Rivera Fracture Zone Research Articles

Seismic Structure of the Southern Rivera Plate and Jalisco Block Subduction Zone

Structural and tectonic features in the Pacific Coast of Mexico generate a high level of seismic activity in the Jalisco block (JB) region, making it one of the most attractive areas of the world for geophysical investigations. The Rivera–North America contact zone has been the object of different tectonic studies in recent years framed within the TsuJal project. To this day, this project is generating numerous crucial geophysical results, which significantly improve our understanding of the region. Our study is focused on the interaction between the south of the JB and Rivera plate (RP), which crosses the Middle America trench. We also cover an offshore–onshore transect of 130 km length between the eastern Rivera fracture zone and La Huerta region, in the Jalisco state. To characterize this region, we interpreted wide-angle seismic, multichannel seismic, and multibeam bathymetry data. The integration of these results, with the local and regional seismicity recorded by the Jalisco Seismic Accelerometric Telemetric Network and by the Mapping the Rivera Subduction Zone experiment, provides new insights into the geometry of the southern RP, which is dipping 12°–14° under the JB in the northeast–southwest direction. Moreover, our results provide new seismic images of the accretionary wedge, the shallow crust, the deep crust, and the upper-mantle structure along this profile.

Seismicity and Stress in a Tectonically Complex Region: The Rivera Fracture Zone, the Rivera-Cocos Boundary, and the Southwestern Jalisco Block, Mexico

Abstract Fault plane solutions for a group of 41 4.9≤ M w ≤8.0 earthquakes between November 1980 and May 2007, extracted from the Global Centroid Moment Tensor Project catalog, are used to investigate spatial and temporal variations of stress in the south part of the Jalisco block (JB). Using rigorous statistical tests we decide on the quality and variability of the earthquake mechanisms and with the use of stress tensor inversion and cumulative misfit analysis find, at the 95% confidence level or more, significantly different regions and periods of stress regime. We conclude that the region located near (105° W, 18.43° N) appears as a particularly diffuse zone affected by various stress styles and that the period following the great M w 8.0 earthquake on 9 October 1995 appears as a short-lived change in stress induced by the great shock. An important difference in seismicity and stress between the southern and northern regions is highlighted by the regular occurrence of earthquakes south of the JB, in stark contrast with the scarcity of seismicity with M w >4.9 north of 19.36° N, a region that is illuminated with microseismicity when local temporal or permanent deployments of seismographs are used, as noted by previous studies.

The seismic and hydroacoustic stations on Socorro Island: Early results


 
 
 A seismic and hydroacoustic network on Socorro Island was installed jointly by the Servicio Sismolo?gico Nacional, Instituto de Geofi?sica, Universidad Nacional Auto?noma de Me?xico, and the Comprehensive Nuclear-Test-Ban Treaty Organization. The detection of ten earthquakes in June and July 2004 is reported in this study. These events occurred at epicentral distances between 209 and 9050 km and ranged in magnitude between 3.9 and 6.8. An event in the Kamchatka Peninsula featured arrivals of teleseismic high-frequency P and pP waves from a distance of 8245 km. An earthquake in the Guerrero-Oaxaca, Mexico, region shows clear seismic and T phases. Three earthquakes in the Rivera Fracture Zone were recorded, thus leading to the expectation of the continued detection of events from nearby fracture zones as well as earthquakes generated by volcanoes on Socorro and San Benedicto Islands. The analysis of the T phases from several events validates the design of the network, with three sites around the island in order to record arrivals from all directions. High frequency seismic waves generated by acoustic waves attenuate as they propagate through the rock from one end of the island to the other. Consequently small or distant events are difficult to detect at the far side of the island.
 
 

The Rivera fracture zone revisited

Available conventional and swath-bathymetric data combined with earthquake slip vectors suggest the existence of three discrete linear segments along the Rivera fracture zone. Comparisons of these segments with small circles about various present-day Euler poles show that the Rivera fracture zone is not a transform fault, as it is currently accepted, but rather an accommodation zone between two different spreading systems. We suggest that the Pacific-Rivera rise and the Western segment of the Rivera fracture zone (trending N54 °W), are parts of the Pacific-North America plate boundary. The Central segment (trending N72 °W) and the Eastern segment (trending N85 °W) belong to a diffuse boundary connecting the Pacific-North America plate boundary to the East Pacific rise system.

Relative motions of the Pacific, Rivera, North American, and Cocos plates since 0.78 Ma

We use magnetic anomaly and fracture zone crossings from 3°N–24°N along the East Pacific Rise to define the boundaries and relative velocities of the rigid Pacific, Rivera, North American, and Cocos plates since 0.78 Ma and to test several hypotheses regarding the nature of deformation between these plates. Crossings of the 0.78 Ma isochron from the Pacific‐Rivera rise north of 22.0°N are significantly better fit by the 0–0.78 Ma Pacific‐North America rotation than by a Pacific‐Rivera rotation, thereby supporting a recently proposed model for accretion of the northern part of the Rivera plate to North America. A significant misfit of the Pacific‐Cocos rotation to crossings of anomaly ln from the East Pacific Rise north of 16.4°N suggests that seafloor north of ∼16°N moves relative to the rigid Pacific or Cocos plates. We also find that nine crossings of the eastern Rivera fracture zone and eight crossings of anomaly ln flanking the Manzanillo spreading segment are fit well by a Pacific‐Rivera rotation. The Rivera‐Cocos angular velocity derived from the best fitting Pacific‐Cocos and Pacific‐Rivera angular velocities predicts 19±3 mm yr−1 (95% confidence limit) of ∼N‐S convergence near the center of the Rivera‐Cocos boundary. Nearby left‐lateral, strike‐slip earthquakes with north trending fault planes suggest that convergence is accommodated via sinistral shear along a north trending zone. The unambiguous kinematic and seismologic evidence for significant Rivera‐Cocos motion indicates that the absence of a well‐defined Rivera‐Cocos plate boundary cannot be interpreted as evidence that Rivera‐Cocos motion is slow or zero, as is postulated in several previous studies. The Rivera‐Cocos angular velocity predicts oblique Rivera‐Cocos convergence at the El Gordo graben and beneath the Manzanillo trough and southern Colima graben. Any extension across these features since 0.78 Ma thus cannot have resulted from divergence between the subducting Rivera and Cococs plates.

Earthquake clustering in shallow subduction zones: Kamchatka and Mexico

This paper studies regularities in earthquake clustering at shallow depths (0–60 km) in two Pacific subduction zones, the Mexican and the Kamchatkan. These zones were formed by subduction under the North American plate of old and fast Pacific plate (Kamchatkan region) and young and slow Rivera and Cocos plates (Mexican region). We use events of magnitude 5.5 and greater (95 events for 1964–1993 from the Kamchatkan region and 52 events for 1974–1993 from the Mexican region). We have two active (Kamchatkan and Cocos) and one passive (Rivera) subduction zones. For the passive Rivera zone, where subduction may be considered aseismic after the great Jalisco event of 1932 (magnitude M s 8.2) and its aftershock activity during 1932–1934, we can study only seismic events along the western border of the plate, related to oceanic rises and the Rivera fracture zone. Here we have a well-developed clustering of earthquakes (47% of all 17 studied events occurred in clusters), mainly of swarm type. For the two active subduction zones, the Kamchatkan and the Cocos, we can formulate the following results: (1) most of the studied events of the Cocos subduction zone (74%) occurred in clusters, and within the Kamchatkan subduction zone, 45% of the events recorded were in clusters; (2) the clustering events occurred mainly at a depth interval from 0 to 40 km (98% of all clustering events in Kamchatka and 61% for the Cocos subduction zone); at the same time, the single events were distributed mainly in a depth interval from 30 to 60 km (73% of all single events in Kamchatka and 100% for the Cocos subduction zone); (3) comparison of the spatial distribution for the different types of clustering in Kamchatka and the Cocos zone shows the absence of a band of swarms along the slope of the Middle American trench, whereas such a band is obvious in the Kamchatkan region; (4) the study of temporal distribution for single events and mainshock-aftershock sequences within the region of large earthquakes shows that there was a change from a long-term sequence of single events to events with aftershocks, 1–4 years before a large earthquake; this regularity was observed before both large (magnitude greater than seven) earthquakes in Kamchatka and before the magnitude 8.1 Michoacan earthquake in Mexico.

Active fragmentation of the North America plate at the Mexican triple junction area off Manzanillo

The identification of 1) the exact location of the East Pacific Rise (EPR)-Rivera Fracture Zone (RFZ) eastern junction, 165 km west of the Acapulco Trench axis, 2) the absence of a clear Rivera-Cocos Plate boundary between the Acapulco Trench axis and the EPR-RFZ eastern junction and, 3) the Jalisco Block (JB) offshore boundary that connects the Colima Graben (CG) to the Tamayo Fracture Zone (TFZ) northward suggests that the Jalisco Block is being transferred from the North America Plate to the Pacific Plate north, rather than south, of the Tamayo Fracture Zone.

The mathematician paleoplate

The Pacific seafloor is littered with small fragments of lithosphere captured from adjacent plates by past plate boundary reorganizations. One of the clearest examples of such a reorganization is documented in the Mathematician Seamounts region, where a distinctive geomorphology and well‐developed magnetic anomalies are present. This reorganization involved a short‐lived microplate between the failing Mathematician Ridge and a new propagating spreading center: the East Pacific Rise. It produced a transfer of a fragment of lithosphere from the Farallon to the Pacific plate, and also created a number of landforms and magnetic patterns, within and on the margins of the captured fragment; these make up the Mathematician paleoplate. In many cases, two sides of a microplate are active spreading ridges. A microplate evolves into a paleoplate when dual spreading ceases and full spreading resumes at the prevailing spreading ridge. We define a paleoplate as the area of the seafloor, from the axis of a failed rift to the boundary of resumed, full spreading. It includes a fragment of captured lithosphere and the lithosphere slowly accreted to it during the period of dual spreading, prior to complete abandonment of the failed rift. The Mathematician paleoplate has the following boundaries and components from west to east: the axis of the Mathematician failed rift, the fragment of captured Farallon plate, a complex rift initiation site at the Moctezuma Trough, a zone of slow spreading, and an as yet ill‐defined eastern boundary where dual spreading stopped and full spreading resumed. The northern boundary of the paleoplate is the Rivera fracture zone; its southeastern boundary a now‐inactive transform fault, the West O'Gorman fracture zone. In this case, as well as in other more poorly documented ones, relict landforms and magnetic patterns are carried on the aging lithosphere, away from the spreading ridge, recording a former geometry.

Seismicity and tectonics of the subducted Cocos Plate

We have examined teleseismic earthquake locations reported by the International Seismological Centre (ISC) for the Middle America region and selected 220 as the most reliable. These hypocenters and other data are used to delineate the deep structure of the subducted Cocos Plate. The results indicate that the subducted plate consists of three major segments: Segment I extends from the Panama Fracture Zone to the Nicoya Peninsula. The structure of this segment is poorly defined. Segment II is the largest and best‐defined segment. This segment consists of two parts, IIA and IIB. Part IIA extends from the Nicoya Peninsula to western Guatemala and is very well defined and continuous in structure. Its strike follows the curvature of the trench and dips at about 60°. Part IIB extends from western Guatemala to Orizaba, Mexico. The dip of this part of the segment decreases slightly toward the northwest, and its strike is more northward than that of the trench. Segment III extends from Orizaba to the Rivera Fracture Zone, and is not well defined due to a lack of earthquake activity beneath about 100 km. Its orientation differs markedly from segment II and strikes somewhat more westward than the trench. Between parts IIA and IIB of segment II the subducted plate seems to be continuous, bending smoothly to accommodate the change in geometry. Local network data from Costa Rica suggest there may be a tear between segments I and II. Between segments II and III there is a gap in the hypocenters which makes it difficult to define the boundary. The change in geometry between these two segments indicates that there may be a tear, and two strike‐slip focal mechanisms in the region support this conclusion. We find no convincing evidence supporting the existence of segments smaller than the three described above. If there is smaller‐scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths. There is no evidence for any major tear in the subducted plate associated directly with either the Tehuantepec Ridge or the Orozco Fracture zone. The shallow subduction at the northwestern end of segment II may be related to the bouyancy of the Tehuantepec Ridge. The Cocos Ridge is probably directly responsible for the change in geometry between segments I and II and may even be slowing or stopping subduction in segment I. The structure of the subducted plate in segment II and the changes in the character of volcanism along the arc can be related to the relative motion of the North American and Caribbean Plates. The present geometry of part IIB of segment II is more consistent with the probable configuration of the trench about 7 Ma ago than with the present configuration, indicating that the North American plate is overriding the subduction zone.Appendices 2, 3, and 4 are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B84‐009; $2.50.

Seismicity and tectonics of the Rivera Plate and implications for the 1932 Jalisco, Mexico, earthquake

Seismicity on and near the boundaries of the Rivera plate is examined in order to determine the subduction regime of the Rivera plate and its influence on the earthquake behavior of the northernmost section of the mid‐America trench. In particular, we address the question of a Rivera‐Cocos boundary.There have been several large historic earthquakes in the coastal areas of the Mexican states Colima and Jalisco, but the last large event was in June 1932 (the 1932 Jalisco earthquake, Ms = 8.1). The present quiescence has lasted longer than the average recurrence interval for the Mexican subduction zone, and the Jalisco area is now categorized as a seismic gap. However, if subduction in this area is dominated by the motion of the Rivera plate, which is subducting at half the rate of the Cocos plate, the deficiency of earthquake activity may not be unusual. In an attempt to better delineate the Rivera‐Cocos boundary,intermediate‐sized earthquakes along and near the Rivera fracture zone and the mid‐America trench were relocated with the joint epicenter determination method. We found that in general, the catalog locations are fairly accurate, with the average change in epicenter upon relocation being 12 km. Seismicity along the Rivera fracture zone follows the physiographic trend of the fracture zone, but east of its intersection with the East Pacific Rise, seismicity is more diffuse. Two intermediate‐sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent Rivera‐Cocos boundary motion. Reexamination of arrival time data for the 1932 Jalisco earthquake yields an epicenter at 19.57°N, 104.42°W, close to the boundary zone inferred from these two earthquakes. We conclude that the possibility that the 1932 Jalisco earthquake broke the northernmost section of the Cocos‐North American plate interface, as opposed to the Rivera‐North American plate interface, cannot be ruled out.

Microearthquakes on the East Pacific Rise at 21°N and the Rivera Fracture Zone

During March and April of 1974, three ocean bottom seismometer (OBS) capsules were deployed in arrays on the East Pacific Rise (EPR) at 21°N for 8 days, on the Rivera Fraction Zone (RFZ) for 9 days, and at the EPR/RFZ intersection for 12 days. Seismicity on the EPR is characterized by extremely small events, none large enough to trigger all of the instruments simultaneously. A weak 24‐hour periodicity in OBS event trigger rate is observed. This could be related to earth strain tides or a modulation in the trigger sensitivity due to a varying noise level. On the Rivera Fracture Zone, an en echelon pattern of earthquakes with a maximum depth of 10 km was observed. Seismic activity at the junction of the EPR and RFZ becomes more diffuse and terminates abruptly at the EPR. The junction array was slightly offset from the active zone of seismicity, and the depth of only one event is reliably determined at 5.4±2 km. The S wave spectra of microearthquakes at the junction and on the RFZ are computed, and seismic moments and corner frequencies determined. Stress drops are between 0.1 and 10 bars and show no systematic difference at the two deployment sites. Larger earthquakes observed at teleseismic distances do show markedly lower stress drops when they originate near to spreading centers, but these differences are not observed at the microearthquake level in this study. It is suggested that this is due to a natural tendency for small earthquakes to have small stress drops and a lack of large earthquakes during the RFZ deployment.

Rivera Ocean Seismic Experiment (ROSE) overview

The Rivera Ocean Seismic Experiment (ROSE) was designed as a combined sea and land seismic program to utilize both explosive sources and earthquakes to study a number of features of the structure and evolution of a mid‐ocean ridge, a major oceanic fracture zone, and the transition region between ocean and continent. The primary region selected for the experiment included the Rivera Fracture Zone, the crest and eastern flank of the East Pacific Rise north of the Rivera, and adjacent areas of Baja California and mainland Mexico. These areas were to be instrumented with land and ocean bottom seismographs in order to determine good source parameter and location data for natural events and to record these events along a large number of paths crossing various parts of the region. Explosive charges were to be detonated at sea to supplement the natural events. However, the necessary permission to conduct the experiment was not received from Mexican authorities; therefore an alternate plan was implemented whereby the marine program had to be moved southward outside of territorial waters. This had the effect of transforming this experiment into three, almost independent components: (1) an experiment to study the East Pacific Rise south of the Orozco Fracture Zone primarily using ocean bottom recording and explosive sources, (2) a seismicity program at the Orozco, and (3) a land‐based program of recording natural events along the coastal region of Mexico. A considerable amount of useful data was obtained in each of the three subprograms. In the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle, structure and dynamics of the East Pacific Rise, seismicity of the Orozco Fracture Zone, and partitioning of energy transmission between the ocean volume and the crust/lithosphere. On land, the fortuitous occurrence of the Petatlan M7.6 earthquake of March 14, 1979, permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event, which provide new insight into the filling of a major seismic gap in the region. This overview describes the scientific rationale and the design of the experiments, along with some general results. Other articles in this volume give preliminary scientific results from certain components of the overall experiment or, in some cases, report on other data pertinent to the scientific goals of ROSE.

Northern East Pacific Rise: Magnetic anomaly and bathymetric framework

The oceanic crust in the eastern Pacific between 7°N and 30°N and east of 127°W contains a fairly complete history of the spreading centers associated with the East Pacific Rise since 25 m.y. B.P. (late Oligocene). In this paper, we have summarized the seafloor spreading magnetic‐anomaly data and the bathymetric data that reflect the record of this tectonic history. The well‐defined magnetic lineations north of the Clarion fracture zone, in the mouth of the Gulf of California, and on the east flank of the East Pacific Rise (EPR) are carefully examined and used to provide a guide for interpreting the spreading pattern between the Clarion and Clipperton fracture zones, southward of the Rivera fracture zone over the Mathematician Ridge, and over the entire EPR east of the Mathematician Ridge between the Rivera and Siqueiros fracture zones. The bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas. The fracture zone bathymetry and the seafloor spreading magnetic lineations on the EPR south of the Rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations. All these data provide a good record of the plate reorganizations in the middle Miocene at magnetic anomaly 5 A time (12.5 to 11 m.y. B.P.), in the late Miocene at magnetic anomaly 3′−4 time (6.5 m.y. B.P.), and in the Pliocene at magnetic anomaly 2′−3 time (3.5 m.y. B.P.). Several abandoned spreading centers, including the Mathematician Ridge, were left behind as a result of these reorganizations. The Mathematician Ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5A.

An earthquake sequence studied with ocean-bottom seismographs

Summary. A tripartite ocean-bottom seismograph array at the junction of the East Pacific Rise and Rivera Fracture Zone recorded an eathquake sequence, consisting of three main shocks (mB= 4.3, 4.3 and 4.8) and numerous aftershocks from the fracture zone, in the distance range 35–50 km. Delineation of the rupture zones by aftershocks indicates that the first two main shocks took place on overlapping fault areas, while the third occurred over a fault area separated from the first by several kilometres. Both rupture zones were about 4 km long. Surface wave spectra indicate a shallow (about 3 km below the sea floor) source, as does OBS array phase velocity data. The seismic moments, obtained from teleseismic surface wave data, of 1.3, 2.1 and 2.8 × 1023 dyn cm, with the fault areas as delineated by aftershocks, imply a stress drop of about 8 bars for the main shocks. Aftershock sequences of each of the main shocks are similar, with a b-value of about 0.65. Teleseismic P travel times are similar to those from near-surface sources in Nevada.

Neogene reorganization of spreading between the Tamayo and the Rivera fracture zone

Bathymetric and magnetic data from the mouth of the Gulf of California show an early episode of spreading between ±6 mybp and 3.5 mybp which rifted crust at the foot of the Tres Marias Islands from Baja California. Fracture zones from this episode do not offset the present East Pacific Rise (EPR). A reorganization of spreading beginning 3.5 mybp has led to the present configuration; its initiation is marked in the topography by a valley partially filled by a NS trending turbidite plain.

Continental margin and lower slope structures of the Middle America Trench near Acapulco (Mexico)

The continental margin of southern Mexico north of the Tehuantepec Ridge and south of the Rivera Fracture Zone is a narrow crustal transition zone. The margin lacks a well-developed forearc basin and the active volcanic arc is subparallel to the trench and not clearly related in a normal fashion to the Benioff Zone. A geological survey of a small part of the margin 100 km south of Acapulco reveals significant variations in structure over short distances. Multi-channel seismic reflection data, sonobuoy refraction data and dredging indicate that the shelf and upper slope are underlain by continental crust. Geometry derived from the reflection data suggests that the continental crust is less than 10 km thick at its seaward termination. Migrated reflection data reveal structures on the lower slope and within trench axis turbidite fill which are interpreted as folds and thrust faults. Structures and landward dipping reflections suggest that the base of the slope is an accreting zone with trench turbidite fill being uplifted and emplaced. Both reflection and refraction data suggest the trench inner wall contains both slope deposits and uplifted trench deposits. There is no evidence of significant amounts of oceanic crust within the slope. Volume calculations reveal that some of the sedimentary rocks are being consumed along with the oceanic crust.

Seismic evidence for a narrow zone of partial melting underlying the East Pacific Rise at 21°N

Analysis of a seismic-refraction profile on the crest of the East Pacific Rise at 21 °N reveals considerable lateral inhomogeneity in the velocity structure. A split profile, of a total length of 120 km, was shot to a tripartite array of ocean-bottom seismometer capsules on or close to the spreading axis. The recorded travel times and amplitudes indicate the presence of a low-velocity zone in the immediate vicinity of the spreading axis, at a depth of about 2.5 km below the sea floor. This low-velocity zone is not evident a few (∼10) kilometres from the axis. Additional structural evidence was obtained from a deployment of the capsules at the intersection of the East Pacific Rise crest with the Rivera Fracture Zone, at 20 °N. A group of small earthquakes was recorded by the seismometers, and, for ray paths traversing or close to the rise crest axis, the shear waves appear to be anomalously attenuated, particularly at shorter periods. While the evidence is not conclusive, it suggests the presence of a shear-attenuative region beneath the rise crest, presumably identical with the low-velocity zone and consistent with the interpretation of this zone as a region of partial melting.

Ocean bottom seismic measurements on the East Pacific Rise and Rivera Fracture Zone

WE describe preliminary results of a seismic experiment using ocean bottom seismometer capsules (OBS) in three separate tripartite arrays. The arrays were located on the central topographic high of the East Pacific Rise between the Rivera and Tamayo Fracture Zones (8 d in position), at the junction of the ridgecrest and the Rivera fracture zone (12 d), and on the Rivera fracture zone well away from the ridgecrest (9 d). Accurate locations and depths could be computed for events sufficiently close to each array, and showed that the main transform fault lay within the central trough. OBS capsules have been successfully used for refraction work1,2 (also B. Lewis and C. Lister, unpublished) and microseismicity studies3, but we report here the first use of arrays on a spreading centre–transform fault system for event locations. Other questions of importance to seismology can also be answered from these data.

Sonobuoy measurements of seaquakes on the East Pacific rise near San Benedicto island

Sonobuoy modified to receive low (< 100 Hz) frequency sounds in the deep ocean were deployed from the R.V. Yaquina on the Rivera fracture zone and 4·3 kmoff San Benedicto Island. An array of at least two operating sonobuoys was maintained for 24 h at each station. Calculated earthquake magnitudes were 2·2 to 3·7 on the Richter scale. Most of the acoustic energy in the signals was below 40 Hz and recorded well on the modified sonobuoys but poorly on simultaneously monitored unmodified buoys. A swarm of microearthquakes following a magnitude 4·3 earthquake was recorded on the Rivera fracture zone site while seismic activity at the San Benedicto Island was more sporadic. Epicentral distance from the sonobuoys, calculated from the T-P interval (when present), ranged from 10 to 120 km, most less than 20 km. The earthquake magnitude versus occurrence rate curve for the Rivera fracture zone events has b value of 0·6, indicating that the earthquakes were associated with the fracture zone rather than the median rift of the East Pacific Rise.

Detailed study of relative plate motion in the Gulf of California

Detailed comparisons of a two-plate tectonic theory with evidence of transform faulting on the North American-Pacific plate boundary in the Gulf of California and on the Rivera Fracture Zone show individual data with as much as 30° of angular discrepancy. Despite systematic deviations of as much as 20° in certain areas and the consequent suggestion of possible minor platelets along the major boundary, uncertainties in models and data prevent the resolution of more than two plates.