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[1] On 6 April (01:32 UTC) 2009 a MW 6.1 normal faulting earthquake struck the axial area of the Abruzzo region in central Italy. We study the geometry of fault segments using high resolution foreshock and aftershock locations. Two main SW dipping segments, the L'Aquila and Campotosto faults, forming an en echelon system 40 km long (NW trending). The 16 km long L'Aquila fault shows a planar geometry with constant dip (∼48°) through the entire upper crust down to 10 km depth. The Campotosto fault activated by three events with 5.0 ≤ MW ≤ 5.2 shows a striking listric geometry, composed by planar segments with different dips along depth rather than a smoothly curving single fault surface. The investigation of the spatiotemporal evolution of foreshock activity within the crustal volume where the subsequent L'Aquila main shock nucleated allows us to image the progressive activation of the main fault plane. From the beginning of 2009 the foreshocks activated the deepest portion of the fault until a week before the main shock, when the largest foreshock (MW 4.0) triggered a minor antithetic segment. Seismicity jumped back to the main plane a few hours before the main shock. Secondary synthetic and antithetic fault segments are present both on the hanging and footwall of the system. The stress tensor obtained by inverting focal mechanisms of the largest events reveals a NE trending extension and the majority of the aftershocks are kinematically consistent. Deviations from the dominant extensional strain pattern are observed for those earthquakes activating minor structures.

International audience; Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 M w = 6.9 Zemmouri, Algeria, earthquake [1] The essential features of stress interaction among earthquakes on en echelon thrusts and tear faults were investigated, first through idealized examples and then by study of thrust faulting in Algeria. We calculated coseismic stress changes caused by the 2003 M w = 6.9 Zemmouri earthquake, finding that a large majority of the Zemmouri afterslip sites were brought several bars closer to Coulomb failure by the coseismic stresses, while the majority of aftershock nodal planes were brought closer to failure by an average of ∼2 bars. Further, we calculated that the shallow portions of the adjacent Thenia tear fault, which sustained ∼0.25 m slip, were brought >2 bars closer to failure. We calculated that the Coulomb stress increased by 1.5 bars on the deeper portions of the adjacent Boumerdes thrust, which lies just 10–20 km from the city of Algiers; both the Boumerdes and Thenia faults were illuminated by aftershocks. Over the next 6 years, the entire south dipping thrust system extending 80 km to the southwest experienced an increased rate of seismicity. The stress also increased by 0.4 bar on the east Sahel thrust fault west of the Zemmouri rupture. Algiers suffered large damaging earthquakes in A.D. 1365 and 1716 and is today home to 3 million people. If these shocks occurred on the east Sahel fault and if it has a ∼2 mm/yr tectonic loading rate, then enough loading has accumulated to produce a M w = 6.6–6.9 shock today. Thus, these potentially lethal faults need better understanding of their slip rate and earthquake history. among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 M w = 6.9 Zemmouri, Algeria, earthquake

[1] A series of complex events at Kīlauea Volcano, Hawaii, 17 June to 19 June 2007, began with an intrusion in the upper east rift zone (ERZ) and culminated with a small eruption (1500 m3). Surface deformation due to the intrusion was recorded in unprecedented detail by Global Positioning System (GPS) and tilt networks as well as interferometric synthetic aperture radar (InSAR) data acquired by the ENVISAT and ALOS satellites. A joint nonlinear inversion of GPS, tilt, and InSAR data yields a deflationary source beneath the summit caldera and an ENE-striking uniform-opening dislocation with ∼2 m opening, a dip of ∼80° to the south, and extending from the surface to ∼2 km depth. This simple model reasonably fits the overall pattern of deformation but significantly misfits data near the western end of an inferred dike-like source. Three more complex dike models are tested that allow for distributed opening including (1) a dike that follows the surface trace of the active rift zone, (2) a dike that follows the symmetry axis of InSAR deformation, and (3) two en echelon dike segments beneath mapped surface cracks and newly formed steaming areas. The en echelon dike model best fits near-field GPS and tilt data. Maximum opening of 2.4 m occurred on the eastern segment beneath the eruptive vent. Although this model represents the best fit to the ERZ data, it still fails to explain data from a coastal tiltmeter and GPS sites on Kīlauea's southwestern flank. The southwest flank GPS sites and the coastal tiltmeter exhibit deformation consistent with observations of previous slow slip events beneath Kīlauea's south flank, but inconsistent with observations of previous intrusions. Slow slip events at Kīlauea and elsewhere are thought to occur in a transition zone between locked and stably sliding zones of a fault. An inversion including slip on a basal decollement improves fit to these data and suggests a maximum of ∼15 cm of seaward fault motion, comparable to previous slow-slip events.

A recent gravity survey composed of 317 bench marks all over the island of La Palma (Canary Islands) is used, in combination with satellite data for regional aspects, to obtain results about structural properties of the island connected with the tectonic environment and local volcanism. To that end, a nonlinear three-dimensional gravity inversion approach is considered. The inversion scheme provides, in a nonsubjective form, the geometry of the anomalous bodies constructed in a random growth process. Results from the inversion can be interpreted in the framework of the geologic evolution of this ocean island volcano as a complex composite volcano with a large central body with high-density corresponding to the older intrusive part of the basalt complex. New unexpected features are enlightened, such as large thermal anomalies in the upper mantle southward of La Palma, as well as fracture en echelon zones associable to a slow active process of dislocation related to the recent volcanism in the southern half of the island. The results obtained for La Palma as a test site testify to the usefulness of the developed gravity inversion methodology for structural studies on islands in general.

[1] We use high-resolution seismic reflection profiles, boring transects, and mapping of fold scarps that deform late Quaternary and Holocene sediments to define the kinematic evolution, subsurface geometry, coseismic behavior, and fault slip rates for an active, basement-involved blind thrust system in central Japan. Coseismic fold scarps on the Yoro basement-involved fold are defined by narrow fold limbs and angular hinges on seismic profiles, suggesting that at least 3.9 km of fault slip is consumed by wedge thrust folding in the upper 10 km of the crust. The close coincidence and kinematic link between folded horizons and the underlying thrust geometry indicate that the Yoro basement-involved fold has accommodated slip at an average rate of 3.2 ± 0.1 mm/yr on a shallowly west dipping thrust fault since early Pleistocene time. Past large-magnitude earthquakes, including an historic M∼7.7 event in A.D. 1586 that occurred on the Yoro blind thrust, are shown to have produced discrete folding by curved hinge kink band migration above the eastward propagating tip of the wedge thrust. Coseismic fold scarps formed during the A.D. 1586 earthquake can be traced along the en echelon active folds that extend for at least 60 km, in spite of different styles of folding along the apparently hard-linked Nobi-Ise blind thrust system. We thus emphasize the importance of this multisegment earthquake rupture across these structures and the potential risk for similar future events in en echelon active fold and thrust belts.

[1] We examined fracture features within arctic sea ice as revealed by Landsat-7 and RADARSAT imagery, by an aerial survey, and through observations in the laboratory of specimens loaded to brittle failure under biaxial compression. Regardless of scale, which spans the range from kilometers to millimeters, the features look alike. They consist mainly of narrow lineaments that traverse the field of view, plus wing-like and comb-like secondary cracks. The lineaments occasionally intersect and generally exhibit either right-lateral or left-lateral relative movement, like strike-slip faults within Earth's crust. We term them brittle compressive shear faults. They form through the linking of en echelon arrays of deformation-induced secondary cracks. From an application of wing-crack and comb-crack mechanics, we estimated the maximum compressive stress near the onset of faulting and found that our estimates compare favorably with in situ measurements by earlier investigators of ice sheet failure stresses. To account for the brittle behavior of the sea ice cover, we applied a recent model of the ductile-to-brittle transition in which the key idea is the competition between stress buildup and stress relaxation at stress concentrators. In identifying the nature of sea ice fracture features, we advance the view that failure occurs on many scales through highly localized as opposed to uniformly distributed deformation, via the operation of scale-independent mechanisms.

[1] This study investigates whether the three-dimensionality of erosion and deposition can influence three-dimensional folding, even when compression is purely convergent. This problem is studied by employing a three-dimensional mathematical model incorporating dynamic thin-plate deformation coupled with fluvial and hillslope sediment transport. The approach taken is to impose several fundamentally different sediment routing patterns (by changing the initial regional topographic slope, β, and the ratio of the characteristic timescales for deformation to fluvial surface processes, R) and to investigate how deformation responds. Numerical results indicate that when β and/or R are small (i.e., initial topography is regionally flat and/or rates of surface processes are slow compared to rate of imposed deformation), folding is two-dimensional and unaffected by surface processes since loads created by surface processes are imposed after deformation. This situation favors development of small-scale drainage networks transporting sediment from anticlines to adjacent basins. When β and/or R are relatively large, the development of a large-scale transverse drainage network, which can be maintained during folding, is able to strongly amplify and localize folding. This leads to interesting features where the largest transverse rivers coincide with the greatest structural and topographic amplitude of doubly plunging folds, which is caused by erosion-enhanced deformation. For intermediate parameter values, folding and surface processes interact in a complex manner, giving rise to three-dimensional en echelon structures that migrate in a transient manner. This study demonstrates that under some circumstances, one can anticipate major two-way interaction between surface processes and three-dimensional deformation.

Tectonic deformation rates in and around the Apennine mountains of Italy are studied using seismic moments estimated from macroseismic effects of historical earthquakes. Northeastward extension in the northern Apennines (north of ∼42.5°N) accompanies shortening along their northeast flank. Since the seventeenth century, the sparse seismicity in these two zones has included no earthquake with magnitude >6.5 or seismic moment above ∼6×1018 N m. Their spatially averaged deformation rates are only ∼0.3 mm yr−1, but are equal, such that the extension and shortening balance with no relative motion between their external surroundings. In contrast, the numerous historical earthquakes in the central and southern Apennines with magnitude ∼7 and seismic moment ∼20×1018 N m require northeastward relative velocity across the deforming zone up to ∼5 mm yr−1, matching the expected relative motion of their surroundings. This relative velocity is taken up in part by extension and in part by strike-slip, depending on the trend of the deforming zone, and is not accompanied by en echelon shortening elsewhere in Italy. The northern Apennines thus show different senses and rates of deformation from localities farther south, and are thus technically distinct at present, in contrast with previous interpretations.

The November 30, 1974, ML = 5.5 and November 16, 1983, ML = 6.6 earthquakes generated left-stepping, en echelon ground cracks within the Kaoiki seismic zone, on the southeast flank of Mauna Loa volcano, Hawaii. The general trend of the ruptures, N48°–55°E, parallels a nodal plane of the main shocks' focal mechanisms. The ruptures themselves consist of short, predominantly extension cracks, which are up to 20 m long and strike roughly E-W, 30°–50° clockwise from the overall trend of the zones. Some of the cracks are linked by secondary fractures and rubble breccia to form left-stepping crack arrays, which are themselves linked to form longer en echelon systems of ground rupture. Geologic maps and field observations indicate that these features emerge from an underlying strike-slip fault, and they form a “fracture-process zone” above its tip. The maximum displacement measured across cracks in the 1983 rupture zone is 0.5 m. Trilateration data, however, suggests that the overall shear displacement was about 1.5 m at depth. Elastic solutions indicate that a region of significant tensile stress can exist above buried strike-slip faults. We suggest that these stresses generated the extensional ground cracks and that shear displacements were transmitted to the Earth's surface by subsequent growth and linkage of these cracks into the observed arrays. We infer that the crack arrays accommodate increased displacement with depth and they merge downward into the “parent” strike-slip fault at an estimated 1–2 km depth, where strike-slip displacement was probably more or less continuous along the ∼7 km length of the rupture. In the Kaoiki region, only three major ground ruptures traverse a series of basaltic lava flows that date back 1500 years. This suggests that the recent ∼10-year periodicity of moderate-magnitude Kaoiki strike-slip events may not have extended far into the past. The tectonic significance of strike-slip faulting on Mauna Loa volcano remains enigmatic.

A model for earthquake swarms in volcanic regions consists of the following concepts: (1) clusters of magma-filled dikes exist within brittle volumes of the crust, (2) dikes within a cluster are systematically oriented with their long dimension in the direction of the regional greatest principal stress, and (3) a sequence of shear failures (an earthquake swarm) occurs along a system of conjugate fault planes joining en echelon offset dike tips at oblique angles. This model accounts for commonly observed geometric relations between surface faulting patterns, the hypocentral distribution of swarm earthquakes, and fault plane solutions in a variety of situations. Swarm areas dominated by strike-slip faulting, however, provide the most compelling examples of the utility of the model. Specific examples considered here include a swarm on the east rift zone of Kilauea volcano, Hawaii, and swarms in the Imperial Valley, California, and the Reykjanes Peninsula, Iceland, which represent transitional zones between spreading centers and transform faults.

The Canton Trough is an extremely deep linear feature flanked by large en echelon ridges. This system, located in the equatorial Central Pacific Basin, can be traced ENE from the Phoenix Islands for a distance of more than 500 km. The surrounding topography is lineated parallel to the ridge-trough-ridge system and probably originated by block faulting. A similar mode of formation is inferred for the main trough itself, the implication being that the vertical displacement may exceed 3000 m in some localities. This interpretation is reinforced by the occurrence near the base of the trough of an unusually diverse assemblage of igneous rocks, including normal oceanic tholeiitic basalts, coarse-grained diabases, anorthositic gabbros, pegmatitic gabbros, and cumulate gabbros. Simple vertical displacements of crustal blocks also could produce the major components of the observed gravity anomaly signatures as well as explain the necessity in the gravity models for a crustal root beneath the Canton Trough. Sediments within the Canton Trough region are predominately radiolarian oozes and calcareous turbidites, which correlate with transparent layers and highly stratified units observed on reflection profiler records. Magnetic studies of the region reveal 650-γ residual anomalies, which are at least partly produced by the great topographic relief of the ridge-trough-ridge zone. However, the anomalies appear to obliquely cross the structural fabric and extend beyond the bathymetrie terminus of the trough and ridges. This configuration indicates that some components of the magnetic signatures are generated by another mechanism. Although there is some evidence that the Canton Trough and its surrounding structures represent an extension of the Clipperton Fracture Zone into the Central Pacific Basin, the absence of structural continuity, the lack of regional depth changes, and the limitations imposed by the regional tectonic patterns argue against this hypothesis. It is possible that the Canton Trough zone was created by an abrupt change from the Phoenix spreading direction to that inferred across the Manihiki Plateaus.

Data pertaining to the instantaneous relative motions of the Caribbean plate with respect to the North American and Cocos plates have been inverted by using the iterative fitting algorithm formulated by Minster et al. (1974). The best-fitting Caribbean-North American pole is located at 50°N ± 18°, 116°E ± 9°, and the computed angular rate is 0.20° ± 0.07°/m.y., which corresponds to a spreading rate of 2.1 cm/yr across the mid-Cayman rise. The model predicts that the present-day motion of South America with respect to the Caribbean is northwestward and suggests that since the late Tertiary this motion has been accommodated along an en echelon series of northwest-trending right lateral strike-slip faults and northeast-trending thrust faults which occupy a broad zone of deformation extending from the Curacao Ridge and its westward extension into the South Caribbean Basin to the frontal thrusts along the southern boundary of the Venezuelan Coast Ranges. The model predicts that the relative motion between the Caribbean and Nazca plates is parallel to the continental margin south of the Gulf of Panama. The rate of motion of the Caribbean plate with respect to the mesosphere, deduced from the fixed hot spot hypothesis, is very small, and this plate may in fact be stationary over the mantle.

Many linear island chains in the Pacific appear to consist of individual volcanic shields that lie on relatively short, curved loci sometimes of sigmoidal form. These loci, in turn, lie at an angle to the directions of propagation of the chains. Those chains that now trend just south of east (the Pratt-Welker, Hawaiian, Tuamotu, and Austral chains) consist of en echelon sets of loci with right lateral (clockwise) sense of stepwise overlap, whereas the supposed older extensions of these chains that lie nearly north-south (the Emperor and Ellice-Gilbert-Marshall chains) consist of sets of loci with left lateral (counterclockwise) sense of stepwise overlap. Rift zones of isolated volcanoes in the chains, that is, those volcanoes whose local stress fields were not influenced by the buttressing effects of near-neighbor volcanoes at the time of their formation, show the same orientation as the loci. First-motion studies of earthquakes in the Pacific lithosphere to date are few in number and give a variety of solutions with inferred directions of maximum principal compression (P) nearly horizontal and trending in both northeasterly-southwesterly and northwesterly-southeasterly directions. However, the number of such solutions is small, in some cases related to edifice effects, and interpretations relating compression and dilatation to principal stress directions in magmatic source regions are open to question. We consider it likely that the dominant orientation patterns have been due to dynamic effects related to the overall kinematic patterns of Pacific plate motions. Although a large number of different factors can influence the inherent and transmitted stresses in the lithosphere, and thereby influence the locally dominant stress field, we conclude that the effective stress orientations in the very recent past and for about the last 40–50 m.y. can be considered to have been caused by dynamic forces reflected in a right lateral rotational couple acting within the plane of the Pacific plate. These forces have induced maximum (S1) principal compressional stress directions that have had surface traces trending about northwest-southeast, relating to minimum (S3) principal stress directions that have trended northeast-southwest. Prior to 40–50 m.y. ago the dominant stress orientations in the plate were caused by a tendency for left lateral stress rotations, which induced maximum (S1) principal compressional stress directions with surface traces that trended just west of north and minimum (S3) principal stress directions that trended just north of east. These data and interpretations are independent of whether or not a change in direction of motion of the Pacific lithospheric plate took place roughly at the time represented by the bends in the island chains and are also independent of arguments as to whether melting anomalies of the Pacific are rigidly fixed or whether they are better explained in terms of thermal plumes or gravitational anchors. We conclude that the trends and age correlations of volcanic loci in the Pacific accurately track and identify the evolution of states of stress in the Pacific lithosphere with time. As more age data for linear island edifices become available, it should be possible to construct a Pacific-wide chronology of volcanism independent of but similar to that developed from magnetic reversals. Further, it appears that changes in stress directions in the plate have been episodic and that they may correlate with episodic magma generation not only in the central part of the Pacific plate but also around its margins.

The growth of cracks in photoelastic material and glass under compression is being studied as part of an investigation of brittle fracture of rock. In compression the most severely stressed crack is inclined at about 30° to the axis of compression. Such cracks, when either isolated or placed in an array, grow along a curved path which becomes parallel with the direction of compression. When this direction is attained, growth stops, unless applied compression is increased considerably. Cracks in certain en echelon arrays start to grow at much smaller applied stress than that required to enlarge an isolated crack.

The Mexia-Talco fault zone is one segment of a structural zone that forms the northern rim of the Gulf of Mexico basin. A gross graben structure is commonly developed in the belt of en echelon faults; thus, the zone is an extensional feature. In northeast Texas the Mexia-Talco zone bends abruptly by 75° in Hunt County, in an area largely covered by Holocene fluviatile deposits. The near-surface position of individual faults is weakly expressed in the gravity field. The faults apparently maintain their trends into the great bend and in an alternate fashion mutually terminate against fault members of the opposite trend; however, because of the en echelon arrangement in the two segments of the zone that meet at the bend, the change in the trend of individual fault strikes is less than half that of the zone as a whole. The zone coincides with a broad ‘step’ anomaly wherein gravity increases 6.8 mgal relatively abruptly from the southeast to the north and west sides of the zone. Model computations suggest that the Jurassic Louann salt thins from a thickness of 350 meters on the Gulfward side to zero within the zone. The geometry of individual faults and the zone as a whole is compatible with an extensional mechanical scheme wherein Mesozoic and Tertiary sediments were translated passively a short distance basinward, facilitated by long-duration salt flowage.

A gravity survey was made over the western Snake River Plain, Idaho, during the summers of 1959 and 1960. The data were reduced to sea level and expressed as simple Bouguer gravity anomalies for an assumed density of 2.67 g/cm3. Three elongated, northwest-trending, en echelon gravity highs were defined by the survey. The largest high is about 150 km long and 40 km wide; its maximum amplitude is 70 mgal. Application of limiting-depth methods implies that the tops of the disturbing bodies can be no deeper than 5 km below sea level. Two-dimensional analyses, based on a density contrast of 0.3 g/cm3, indicate that the anomaly-causing bodies extend from near sea level to at least 6 km below sea level and possibly to as much as 18 km below sea level. The total mass excess of the disturbing bodies is estimated to be 1×1019 g, and the corresponding volume for material 0.3 g/cm3 more dense than the surrounding material is approximately 33,000 km3. If the highlands on either side of the plain are regionally compensated, the crust under the plain is probably undercompensated by an amount proportional to the local anomalous bodies.

A GLORIA side scan sonar survey along the crest of the midocean ridge in the Gulf of Aden has provided information on the small-scale morphology of its spreading centers and fracture zones. Spreading center offsets increase in length to the east. This is thought to result from changes in the behavior of the mantle with increasing distance from the Afar hot spot. A 300-km-long spreading center in the western Gulf of Aden is oriented obliquely to the spreading direction. Nested within the obliquely oriented median valley is an en echelon arrangement of a number of smaller, elongated basins, each oriented approximately normal to the spreading direction. Close to the axis of spreading, volcanic and closely spaced tectonic structures run approximately normal to the spreading direction. Higher on the walls of the median valley, more-widely-spaced normal faults have formed subparallel to the valley. In the western Gulf of Aden there are no indications of sonar targets that might represent transform faults parallel to the NNE spreading direction. In the east, spreading center offsets of 15–25 km are made at fracture zones which are best described as narrow zones of highly oblique spreading. Offsets of greater than 25 km are achieved on single transform faults, and the associated tectonic fabric has a spreading direction trend. The boundaries between crusts of different ages along inactive transform fault extensions lie along the bases of fracture zone scarps. These scarps are situated in the older crust and face the younger. This is the opposite of the situation predicted on the basis of simple plate cooling models. At some places in the Gulf of Aden the fracture zone topography is very subdued, and this is attributed to the failure of the two slabs of lithosphere at the end of the transform fault to couple completely before the younger lithosphere on the nontransform side of the spreading center completes its ascent of the median valley wall. There is evidence of a change of spreading direction 3.5 m.y. ago, from about 040° to 022°. It is argued that the small-scale morphology of the transform fault/spreading center intersections can be understood in terms of the transfer of shear fracture on a transform fault to tensile fracture on a spreading center propagating away from the intersection. It is concluded that the transform fault must be weaker than the spreading center.

The Chang Ma fault, in the western Qilian Shan, Gansu province, shows evidence of consistent left-lateral displacements despite a complex en echelon geometry. We have measured short- and long-term offsets on two N70°–80°E striking segments of the fault, along the southern edge of the Ta Quen Kou basin, where it cuts late Quaternary terraces, transverse ridges, and drainage channels. To analyze the offsets of such structures at one site in the eastern part of the basin, we ran topographic profiles parallel and perpendicular to the fault over a distance of about 5 km. Spectral analysis and comparison of longitudinal profiles north and south of the fault allowed us to separate horizontal offsets of structures of different wavelengths, apparent on air photos and satellite images. Transverse profiles were used to determine the vertical throw, whose variations appear to result from changes in the fault strike. Small horizontal offsets of ridge crests and valley slopes are about 55 m. Along the N70°E striking segment of the fault, this displacement corresponds to an uplift of the northern block of about 27 m implying that the fault dips about 73° to the South. The measurements also constrain horizontal projection of the slip vector to about N78°E. A larger horizontal offset (≈500 m) of morphological swells of longer wavelength (≈1.6 km) is visible on the longitudinal profiles. Assuming that periglacial erosion has reshaped the surface of the lateral slopes and tops of ridges during the Wurm glacial period, the smallest offsets of adjacent ridge crests may postdate the begining of the Holocene, yielding an average rate of left-lateral slip on the fault of 5.5±2.2 mm/yr in the last 10±2 kyr. In the western part of the Ta Quen Kou basin, a somewhat slower rate of Holocene slip (4.6±.4 mm/yr) is obtained. Measurement of the horizontal displacement due to the 1932 earthquake there (6.2±0.6 m) would imply that the recurrence interval for similar earthquakes at this second site is 1350±540 years.

Seismicity along the San Jacinto fault suggests that a 40-km-long section near the town of Anza may constitute a gap in the occurrence of ML = 6–7 earthquakes. The potential for a gap-filling shock and the high rate of seismicity at the southern end of the gap (five events of 4.0 ≤ ML ≤ 5.5 since 1970) provided the impetus for deploying a digital seismic array to collect high-quality ground motion recordings of all events 2 ≤ ML ≤ 4.5 (on-scale recording for shocks with magnitudes above ML = 4 can be obtained from an existing array of eight strong motion accelerographs). The Anza site also had the advantage of being in the southern California batholith, which appears to be relatively homogeneous compared to the Franciscan/Gablian contrast of the central San Andreas; we expected that the granitic rocks of the batholith would yield relatively accurate earthquake locations and efficiently propagate high frequencies. The field instrumentation is specifically designed for broadband recording (up to 70 Hz) and high dynamic range (96 dB in the on-site digitizer alone), since both are necessary for determining the rupture history of earthquakes. Both local VHF and microwave digital telemetry transmit the data from the Anza region to San Diego for computer data logging. In the first 30 months of the array's operation, approximately 292 events have been recorded, located, and processed for source parameters. Most events occur in one of five clusters or in a diffuse zone near the Buck Ridge fault. Two of these clusters are located at right-stepping en echelon offsets (Coyote Creek-San Jacinto and San Jacinto-Hot Springs); two others are directly below and about 8 km west of Anza, respectively. The fifth cluster is just to the northeast of the Hot Springs fault. Although event depths are generally between 11 and 14 km, at the southern end of the Hot Springs fault, depths extend to 18 km; these are some of the deepest strike-slip earthquakes on the San Andreas system. We calculate source parameters such as the scalar moment and stress drop for the analysis of high-frequency waves, scaling relations, and earthquake interaction. The largest event recorded thus far had a moment of 4.4 × 1021 dyn cm (M = 3.8) and a stress drop of 55 bars. Both arms and Brune stress drops increase with moment; source radius increases only slowly with moment. The maximum values of both the Brune and arms stress drops increase with depth down to 10 km, remain approximately constant to 14 km, and may decrease below 14 km. The data suggest that stress drops of a group of earthquakes can be related to the strength of the upper crust calculated from frictional and quasi-plastic flow laws, although individual events may not be related. Stress drops on the San Jacinto fault are high compared to those on the central creeping section of the San Andreas, where stress drops are about 10 bars or less. This observation is consistent with the relative rupture area of events of equal ML (5.5) and may be related to the lithologie differences between the two fault segments and the amount of normal stress compared to shear stress.

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.

In this paper we describe a Gloria long-range sidescan sonar survey of the area around 95.5°W on the Cocos-Nazca Rise, where a propagating rift has been predicted to occur. The sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1-Ma -old crust. The new crust is up to 800 m deeper than the surrounding seafloor, although the elevation difference appears to decay with time. The ‘pseudofaults’ which bound this new crust have been observed from the rift tip back to crustal isochron 0.65 Ma. Near the rift tip they consist of scarps several hundred meters high with narrow, shallow valleys at their feet. The ‘sheared zone’ between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and NNW–SSE trending lineaments. These may have been produced by the rotation of tectonic fabric formed at the dying rift, perhaps accompanied by other deformation. The youngest tectonic and volcanic lineaments on both spreading segments strike 273°, but on the dying rift, short spreading segments are offset en echelon so that the regional trend of the rise axis is 265°. Older lineaments throughout the area strike 265°. The propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction, and it is suggested that this is occurring in response to a recent change in spreading direction from 355° to 003°. Variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273° direction, which we take to be normal to the new spreading direction.

A general study of Caribbean plate tectonics is first focused on the determination of fault parameters and source processes of the Caracas (Venezuela) earthquake of July 29, 1967 (mb = 6.5, Ms = 6.7). Synthetic seismograms which closely reproduce the observed P, SH, and Love wave seismograms were generated using generalized ray and mode theories. The results indicate a complicated faulting process, consisting of at least three separated sources aligned along a Nl0°W trending ‘en echelon’ vertical left lateral strike slip system of three faults that ruptured from north to south, at three discrete places with an extreme separation of 90 km. The process of rupture progressed southward with a mean velocity of 3 km/s. The focal depths of the individual sources varied between 8 and 27.5 km. The total dislocation was calculated as 120 cm along the direction N 10°W, and the total average moment as 4 × 1026 dyn cm. The multiple character of the event severely constrains the number of suitable source models that can be inferred, thus facilitating the process of inversion. Tectonic implications are briefly discussed, and local geology is successfully invoked to support the source model.

As part of the Rivera Ocean Seismic Experiment, a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the Orozco transform fault in the central eastern Pacific. We present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network. All epicenters are within the transform region of the Orozco Fracture Zone and clearly delineate the active plate boundary. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone (region 1). Most focal depths for these events are very shallow, within 4 km of the seafloor; several well-determined focal depths, however, are as great as 7 km. No shallowing of seismic activity is observed as the rise-transform intersection is approached; to the contrary, the deepest events are within 10 km of the intersection. First motion polarities for most of the earthquakes in region 1 are compatible with right-lateral strike slip faulting along a nearly vertical plane, striking parallel to the spreading direction. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity in this region is more scattered than in the first, and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. Pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions. The effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three-dimensional velocity grid. While findings for events in region 1 are not significantly affected, in region 2, epicentral mislocations of up to 10 km and azimuthal deflections of up to 45° may result from assuming a laterally homogeneous velocity structure. When corrected for the effects of lateral heterogeneity, the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending NW–SE; the focal depths, however, are poorly constrained. These results suggest an en echelon spreading center or leaky transform regime in the central transform region.

The Ninetyeast Ridge, far from being ‘aseismic’, has historically been a region of substantial seismicity. Since 1913, 4 magnitude 7 or greater earthquakes (including one with Ms = 7.7) and 10 magnitude 6 events have occurred in this general area. We have determined the mechanisms of several of these earthquakes, which suggest that the Ninetyeast Ridge area is presently a complex zone of deformation within the Indian plate. The northern portion (3°N–10°S) of the ridge is the active seismic zone, where both vertical and strike slip motion occur, while further south the ridge is far less seismic. This transition roughly coincides with a change in the ridge's morphology from irregular en echelon blocks to a smooth flat-topped high. The strike slip motion is left lateral, which is consistent with the Indian (west) side encountering resistance due to the collision with Asia while the Australian (east) side is subducting smoothly at the Sumatra trench. South of about 9°S the style of deformation differs on the two sides of the ridge. To the east, normal faulting occurs, which may be related to the formation of grabenlike structures. To the west the topography can be interpreted as the result of NW-SE compression which takes place largely aseismically but is observed for one large earthquake. This signficant intraplate deformation may explain the difficulties that occur in attempts to close the India-Africa-Antarctica triple junction using a rigid Indian plate.

The San Clemente fault, located in the California Continental Borderland, is an active, northwest trending, right-lateral, wrench fault. Sea Beam data are used to map the major tectonic landforms associated with active submarine faulting in detail unavailable using conventional echo-sounding or seismic reflection data. In the area between North San Clemente Basin and Fortymile Bank, the major late Cenozoic faults are delineated by alignments of numerous tectonic landforms, including scarps, linear trenches, benches, and sags. Character and spatial patterns of these landforms are consistent with dextral wrench faulting, although vertical offsets may be substantial locally. The main trace of the San Clemente fault cuts a straight path directly across the rugged topography of the region, evidence of a steeply dipping fault surface. Basins or sags located at each right step in the en echelon pattern of faults are manifestations of pull-apart basin development in a right-slip fault zone. Seismic reflection profiles show offset reflectors and a graben in late Quaternary turbidites of the Navy Fan, where the fault zone follows a more northerly trend. Modern tectonic activity along the San Clemente fault zone is demonstrated by numerous earthquakes with epicenters located along the fault's trend. The average strike of the San Clemente fault is parallel to the predicted Pacific-North American relative plate motion vector at this location. Therefore we conclude that the San Clemente fault zone is a part of the broad Pacific-North American transform plate boundary and that the southern California region may be considered as a broad shear zone.

A continuous Sea Beam swath along 2500 km of the crest of the fast-spreading East Pacific Rise (EPR) defines the location and morphology of seven systems of spreading centers. They are bounded by six transform fault systems, where reconnaissance swaths mapped additional short axes of plate accretion. The spreading center systems are subdivided by nontransform offsets, most of which step right on the central part of the plate boundary (12°S–4.5°S) and left on the northern and southern parts. These staircases of small lateral offsets were probably created by changes in spreading direction, the right steps by a counterclockwise rotation as the 12°S–4.5°S EPR matured after 6 Ma from the propagating western boundary of the Bauer microplate. Because the evolution of this microplate also eliminated transform offsets inherited from the Pacific-Farallon boundary (the pre-24-Ma ancestor of the Pacific-Nazca EPR), led to segmentation of the (fossil) Galapagos Rise, and created the present right-offset transform fault systems on the EPR, it is identified as the principal event in the development of the mapped plan pattern. A recent small clockwise rotation in Pacific-Nazca relative motion is inferred to be responsible for most of the small left steps along the boundary and for segmentation of the transforms into systems of en echelon fault zones linked by short intratransform spreading centers. The plan segmentation that results from the plate and microplate histories obstructs the shallow horizontal distribution of magma along the rise crest, allowing spatial variations in the rate of vertical magma delivery to be topographically expressed by the variable cross section of the axial ridge and by the along-strike relief of the neovolcanic zone. The pattern of variations is much more complex than that predicted by models of transform-bounded asthenosphere upwelling and regularly spaced melt diapirs. For example, the shallowest spreading segments, as well as the deepest, are short links between long fault zones in transform fault systems, and some long straight parts of the rise crest have humped long profiles consistent with a diapir-fed model, whereas others have a remarkably flat crest line with no topographic evidence for spreading cells.

Water levels have been continuously recorded since March 1978 in a well in Fremont Valley, where several strands of the adjacent Garlock fault zone have exhibited both left-lateral displacement and components of normal displacement. Differences in water levels indicate that a fault segment lies between the observation well and a nearby irrigation well. During the 4-year recording period, six sharp fluctuations, or “spikes,” were noted. These fluctuations, occurring over 2- to 4-day periods, have amplitudes of 15–30 cm. They appear to be the result of creep events on a nearby fault. Two types of creep events are plausible: (1) normal slip on an en echelon trace of the Garlock fault less than 300 m south of the well, with the north side up relative to Fremont Valley or (2) left-lateral slip on the same fault. Because of the nature of the fluctuations we favor the latter interpretation. Dislocation models utilizing exponential, arc tangent, and skewed cosine functions were used to analyze the water level fluctuations, associated pressure distribution, and fault displacements. The results suggest that creep on the fault ranges from several millimeters to a centimeter for individual events. Estimates of cumulative creep for the period 1978–1982 range from 20 to 50 mm, depending on the particular model employed.

Geophysical and petrologic data are presented on the northern extent of the Reykjanes Ridge. As it nears Iceland the crest of the Reykjanes Ridge is a small belt of rough topography superimposed on an elevated plateau. The rough topography consists of a series of en echelon ridges. Immediately to the southwest of the Reykjanes Peninsula the ridges strike about N35°E and apparently represent a rotational adjustment toward the N40°E trends on Iceland. To the south the en echelon ridges strike about N25°E. Magnetic profiles place the peak of the magnetic anomaly either directly below or along the eastern side of the topographic highs. Eight distinctive en echelon volcanic systems (swarms) are identified and are structurally and petrologically indistinguishable from those of the western Reykjanes Ridge. Petrologically, three rock types (picrite basalts, olivine tholeiites, and tholeiites) were recovered. These form distinctive groups which match those described from the western Reykjanes Peninsula.

In the standard mission the low-low Geopotential Research Mission (GRM) pair fly close coplanar orbits in tandem at nearly constant distance and with a fixed mutual orientation for extended periods of time. Sensitivity to the gravity field is predominantly along-track at a single altitude. Increased geopotential accuracy and discrimination can be obtained in other configurations for which radial and cross-track information are also sensed strongly. One such configuration which is only a small change from the standard has the pair flying en echelon in slightly different orbit planes. The addition of cross-track information in this approach is shown to yield considerable improvement in the geopotential, especially in near-sectorial terms. Overall field accuracy is estimated to almost double with plane separations of only a few tenths of a degree (permitting fixed antenna operation). Many near-sectorial terms show gains of up to an order of magnitude over the equivalent standard mission. These benefits are particularly important for low-degree terms (l < 6) expected to show significant changes in the GRM lifetime due to past and present glacial activity.