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Documentation for the 2008 update of the United States National Seismic Hazard Maps

Subduction zones produce the largest earthquakes. Over the past two decades, space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes. The short time span of modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle. Piecing together geodetic 'snapshots' from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short-term (years) and long-term (decades and centuries) viscous behaviour of the mantle. Traditional views based on elastic models, such as coseismic deformation being a mirror image of interseismic deformation, are being thoroughly revised.

/pdf/deformation-cycles-of-subduction-earthquakes-in-a-4qjxwcj2pi.pdf

Deformation cycles of subduction earthquakes in a viscoelastic Earth

[1] Interseismic deformation in Japan results from the combined effects of tectonic processes including rotation of crustal blocks and the earthquake cycle process of elastic strain accumulation about upper plate faults and subduction zone interfaces. We use spherical linear block theory constrained by geodetic observations from densely spaced Global Positioning System (GPS) stations to estimate plate motions, fault slip rates, and spatially variable interplate coupling on the Japan-Kuril, Sagami, and Nankai subduction zones. The reference model developed in this paper consists of 20 blocks, produces a mean residual velocity magnitude of 1.84 mm/yr at 950 stations, and accounts for 96% of the observed interseismic deformation signal. We estimate fault slip rates in excess of 15 mm/yr along the Niigata-Kobe Tectonic Zone and Itoigawa-Shizuoka Tectonic Line through central Japan, confirming their hypothesized roles as major tectonic boundaries. Oblique convergence across the Nankai Trough is partitioned, with 3/4 of the ∼30 mm/yr of trench-parallel motion accommodated by strike-slip motion on the subduction interface and the remaining 1/4 accommodated by right-lateral slip on the Median Tectonic Line. In contrast, our models suggest negligible slip partitioning in eastern Hokkaido, where oblique slip on the Japan-Kuril subduction interface accommodates all of the trench-parallel component of relative plate motion. Inferred spatial variations in the rake and magnitude of slip deficit on subduction zone interfaces reflect the influences of megathrust geometry and earthquake cycle processes such as enhanced elastic strain accumulation about seismic asperities and coseismic sense fault motion indicative of silent slip events or afterslip following large earthquakes.

/pdf/geodetic-imaging-of-plate-motions-slip-rates-and-14m1kjm0pb.pdf

Geodetic imaging of plate motions, slip rates, and partitioning of deformation in Japan

/pdf/quaternary-magmatism-in-the-cascades-geologic-perspectives-3cq91jahqw.pdf

Quaternary magmatism in the Cascades : geologic perspectives

[1] Surface velocities derived from GPS observations from 1993 to 2011 at several hundred sites across the deforming northwestern United States are used to further elucidate the region's active tectonics. The new velocities reveal that the clockwise rotations, relative to North America, seen in Oregon and western Washington from earlier GPS observations, continue to the east to include the Snake River Plain of Idaho and south into the Basin and Range of northern Nevada. Regional-scale rotation is attributed to gravitationally driven extension in the Basin and Range and Pacific-North America shear transferred through the Walker Lane belt aided by potentially strong pinning below the Idaho Batholith. The large rotating section comprising eastern Oregon displays very low internal deformation rates despite seismological evidence for a thin crust, warm mantle, organized mantle flow, and elevated topography. The observed disparity between mantle and surface kinematics suggests that either little stress acts between them (low basal shear) or that the crust is strong relative to the mantle. The rotation of the Oregon block impinges on Washington across the Yakima fold-thrust belt where shortening occurs in a closing-fan style. Elastic fault locking at the Cascadia subduction zone is reevaluated using the GPS velocities and recently published uplift rates. The 18 year GPS and 80 year leveling data can both be matched with a common locking model suggesting that the locking has been stable over many decades. The rate of strain accumulation is consistent with hundreds of years between great subduction events.

Active tectonics of northwestern U.S. inferred from GPS-derived surface velocities

SUMMARY

We interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr–1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr–1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.

/pdf/fault-locking-block-rotation-and-crustal-deformation-in-the-30nmudqmct.pdf

Jesse J. Vollick

Papers

Fault locking, block rotation and crustal deformation in the Pacific Northwest