteeply dipping fault system might have beenresponsible for both the scarp and the earth-quake (Fig. 5C). The 1997 earthquake, how-ever, was only M 4.9, and the resulting rupturesurface probably did not reach the topographicsurface and may not have cut the frontal fault.Kinematically relating the Toe Jam Hill scarpto the 1997 earthquake remains problematic.

Alternatively, the Toe Jam Hill scarp on Bain-bridge Island may represent a back thrust to themain strand of the Seattle fault zone (Fig. 5D).In this model, the scarp reflects movement onthe frontal fault and is related to the long-termslip rate of the Seattle fault zone. This modelimplies that the 1997 Bainbridge Island earth-quake and aftershocks are not directly related tothe Toe Jam Hill scarp but rather occurred withinthe lower plate of the frontal fault, perhaps on afault stressed by the advancing wedge above thethrust sheet (Fig. 5D).

The focal mechanism of the 1997 Bain-bridge Island earthquake is also consistentwith a nearly horizontal plane of slip, with theupper block having moved northward relativeto the lower block. The zone of aftershocksabove the main shock may reflect deformationalong an active axial surface in the bending,overriding block, in the manner described byShaw and Suppe (1996) for blind thrusts inthe Los Angeles basin.

The first model (Fig. 5C) has the advan-tage of kinematically linking the Toe JamHill scarp to the 1997 Bainbridge Islandearthquakes, whereas the second (Fig. 5D)has the potential to quantitatively relate pa-leoseismic slip rates to crustal structure ofthe Seattle fault zone. The different tectonicmodels describe possible fault rupture di-mensions and kinematic links with importantimplications for evaluating the location, po-tential magnitude, and recurrence rates ofmoderate to large earthquakes on the Seattlefault zone.

The 1997 Bainbridge Island earthquakewas representative of a general pattern ofcrustal earthquakes within the Seattle faultzone. Figure 6A shows epicenters of upper-plate earthquakes of M 1.5 or larger occur-ring in the vicinity of the Seattle fault zonesince January 1980. Figure 6B shows hypo-centers of selected earthquakes projectedonto a vertical plane that lies normal to thefault zone, taking into account the sinuoustrace of the fault zone. Most earthquakes in



Figure 5. Magnetic model of the Seattlefault zone. (A) Calculated and observedmagnetic profiles. See Figure 2A for profilelocation. Profile extends north-south alongthe east shore of Bainbridge Island (see Fig.4 for location). A, B, and C indicate anom-alies discussed in text. (B) Magnetic model.

Sources are assumed to be infinitely extended in the east and west directions. Dip of frontalfault (50�S) determined from seismic-reflection data (ten Brink et al., 1999; Johnson etal., 1999). Dip (74�N) of Miocene conglomerate (labeled by its magnetization M � 0.8 A/m) from geologic mapping (Yount and Gower, 1991). The Crescent Formation is the unitwith a magnetization of 1.7 A/m. (C) Interpretation favoring a structural connection be-tween Bainbridge Island earthquakes and Holocene scarp. OPF—Orchard Point fault,BHF—Blakely Harbor fault, FF—frontal fault, S—Holocene scarp seen in lidar topo-graphic data. (D) Interpretation favoring the Holocene scarp as a back thrust to the mainthrust sheet. First-motion solution and earthquake locations from Weaver et al. (1999).

this part of the Puget Sound occur deeperthan�15 km (emphasized by the dashed linein Fig. 6B). On the basis of a larger set ofrelocated hypocenters, T.M. Van Wagoner,R.S. Crosson, K.C. Creager, G. Medema, andL. Preston (2001, personal commun.) foundthat nearly 60% of hypocenters fall between15 and 25 km, with a mean depth of 17.6km, placing most crustal earthquakes in theSeattle region within Crescent Formationbasement. Within the Seattle fault zone, hy-pocenters depart significantly from this depthrange, forming a near-vertical zone that ex-tends well above the 15 km depth to near thetopographic surface. These shallow earth-quakes do not lie along the thrust fault pre-dicted from our magnetic model, nor alongthrust faults described in earlier studies (e.g.,Johnson et al., 1994; Pratt et al., 1997;Brocher et al., 2001). This recent seismic ac-tivity may reflect one or more basin-loadingfaults stressed by the advancing wedge of thethrust sheet, or deformation within an upperplate that moves along a nearly horizontalplane of slip. In any case, the pattern of re-cent seismicity appears to be a departurefrom the long-term deformational history ofthe Seattle fault zone.

SUMMARY

New, high-resolution aeromagnetic dataprovide constraints on the location, length,and geometry of the Seattle fault zone. Thecorrelation of aeromagnetic anomalies withtilted upthrown-block stratigraphy definesthe location of the Seattle fault zone withinnarrow limits over a distance of 50 km.Thus determined, the fault zone on Bain-bridge Island coincides with recent seismic-ity, a postglacial fault scarp, and the M 7earthquake that occurred�1100 yr ago(A.D. 900–930) on the Seattle fault zone.The details of the relationship betweenearthquake sources in the footwall and thosein the hanging wall and the role of the fron-tal fault at depth remains to be resolved, aswell as the kinematic relationships amongcurrent earthquakes, paleoseismic evidence,and fault geometry.

APPENDIX

The aeromagnetic survey (Blakely et al., 1999) wasflown along north-south lines spaced 400 m apart andalong east-west control lines spaced 8 km apart. Flightaltitude was 250 m above terrain, or as low as per-mitted by safety considerations. A theoretical flightsurface, based on a digital topographic model, wascomputed in advance of the survey, and real-time, dif-ferentially corrected Global Positioning System (GPS)navigation was used during flight to maintain the de-


BLAKELY et al.

Figure 6. Upper-plateearthquakes of the Seat-tle fault zone. (A) Epicen-ters of M 1.5 and largerearthquakes occurringsince January 1980 areshown by circles, wheresize of circle indicates rel-ative magnitude. Dottedpurple line shows pre-dicted traces of the Seat-tle fault zone, taken fromFigure 4. (B) Cross sec-tion across the Seattlefault zone. Earthquakeswithin pink box of Figure6A projected parallel tofault zone. S—Holocenescarp seen in lidar topo-graphic data, FF—fron-tal fault (see also Fig. 5,C and D, for relationshipto other faults). Thedashed line at 15 km em-phasizes the fact thatmost hypocenters are be-low this depth.

sired flight surface. Two ground-based magnetometerswere used to monitor and correct for time-varyingmagnetic fields. Total-field anomalies were computedbased on the International Geomagnetic ReferenceField updated to the date of the survey.

ACKNOWLEDGMENTS

We are grateful to Ralph Haugerud, Tom Pratt,Brian Sherrod, Derek Booth, Kathy Troost, TomBrocher, and Uri ten Brink for discussions that

helped formulate many of the ideas in this paper.Early reviews by Bob Jachens and Tom Brocher andlater reviews by Bob Crosson, Silvio Pezzopane,Peter La Femina, and Chuck Connor were particu-larly helpful.



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