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Rotation and Plate Locking at the Southern Cascadia Subduction Zone

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Rotation and Plate Locking at the Southern Cascadia Subduction Zone GEOPHYSICALRESEARCH LETTERS, VOL. 27, NO. 19, PAGES3117-3120, OCTOBER 1, 2000 Rotation and plate locking at the southern Cascadia subduction zone RobertMcCaffrey, 1 Maureen D. Long,1 ChrisGoldfinger, 2 Peter C. Zwick,3 JohnL. Nabelek,2 CherylK. Johnson,1 and Curt Smith4 Abstract. Global Positioning System vectors and surface using the GAMIT/GLOBK analysis software [King and tilt rates are inverted simultaneously for the rotation of west- Bock, 1999; Herring, 1998]. Site velocitiesand their covari- ern Oregon and plate locking on the southern Cascadia sub- ances were estimated by linear regression of the time series duction thrust fault. Plate locking appears to be largely of positions. Velocities were put in the North American offshore, consistent with earlier studies, and is sufficient to (NA) referenceframe by removingthe NA-ITRF96 rotation allow occasional great earthquakes inferred from geology. [DeMets and Dixon, 1999],which agrees with our solutionat Clockwise rotation of most of Oregon about a nearby pole is the mm/a level for 16 NA sites. Convergenceof the Juan de likely driven by collapse of the Basin and Range and results Fuca (JF) plate with NA is the sum of JF-Pacific [Wilson, in shortening in NW Washington State. The rotation pole 1993]and Pacific-NA [DeMetsand Dixon, 1999]poles. lies along the Olympic- Wallowa lineament and explains the The 71 horizontalGPS vectors(2 componentseach) and predominance of extension south of the pole and contraction 4 tilt rates provide 146 observations for the inversion. Misfit north of it. is defined by 1. Introduction - v-P Oblique convergenceof the young, oceanic Juan de Fuca where N is the number of observations, P is the number plate with the westernmargin of North America (Fig. 1) re- of free parameters, r• is the residual (observedminus cal- sults in both elastic deformation of coastal regions and dis- culated velocity), cr• is the formal velocity uncertainty, and tributed permanent deformation within the overriding plate. f is a scaling factor. Formal GPS velocity uncertainties are Evidence for large thrust earthquakes at the subduction well-knownto be underestimated[Mao et at., 1999]and f= 3 zoneis plentiful [Adams,1990; Atwater and Hemphill-Haley, returnsa minimumX2• • 1. The final, unscaledstandard de- 1997; Goldfingeret al., 1999] yet details of the earthquake viations of residualsare 1.2 mm/a for the north component history and potential are sketchy. Upper plate deforma- and 2.1 mm/a for the east. Velocity differencesbetween our tion, while clearly significant[Pezzopane and Weldon,1993; solutionand $ava#e et at.'s [2000]were 1.2 and 1.3 mm/a Wells et al., 1998], remainspoorly quantified. We are us- for two common sites. ing the Global PositioningSystem (GPS) to measurecrustal deformation in western Oregon from which we infer strain 3. Inversion Approach rates in the overriding plate, the increase in the potential for earthquake slip on the Cascadia subduction fault, and Our goal is to estimate the rotation of Oregon and plate how Oregon moves relative to North America. locking on the Cascadia thrust from the geodetic data. Be- cause plate locking strain extends hundreds of kilometers 2. Data and Analysis landward from the coast, the rotation pole and plate lock- ing are estimated by a simultaneous inversion. Like many We use horizontal vectors from 50 GPS sites in northwest others, we describeplate locking as the fractional part (•) Oregon analyzed by us, 21 published vectors from southern of fault area that undergoes stick-slip and is currently stuck. Oregon [Savageet al., 2000] (Fig. 2), and four tilt rates The rate at which potential seismic moment (i.e.. mo- near the Oregon coast [Reilingerand Adams, 1982]. We ment that could later be releasedin earthquakes)builds is re-processedGPS campaign data collected by the US Geo- ]l)Io= qS!•AVwhere A is fault area, V is the long-termslip logicalSurvey (1992-1994),by the CascadesVolcano Obser- rate on the fault, and/• is the shear modulus of rocks (40 vatory (1992-1997),by us (1996-1999),and by a consortium GPaused here). Geodetic data constrain g;/o because in an of local observers under direction of the National Geodetic elastic medium surface deformation rates are proportional Survey (1998). Site positionswere calculatedin the ITRF96 to •V. referenceframe [Sillard et al., 1998]by combiningcampaign To parameterize plate locking, we specify nodes every data with regional and global sites and precise satellite orbits 10 km in depth and about every 100 km along strike on the thrust fault using the fault geometry of Hyndman and Wang [1995](Fig. 3a). The convergencevector Vi at node •RensselaerPolytechnic Institute, Troy, New York i is calculated from the Juan de Fuca (JF) - western Ore- 2OregonState University,Corvallis, Oregon gon (WO) pole of rotation which is the sum of JF - North aSeafloorSurveys International, Seattle, Washington America (NA) and NA-WO poles. Plate locking, in the di- 4National GeodeticSurvey, Salem, Oregon rection of JF-WO convergence, is •i V,. Interseismic surface deformation is estimated with an elastic, half-space dislo- Copyright2000 by theAmerican Geophysical Union. cation model [Okada, 1985] by integratingover small, finite Papernumber 2000GL011768. fault patches between nodes. • at each patch is estimated 0094-8276/00/2000GL011768505.00 by bilinear interpolation between the four enclosingnodes. 3117 3118 MCCAFFREY ET AL.- ROTATION AND PLATE COUPLING IN OREGON 50*N to NA. Our preferred model includes both coupling and ro- Canada tation (Fig. 3) with 17 setsof free nodes,3 pole parameters, 49'N andgives X• - 1.07(126 degrees of freedom).All fourtilt ß ratesare fit to within onestandard error. ,.Formal errors in ß 45'N all •b are less than 0.5. eattieWashington 4TN 4. Plate Locking 46øN The final model suggestsa double locked zone - one zone Juan de Fuca offshore and one near the down-dip edge of the fault at 30 to 450N 40 km depth (Fig. 3a). We suspectthat the inland locked • i ß c• c•' zone might be an artifact for the following reason. Half- • i•, '• Oregon44'N space dislocation models treat the base of the lithosphere !:.t ß •, c•. effectively as a no-slip boundary in an elastic medium, re- ß ß 430N ß:ii ß sulting in unrealistically high resistance of the lithosphere Pacific to trench-normalcontraction [Thatcher and Rundle, 1984] 42'N and a steep dropoff in surface velocities above the downdip edge of the coupled zone. Preliminary finite-element models 41'N in which low basal stress is allowed produce a gentler de- 2•30:• 40•• km• a Nevada cay in velocities[Williams and McCaffrey, 1999] landward 20 mm/a• 400N C•ifomia of the locked zone as seen in the data. Hence, we suggest I100km • that the majority of plate locking along the southern Cas- cadia subduction zone occurs offshore, in general agreement 232'E 234'E 236'E 235'E 240'E 2420E Figure 1. Pacific Northwest showingactive volcanoes(trian- gles) and depth contoursof the top of the subducting Juan de Fuca plate (dashed lines). Large arrows show convergenceof Juan de Fuca plate with both North America (JF-NA) and the 46•N rotating Oregon block (JF-WO) at Cascadia deformation front. Ellipse in NE Oregon shows pole and l•r uncertainty for rota- tion of WO relative to NA. Small arrows show velocities and uncertainties relative to NA predicted by this pole. Large op- posing arrows show predicted sense of relative motion across the 45'N Olympic-Wallowalineament (OWL). We set •b -- 0 at the deepest nodes at 50 km depth, noting that interplate seismicity, and presumably locking, at most ,•i2 44•N subductionzones ceases at about this depth [Tichelaar and Ruff, 1993]. To increasethe sensitivityof the data for the •'3C• • .... •'•:" estimated locking parameters, in some caseswe grouped ad- jacent nodes into a single free parameter. For example, be- cause land geodetic observationsare insensitive to the near- trench locking, nodes at the trench were forced to have the ...................• Calculated same •b as adjacent nodes 10-km downdip. Model edges were handled by forcing nodes outside the GPS network to ' 100km ' 42'N have the same coupling as adjacent nodes at the edge of Relative to North America the network. Other nodes were grouped if their uncertain- •..• Observed ties were large based on initial inversionsin which all nodes 236øE 237•E 238'E 239øE 240øE were free and independent. To find parameter values that minimize misfit, we apply simulated annealing to downhill SouthOregon12ø ,.. CentralOregon12øI NorthOregon] 20 41.7*N-43.4*NI ' ".. , 43.5øN-44.9øNI ['".. 44.9*N-46.3øN{ simplexminimization [Presset al., 1989]. A penalty func- tion keeps •b between 0 and i to prevent backslip or locking at a rate faster than plate convergence. At the minimum X2•,we checkfor unconstrainedparameters and calculate the covariance matrix with singular value decomposition of linearized normal equations. Longitude,øE Longitude,•E Longitude,*E We tested a range of models by varying the parameteri- zation and constraints. With no motion of WO relative to Figure 2. (a) Observedand calculatedOregon GPS site veloci- NA and no plate coupling(no free parameters),minimum ties relative to North America (NA) with 3a ellipses.Black lines X2•- 26.1;when locking alone is allowedwithout rotation of show locations of tilt lines. Dashed lines enclose profile regions. (b-d) West-to-East profilesof the North (open) and East (closed) Oregon(25 freeparameters), X2• = 7.2;and rotation with- componentsof the GPS vectors relative to NA (3(r error bars). out platelocking (3 freeparameters) gives X2• - 2.5. Hence, Curves show predictions of rotation - locking model. Triangles most of the data are explained by rotation of WO relative show where profiles cross volcanic arc. MCCAFFREY ET AL.- ROTATION AND PLATE COUPLING IN OREGON 3119 , 40• the largestearthquake [Rundle, 1989], is approximately1-/3 [McCaffrey,1997].
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