Relative Plate Motion Research Articles

Cretaceous‐Tertiary tectonic history of the Fiordland margin, New Zealand

Integration of regional seismic reflection data with swath bathymetry and imagery data and onshore geology has revealed the complex sequence of tectonic events that have affected the development of the offshore Fiordland margin. There are no core or dredge samples to constrain the timing of these events, but comparison with recent work on the Tertiary evolution of the Australia‐Pacific plate margin provided a temporal framework for the subsurface interpretation. The integrated dataset allows the prediction of the age and likely lithology of the seismic sequences, the timing of tectonic events, and details of how structures along the Fiordland plate margin developed in response to changes in relative plate motion direction. We conclude that Cretaceous rifting in the Tasman Sea formed half‐grabens along Caswell High and beneath the Fiordland Basin. Up to several kilometres of Cretaceous to Early Eocene sediments, probably terrestrial and marginal marine sediments derived from the Campbell Plateau, are preserved in these grabens. A second phase of rifting in the Eocene, associated with the formation of the Southeast Tasman Sea, resulted in the separation of the Caswell High and Fiordland Basin block from the Campbell Plateau and the start of its journey northwest, northeast, and finally east to its present position adjacent to Fiordland. The rift sediments are overlain by a relatively uniform sequence, probably Late Eocene to Mid Miocene deep‐water carbonates. Strike‐slip faults developed along the continental shelf and slope in the Miocene, and increasing margin‐normal shortening in the Pliocene‐Pleistocene, led to the development of thrust faults along the margins of sediment lobes in the Fiordland and Milford Basins. The youngest sediments are up to several kilometres thick, probably Late Miocene‐Quaternary elastics derived from Westland and Fiordland.

Plumes in a convecting mantle: Models and observations for individual hotspots

The motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented. A new list of 44 possible hotspots with associated tracks has been compiled. For all of them, calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow. Plume source depth has usually been assumed to be the top of D”, but an alternative source depth at the 670‐km discontinuity has also been considered. Using models of relative plate motions and boundaries, hotspot tracks on plates have been calculated and compared with age data, ocean floor topography, and distribution of volcanics on continents. Absolute plate motions have been redetermined under consideration of hotspot motion, using a new least squares method. For the Hawaiian and Yellowstone hotspots, source locations and hotspot motion have been computed for a total of up to 23 different models. The results show plume conduits being tilted, with source regions at the D” moving in the lowermost mantle flow, generally toward large‐scale upwellings under southern Africa and the south central Pacific. Hotspot surface motion often represents the horizontal component of midmantle flow, which is frequently opposite to plate motion, toward ridges and away from subduction zones. In particular, almost all models tested predict southward motion of the Hawaii and Kerguelen hotspots and westward motion of the Iceland hotspot. For models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as, or better than, for the fixed hotspot model, but sometimes fixed hotspots give the best fit. In some cases where the track ends at a subduction zone, e.g., for the Bowie hotspot, results can give indications about the otherwise unknown age of the hotspot. In other cases, especially for the Tahiti hotspot, results suggest an origin shallower than D”, and in yet other cases, particularly East Africa, the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes.

Timing of eastern North American kimberlite magmatism: continental extension of the Great Meteor hotspot track?

Twenty-nine new high precision U–Pb perovskite ages for kimberlite and other CO 2-rich ultrabasic rocks, primarily from five fields or clusters in eastern North America (Rankin Inlet, Attawapiskat, Kirkland Lake, Timiskaming and Finger Lakes), indicate that at least five periods of Mesozoic kimberlite magmatism can be distinguished. These new data document four periods of kimberlite magmatism previously unrecognized in eastern North America: at 196, 180–176, 148–146 and 142–134 Ma. In addition, the detailed emplacement history of Jurassic kimberlites in the Kirkland Lake field indicates magmatism spanned a period of 13 Myr from 165 to 152 Ma with approximately half of the kimberlites in this field emplaced in less than 2 Myr between 156.9–155.3 Ma. These U–Pb results demonstrate for the first time that there is a NW–SE Triassic to Cretaceous age progression in kimberlite magmatism, which is consistent with the relative plate motions of North America during this interval. This age progression in kimberlite magmatism extends for more than 2000 km from Rankin Inlet through to the Attawapiskat, Kirkland Lake and Timiskaming fields and is interpreted, in part, to be the continental expression of magmatism linked to the Great Meteor mantle plume hotspot track. If correct, then the timing and location of this magmatism provides a more rigid constraint for both the exact position of the hotspot and the relative direction and rate of North American plate motion during the Mesozoic opening of the North Atlantic Ocean.

Geodetic constraints on the kinematics of the Alpine Fault in the southern South Island of New Zealand, using results from the Hawea‐Haast GPS Transect

Repeated GPS measurements at 21 sites along a profile extending 100 km southeast of the Alpine Fault demonstrate that active deformation occurs across much of the South Island of New Zealand and that slip on the Alpine Fault cannot accommodate all of the Pacific‐Australian plate motion. Projecting the velocity field as normal and parallel components to the Alpine Fault, shows that the fault‐parallel component of motion falls on a characteristic dislocation curve that suggests elastic strain accumulation in the upper crust. This type of deformation is associated with deep aseismic slip in the ductile regions below the brittle‐ductile transition. The derived slip rate, of greater than 20 mm/yr, is in reasonable agreement with geological observations. The best fitting locking depth is about 10 km. There is also evidence that approximately 25% of the relative plate motion is accommodated either by distributed deformation or by slip on a second structure located within the Southern Alps, approximately 80 km east of the Alpine Fault.

Spatial variations in present‐day deformation, Kenai Peninsula, Alaska, and their implications

From four years of Global Positioning System (GPS) measurements, we find significant spatial variations in present‐day deformation between the eastern and western Kenai Peninsula, Alaska. Sites in the eastern Kenai Peninsula and Prince William Sound move to the NNW relative to North America, in the direction of Pacific‐North America relative plate motion. Velocities decrease in magnitude from nearly the full plate rate in southern Prince William Sound to about 30 mm/yr at Seward and to about 5 mm/yr near Anchorage. In contrast, sites in the western Kenai Peninsula move to the SW, in a nearly trenchward direction, with a velocity of about 20 mm/yr. The data are consistent with the shallow plate interface offshore and beneath the eastern Kenai and Prince William Sound being completely locked or nearly so, with elastic strain accumulation resulting in rapid motion in the direction of relative plate motion of sites in the overriding plate. The velocities of sites in the western Kenai, along strike to the southwest, are opposite in sign with those predicted from elastic strain accumulation. These data are incompatible with a significant locked region in this segment of the plate boundary. Trenchward velocities are found also for some sites in the Anchorage area. We interpret the trenchward velocities as being caused by a continuing postseismic transient from the 1964 great Alaska earthquake. There may be significant along‐strike differences in the long‐term behavior of the plate interface between the western and eastern Kenai, based on roughly coincident boundaries in the coseismic slip distribution, cumulative postseismic uplift, present‐day plate coupling, and stress field. The present postseismic response appears to generate purely trenchward motion, suggesting a creep process that is purely dip slip. Our observations suggest that postseismic processes after the largest earthquakes can influence patterns of deformation for decades after the event.

Shear wave anisotropy beneath the Andes from the BANJO, SEDA, and PISCO experiments

We present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western South America: the Broadband Andean Joint experiment (BANJO), a 1000‐km‐long east‐west line at 20°S, and the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) and Seismic Exploration of the Deep Altiplano (SEDA), deployed several hunderd kilometers north and south of this line. We determined the splitting parameters ϕ (fast polarization direction) and δt (splitting delay time) for waves that sample the above‐ and below‐slab regions: teleseismic *KS and S, ScS waves from local deep‐focus events, as well as S waves from intermediate‐focus events that sample only the above‐slab region. All but one of the *KS stacks for the BANJO stations show E‐W fast directions with δt varying between 0.4 and 1.5 s. However, for *KS recorded at most of the SEDA and PISCO stations, and for local deep‐focus S events north and south of BANJO, there is a rotation of ϕ to a more nearly trench parallel direction. The splitting parameters for above‐slab paths, determined from events around 200 km deep to western stations, yield small delay times (≤0.3 s) and N‐S fast polarization directions. Assuming the anisotropy is limited to the top 400 km of the mantle (olivine stability field), these data suggest the following spatial distribution of anisotropy. For the above‐slab component, as one goes from east (where *KS reflects the above‐slab component) to west, ϕ changes from E‐W to N‐S, and delay times are substantially reduced. This change may mark the transition from the Brazilian craton to actively deforming (E‐W shortening) Andean mantle. We see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion. The small delay times for above‐slab paths in the west require the existence of significant, spatially varying below‐slab anisotropy to explain the *KS results. The implied anisotropic pattern below the slab is not easily explained by a simple model of slab‐entrained shear flow beneath the plate. Instead, flow induced by the retrograde motion of the slab, in combination with local structural variations, may provide a better explanation.

Transition from strike-slip faulting to oblique subduction: active tectonics at the Puysegur Margin, South New Zealand

South of New Zealand the Pacific–Australia (PAC–AUS) plate boundary runs along the intracontinental Alpine Fault, the Puysegur subduction front and the intraoceanic Puysegur Fault. The Puysegur Fault is located along Puysegur Ridge, which terminates at ca. 47°S against the continental Puysegur Bank in a complex zone of deformation called the Snares Zone. At Puysegur Trench, the Australian Plate subducts beneath Puysegur Bank and the Fiordland Massif. East of Fiordland and Puysegur Bank, the Moonlight Fault System (MFS) represents the Eocene strike-slip plate boundary. Interpretation of seafloor morphology and seismic reflection profiles acquired over Puysegur Bank and the Snares Zone allows study of the transition from intraoceanic strike-slip faulting along the Puysegur Ridge to oblique subduction at the Puysegur Trench and to better understand the genetic link between the Puysegur Fault and the MFS. Seafloor morphology is interpreted from a bathymetric dataset compiled from swath bathymetry data acquired during the 1993 Geodynz survey, and single beam echo soundings acquired by the NZ Royal Navy. The Snares Zone is the key transition zone from strike-slip faulting to subduction. It divides into three sectors, namely East, NW and SW sectors. A conspicuous 3600 m-deep trough (the Snares Trough) separates the NW and East sectors. The East sector is characterised by the NE termination of Puysegur Ridge into right-stepping en echelon ridges that accommodate a change of strike from the Puysegur Fault to the MFS. Between 48°S and 47°S, in the NW sector and the Snares Trough, a series of transpressional faults splay northwards from the Puysegur Fault. Between 49°50′S and 48°S, thrusts develop progressively at Puysegur Trench into a decollement. North of 48°S the Snares Trough develops between two splays of the Puysegur Fault, indicating superficial extension associated with the subsidence of Puysegur Ridge. Seismic reflection profiles and bathymetric maps show a series of transpressional faults that splay northwards across the Snares Fault, and terminate at the top of the Puysegur trench slope. Between ca. 48°S and 46°30′S, the relative plate motion appears to be distributed over the Puysegur subduction zone and the strike-slip faults located on the edge of the upper plate. Conversely, north of ca. 46°S, a lack of active strike-slip faulting along the MFS and across most of Puysegur Bank indicates that the subduction in the northern part of Puysegur Trench accounts for most of the oblique convergence. Hence, active transpression in the Snares fault zone indicates that the relative PAC–AUS plate motion is transferred from strike-slip faulting along the Puysegur Fault to subduction at Puysegur Trench. The progressive transition from thrusts at Puysegur Trench and strike-slip faulting at the Puysegur Fault to oblique subduction at Puysegur Trench suggests that the subduction interface progressively developed from a western shallow splay of the Puysegur Fault. It implies that the transfer fault links the subduction interface at depth. A tectonic sliver is identified between Puysegur Trench and the Puysegur Fault. Its northwards motion relative to the Pacific Plate implies that is might collide with Puysegur Bank.

Tectonostratigraphic study of the Chichibu and Shimanto Belts in the Kanto Mountains, central Japan.

On the basis of 120 radiolarian biochronologic data and detailed geologic mapping, fifteen tectonostratigraphic units have been recognized in the Southern Chichibu and Northern Shimanto Belts of the Kanto Mountains, central Japan. All units are in thrust contact with each other and are numbered from north to south as Units 1 to 15. The fifteen tectonostratigraphic units are composed of accretionary deposits which were successively accreted southward during Early Jurassic to Late Cretaceous time. Each unit in the study area primarily formed a stratigraphic succession from greenstone or limestone through chert, siliceous mudstone, and mudstone to sandstone (=oceanic plate stratigraphy). The combination of those rocks is recognized not only in coherent units but in melange units. Reconstruction of the plate stratigraphy of each unit helps our understanding of the pulse of the formation of accretionary complexes in the study area. There is a lack of Early Cretaceous accretionary complexes of the Hauterivian and the Aptian to middle Albian. The drastic changes from the formation of coherent units (Middle-Late Jurassic) through the formation of melange units (latest Jurassic-earliest Cretaceous) to the accretionary hiatus (Early Cretaceous) are recognized near the boundary between the Chichibu and Shimanto Belts. The formation process for the Jurassic to Cretaceous accretionary complexes in the Kanto Mountains is therefore divided into the following three tectonic phases : the Early to middle Late Jurassic (Phase I), late Late Jurassic to Early Cretaceous (Phase II), and Late Cretaceous (Phase III). This is possibly a record of the process and the historical changes in the ancient accretionary tectonics and the relative plate motions. Phase I (the accretion of complexes in the Chichibu Belt) and Phase III (the accretion of complexes in the Shimanto Belt) may be attributed to a difference in the two subducted oceanic plates : the former was old (Izanagi Plate) while the latter was young (Kula Plate) ; moreover, the conversion of the subduction of the different oceanic plates is likely to have caused the accretionary hiatus (Phase II) in the study area.

サハリン（樺太）島の地質と地形 サハリン最北部の地質構造からみた北米・ユーラシアプレート収束境界のテクトニクス

The region from Sakhalin in Russia to the eastern margin of the Japan Sea has been regarded as a convergent plate boundary zone between the Eurasia and the North America Plates because large earthquakes and active crustal movements are prominent in this zone. These activities along the eastern margin of the Japan Sea to southern Sakhalin are almost consistent with the expected relative plate motion deduced from the Euler pole which is estimated from the magnetic anomaly lineations in the Atlantic Ocean, but the modern tectonic aspect in northern Sakhalin is inconsistent; e.g., 1995 Neftegorsk Earthquake is one of the typical events.As a result of a structural and tectonic study in northern Sakhalin, the NE-SW compressive tectonic feature since the Late Miocene was clarified. In the northernmost area of the Schmidt Peninsula, the early Cretaceous ophiolite thrusts upon the Late Cretaceous sedimentary rocks. The ophiolite has an overturned sequence: serpentinites, gabbros, basaltic rocks, and hemipelagic sediments in descending order. The Cretaceous sediments form a map-scale and NW vergent synclinorium. All the deformation structures in outcrop scale, such as micro-folds, minor reverse and normal faults, axial plane cleavages and bedding slips, are consistent with a large scale folding. These structural relationships suggest only one event of deformation, which appears to be linked to overthrusting of the ophiolitic rocks.A structural investigation of the southeastern part of the Schmidt Peninsula revealed that the Middle Miocene sediments composed of siltstone and sandstone are folded with wavelength of several tens to several hundred meters. Their fold axes trend in the NW direction, which indicates that deformation under the NE compression is the same as the deformation in the northernmost area. The thickness of sedimentary layers are constant everywhere in folding. This fact indicates that the timing of the deformation is after sedimentation, that is, after the Middle Miocene. This deformation event is consistent with modern activity in northern Sakhalin. Therefore, the modern tectonic framework might have started in the Late Miocene time.A plate tectonic model indicates that the modern relative motion between the Eurasia and the North America Plate started at about 11 m. y. ago. The Late Miocene onset of the modern tectonic framework in northern Sakhalin occurred at almost the same time. The most reliable model to explain the discrepancy of sense of movement from the Eurasia-North America retative motion, may be the “extrusion” of the Okhotsk Block toward the Pacific Ocean. The dextral extrusion boundary in northern Sakhalin may be traced along theeastern coast of Sakhalin to the northern edge of the Kuril Basin, where active seismicityhas been observed although a detailed study of focal mechanisms and other tectonic aspects is needed in the future.

Glacial isostatic adjustment observed using very long baseline interferometry and satellite laser ranging geodesy

In global space geodetic solutions, radial site motions are usually estimated relative to the geocenter (the center of figure of the solid Earth). Most geodesists estimate the motion of the geocenter assuming both that sites do not move radially and that sites move laterally as predicted by plate motion model NUVEL‐1A [DeMets et al., 1990, 1994]. Here we estimate the motion of the geocenter assuming that the plate interiors deform radially and laterally as predicted by the postglacial rebound model of Peltier [1994] or that of Peltier [1996] without assuming a priori knowledge about relative plate motion. Radial site motions estimated relative to this rebound‐adjusted geocenter are in the same reference frame as the rebound model predictions, whereas site motions estimated without adjusting for rebound are not. We further constrain the motion of the rebound‐adjusted geocenter using satellite laser ranging's sensitivity to the center of mass (of the solid Earth, the oceans, and the atmosphere) by assuming that the mean velocity between the rebound‐adjusted geocenter and the center of mass is negligible over the time period of geodetic measurement. Twenty years of observation with satellite laser ranging and very long baseline interferometry record the isostatic response of the solid Earth to the unloading of the late Pleistocene ice sheets. The misfits of the postglacial rebound model of Peltier [1994] and that of Peltier [1996] are 34% and 16% less, respectively, than the misfit of the rigid plate model. Sites at Onsala (Sweden) and Algonquin Park (Ontario) are observed to be rising at 3 mm/yr and 2 mm/yr, respectively, reflecting unloading of the Fennoscandian and Laurentide ice sheets. Sites along the east coast of the United States are subsiding at <2 mm/yr, indicating that the forebulge produced by the Laurentide ice sheet is currently collapsing very slowly. Sites beneath the margins of the ice sheets during the last glacial maximum are currently moving laterally away from the ice sheet centers at <1.5 mm/yr, in disagreement with the moderately fast outward motion predicted by the model of Peltier [1996].

Crustal deformation during 1994–1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault

The positions of 115 ground marks in a 150 × 100 km area of oblique continental collision in the central Southern Alps, New Zealand, have been measured by Global Positioning System (GPS) two to four times between 1994 and 1998. Contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and Alpine fault. Across strike, more than 60% of the strain occurs within a band from 5 km NW to 20 km SE of the Alpine fault, but significant strain continues at least a further 60 km SE to near the edge of the Southern Alps foothills. Projections of the fault‐parallel and fault‐normal components of velocity onto an Alpine faultnormal profile show that about 85% of the NUVEL‐1A model relative plate motion is observed within the GPS network. The surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50–70% of the relative plate rate in a region deeper than about 5–8 km on the down‐dip extension of the SE dipping Alpine fault. Material shallower than this is behaving elastically and thus storing elastic strain in the region of the Alpine fault. The longer‐wavelength displacements can be modeled either as distributed deformation beneath the Southern Alps, or by localization of elastic strain around the upper end of a discrete NW dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the SE boundary of the mountains if extrapolated to the surface. Strain determined from a small‐scale survey network crossing the Alpine fault indicates no significant near‐surface aseismic fault slip on the central Alpine fault over the past 25 years. Our results are consistent with independent geological evidence that the central section of the Alpine fault is capable of producing large to great earthquakes.

Age and stratigraphic relationships of pre- and syn-rift volcanic deposits in the northern Puertecitos Volcanic Province, Baja California, Mexico

Geologic mapping of volcanic strata of the northern Puertecitos Volcanic Province (PVP) in northeastern Baja California, Mexico, performed in conjunction with 40 Ar/ 39 Ar analysis and petrochemical study, documents the Miocene geologic history of a well-preserved volcanic succession within the northern Sierra Santa Isabel and its relationship to the evolving Pacific–North America plate boundary. Subduction-related volcanic deposits, well-exposed in profile along the northern margin of the PVP in the informally named Santa Isabel Wash region, span pre-17 to 15 Ma. Minor rift-related volcanism occurred at ∼12.5 and ∼9 Ma, prior to voluminous PVP-forming volcanism at ∼6–6.5 Ma. Isochron ages typically exhibit precision (1 σ) for plagioclase of ±2–7% and for anorthoclase and sanidine of ±2–5%, and replicate analysis of an internal anorthoclase standard indicate ∼1–2% reproducibility within a given irradiation and ∼2.5% for samples irradiated separately. Improved local correlations made possible by the rich stratigraphic section preserved in Santa Isabel Wash help constrain the relationships of several widespread pyroclastic flow deposits in northeastern Baja California. These correlations are important for both paleomagnetic studies within the region and for establishing geologic ties across the Gulf of California. The combined mapping and age results imply that most extensional deformation in the study area is post-6 Ma, although some earlier faulting and the development of the pre-6 Ma Matomı́ accommodation zone are also documented. Results support a transitional plate boundary model which implies that much of the Pacific–North America relative plate motion north of Delfı́n basin (i.e., the northernmost Gulf of California) is accommodated on N- to NNW-striking faults developed during Late Miocene ENE-directed extension. The model predicts a zone of divergence east of the PVP which provides a structural mechanism for the positions and jumps of nearby Gulf of California spreading centers (Upper and Lower Tiburón and Delfı́n basins) since 6 Ma, and relates major pulses of PVP volcanism at ∼6 and ∼3 Ma to these offshore spreading center adjustments. Results also imply that most extensional deformation in Santa Isabel Wash is the result of incorporation of the PVP into the Gulf Extensional Province ∼2–3 Ma due to northwestward propagation of the Guaymas fracture zone. Rotational deformation north of the PVP may have begun contemporaneously with this adjustment along the Gulf Extensional Province rift margin.

Mechanical control of oceanic plate boundary geometry

We present a global analysis of oceanic plate boundary geometry based on the mechanics of relative plate motion at mid-ocean ridges and transform faults. If the observed geometry formed by the first-order segmentation of oceanic plate boundaries represents a state of mechanical equilibrium, we find the relative strength of spreading ridges, and their bounding transform faults to be fundamental to its organisation. A consideration of power dissipation along adjacent lengths of spreading ridge and transform fault leads to a simple relationship between spreading obliquity and relative strength. Increased spreading rate is found to decrease the strength of spreading segments relative to transform faults. Proximity to an active hotspot reduces the relative strength of spreading ridges.

Interplate coupling in the Tokai District, central Japan, deduced from continuous GPS data

I invert GPS displacement rates in the Tokai district to investigate strain accumulation associated with the anticipated Tokai earthquake. I apply a representation of interseismic strain accumulation at subduction zones by Savage [1983] and estimate a distribution of virtual normal fault slip (back‐slip) on the plate boundary surface. Estimated back‐slip takes its maximum value of 30–40mm/year under the Sea of Enshu. This value is nearly equal to the relative plate motion there, implying almost complete coupling. Back‐slip is smaller under Suruga Bay, and its direction is different from the relative motion between the Eurasian and the Philippine Sea plates. Localized deformation at the northern tip of the Philippine Sea plate may be responsible for this discrepancy, partitioning tectonic strain due to the plate convergence into faults on both sides of the Izu peninsula and also distributed strain inside the peninsula.

Seismic Hazard at the New Madrid Seismic Zone

Seismic Hazard at the New Madrid Seismic Zone

Interplate coupling in southwest Japan deduced from inversion analysis of GPS data

Recently, the Geographical Survey Institute of Japan completed the installation of a GPS continuous observation network in Japan, which has enabled us to investigate real-time crustal movements. In this study, we attempt to obtain spatial distribution of interplate coupling and relative plate motion between subducting and overriding plates in southwest Japan, using horizontal and vertical deformation rates, which were observed at 247 GPS observation stations during the period from April 6, 1996 to March 20, 1998. For this purpose, we carried out an inversion analysis of geodetic data, incorporating Akaike's Bayesian Information Criterion (ABIC). As a result, strong interplate coupling was found off Shikoku and Kumanonada regions, which corresponds well with the fault regions of the 1946 Nankai (M 8.1) and the 1944 Tonankai (M 8.0) earthquakes, respectively. We also found that interplate coupling becomes weak at depths deeper than about 30 to 40 km beneath the Shikoku and Kii peninsula. The recurrence time of great trench-type earthquakes was roughly estimated as 107 years, which is consistent with previous research. The direction of relative plate motion is oriented N53°W, which is close to the direction predicted from the plate motion model. On the other hand, a large forward slip was found in the Hyuganada region off southeast of Kyushu. Since the coseismic displacements associated with the two 1996 Hyuganada earthquakes (M 6.6, M 6.6) are removed from the GPS data, this suggests that after-slip occurred near the source region and/or that Kyushu moves southeastward stationarily due to other tectonic forces.

California Coast Range Tectonics in Light of Geophysical Constraints: A Tribute to Ben Page

The central and southern Coast Ranges, extending from the San Francisco Bay area 400 km southeastward to the Transverse Ranges, form part of a wide, soft plate boundary between the Pacific plate and the Sierra Nevada subplate, a boundary that was inherited from the Mesozoic Franciscan subduction prism. Cutting diagonally through the Coast Ranges from their west side on the north to their east side on the south, the San Andreas strike-slip fault system accommodates more than 90% of the relative plate motion; the remaining small component, convergence normal to the fault, produced Miocene—present intense folding, reverse faulting, and, finally, the present ranges. A representative crustal cross section at the latitude of San Francisco Bay displays these structures along with the geophysical evidence. Structures now juxtaposed across the San Andreas system were transported through restricting and releasing bends in the complex fault system, but regionally the central and southern Coast Ranges exhibit areal contraction of at least 20%.

Kinematics of the Pacific‐North America Plate Boundary Zone, northern California

We measured motions of 54 sites in an east‐west transect across northern California at 38°–40° north by Global Positioning System (GPS) observations over a 4 year. We estimate the total slip rate on the San Andreas fault system to be 39.6−0.6+1.5 mm/yr (68.6% upper and lower confidence intervals from a nonlinear inversion are indicated by superscripts and subscripts). Slip rates on the individual faults are determined less precisely due to the high correlations between the estimated parameters. Our best fitting model fits the fault‐parallel velocities with a mean square error of 1.04 and the following estimated fault slip rates (all in mm/yr): San Andreas 17.4−3.1+2.5, Ma'acama 13.9−2.8+4.1, and Bartlett Springs 8.2−1.9+2.1. The data are fit best by models in which the San Andreas fault is locked to 14.9−7.1+12.5 km, the Ma'acama fault locked to 13.4−4.8+7.4 km except for shallow creep in the upper 5 km, and the Bartlett Springs fault creeping at all depths. The Ma'acama fault most likely poses a significant seismic hazard, as it has a high slip rate and has accumulated a slip deficit large enough to generate a magnitude 7 earthquake. We find little evidence for contraction across Coast Ranges, except at western edge of Great Valley where 1–3 mm/yr of shortening is permitted by the data. No strain is observed within the Great Valley or Sierra Nevada except that associated with right‐lateral strike slip on the San Andreas fault system. This is consistent with models of the Pacific‐North America plate boundary zone in which the relative plate motion is partitioned into two domains, one strike‐slip and one dominantly extensional, separated by the elastically deforming Sierra Nevada‐Great Valley block.

Interplate coupling along the Nankai Trough off southwest Japan derived from GPS measurements

We investigate interplate coupling between the Philippine Sea and the Eurasian plates at the Nankai Trough off southwest Japan during the interseismic period. Plate coupling is modeled by imaginary backward normal slip superposed on the steady‐state subduction. We divide the plate interface into several segments, and estimate coupling on each segment from GPS horizontal velocity data. Estimated coupling is in general agreement with the rate of steady‐state subduction predicted from a plate motion model. This implies that current coupling at the Nankai Trough is so strong that much of the relative plate motion is taken up by shortening of the leading edge of the overriding plate. It seems that stronger interseismic coupling is realized on the segment where larger coseismic dislocation occurred, and vice versa. Time fluctuation of plate coupling from 1995 to 1996 is found on a small segment.

From rifting to drifting

Africa is splitting apart down the East African Rift. An analysis of the relative motion of the tectonic plates involved allows the rate of continental rifting to be estimated.