Rate Of Plate Motion Research Articles

Dislocation model for crustal deformation in the Longitudinal Valley area, Eastern Taiwan

Three trilateration and leveling networks traversing the Longitudinal Valley, eastern Taiwan have been surveyed frequently during 1983–1988. Average rates of length change on each line and height difference on each leveling section are used to study the crustal deformation of this area. We model the aseismic deformation near the fault as the sum of a steady rigid block motion and the effect of an upper locked crust due to frictional resistance. The effect of the locked part is equivalent to that of a negative dislocation on the fault. For the northernmost Hualien and central Yuli networks, we simply assume two blocks and one fault patch, while the southernmost Taitung network is modeled by three blocks and three fault patches. The fault aforementioned is presumed to be the main trace of the Longitudinal Valley fault (LVF). We invert the geodetic data to determine the block motion and dislocation model parameters by utilizing a Bayesian nonlinear inversion method. Parameters to be estimated include the three components of velocity for each block and the vertical and horizontal dislocation rates, dip angle, and width of each rectangular fault patch. The estimated block motion parameters and fault widths are mostly well resolved by the observed data. The fault widths are all very shallow (< 3 km). This implies the Longitudinal Valley fault is weakly locked. The blocks on both sides of the fault may be essentially sliding freely. The Pinanshan Conglomerate seems to be situated in an isolated block which moves at a velocity of 26 mm/yr in the direction N109°W relative to the Central Range. The net horizontal motion across the Longitudinal Valley changes from 34 mm/yr in N46°W at Taitung network to 25 mm/yr in N-S direction at Hualien network with intermediate value of 33 mm/yr in N27°W at Yuli network. It indicates that due to resistance of the Central Range the northwestward moving direction of the Coastal Range systematically changes to northward moving. About half the rate of plate motion in this area estimated by Seno ( Tectonophysics, Vol. 42) is concentrated on a quite narrow zone several kilometers wide centered at the LVF. However, due to the limited aperture of our geodetic networks, the total width of the deformation zone and the velocity of relative plate motion at this active collision boundary are not resolved. The new GPS satellite survey technique can be used to solve this ambiguity.

The mechanics of the South Iceland Seismic Zone

The South Iceland Seismic Zone appears to act as a transform fault between offset sections of the Mid‐Atlantic Ridge. However, there is no morphological expression of a throughgoing transform between the ridges. The anticipated throughgoing east‐west transform is represented by a series of north‐south strike‐slip faults. We study the mechanics of the South Iceland Seismic Zone using boundary element methods. First, we examine the implications of the differences between the geometry of an orthogonal ridge‐transform‐ridge system and the geometry of the structures observed in Iceland. Then we consider the role of the north‐south faults in accommodating transform motion in an east‐west direction. We conclude that the north‐south faults can accommodate deformation if they are longer or more spatially frequent than mapped. The South Iceland Seismic Zone is subject to sequences of large earthquakes every 45–112 years. In comparing the seismic moment release derived from earthquake magnitudes with that predicted by our models, we confirm that the system of north‐south faults can act as a transform fault. The seismic moment release derived from the earthquake magnitudes is 1.4 times greater than that predicted by our models. If our application of the moment‐magnitude relation is correct, the above could imply that the brittle layer extends to 14 km (rather than the 10 km assumed in our models). Other possibilities are that the magnitudes of the earthquakes have been slightly overestimated or that the historical seismic record documents a period of seismicity greater than that expected from rates of plate motion inferred from geological and Very Long Baseline Interferometry (VLBI).

Equivalent strike‐slip earthquake cycles in half‐space and lithosphere‐asthenosphere earth models

By virtue of the images used in the dislocation solution, the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike‐slip fault in an Earth model consisting of an elastic plate (lithosphere) overlying a viscoelastic half‐space (asthenosphere) can be duplicated by prescribed slip on a vertical fault embedded in an elastic half‐space. For the case in which each earthquake ruptures the entire lithosphere (thickness H), the half‐space equivalent slip rate is as follows: Depth interval 0‐H, slip identical to that in lithosphere‐asthenosphere model (i.e., abrupt coseismic slip and no subsequent slip); depth interval (2n−1)H to (2n+1)H (n = 1,2,…), slip rate uniform in space and dependent upon time as Fn(t) exp (−t/τa) where Fn is a (n ‐ 1) degree polynomial in t, τa is twice the asthenosphere relaxation time (η/μ), and t is measured from the instant after the preceding earthquake. The slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion. For reasonable values of τa, slip rates below 5H do not vary much from that mean value and can be treated as constant. Thus the surface deformation due to the earthquake cycle in the lithosphere‐asthenosphere model can be calculated very simply from the half‐space model with time‐dependent slip in the two depth intervals H−3H and 3H−5H, and uniform slip at a rate equal to the secular relative plate velocity below depth 5H. Inversion of 1973–1988 geodetic measurements of deformation across the segment of the San Andreas fault in the Transverse Ranges north of Los Angeles for the half‐space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm/yr below 30 km. One equivalent lithosphere‐asthenosphere model would have a 30‐km thick lithosphere and an asthenosphere relaxation time greater than 33 years, but other models are possible.

Seismic potential of the Queen Charlotte‐Alaska‐Aleutian Seismic Zone

The 5000 km long Queen Charlotte‐Alaska‐Aleutian seismic zone is subdivided into 17 unequally sized segments. Their boundaries are delineated based on the prior distribution of large and great earthquakes. The 17 segments are chosen to represent areas likely to be ruptured by “characteristic” earthquakes. This term usually implies repeated breakage of a plate boundary segment by either a large or great earthquake, whose source dimensions remain consistent from cycle to cycle. This definition does not exclude the possibility that occasionally adjacent characteristic earthquake segments may break together in a single “giant” event that is larger than the characteristic size outlined. Conversely, a segment can also sometimes break in a series of smaller ruptures. Formal computations of the conditional probabilities for future large and great earthquakes in the 17 segments of the Queen Charlotte‐Alaska‐Aleutian seismic zone are based on the following data sets and findings: (1) recurrence intervals from historic and geologic data; (2) direct recurrence time estimates based on rates of relative plate motion and the size or displacement of the most recent characteristic event in each segment; and (3) the application of a lognormal distribution of recurrence times for large and great earthquakes. Results of these computations indicate seven areas that have high (i.e., ≥60%) conditional probabilities for the recurrence of either large or great earthquakes within the next 20 years (1988–2008). These areas include Cape St. James, Yakataga, the Shumagin Islands, Unimak Island, and the Fox, Delarof, and Near Islands segments of the Aleutian arc. When a shorter time interval is considered (1988–1998), those segments more likely to rupture in large (MS 7–7.7) rather than great earthquakes have a high conditional probability. These areas include the Unimak, Fox, and Delarof Islands segments. The largest uncertainties in these forecasts stem from the short historic record (providing a single recurrence time estimate for some segments, or widely varying estimates for others); from the unknown importance of aseismic slip; and from a vague definition of “characteristic” earthquake size. In fact, characteristic earthquake size may not be a time‐invariant quantity.

Central and South America GPS geodesy ‐ CASA Uno

In January 1988, scientists from over 25 organizations in 13 countries and territories cooperated in the largest Global Positioning System (GPS) campaign in the world to date (Table 1). 43 GPS receivers collected approximately 590 station‐days of data in American Samoa, Australia, Canada, Colombia, Costa Rica, Ecuador, New Zealand, Norway, Panama, Sweden, United States, West Germany, and Venezuela. The experiment was entitled CASA UNO, an acronym for Central and South America – and uno is the Spanish word for one, designating first epoch measurements. The CASA UNO experiment was the first civilian effort implementing a global GPS satellite tracking network.Scientific goals of the project include measurement of strain in the northern Andes, measurement of subduction rates for the Cocos and Nazca plates beneath Central and South America, and measurement of relative motion between the Caribbean plate and South America. A second set of measurements are planned in 1991 (CASA DOS), and should provide preliminary estimates of crustal deformation and plate motion rates in the region. The CASA series of experiments are intended to be carried out over at least one decade.

Seismic and volcanic activities and aseismic movements as plate motion components in the Aegean area

Slip rates varying between 0.22 and 2.18 cm yr −1 are determined from seismicity in several segments of the Aegean fore-arc and back-arc regions. The rate along the main zone of plate interaction is about 0.5 cm yr −1 while aseismic slip rate, equal to 2.5 or 0.5 cm yr −1 depending on the plate motion rate assumed, indicates that at least 50% of the plate motion is accommodated by creep. These values, with respect to the rate of volcanic eruption (= 0.13 eruptions per year/1000 km of arc length) reinforce the suggestion that active volcanism in subduction zones is related to aseismic subduction. The eruption rate is, however, anomalously low with respect to the degree of plate decoupling (= 0.83 or 0.5) implying that volcanism is not necessarily related to the degree of decoupling and suggesting that the high aseismic slip component of the plate motion is taken up by subcrustal seismicity. A tentative hypothesis is made that the Hellenic subduction zone is of intermediate type between the Marianas and Chile subduction types.

Sedimentary effects of interplay between the Kuroshio Extension and Pacific plate motion

Several apparent sediment drifts of Tertiary age occur in the central northwest Pacific, and some form east-trending bands of thick sediment north and south of the axis of a major near-surface current, the Kuroshio Extension. The intervening area of thin sediment is located directly below the present axis of the Kuroshio Extension. These spatial relationships suggest that the zone of condensed sediment section and the adjacent sediment drifts are causally related to the Kuroshio Extension. The condensed section probably results from the partial insulation of the sea floor from the normal pelagicrain due to the high-energy portion of the Kuroshio and from the erosion of the sea floor by benthic storms associated with the Kuroshio. The bordering sediment drifts may represent the preferential deposition of fine sediments transferred from the high-energy to the low-energy portion of the water column. The interplay between the Kuroshio Extension and the northwestward drift of the Pacific plate should result in predictable variations in the timing and locations of pronounced sedimentation-rate changes. Such variations will depend both on the intensity and spatial stability of the Kuroshio, as well as direction and rate of Pacific plate motion. If the Kuroshio has remained spatially stable and active for at least several million years, then sites presently north of the Kuroshio should exhibit two periods of increased sedimentation. These periods represent the times when the site came into the influence of the high-sedimentation zones, first south and later north, of the Kuroshio. There should also be a diachronous unconformity in areas north of the Kuroshio that separates the two periods of increased sedimentation and represents the effect of passage of the plate directly beneath the Kuroshio. We conclude that some regional unconformities and zones of anomalously high sediment accumulation both in the northwest Pacific and elsewhere may arise largely independent of regional variations in bottom-water flow induced by climate change.

A test of alternative Caribbean Plate relative motion models

Two contrasting models have been proposed to describe the present‐day motions between the Caribbean Plate and neighboring plates. One model, based on both North America‐Caribbean and Cocos‐Caribbean data, assumes that North America‐Caribbean motion is reflected by the spreading rate (approximately 2 cm/yr) inferred from magnetic anomalies at the Cayman Spreading Center and the azimuths of nearby transforms (Jordan, 1975). This geometry was used by Minster and Jordan (1978) in deriving global plate motion model RM2. The other model, based only on North America‐Caribbean data, uses rates and azimuths inferred from the geometry of the Lesser Antilles Wadati‐Benioff zone (Sykes et al., 1982). The Cayman Spreading Center data were discounted assuming that they underestimate the full North America‐Caribbean motion owing to the complex tectonics of the Greater Antilles. The latter model assumes plate convergence at the Lesser Antilles at a rate about twice as fast as, and an azimuth differing by 25° from, the Jordan model. The two models also differ in their predictions of South America‐Caribbean motion; the Jordan model predicts oblique convergence whereas Sykes et al. predict oblique divergence. We use the NUVEL‐1 global relative motion data set, which incorporates recent data not used in earlier studies, to discriminate between the alternative models. We find that the Jordan geometry provides a better fit to the direction of North America‐Caribbean motion. Moreover, we find that models based on this geometry better fit the direction of Cocos‐Caribbean motion observed in earthquake slip vectors along the Middle America Trench. Since the prediction of this direction depends strongly on the rate of North America‐Caribbean motion, we conclude that the Jordan model better describes Caribbean Plate motions. The discrepancy between the two models results from the different methods of estimating plate motions. Sykes et al. (1982) report that they determined a Caribbean‐North America convergence rate of approximately 4 cm/yr from the length of the Lesser Antilles Wadati‐Benioff zone; we employ a similar procedure and obtain a rate closer to 2 cm/yr. We also find that the method they report using to determine the convergence direction is unable to distinguish between the alternative azimuths. We thus conclude that estimates of the rate and direction of plate motion from Wadati‐Benioff zone configurations are insufficiently robust and that such estimates are better derived using rates from magnetic anomalies and azimuths from transform faults and slip vectors. We further conclude that successful estimates of relative motion require internal consistency between the Euler vectors of all relevant plates, as demonstrated by the powerful Cocos‐Caribbean constraint not incorporated in the Sykes et al. model.

Microseismicity and focal mechanisms of the intermediate‐depth Bucaramanga Nest, Colombia

The Bucaramanga nest of intermediate‐depth seismicity in northern Colombia was investigated from microearthquakes (mb ≤ 4.3) recorded from June 20 to July 6, 1979, by a local array of 12 digital three‐component seismographs. Of 161 events studied, 142 are from the nest and 19 define a thin Wadati‐Benioff (W‐B) zone, mainly from 110 to 190 km depth. The W‐B zone, part of the Bucaramanga segment, strikes N10°E with a dip increasing with depth from 30°E to near‐vertical beneath the nest. The 89 best located events from within the nest define a volume of about 8 km (NW) by 4 km (NE) by 4 km (depth) centered at 161 km depth, with relative location accuracies of ±0.5 to ±1.0 km. Double‐couple fault plane solutions of 59 events were determined from an objective procedure using P wave polarities and S/P amplitude ratios. There is no dominant trend in focal mechanisms nor any reasonable correlation with regional stress patterns or previously determined mechanisms from teleseismic data. The subduction and assimilation of a linear feature (such as an island arc undergoing a phase transformation) may explain the existence of the Bucaramanga nest. However, the extraordinarily high rate of seismicity suggests that the nest may be moving at a rate several orders of magnitude faster than the rate of relative plate motion alone. The intense seismicity and variation in focal mechanisms suggest that the nest represents some aspect of stress release associated with the ascent of fluids, perhaps brine or magma, that will ultimately result in the initiation of volcanic activity over this recently emplaced W‐B zone.

Nagssugtoqidian mobile belt of West Greenland: a cryptic 1850 Ma suture between two Archaean continents—chemical and isotopic evidence

New chemical and isotopic data permit the recognition of a cryptic suture zone between two Archaean continental masses within the Nagssugtoqidian mobile belt of West Greenland. This discovery has important implications for Precambrian crustal evolution: suture zones may not always be identifiable from geological field observations, with the consequence that mobile belts in which undetected sutures exist may be mis-identified as ensialic, and thought to require special non-plate tectonic models to account for their development. The Nagssugtoqidian belt consists mainly of Archaean gneisses reworked during the Proterozoic, with metamorphic grade and degree of isotopic disturbance increasing towards the centre of the belt. At the centre of the belt the Nagssugtoqidian includes metasediments and calc-alkaline volcanic and plutonic rocks of Proterozoic age, almost always strongly deformed and metamorphosed. From isotopic evidence (Sr i ca. 0.703; model μ 1 values ca. 8.0; initial ε Nd ca. 0) it is clear that the Proterozoic igneous rocks do not include any significant contributions derived from the Archaean crust, and the chemistry of the rocks, together with the isotope data, suggests that they were formed at a destructive plate margin. The Proterozoic rocks are found in a narrow zone (up to 30 km wide) between the Archaean gneisses to the north and south of Nordre Strømfjord, and are interpreted as reflecting the existence of a suture between two Archaean continental blocks. Zircon U Pb data and other isotope evidence show that subduction started before ca. 1920 Ma ago, and lasted until ca. 1850 Ma when collision occurred, with consequent crustal thickening, high-grade metamorphism and local anatexis. Given the time-span for the operation of subduction, the existence of a wide Nagssugtoqidian ocean can be inferred, even for slow rates of plate motion. The Proterozoic and Archaean gneisses in the Nagssugtoqidian belt are very similar lithologically and chemically, and it has only been possible to distinguish between them using isotopic criteria. Suture zones of this kind are very difficult to detect, and may be present elsewhere within the reworked Archaean terrains of northern Greenland and Canada.

Rates of Relative Plate Motion During the Acadian Orogeny Based on the Spatial Distribution of Black Shales

Devonian-Mississippian black shales apparently were deposited in foreland basins formed in response to deformational loading in the Acadian orogen. These black shales were deposited in a succession of partially overlapping basins separated from each other by coarser clastic units, which migrated episodically south-westward in time parallel to the orogen. If foreland basins are results of an isostatic response to deformational loading in the orogen, migration of successive basins along the orogen must reflect concomitant shift of orogeny, and hence relative plate motion in the same direction. These black-shale deposits, then, not only demonstrate the diachronous nature of the Acadian orogeny, a fact previously indicated by other lines of evidence, but also enable approximation of rates of relative plate motion to be made. Using three different dating schemes, the black-shale deposits indicate Late Devonian plate motion along an 800 km segment of the orogen during approximately 11 Ma, for an average rate of relative plate motion slightly greater than 7 cm/yr. The facts that the black-shale basins do apparently track plate movement and resulting orogeny and that the along-strike component of basin migration exceeds the perpendicular-to-strike component suggest that at least Late Devonian parts of the Acadian orogeny are related to episodes of oblique convergence involving major strike-slip movement with substantial amounts of transpression.

Geologic Evidence for Rate of Plate Convergence during the Taconic Arc-Continent Collision

The rate of plate convergence during arc-continent collision can be estimated from the rate at which the secondary effects of subduction move across the underriding plate in advance of the plate boundary. The following sequence of events is typical: (I) shoaling and/or emergence of the continental shelf, presumably caused by lithospheric flexure; (2) rapid subsidence, by a combination of normal faulting and trenchward tilting; and (3) a change from platformal to flysch sedimentation. Such a sequence has been recognized in the Taconic foreland basin in eastern New York and interpreted as being the result of collision between the ancient passive margin of North America and an island arc terrane at an east-dipping subduction zone during Medial Ordovician times. A plot of age versus distance across strike shows that the diachronous migration of these phenomena across the foreland proceeded at rates of 2 to 3 cm/yr; we regard this as the plate convergence rate during the latter part of the Taconic Orogeny. Our result is comparable with modern rates of plate motion and also agrees with an earlier estimate for the Taconic, which was based on the rate at which a series of locations on the outer trench slope passed through fossil-defined isobaths.

Geodetic measurement of horizontal deformation in the northern San Francisco Bay region, California

Analysis of geodolite observations in the coast ranges of northern California indicates that relative plate motion is spread over a wide zone. Little or no deformation occurs to the southwest of the San Andreas Fault itself. To the northeast of the San Andreas fault, a region transected by the parallel Rodgers Creek and Maacama faults, 25 ± 6 mm/yr of right‐lateral slip is distributed over about 60 km. There is a peak shear strain of 0.6 ± 0.1 μrad/yr near the San Andreas fault. Elsewhere in the deforming zone, shear strain rates are 0.3 ± 0.04 μrad/yr. At the northeastern boundary of the study area the strain rate is less than 0.2 μrad/yr. Except for the high strain rate near the San Andreas fault, there is no evidence of any localization of deformation to the surface traces of the faults, nor is there any evidence of creep or aseismic slip on any of the faults in the area. The data indicate that the relative motion between the Pacific and North American plate occurs primarily to the northeast of the San Andreas fault. The observations are inconsistent with models that include slip at depth on only the San Andreas fault. The data can be fit equally well by models with slip at depth on all three of the major faults or with motion distributed across the entire zone from the San Andreas fault to the Napa Valley fault. Preliminary analysis of observations to the Farallon Islands, about 40 km southwest of the San Andreas fault, suggests that little or no additional relative motion is accommodated on that side of the San Andreas fault. In particular, it is unlikely that the 20‐mm/yr difference between the plate motion rate (about 55 mm/yr) and the San Andreas fault system rate (about 35 mm/yr) can be explained by deformation between the San Andreas fault and the Farallon Islands.

Temporal and spatial variability of seafloor spreading processes in the northern south Atlantic

The paper reports the results of an aeromagnetic study of the Mid‐Atlantic Ridge in the northern South Atlantic. The study consists of 38,000 km of low altitude magnetics tracks centered on the ridge covering nearly 700,000 km2 south of Ascension Island. Most tracks extend out to crust of anomaly 5B age. Large areal coverage coupled with high resolution allows comparison of crustal generation processes over space (along ridge strike) and time (across strike). Temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion. The total spreading rate increased sharply at 10 Ma and then decreased between 6 Ma and the present. The time variation of spreading rate is similar to that seen in other areas such as the North Atlantic and the southern and eastern Pacific. The plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones. In several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones. The periods of plate boundary modification correlate with the increase and decrease in spreading rate. Spatial heterogeneities are caused by the influence of the Ascension hotspot on the Mid‐Atlantic Ridge in the northern portion of the study area, and include two apparent seamount chains formed by the hotspot. Ascension Island and several of the seamounts appear to have been produced some distance away from the hotspot location (presently about 225 km southeast of Ascension Island) by channeled subaxial magma flow beneath the spreading center. The seamount chains, therefore, do not mark hotspot tracks. Large local variations in spreading rate and degree of asymmetry along strike are also found.

Satellite Geodynamics

Satellite geodynamics concerns the use of space technology in the study of the dynamic processes and movements of the solid Earth. The study involves measurements of the Earth's gravity and magnetic fields, the distortions and relative motions of its tectonic plates and the orientation of, and rate of rotation about, its axis. These studies address fundamental questions of geophysics such as the causes of Earth movements and earthquakes, the nature of convection in the mantle, and the origin of the magnetic field. The use of space technology has helped advance our concepts and models in solid-Earth geophysics. Shortly after the dawn of the space age, tracking data were used to precisely define the pear-shaped Earth. Since that time, other data have given several up-to-date representations of the secularly changing magnetic field and global models of the gravity field. Using this technology, it is now possible to determine the contemporary rate of relative plate motion at the level of about 1 cm a year. There are exciting prospects for future missions in satellite geodynamics. The Geopotential Research Mission is designed to improve gravity and magnetic field models and new, highly portable geodetic systems based on the Global Positioning System and spaceborne laser systems should permit rapid determination of positions with 1-cm precision.

Constraints on Pacific Plate Kinematics and Dynamics with Global Positioning System Measurements

Global positioning system (GPS) receivers are capable of generating precise geodetic data that can yield important constraints on both plate kinematics and dynamics. Geodetic measurements can determine plate motion rates over time scales for which little data are currently available. High-quality geodetic data may also allow the investigation of plate driving forces because changes in the intraplate stress field can potentially be inferred from measurement of the resulting crustal strain. A measurement program designed to investigate kinematic and dynamic aspects of plate tectonics should be concentrated in the Pacific region. This area contains the largest and fastest moving plate, up to 17 cm/year. Furthermore, subduction zones, which play a quantitatively important role in plate driving forces, are largely restricted to the Pacific region. We summarize accuracy studies showing that for short (< 100 km) baselines, centimeter-level accuracy can be expected using only mobile stations. For longer baselines, uncertainty in the orbit ephemeris of the GPS satellites is a major error source. Performing simultaneous observations at widely separated (~3000 km) fiducial stations near the region of interest, however, should allow centimeter-level accuracy for baselines up to several thousand kilometers in length. This performance level is predicated upon the assumption that fiducial baselines are known a priori to the centimeter level, for example, from very-long-baseline interferometry, and that corrections for tropospheric path delay are accurate to the centimeter level. The fiducial network location is flexible, limited mainly by the requirement for mutual satellite visibility.

Relative motions between oceanic plates of the Pacific Basin

Appendix tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C., 20009. Document B84‐012; $2.50. Payment must accompany order.Relative motion poles describing the displacement histories between the Pacific plate and once adjacent oceanic plates (Farallon, Kula, Izanagi I, Izanagi II, and Phoenix) were derived for the late Mesozoic and Cenozoic eras. Because fracture zone and magnetic anomaly data are generally available from the Pacific plate but not from adjacent plates, a new method of analysis for onesided data was required. This analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions. The following are the main conclusions drawn from our analysis: (1) For time intervals of the order of 107 years, termed stages, relative motion poles for plate pairs remained nearly fixed. Between stages, shifts in poles were commonly both large and abrupt. Within stages, rates of plate motion were commonly observed to change markedly, indicating that plates changed speed more frequently than they changed direction. (2) The relative motions of all of the plates analyzed changed at about chron M11 (135 Ma), chron 34 (85 Ma), and chron 25 (56 Ma). (3) During the Early Cretaceous there were five oceanic plates in the Pacific basin rather than the four recognized by previous workers. (4) To determine the number of Farallon plates that existed to the east of the Pacific plate during the time interval from chron 34 (85 Ma) to chron 25 (56 Ma), fracture zones and magnetic anomalies that record Pacific‐Farallon spreading from the northern, central, and southern Pacific plate were analyzed separately and collectively. The analysis shows that a single Pacific‐Farallon relative motion pole and a single rate are consistent with all of the data. (5) Spreading rates along the Pacific‐Kula ridge decreased markedly between chrons 32b and 25 (72–56 Ma), probably in response to the arrival of buoyant Kula lithosphere at a subduction zone northwest of the Bering Sea. (6) Soon after chron 25 (56 Ma), a major reorganization is recorded along the Pacific‐Farallon boundary. The Kula‐Farallon boundary is also thought to have changed during this reorganization. (7) Kula‐Pacific anomalies north of anomaly 25 are modeled by assuming continued Kula‐Pacific spreading after chron 25 (56 Ma). (8) Probably by chron 18 (43 Ma), Pacific‐Kula spreading had ceased and the Kula‐Farallon ridge system had evolved into alignment with the Pacific‐Farallon spreading direction. However, the timing of these events is uncertain because Pacific‐Kula data younger than chron 25 (56 Ma) are sparse.

The earthquake deformation cycle, recurrence, and the time‐predictable model

Geodetic and geologic observations from a handful of well‐studied great plate boundary earthquakes provide a basis for exploring those features of the deformation cycle having the strongest influence on recurrence estimation. In ideal circumstances the time‐predictable model requires knowledge of only the seismic slip (and time) of the last earthquake and the rate of relative plate motion or fault slip. Practically, indirect measurements must usually suffice, local coseismic strain changes rather than seismic slip and local deformation rate rather than slip rate. Permanent, nonrecoverable deformation, often roughly half the coseismic strain offset near great thrust earthquakes, complicates the inference of interplate seismic slip. Owing to short‐ and long‐term postseismic transients, commonly 20–40% of the coseismic strain drop, deformation rates are variable and not simply related to the plate motion rate. At strike‐slip plate boundaries these complications can sometimes be avoided and seismic slip and slip rate can be obtained more or less directly. When these favorable circumstances are absent and when knowledge of the deformation cycle is incomplete, recurrence accuracy can be improved by empirically correcting for unknown elements of the cycle. Systematic features in the spatial distribution of the transients and permanent deformation near subduction zones help identify the regions where these corrections are largest and where the shortcomings of the empirical approach are most severe.

Holocene activity of the San Andreas fault at Wallace Creek, California

Wallace Creek is an ephemeral stream in central California, the present channel of which displays an offset of 128 m along the San Andreas fault. Geological investigations have elucidated the relatively simple evolution of this channel and related landforms and deposits. This history requires that the average rate of slip along the San Andreas fault has been 33.9 ± 2.9 mm/yr for the past 3,700 yr and 35.8 + 5.4/−4.1 mm/yr for the past 13,250 yr. Small gullies near Wallace Creek record evidence for the amount of dextral slip during the past three great earthquakes. Slip during these great earthquakes ranged from ∼9.5 to 12.3 m. Using these values and the average rate of slip during the late Holocene, we estimate that the period of dormancy preceding each of the past 3 great earthquakes was between 240 and 450 yr. This is in marked contrast to the shorter intervals (∼150 yr) documented at sites 100 to 300 km to the southeast. These lengthy intervals suggest that a major portion of the San Andreas fault represented by the Wallace Creek site will not generate a great earthquake for at least another 100 yr. The slip rate determined at Wallace Creek enables us to argue, however, that rupture of a 90-km-long segment northwest of Wallace Creek, which sustained as much as 3.5 m of slip in 1857, is likely to generate a major earthquake by the turn of the century. In addition, we note that the long-term rates of slip at Wallace Creek are indistinguishable from maximum fault-slip rates estimated from geodetic data along the creeping segment of the fault farther north. These historical rates of slip along the creeping reach thus do represent the long-term—that is, millennial—average, and no appreciable elastic strain is accumulating there. Finally, we note that the Wallace Creek slip rate is appreciably lower than the average rate of slip (56 mm/yr) between the Pacific and North American plates determined for the interval of the past 3 m.y. The discrepancy is due principally to slippage along faults other than the San Andreas, but a slightly lower rate of plate motion during the Holocene epoch cannot be ruled out.

Motion of Caribbean Plate during last 7 million years and implications for earlier Cenozoic movements

The direction and rate of movement of the Caribbean plate with respect to North America are determined from the slip vectors of shallow earthquakes and from the configuration of downgoing seismic zones in the Greater and Lesser Antilles. A calibration of the relative plate motion for the northeastern Caribbean using data from other subduction zones indicates an average rate of 3.7±0.5 cm/yr for the past 7 million years (Ma). The direction of plate motion inferred from focal mechanisms (ENE) is nearly the same as that deduced from the configuration of downgoing seismic zones going around the major bend in the arc. With respect to North America, the Caribbean plate is moving at an angular velocity of 0.36°/Ma about a center of rotation near 66°N, 132°W. Vector addition using those data and that for the relative motion of North and South America indicates that the Caribbean is moving at an angular velocity of 0.47°/Ma about a center of rotation near 60°N, 88°W with respect to South America. The presence of intermediate‐depth earthquakes beneath Puerto Rico and the Virgin Islands is ascribed to the curvature of the plate boundary and a component of underthrusting that has been going on for at least the past 7 Ma and is likely occurring today. The alternative hypothesis that earthquakes beneath those areas are occurring in materials that were subducted during the Eocene, the last major episode of magmatism, is not tenable from thermal considerations. The lack of recent magmatism in the eastern Greater Antilles is ascribed to the relatively small component of underthrusting. The 2 cm/yr rate of seafloor creation along the mid‐Cayman spreading center for the past 2.4 Ma does not appear to reflect the total Caribbean‐North American plate motion while the 4 cm/yr spreading rate from 6.0 to 2.4 Ma does. Between the mid‐Cayman spreading center and eastern Guatemala, the northern boundary of the Caribbean plate is narrow and follows the southern margin of the Cayman trough. Seismic activity between the spreading center and eastern Hispaniola, however, occurs over a zone about 250 km wide that extends from Cuba to Jamaica and across the entire width of Hispaniola. Individual faults within this broad plate boundary appear to have accommodated differing amounts of motion as a function of geological time while the cumulative plate motion across the zone remained nearly constant. The percentage of total plate motion accommodated near southern Hispaniola and Jamaica is inferred to have increased about 2.4 Ma ago. That change may have been caused by the collision of parts of the Bahama bank and northern Hispaniola. This explanation for the sudden decrease in seafloor creation along the mid‐Cayman spreading center is less catastrophist than the hypothesis that the entire Caribbean plate suddenly changed its velocity with respect to surrounding plates. The Caribbean plate may be regarded as a small buffer plate whose motion is now governed by the movement of the larger North and South American plates which bound it on three sides. The Caribbean plate is either at rest or moving eastward at a rate of no more than 1 cm/yr in the hot spot reference frame. Since the relative motion of the larger plates surrounding the Caribbean has been nearly constant for the last 38 Ma (anomaly 13 time) and since the forces on the Caribbean plate do not appear to have changed greatly during that interval, we extrapolate the motion of the last 7 Ma back to 38 Ma. A reconstruction for the late Eocene places the Caribbean plate about 1400 km west of its present position. The faster rate of plate motion we calculate makes it more likely that the lithosphere beneath the basins of the Caribbean originated in the Pacific. It also has implications for the seismic potential of the region, paleocirculation in the Atlantic Ocean and origin of sediments in the area. Our late Eocene reconstruction aligns the eastern continental margin of Yucatan with that along the southeast side of the Nicaragua rise. This 2500‐km‐long feature may have acted as an arc‐arc transform fault from the late Mesozoic to the late Eocene. Arc‐related rocks of those ages in the Greater Antilles and northern Lesser Antilles define a northwesterly trending subduction zone along the northeastern edge of the former East Pacific‐Caribbean plate. At least three fragments of anomalous seafloor have been sutured onto Hispaniola in the past 50 Ma.