Direction Of Relative Plate Motion Research Articles

The Malpelo Plate Hypothesis and implications for nonclosure of the Cocos‐Nazca‐Pacific plate motion circuit

AbstractUsing global multiresolution topography, we estimate new transform‐fault azimuths along the Cocos‐Nazca plate boundary and show that the direction of relative plate motion is 3.3° ± 1.8° (95% confidence limits) clockwise of prior estimates. The new direction of Cocos‐Nazca plate motion is, moreover, 4.9° ± 2.7° (95% confidence limits) clockwise of the azimuth of the Panama transform fault. We infer that the plate east of the Panama transform fault is not the Nazca plate but instead is a microplate that we term the Malpelo plate. With the improved transform‐fault data, the nonclosure of the Nazca‐Cocos‐Pacific plate motion circuit is reduced from 15.0 mm a−1 ± 3.8 mm a−1 to 11.6 mm a−1 ± 3.8 mm a−1 (95% confidence limits). The nonclosure seems too large to be due entirely to horizontal thermal contraction of oceanic lithosphere and suggests that one or more additional plate boundaries remain to be discovered.

Late Miocene extension in coastal Sonora, México: Implications for the evolution of dextral shear in the proto-Gulf of California oblique rift

The timing, kinematics, and processes responsible for the rapid transition from subduction to oblique rifting and the localization of the Pacific-North America plate boundary in the Gulf of California are not well understood. Well exposed volcanic rocks deposited between ~15 and 10Ma in the Sierra Bacha (coastal Sonora, México) preserve a record of late Miocene deformation on the eastern rifted margin of the Gulf of California and offer new insights into the timing and kinematic evolution of oblique rifting. Detailed geologic mapping, fault kinematic analysis, U–Pb and 40Ar/39Ar geochronology, and paleomagnetic data reveal that the >2km-thick composite volcanic section is cut by a series of southwest-dipping, domino-style normal faults and uniformly tilted down-to-the-northeast. Palinspastic cross-section restoration suggests that the region experienced ca. 55–60% northeast-southwest-directed extension between ~11.7 and ~10–9Ma. Fault kinematic data reflect relatively minor dextral transtension either following or during the later stages of extension. Paleomagnetic results indicating modest clockwise vertical-axis block rotation suggest that dextral shear was concentrated in the southwest of the study area near the modern coastline.These results support an emerging model in which dextral strain was not ubiquitous across Sonora and did not initiate immediately following the ~12.5Ma transition from subduction to oblique rifting. Instead, strain east of the Baja California microplate at this latitude evolved from extension-dominated transtension prior to ~8Ma to dextral shear-dominated transtension by ~7–6Ma. The onset of dextral shear in coastal Sonora likely resulted from an increase in rift obliquity due to a change in relative plate motion direction at ~8Ma. The increase in rift obliquity and resultant onset of significant strike-slip faulting played a crucial role in facilitating subsequent plate boundary localization and marine incursion in the northern Gulf of California by ~6Ma and continental rupture at ~2–1Ma.

An approximately 9-yr-period variation in seismicity and crustal deformation near the Japan Trench and a consideration of its origin

It is well known that the statistical probability of earthquake occurrence changes over the course of a day due to periodic variations in the tidal stress acting on faults. However, periodicity on a decadal scale has been studied by relatively few researchers. It has been reported that an approximately 10-yr periodicity is observed globally for the seismicity of M-8-class large earthquakes. However, the mechanism underlying this periodicity has not yet been revealed. In this study, the decadal-scale periodicity of earthquakes along the Japan Trench is investigated. A new finding is presented that in northeast Japan, the probability of the occurrence of historical earthquakes with an M ≥ 6 that have occurred during the past 1000 yr has increased approximately every 9 yr. Periodicity becomes even more apparent for large earthquakes with an M > 7.5 and approximately half the recorded events intensively occurred within two successive years on a cycle of approximately 9 yr. This implies the presence of a periodic stress disturbance at an appreciably regular interval. The past strain and tilt observations conducted in Japan during the 1950s through the 1970s indicate that, nationwide, gradual compression repeated every 8–10 yr in the direction of relative plate motion. These compression periods are in accordance with the periods of higher seismic activity discussed above. As a first step in investigating the origin of earthquake periodicity, periods associated with lunar motion are considered. It is shown that long-term motion primarily governed by the period of the lunar perigee is synchronized with the cyclic variation in seismicity and crustal deformation described above. Decadal changes in tidal stress, as calculated using an ordinary theory of solid Earth tides, are too small to cause periodic variations in seismicity. Therefore, the conditions by which tidal stress is sufficiently amplified to trigger an earthquake are investigated. The results show that, if one assumes that a tidal force acts on a spherically asymmetric block-like upper mantle beneath the Pacific Plate, the computed phase and amplitude can explain the observations. Otherwise, it is difficult to consider direct tidal force alone as the main source of periodic variations in seismicity. Other possibilities should be considered, such as unknown interactions between the plate boundaries and the ocean/atmosphere with a period of approximately 9 yr or a resonance between the period of the tidal force and a recurrence period of slow slip events in the transition zone on the plate boundary. Apart from understanding the origin, the important fact confirmed in this study is that in some areas, the occurrence of large earthquakes, if considered as a group, appears to be strongly governed by a periodic stress disturbance rather than by completely random processes. Elucidating the wide-range approximately 9-yr mode helps us narrow a range in occurrence time in a probabilistic mid-term prediction of large interplate earthquakes.

Active faulting on the Ninetyeast Ridge and its relation to deformation of the Indo‐Australian plate

The ~4500 km long Ninetyeast Ridge (NER) in the northeastern Indian Ocean crosses a broad zone of deformation where the Indo‐Australian plate is fracturing into three smaller plates (India, Capricorn, Australia) separated by diffuse boundaries whose extents are poorly defined. New multichannel seismic reflection profiles image active faults along the entire length of the NER and show spatial changes in the style of deformation along the ridge. The northern NER (0°N–5°N) displays transpressional motion along WNW‐ESE faults. Observed fault patterns confirm strike‐slip motion at the western extent of the April 2012 Wharton Basin earthquake swarm. In the central NER (5°S–8°S), deformation on WNW‐ESE‐trending thrust faults implies nearly N‐S compression. An abrupt change in fault style occurs between 8° and 11°S, with modest, extension characterizing the southern NER (11°S–27°S). Although extension is dominant, narrow zones of faults with strike‐slip or compressional character also occur in the southern NER, suggesting a complex combination of fault motions. At all sites, active faulting is controlled by the reactivation of original, spreading‐center formed, normal faults, implying that deformation is opportunistic and focused along existing zones of weakness, even when original fault trend is oblique to the direction of relative plate motion. Observed faulting can be interpreted as India‐Australia deformation in the northern NER and Capricorn‐Australia deformation in the southern NER. The India‐Capricorn boundary is directly adjacent to the northern NER and this juxtaposition combined with a different style of faulting to the east of the NER imply that the ridge is a tectonic boundary.

How do long‐offset oceanic transforms adapt to plate motion changes? The example of the Western Pacific‐Antarctic plate boundary

Oceanic transform faults respond to changes in the direction of relative plate motion. Studies have shown that short‐offset transforms generally adjust with slight bends near the ridge axis, while long‐offset ones have a remarkably different behavior. The western Pacific‐Antarctic plate boundary highlights these differences. A set of previously unpublished seismic profiles, in combination with magnetic anomaly identifications, shows how across a former, ~1250 km long transform (the Emerald Fracture Zone), plate motion changes have produced a complex geometric readjustment. Three distinct sections are recognized along this plate boundary: an eastern section, characterized by parallel, multiple fault strand lineaments; a central section, shallower than the rest of the ridge system, overprinted by a mantle plume track; and a western section, organized in a cascade of short spreading axes/transform lineaments. This configuration was produced by changes that occurred since 30 Ma in the Australia‐Pacific relative plate motion, combined with a gradual clockwise change in Pacific‐Antarctic plate motion. These events caused extension along the former Emerald Fracture Zone, originally linking the Pacific‐Antarctic spreading ridge system with the Southeast Indian ridge. Then an intra‐transform propagating ridge started to develop in response to a ~6 Ma change in the Pacific‐Antarctic spreading direction. The close proximity of the Euler poles of rotation amplified the effects of the geometric readjustments that occurred along the transform system. This analysis shows that when a long‐offset transform older than 20 Ma is pulled apart by changes in spreading velocity vectors, it responds with the development of multiple discrete, parallel fault strands, whereas in younger lithosphere, locally modified by thermal anisotropies, tensional stresses generate an array of spreading axes offset by closely spaced transforms.

An alternative early opening scenario for the Central Atlantic Ocean

The opening of the Central Atlantic Ocean basin that separated North America from northwest Africa is well documented and assumed to have started during the Late Jurassic. However, the early evolution and the initial breakup history of Pangaea are still debated: most of the existing models are based on one or multiple ridge jumps at the Middle Jurassic leaving the oldest crust on the American side, between the East Coast Magnetic Anomaly (ECMA) and the Blake Spur Magnetic Anomaly (BSMA). According to these hypotheses, the BSMA represents the limit of the initial basin and the footprint subsequent to the ridge jump. Consequently, the evolution of the northwest African margin is widely different from the northeast American margin. However, this setting is in contradiction with the existing observations. In this paper, we propose an alternative scenario for the continental breakup and the Mesozoic spreading history of the Central Atlantic Ocean. The new model is based on an analysis of geophysical data (including new seismic lines, an interpretation of the newly compiled magnetic data, and satellite derived gravimetry) and recently published results which demonstrate that the opening of the Central Atlantic Ocean started already during the Late Sinemurian (190 Ma), based on a new identification of the African conjugate to the ECMA and on the extent of salt provinces off Morocco and Nova Scotia. The identification of an African conjugate magnetic anomaly to BSMA, the African Blake Spur Magnetic Anomaly (ABSMA), together with the significant change in basement topography, are in good agreement with that initial reconstruction. The early opening history for the Central Atlantic Ocean is described in four distinct phases. During the first 20 Myr after the initial breakup (190–170 Ma, from Late Sinemurian to early Bajocian), oceanic accretion was extremely slow (∼ 0.8 cm/y). At the time of Blake Spur (170 Ma, early Bajocian), a drastic change occurred both in the relative plate motion direction (from NNW–SSE to NW–SE) and in the spreading rate (an increase to ∼ 1.7 cm/y). After a small increase between Chron M25 (∼ 154 Ma, Kimmeridgian) and Chron M22 (∼ 150 Ma, Tithonian), the spreading rate slowed down to about 1.3 cm/y and remained fairly constant until Chron M0 (125 Ma, Barremian–Aptian boundary). In addition, kinematic reconstructions illustrate a significant spreading asymmetry during the early history of the Central Atlantic Ocean; the accretion rates were higher on the American side and led to the formation of more oceanic crust on this plate. We infer that this asymmetry could be related to the fact that the thermal anomaly responsible for the significant magmatism of the Central Atlantic Magmatic Province (CAMP) was preferentially located below the African plate.

Geodetic constraints on active tectonics of the Western Mediterranean: Implications for the kinematics and dynamics of the Nubia-Eurasia plate boundary zone

We present GPS observations in Morocco and adjacent areas of Spain from 15 continuous (CGPS) and 31 survey-mode (SGPS) sites extending from the stable part of the Nubian plate to central Spain. We determine a robust velocity field for the W Mediterranean that we use to constrain models for the Iberia-Nubia plate boundary. South of the High Atlas Mountain system, GPS motions are consistent with Nubia plate motions from prior geodetic studies. We constrain shortening in the Atlas system to <1.5 mm/yr, 95% confidence level. North of the Atlas Mountains, the GPS velocities indicate Nubia motion with respect to Eurasia, but also a component of motion normal to the direction of Nubia-Eurasia motion, consisting of southward translation of the Rif Mountains in N Morocco at rates exceeding 5 mm/yr. This southward motion appears to be directly related to Miocene opening of the Alboran Sea. The Betic Mountain system north of the Alboran Sea is characterized by WNW motion with respect to Eurasia at ∼1–2 mm/yr, paralleling Nubia-Eurasia relative motion. In addition, sites located in the Betics north of the southerly moving Rif Mountains also indicate a component of southerly motion with respect to Eurasia. We interpret this as indicating that deformation associated with Nubia-Eurasia plate motion extends into the southern Betics, but also that the Betic system may be affected by the same processes that are causing southward motion of the Rif Mountains south of the Alboran Sea. Kinematic modeling indicates that plate boundary geometries that include a boundary through the Straits of Gibraltar are most compatible with the component of motion in the direction of relative plate motion, but that two additional blocks (Alboran-Rif block, Betic Mountain block), independent of both Nubia and Eurasia are needed to account for the motions of the Rif and Betic Mountains normal to the direction of relative plate motion. We speculate that the southward motions of the Alboran-Rif and Betic blocks may be related to mantle flow, possibly induced by southward rollback of the subducted Nubian plate beneath the Alboran Sea and Rif Mountains.

Influence of rheology on realignment of mantle convective structure with plate motion after a plate reorganization

Small‐scale convection (SSC) rolls below the oceanic lithosphere have the tendency to align with the large‐scale shearing direction and thus with the plate motion direction relative to the deep mantle. Understanding the timescales of and processes responsible for realignment would contribute significantly to our understanding of the unresolved phenomena in the Pacific such as gravity lineations, small‐scale seismic velocity variations, and intraplate volcanism that cannot be explained by hot spots. In this study we examine the evolution of those convection rolls when this relative plate motion direction is suddenly changed, as suggested by the kink in the Hawaii‐Emperor seamount chain. Using three‐dimensional numerical flow models, we investigate the realignment of SSC rolls after a change in plate motion direction. From the nature of the SSC, it is expected that rheological parameters dominate the characteristics of this realignment. Our results show that this is indeed the case. We find that (1) using constraints from onset timing of SSC, realignment of rolls can occur as fast as within 20 Ma, but might also take much longer, dependent on the rheology; (2) the realignment period is strongly correlated to the sum of large‐scale shear stress induced by plate motion and small‐scale shear stress from the SSC itself; (3) in a mantle deforming by dislocation creep, realignment occurs faster than by diffusion creep, because dislocation creep SSC is more vigorous; and (4) activation energy has little influence on the realignment time. Possible evidence for the realignment period might come from precise age determination of intraplate volcanism or azimuthal seismic anisotropy.

Deformation and stress regimes in the Hellenic subduction zone from focal Mechanisms

Fault plane solutions for earthquakes in the central Hellenic arc are analysed to determine the deformation and stress regimes in the Hellenic subduction zone in the vicinity of Crete. Fault mechanisms for earthquakes recorded by various networks or contained in global catalogues are collected. In addition, 34 fault plane solutions are determined for events recorded by our own local temporary network on central Crete in 2000–2001. The entire data set of 264 source mechanisms is examined for types of faulting and spatial clustering of mechanisms. Eight regions with significantly varying characteristic types of faulting are identified of which the upper (Aegean) plate includes four. Three regions contain interplate seismicity along the Hellenic arc from west to east and all events below are identified to occur within the subducting African lithosphere. We perform stress tensor inversion to each of the subsets in order to determine the stress field. Results indicate a uniform N-NNE direction of relative plate motion between the Ionian Sea and Rhodes resulting in orthogonal convergence in the western forearc and oblique (40–50∘) subduction in the eastern forearc. There, the plate boundary migrates towards the SE resulting in left-lateral strike-slip faulting that extends to onshore Eastern Crete. N110∘E trending normal faulting in the Aegean plate at this part is in accordance with this model. Along-arc extension is observed on Western Crete. Fault plane solutions for earthquakes within the dipping African lithosphere indicate that slab pull is the dominant force within the subduction process and responsible for the roll-back of the Hellenic subduction zone.

北海道および東北日本沈み込み帯におけるS波偏向異方性とマントルウェッジ内の二次対流

We analyzed shear-wave splitting beneath Hokkaido and Tohoku, Japan. Waveforms of 912 intermediate-depth earthquakes recorded at 239 seismic stations were used, and 2276 splitting parameters, the leading shear-wave polarization direction (fast direction) and delay time between two split waves, were observed. Obtained results show that most of fast directions observed at stations in the back-arc side are perpendicular to the trench-axis, whereas those at stations in the fore-arc side are sub-parallel to it. Average delay times observed at stations in the back-arc side are 0.1-0.5 s, and those at stations in the fore-arc side are smaller (0.05-0.1s). This systematic spatial variation is observed for the whole study area, from the southern part of Tohoku to the eastern part of Hokkaido. We infer that the anisotropy caused by lattice-preferred orientation of olivine, which is probably attributable to the upwelling flow portion of the mechanically-induced convection in the mantle wedge, is a likely candidate for the shear-wave splitting in the back-arc mantle wedge. If this is the case, the present observations for the east of the arc-arc junction may indicate that the upwelling flow direction is sub-parallel not to the relative plate motion direction but to themaximum dip-direction of the subductiog slab, in contrast with that in the south of the arc-arc junction where the upwelling flow sub-parallel to the relative plate motion is expected. The anisotropy structure revealed in this study plays a crucial role on understanding ongoing mantle dynamics in subduction zones.

Shear-wave polarization anisotropy and subduction-induced flow in the mantle wedge of northeastern Japan

Shear-wave polarization anisotropy in the central part of northeastern (NE) Japan was investigated. We analyzed S phases of 293 intermediate-depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters. The obtained results show the systematic variation in splitting parameters across the arc. The leading shear-wave polarization directions (fast direction) obtained at stations in the western side of the study area are oriented nearly E–W, which is sub-parallel to the direction of relative plate motion between the Pacific plate and the North American/Okhotsk plate. In contrast, fast directions obtained at stations in the eastern side of the study area are oriented approximately N–S. Stations in the western side have a larger average delay time compared to those in the eastern side. These variations suggest that the nature of anisotropy is quite different between the eastern and the western sides. Both experimental studies on the deformation of olivine and numerical simulations of plate-driven flow predict that the lattice-preferred orientation of olivine causes the fast direction sub-parallel to the flow direction for the conditions in the mantle wedge of NE Japan. We thus infer that the lattice-preferred orientation of olivine generated by flow-induced strain is the most likely candidate for anisotropy that produces splitting with E–W fast direction. Three possible causes of anisotropy in the eastern side are considered: deformation of water-rich olivine in the mantle wedge, trench-parallel flow in the mantle wedge due to along-strike dip variations in the slab and anisotropy in the crust and the slab. We could not, however, determine which causes are dominant. Further systematic study is required to reveal the cause of anisotropy in the eastern side.

The Significance and Application of Foliation Intersection/Inflection Axes (FIA( within Porphyroblasts: A review

An increasing number of structural and metamorphic studies throughout the past sixty years have shown the importance of using porphyroblast microstructures to gain further understanding of deformation and metamorphic history and mechanisms. A recent very significant advance has been the development of techniques for the measurement of Foliation Intersection/ Inflection Axes (FIAs) preserved within porphyroblasts. FIAs provide a quantitative method of microstructural analysis that enables the direct linkage and integration of the deformation, metamorphic and tectonic history. The FIA technique determines the orientation of or the axis of curvature of a foliation effected by an overprinting event, or the intersection lineation between successive foliations, preserved as inclusion trails within porphyroblasts. Because the axis is determined completely independent of the model of formation of these included foliations, it allows such models to be thoroughly tested. The method involves three-dimensional examination of microstructures within porphyroblasts in several vertically orientated thin sections that fan around the compass and which have been cut from spatially orientated samples. FIA measurements allow reconstruction, correlation and dating of regional and large-scale deformation and metamorphic histories. Such reconstructions of the tectonic evolution had been conducted in several orogenies around the world. New insights include (1) the intimate control of deformation partitioning on metamorphic reactions, (2) a possible technique for determining the direction of relative plate motion within ancient orogens, and (3) the role of rotation versus non-rotation of porphyroblasts relative to geographical coordinates during deformation have also been raised.

次の南海トラフ巨大地震に備えて 東海地域における近年の地殻変動及び地震活動の変化に関する数値シミュレーションによる検討

Recently, Tada (1996) and Matsumura (2000) reported that crustal deformation rate and seismicity pattern have interestingly changed in the Tokai area, central Japan. We formulate recurring earthquake model in the Tokai area based on the rate- and state dependent frictional law derived from laboratory experiments of rock friction and on the location of a seismically locked region. Using the model parameters based on previous studies, we estimate the time when the next Tokai Earthquake will occur and the location of plate interface by fitting simulated calculation to observed data. Some of the results explain the recent decrease in the rate of uplift change especially at the three observational points.We find out that the mechanism of microearthquakes is well explained by the rate change of stress field. The present model shows the vertical rotation of the axes of principal stresses and predicts the precursory change of seismic activity in the region, where the distance from the assumed trench along the relative plate motion direction is about 150 km, in the depth range of 10-20 km. The present seismicity with P axis almost horizontal will become more inactive several years before the occurrence of a great earthquake, and/or the direction of P axis rotates abruptly about 80 degrees so as to be orthogonal to plate interface.

Structural controls on the continent‐ocean transition in the northern Gulf of California

In the Gulf of California the Pacific‐North America plate boundary changes character from an oceanic‐type spreading center and transform fault system (to the south) to a region of diffuse continental deformation (to the north). The presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric, heat flow, gravity, or seismic data which indicate significant differences north and south of latitude ∼30°N. We suggest instead that north of ∼30°N a continent‐ocean transition begins which we name the Wagner Transition Zone (WTZ). Diffuse deformation characterizes the WTZ where slip occurs along reactivated north to NNW striking normal faults developed during late Miocene or Pliocene ENE directed extension. Transtensional deformation varies from ENE directed extension along dip‐slip faults in the west to dextral shear along the coast to dextral‐oblique slip along inferred north to NNW striking faults submerged in the northern gulf. By accounting for rotational and extensional plate motion deformation in northeastern Baja California, vector constraints require that submerged structures accommodate ∼30 mm/yr of slip in a direction slightly clockwise of the relative plate motion direction. The juxtaposition of the discrete spreading center system in the central gulf with the diffuse WTZ appears to have been a stable configuration since 4–6 Ma, perhaps controlling the evolution of spreading center jumps between Upper and Lower Tiburón and Delfin basins due to the juxtaposition of kinematically partitioned structural domains. Different histories of prerift extension and subduction‐related arc magmatism along the length of the gulf, partly related to the migration of the Rivera triple junction, may explain the location of the continent‐ocean transition.

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.

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.

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.

Paleomagnetic evidence of localized vertical axis rotation during Neogene extension, Sierra San Fermín, northeastern Baja California, Mexico

Paleomagnetic data from Sierra San Fermín in the Gulf of California Extensional Province indicate that localized clockwise rotations about vertical axes occurred during Pliocene through Recent extension and dextral shear. Relative declination discordances in upper Miocene and Pliocene ash flow tuffs indicate a net clockwise rotation of 30°±16°. Clockwise rotation between 12.5 and 6 Ma is statistically insignificant (11°±17°). Structural observations and geochronological data suggest that rotations in this area began post‐6 Ma, comprising uniform‐sense block rotations (oblique divergence) associated with extension and dextral slip in the northwest striking boundary between the Pacific and North American plates. Northeast striking sinistral‐slip faults and north striking normal faults accommodate distributed dextral shear in this area, allowing fault blocks to rotate in a clockwise sense. A model for oblique divergence predicts ∼21 km of shear in the direction of relative plate motion and ∼20% (∼7 km) ENE directed extension, perpendicular to the Main Gulf Escarpment. A broad region of northeastern Baja California may have undergone similar distributed shear. Two possible dynamic models may explain this shear. In one model, rotation accumulates above a deep, subhorizontal, basal shear zone. Rotating blocks may extend downward to a detachment beneath the extensional province, either a low‐angle eastward continuation of the San Pedro Mártir fault or to a basal shear surface on top of a subducted remnant of the Farallon plate. Alternatively, distributed dextral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults. In either case, shear may have transferred northward onto faults west of the San Andreas fault, contributing to late Miocene to Recent clockwise rotation of the Western Transverse Ranges. This shear is not accounted for in the 300 km of dextral slip computed from cross‐gulf geologic tie points.

The 1990 Lake Tennyson earthquake sequence, Marlborough, New Zealand

Aftershocks from the 1990 Lake Tennyson earthquake (ML 5.8) recorded at nine temporary portable seismographs have been used to invert travel‐time data simultaneously for both hypocentre and velocity parameters, resulting in a 1‐D velocity model and station terms for the Lake Tennyson region. The distribution of the best relocated aftershocks outlines a main fault lineation in a ENE direction, and several off‐fault clusters. The main fault lineation is 8 km long, with a strike of about 60° and a dip that is nearly vertical. It is located between and subparallel to the Awatere and Fowler Faults, on a previously unknown fault. The mainshock has been relocated in the middle of this lineation zone, which suggests that the fault ruptured bilaterally. The distribution of aftershocks matches that expected from the Coulomb failure criterion, which identifies areas of increased and decreased stress levels due to the occurrence of the mainshock. Focal mechanisms for the mainshock and aftershocks that make up the main fault lineation are consistent with right‐lateral strike‐slip movement on this fault. Clusters that extend from each end of the main fault lineation predominantly do not have strike‐slip focal mechanisms. Aftershocks located in the clusters to the north of the main fault lineation have various thrust mechanisms with no consistent orientation. Most focal mechanisms from this sequence had their P axes closely aligned with the regional axis of compression, and the main fault lineation is consistent with the relative plate motion direction in the Lake Tennyson region.

Paleogene seafloor spreading in the southeast Tasman Sea

Magnetic anomalies 11–18 have been identified in the southeast Tasman Sea, the area of ocean crust southwest of Fiordland, New Zealand, west of the Puysegur Trench (the present plate boundary) and southeast of the older (80–58 Ma) ocean crust of the Tasman Sea basin. Structures associated with two changes in spreading direction are preserved in the area. Spreading between the Australian and Pacific plates began in the Eocene, about 40 Ma in this area, orthogonal to Cretaceous‐Paleocene spreading between them in the Tasman Sea. The boundary between the two ages of ocean crust is abrupt and associated with what we have interpreted as marginal uplift blocks and rift basins of the Resolution Ridge system. Rifting appears to have propagated northeast along a fracture zone. The northern blocks of the Resolution Ridge system may be isolated fragments of continental crust of the Campbell Plateau. Between anomalies 18 and 11 (40–30 Ma) the spreading rate was about 1.5 cm/yr. Swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11, about 30 Ma, and led to a change from tension to transcurrent motion along the plate boundary. The timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data.