Component Of Strike-slip Motion Research Articles

Geomorphological and geophysical analyses of the Hebron Fault, SW Namibia: implications for stable continental region seismic hazard

SUMMARYThis study explores the geomorphological expression and geological context of a normal fault scarp in a stable continental region (SCR) which we interpret as having failed in large (Mw >7) earthquakes. Records of such large normal faulting events in an SCR (or even in more rapidly deforming regions) are extremely rare, and so understanding this feature is of international interest. The scarp is exceptionally well-preserved due to the extensive calcrete/silcrete cementation. In areas where this cementation is reduced or absent the scarp is more diffuse, as expected for a feature formed by one or more paleoearthquakes. The exceptional preservation aids comparison with data sets based on scarps which have formed more recently. Our analysis is based on a high-resolution digital elevation model of the Hebron Fault scarp in southern Namibia using pan-sharpened Worldview-3 satellite stereophotos (0.31 m resolution). We make scarp height measurements at 160 locations providing improved estimates of the average displacement (5.9 m), maximum displacement (10.1 m), and the minimum fault length (45 km). No consistent evidence of lateral displacements in water courses or alluvial fan margins were found implying predominantly normal displacement. A newly described section in the northwest has en-echelon scarps consistent with a component of strike-slip motion that may be explained by its difference in strike from the central section. Most channels crossing the fault show a single knick-point. The displacement varies smoothly as it crosses a number of different generations of alluvial fan surfaces. No evidence of a multiscarp or a composite scarp were observed. We have therefore found no evidence for a mutiple-event origin for the scarp, although, this lack of evidence does not conclusively demonstrate a single-event origin. Published regressions, based on the limited data available for SCRs, suggest that the mean expected average displacement ($\bar{D}_{\rm av}$) for a faults of this length is 1.2–3.1 m implying that the scarp is likely to have formed in 2–5 events with an expected Mw = ∼7.1 though displacements in individual events may exceed these average values. Comparison with the regional geology and aeromagnetic data sets suggests that the fault reactivates a Mesoproterozoic ductile structure, the Nam Shear Zone, and that the location, orientation and segmentation of the scarp is controlled by the alignment of pre-existing structurally weak zones with the present-day stress regime. The fault has undergone repeated brittle reactivation, accumulating ∼110 m of vertical offset since the deposition of the Ediacaran-to-Cambrian Nama Group. This is less than expected from global compilations of total displacement and fault length data, suggesting that the fault rapidly attained its current length by recruiting an existing weak zone and is expected to accumulate displacement at a relatively constant length in the future.

Structural Constraints and Localization of Gold Mineralization in Leather Jacket Lodes, Ballarat, Victoria, Australia

Gold mineralization at the Ballarat East deposit, central Victoria, Australia, is hosted in lodes that are historically known as “leather jackets.” These are quartz-dominant vein arrays related to low-displacement, W-dipping faults (≤45°) that transect the core and/or eastern limb of tight, asymmetric N-S-trending anticlines. The leather jacket lodes typically have dip extents from 5 to 65 m, widths of ≤20 m, and strike lengths up to hundreds of meters, but their along-strike continuity is disrupted by oblique, low-displacement listric faults known as “cross courses.” The gold lodes are characterized by distinct phases of sulfide paragenesis with minor gold + arsenopyrite + pyrite defining the early sulfide stage. Late-stage coarse gold was precipitated with galena + sphalerite ± pyrrhotite ± chalcopyrite (late pyrite also occurs). The gold mineralization events are linked to low-strain mineralized fracture networks, which are closely related to the final deformation stages and the amplification of the major folds enclosing the lodes. This amplification produced domal fold culminations, with plunges ≤30°, and localized minor parasitic folds with shallower plunges (≤10°). A network of dilation sites, on the W-dipping faults, preferentially developed in the cores of anticlines, particularly in zones where there are changes in strike of bedding or fault bifurcation and refraction through contrasting sandstone and interbedded packages of sandstone and shale. Numerical three-dimensional simulations were undertaken to test our geologic observations and replicate conditions controlling the emplacement of the leather jacket lodes. Two different scenarios were investigated: first, to determine how changes in the local stress field orientation influences dilation and fluid infiltration; secondly, to test variations in fault geometry during the last stages of deformation—that is, within the final 2% of shortening, when most of the mineralized sites were created. Results show that strain and fluid flow localized along refracted sections of faults and around changes in dip, specifically on the shallower dipping sections within subvertical sandstone units. This is consistent with the observation that high-grade gold-bearing quartz is associated with localized changes in fault dip in thicker sandstone and sandstone-shale packages. There was also a component of strike-slip motion and near-field NW-to-SE or N-to-S stress fields, which can be attributed to the development of a component of out-of-plane motion during the development of fold culminations. The preferred model for the distribution of the high-grade auriferous vein arrays defining the leather jacket lodes is one of fold amplification and extension parallel to the fold axes, which produced an increasing out-of-plane relaxation. The main fluid conduits responsible for the leather jacket style of mineralization involve infiltration along steep bedding-parallel faults and veins that link up with the arrays of low-displacement W-dipping faults.

Fault Geometry and the Dynamics of the 1999 Chi-Chi (Taiwan) Earthquake

The 1999 M 7.6 Chi-Chi (Taiwan) earthquake produced a data set of unparalleled size and quality, particularly in the near-source region where data have been previously quite scarce. The large amount of near-source data allows the verification of many predictions of thrust-fault behavior for faults that intersect the surface of the earth. Through rigorous three-dimensional dynamic models of the Chi-Chi earthquake, it can be shown that many aspects of the observed near-source ground motion in this event are direct effects of the asymmetrical dipping fault geometry. These effects include the hanging wall moving more than the footwall (with strongly peaked velocities right at the fault trace) and a transition from predominantly thrust motion in the south of the fault to largely left-lateral motion in the north. Building on the work of Oglesby and Day (2001), the current work helps to delineate the effects of fault geometry, nonuniform prestress, and dynamic waves on the physics of the Chi-Chi earthquake and dip-slip faults in general. In particular, we find that a completely homogeneous prestress pattern still fits the gross features of the near-source ground motion quite well. Additionally, the strike-slip component of motion near the fault trace is seen to be a combination of dynamic and static effects. Finally, dynamic overshoot is seen to be much larger for dip-slip faults than for otherwise identical vertical faults. The results emphasize the necessity of rigorous models that correctly account for both the effects of fault geometry and dynamic waves in the rupture and slip processes. Manuscript received 31 July 2000.

Effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin, California: Structural evolution

Offshore northern California, the Gorda plate is subducting obliquely beneath North America; the resulting complicated tectonic setting forms the southern end of the Cascadia subduction zone. The southern Cascadia subduction zone and overlying Eel River forearc basin lie just north of the unstable Mendocino triple junction. The Neogene strata of the Eel River basin record structural deformation caused by the underthrusting of the Gorda plate as well as deformation generated by northward migration and encroachment of the Mendocino triple junction. Three distinct deformation regimes are present in the Eel River forearc basin. (1) Along the western margin of the forearc basin and within the foreslope of the accretionary prism, thrust faults and anticlines record Pliocene– Pleistocene shortening caused by subduction of the Gorda plate. (2) The southern part of the basin rotated counterclockwise in the late Pleistocene, resulting in modern transpressional deformation offshore Humboldt Bay. The rotation and deformation are caused by north-south convergence across the boundary between the Pacific plate and the southernmost part of the forearc basin at the triple junction. (3) The northeastern margin of the Eel River basin is deformed by high-angle faults with a component of strike-slip motion that may represent the incipient northward propagation of the Pacific–North American transform system north of the triple junction.

A simplified model of stress-slip relationships: application to the Frøy field, northern North Sea

We apply an analytical method, Sortan, to determine fault slip as induced by far-field stress. The Sortan method is based on the Wallace–Bott hypothesis. The method is applied to all major faults of the Frøy oil field located at the eastern margin of the southern Viking Graben, North Sea. The purpose was to quantify the most likely senses of motion experienced by the faults during the Late Jurassic–Early Cretaceous rifting event.Because strike-slip displacement is generally difficult to determine based on classical exploration methods, particular attention was paid to the simulated strike-slip component along the major faults. Depth-converted structural maps were used to determine fault attitudes. The presence at the northern edge of the field of a major syn-rift dextral strike-slip fault, the North Frøy Fault, allowed us to constrain the most likely characteristics of the prevailing stress field during rifting. According to different possible stress states, different models were run. The results from the Sortan modelling indicate that the strike-slip component of motion was in general minor on the major faults within the oil field. The inferred stress state is found consistent with rifting involving fault-related displacement strongly dependent on σ 3 orientation. This stress state prevailed at least during the latest stages of rifting (Latest Jurassic–Early Cretaceous) of the northern North Sea. This case study shows that where paleostress inversion methods cannot be applied in the absence of direct observation, the Sortan method has the potential to constrain the slip behaviour of the brittle structure. It also enables one to rapidly check the overall mechanical consistency of tectonic interpretations.

Modelling the contemporary stress field and its implications for hydrocarbon exploration

The forces that act on the Earth’s lithospheric plates are responsible for the stress regime within the plates at a regional scale, and thus influence issues pertinent to hydrocarbon exploration – such as the nature of fault reactivation, hydraulic seal integrity, natural and induced fracture orientation and wellbore stability. Four models of the intraplate stress field of the Australian continent have been produced by elastic finite element modelling of the forces acting on the Indo-Australian plate. All four models incorporate the push of mid-ocean ridges and of continental margins. In the four models the magnitude of the poorly constrained convergent boundary and basal drag forces are varied within reasonable limits. Despite being based on significantly different force magnitudes, regional stress orientations predicted by the three models that recognise the heterogeneity of forces acting along the convergent northeastern boundary of the Indo-Australian plate are considered reliable because they are consistent over much of the Australian continent and show broad agreement with the available in situ stress measurements.In the absence of in situ stress measurements, for example from borehole breakouts, modelled stress orientations should be incorporated into the assessment of issues pertinent to hydrocarbon exploration that are influenced by the contemporary stress field. In the context of fault reactivation, pre-existing vertical faults striking at 30° to 45° to the maximum horizontal stress direction are the most prone to at least a component of strike-slip motion. Planes dipping in the minimum horizontal stress direction are the most suitably oriented to be reactivated in extension, and planes dipping in the maximum horizontal stress direction are the most suitably oriented to be reactivated in compression. Modelled stress orientations can also be used to predict open natural fracture orientation, with the preferred orientation being normal to the minimum horizontal stress, and to help assess the hydraulic integrity of reservoir seals, with faults and fractures normal to the minimum horizontal stress least likely to be sealing.

From oblique subduction to intra-continental transpression: Structures of the southern Kermadec-Hikurangi margin from multibeam bathymetry, side-scan sonar and seismic reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4–5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scrap of the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10–25 km for more than 200 km, and this is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone.

Quantitative Analysis of a Transpressional System, El Biod Arch, Ghadames Basin, Algeria: ABSTRACT

Trap definition within the northern extension of the Hassi Touareg - Rhourde El Baguel fault zone in the western Ghadames Basin of Algeria is difficult due to complex structural geometries. The fault zone consists of a narrow system of discontinuous. locally en echelon faults. Although north-trending to the south, the zone curves to a northeast trend to the north. Reserves associated with the southern portion of the system total 1500 MMBOR and 2 TCFG. Several lines of evidence support a strike-slip component of motion for the northern segment. Horizontal slickensides have been described in cores taken from wells within the fault trend. Fracture patterns measured from logs taken within the NE-SW fault trend show clusters expected for right-lateral Reidel shears. Although complicated by all evaporate sequence at mid-level in the stratigraphic section, we interpret downward converging faults imaged on recent 2D seismic as positive flower profiles. Map patterns are also interpreted as right-lateral, recognizing that the 2D grid cannot resolve all of the structural complexity. To confirm the component of strike-slip fault displacement, we applied a new quantitative method relating map view structural orientations to the shear magnitude, the degree of convergence or divergence, and the magnitudes of horizontal andmore » vertical strains. Strike-slip to convergence ratios ranging from 2:1 to 3:1 were measured in the study area. Higher ratios (10:1) measured above the salt may indicate a detachment. These ratios also fit the regional tectonic pattern: to the south, where the fault zone trends due north, structural geometries support dip-slip inversion indicative of east-west shortening. Applying the same shortening vector to the northeast-trending part of the zone suggests oblique right-lateral motion, with a strike-slip to convergence ratio of 2:1.« less

Structural and hydrogeological features of Pleistocene shear zones in the area of Rome (Central Italy)

The last tectonic episode observed in the Latium Tyrrhenian margin (Central Italy), few km cast of Rome, is represented by a set of middIe-upper Pleistocene N-S shear zones, characterised by complex geometric and kinematic setting. The easternmost of these shear zones displays a strike-slip component of motion and is located at the boundary between the Apennine carbonate chain and the volcanic areas. The distribution of travertine deposits and hydrothermal springs suggests that this fault zone acts as an impermeable barrier for lateral flow derived from superficial karstic circuit, and as a preferential upwelling surface for deep hydrothermal fluids. We propose that high fluid pressure could develop inside these fault zones favouring the reactivation of buried pre-existing crustal discontinuities and the local re-orientation of the stress field, as testified by the geometry and the kinematics of the surface fault pattern.

Tectonic effects of a subducting aseismic ridge: The subduction of the Nazca Ridge at the Peru Trench

A 1987 survey of the offshore Peru forearc using the SeaMARC II seafloor mapping system reveals that subduction of the Nazca Ridge has resulted in uplift of the lowermost forearc by as much as 1500 m. This uplift is seen in the varied depths of two forearc terraces opposite the subducting ridge. Uplift of the forearc has caused fracturing, minor surficial slumping, and increased erosion through small canyons and gullies. Oblique trending linear features on the forearc may be faults with a strike-slip component of motion caused by the oblique subduction of the Nazca Ridge. The trench in the zone of ridge subduction is nearly linear, with no re-entrant in the forearc due to subduction of the Nazca Ridge. Compressional deformation of the forearc due to subduction of the ridge is relatively minor, suggesting that the gently sloping Nazca Ridge is able to slide beneath the forearc without significantly deforming it. The structure of the forearc is similar to that revealed by other SeaMARC II surveys to the north, consisting of: 1) a narrow zone (10 to 15 km across) of accreted material making up the lower forearc; 2) a chaotic middle forearc; 3) outcropping consolidated material and draping sediment on the upper forearc; and 4) the smooth, sedimented forearc shelf.

The landscape geometry and active tectonics of northwest Greece

When a region is tectonically deformed, its geometry changes. Some of these changes produce easily identified and often readily datable morphological features such as regions of rapid uplift and subsidence, sediment ponds, or river terraces. These features are usually secondary to the main active structures and consequently do not provide information about them directly. We show, however, that simple models using boundary-element methods can be adapted to relate the evolution of these minor features to motion on major structures. This adds substantially to the information available to determine motion on such features. In this paper, we apply the technique to northwest Greece. We show that, in addition to the long recognized compressional component of motion in Epirus, a substantial left-lateral strike-slip component of motion must be present. The modeling allows us to identify a regional slip vector of N275°E. This slip vector suggests that the plate configuration assumed by previous workers must be modified. A new configuration is proposed that remains consistent with the data used for earlier interpretations and our new data. It is proposed that the Medina Wrench is a significant active structure and that a period of quiescence explains the low seismic activity at present. The plate configuration and angular rotation rates that we suggest require that the Gulf of Arta is associated with a triple junction. Using the slip vectors we have defined for the plates meeting at this junction, we model the regions of uplift and subsidence in the region. The ability of this model to explain many features associated with the Gulf provides assurance that our overall tectonic model is correct.

Geology of Reno and Truckee Meadows, Nevada, United States of America

Reno, situated within the Truckee Meadows, just a few miles east of the California state line, is located between two markedly different physiographic provinces. One province is the Sierra Nevada (to the west), a region characterized with high mountains and heavy winter snow. To the east is the semi-arid Basin and Range province. Consequently, Reno, and the surrounding area, contains a diverse geology and a highly varied and interesting suite of geotechnical challenges, which greatly influence land-use considerations in this rapidly growing urban area. Reno owes its origins to the immigrant trails from the Midwest to California in the 18409s and to the Comstock Lode silver discovery in the nearby Virginia Mountains, in the 18509s. Officially, it9s existence began in 1868, with the arrival of the Central Pacific Railroad and the need for a station to join with the future Virginia and Truckee Railroad to service the Comstock Lode. Truckee Meadows is a fault-bounded basin surrounded by mountain ranges composed mainly of Tertiary volcanic rocks and inliers of pre-Tertiary volcanic and granitic rocks. The Virginia Range, to the east, is composed entirely of hydrothermally altered Oligocene and Miocene volcanic rocks forming the famous Comstock silver mining district. On the west, the Carson Range segment of the Sierra Nevada is composed largely of Tertiary volcanic rocks, overlying older granites, granodiorites, and metamorphic rocks. A thick sequence (in excess of 330 m [1,000 ft]), of glacial-fluvio, alluvial fan, and lacustrine deposits of late Tertiary to Quaternary age underlies the Truckee basin. At the southern end of the Truckee Meadows is located Steamboat Hot Springs, which has been utilized for commercial and recreational purposes since the 18609s. Presently three small (12.5, 7 and 5.5 MW) electric power plants make use of this geothermal resource. Seismologically active, Reno lies along the northern segment of the Sierra Nevada fault zone, a series of north-south trending faults, separating the Sierra Nevada from the Great Basin. Fault movement is predominantly dip-slip, based on surface displacements, although focal plane solutions suggest a component of strike-slip motion. No large magnitude earthquakes (MS>7) have been noted in the Reno area during historic time. However, several moderate-sized earthquakes with surface rupture events were associated with the 1869, 1950, and 1966 events. Long-term recurrence intervals for large-magnitude earthquakes on this segment of the Sierra Nevada fault zone are believed to be of the order of several thousand years. Several rock and soil formations pose potential geotechnical problems or hazards. Foundation problems related to expansive soils are mainly due to the hydrothermal alteration of andesites of the Alta Formation and surface weathering of other volcanic rocks. Floodplain and lacustrine deposits in the Truckee basin are composed of clayey silts, silty sands and large areas of fine sands which are susceptible to liquefaction where the ground-water surface is close to the topographic surface. Two major slope stability hazards in the hills surrounding the basin are recognized: rockfall from the well-jointed and slabby volcanic units and debris flows in the weathered rock, lahars, and surficial deposits which mantle the hill slopes. Major slope instability is restricted to steeper hill slopes and the Truckee River canyon. Snow avalanche hazards exist in the winter on the highways leading from Reno to Lake Tahoe and into California. Flash floods and floodplain hazards are historically a common occurrence. These events are due to severe summer thunderstorms, heavy spring rains, or rapidly melting snow packs. Even though a 1963 channelization of the Truckee River has reduced the hazard, over $16 million dollars of flood-related damage occurred as recently as 1986. Numerous small and large-scale metal mines exist in the hills surrounding Reno, producing cinnabar, lead, silver and gold. The mines are now inactive but their presence produces mine-related hazards of ground subsidence or the dramatic and rapid collapse of their shafts, especially where workings underlie buildings and other structures. In common with many other cities in the west, a dependable and adequate water supply is at a premium. Reno9s water reservoir is the winter snow pack of the Sierra Nevada, which, in a good year, can exceed 1,000 cm (± 480 in.). The Truckee River is fed from snowmelt and Reno derives its water either directly from the river or from wells in the alluvium which borders the river. Overall water quality is good, but localized geothermal discharges can adversely affect ground-water quality. Solid waste, in excess of 220 tonnes (1,200 tons)/day, is placed in two sanitary landfills east of the city. Liquid aqueous waste requires special handling and after waste-water treatment, its sludge is taken to one of the landfills.

A block model of distributed deformation by faulting

It is clear from palaeomagnetic evidence that large and hitherto unexpected rotations about a vertical axis are common in regions of distributed continental deformation. We propose a simple two-dimensional model to illustrate how fault movement and block rotation within a zone of distributed deformation may be related to the relative motion of the rigid plates that bound the zone. One surprizing feature of this model is that the component of strike-slip motion on the faults within the deforming zone is in the opposite sense to the strike-slip component across the zone as a whole. Thus slip vectors within the zone are not the same as that between the bounding rigid plates: in contrast to deformation at oceanic plate boundaries. Examples of active fault geometries similar to those of our model can be seen in Greece, Iran and the western USA.

Evidence for Dominance of Normal Dip-Slip Motion on Segment of Flemington Fault in Newark Basin of New Jersey: ABSTRACT

A fracture study conducted on a series of outcrops along U.S. Route 202, beginning approximately 1.5 km east of the Delaware River and extending eastward for a distance of approximately 1.5 km, provides ample evidence for normal dip-slip faulting and virtually no evidence of strike-slip faulting. The study area is located approximately 1 km south of the Dilt's Corner fault, the southern splay of the Flemington fault, and along a line nearly perpendicular to the axis of a broad, open anticline plunging approximately 10°S, 20°E. This anticline is one of several transverse folds on the hanging-wall side of the Flemington-Dilt's Corner fault system. Measurement of more than 150 small faults revealed fewer than 10 with a significant strike-slip component of motion. Most of the small faults in the area strike northeasterly and dip steeply either to the northwest or to the southeast. Of particular interest are faults that strike N40°E and dip approximately 70° to the northwest or southeast. These faults appear to define a conjugate set, and as such would require ^sgr1 to be nearly vertical, ^sgr2 to be horizontal and trending N40°E, and ^sgr3 to be horizontal and trending S50°E. In 1962, Sanders suggested that in addition to considerable dip-slip displacement, the Flemington fault might have a major right-lateral strike-slip component of motion. Manspeizer used right-lateral strike-slip motion on the Flemington fault as part of his rhomb-graben model for the Newark basin. Recently, Burton and Ratcliffe have suggested that the Flemington fault has both right-lateral and normal components of motion on it. Strike-slip motion, at least on the Dilt's Corner splay of the Flemington fault, is not compatible with the observed field data. End_of_Article - Last_Page 1438------------

Earthquake focal mechanisms and the intraplate setting of the Bermuda Rise

A number of intraplate earthquakes occurring in the western North Atlantic Ocean are located near the perimeter of the Bermuda rise. Focal mechanisms and depths of two earthquakes, November 24, 1976 (mb 5.1; M0 = 2.96 × 1023 dyne cm) and March 24, 1978 (mb 6.1; M0 = 3.58 × 1025 dyne cm), were determined using Rayleigh wave amplitude data in the period range 20–50 s. The 1978 earthquake occurred approximately 380 km southwest of Bermuda, near magnetic anomaly M4 (≈118 m.y. B.P.). The focal mechanism for the 1978 event is of thrust type and has nodal planes striking 340°. The depth of this event is 6 km below the seafloor, near the local depth to Mono. The strike of the fault planes does not parallel the trends of either fracture zones (300°) or magnetic lineations (035°) in the area. The fault planes do, however, parallel the strike of a magnetic gradient in the epicentral area. The 1976 earthquake occurred approximately 300 km northeast of Bermuda, near Muir seamount. The depth of this event is 10 km below the seafloor. The available data are suggestive of one nodal plane striking between 320° and 340° and nearly parallel to the trend of Muir seamount and other volcanic features in the region. In contrast to the 1978 event, the 1976 earthquake appears to exhibit a significant component of strike slip motion. P axes of both mechanisms are subparallel to the direction of absolute plate motion for North America. We suggest, however, that strain release in the Bermuda rise area is not occurring along major fracture zones or topography parallel to seafloor spreading anomalies but rather on smaller‐scale structures. The stresses induced by variations of crustal thickness may be responsible for triggering intraplate seismicity in this region.

Surface wave focal mechanisms for eastern North American earthquakes with tectonic implications

Using P wave first motions together with amplitude and phase data from long‐period Love and Rayleigh waves, the focal mechanisms of 22 earthquakes occurring in eastern North America between 1962 and 1976 are determined. The techniques used yield good results for events with body wave magnitudes as small as 4.0. Eighteen of the events have depths between 5 and 16 km; 15 have significant components of strike slip motion; and for focal mechanisms having nearly horizontal pressure axes, 70% of the pressure axes trend northeast or east. Though the earthquake sample may be biased toward the central United States, it is significant that no focal depths greater than 25 km have been encountered. A confidence factor is assigned to each focal mechanism for use in future tectonic syntheses.