SHmax Direction Research Articles

Tectonic stress within the New Madrid seismic zone

Refraction data indicate a significant high‐density rift pillow beneath the New Madrid seismic zone. We present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present‐day stress field, and we consider the implications for intraplate seismicity. These models were run for a loading time of 100 m.y. to account for relaxation and transfer of stress since the last reactivation of the rift in the mid‐Mesozoic. Results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes. The model predicts a maximum compression of 30–40 MPa above the rift pillow in the center of the rift axis. If the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10°–30° in the direction of SHmax (maximum horizontal compression) near the rift axis, the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces. The addition of the local stress associated with the rift pillow, however, results in an approximately 30% reduction in the resolved maximum horizontal shear stress. Thus, while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure, reduction in the resolved shear stress requires a separate mechanism for strength reduction. Results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m.y. and may still play a role in present‐day deformation and seismicity of the New Madrid seismic zone. Local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones.

Fault plane solutions and relocations of recent earthquakes in Middleton Place Summerville Seismic Zone near Charleston, South Carolina

Abstract The Middleton Place Summerville Seismic Zone (MPSSZ), located about 20 km northwest of Charleston is the most active seismic zone in South Carolina. Between 1980 and 1991, 58 events with Md 0.8 to 3.3 were recorded in MPSSZ. They lie in a diffuse area of 11 km by 14 km of which over two-thirds are located in a narrow 5 km by 6-km zone. The hypocentral depths range from 2 to 11 km with over 90% deeper than 4 km. Single fault plane solutions were obtained for 35 events. Based on the focal mechanisms the earthquakes were grouped into five subsets. The mean P-axis of all fault plane solutions is oriented N63°E, in general agreement with the direction of SHmax obtained from in situ stress measurements. Of the 35 events, 18 are associated with reverse faulting on NW - SE striking and SW dipping fault planes. These events were inferred to be associated with the Ashley River fault zone, which is not a planar feature, but is composed of short segments of varying strikes (N20°W to N70°W) and dips (40° to 70°SW). Eleven events were associated with strike-slip motion on NNE - SSW striking vertical faults and with thrust faulting on N - S oriented faults dipping to the west, respectively. These two sets are identified as being parts of the Woodstock fault zone. The concentrated zone of seismicity included events associated with both the ARF and WF zones suggesting that it is at the intersection of these two fault zones.

The regional intraplate stress field in South America

A compilation of lithospheric stress directions for continental South America and the inferred major patterns of the regional intraplate stress field are presented. Stress orientations are based primarily on earthquake focal mechanisms and Quaternary fault slip inversion published in the literature. Four new focal mechanisms based on short‐period P wave modeling, and selected centroid‐moment‐tensor solutions (published by U.S. Geological Survey) consistent with P wave first motions at South American and other World‐Wide Standard Seismograph Network stations are also included in the data base. The observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the South American plate. In the Andean plateau (altitudes greater than 3000 m), N‐S extensional stresses predominate. E‐W compressional stresses are observed in the sub‐Andean and platform regions extending up to about 1000 km east of the Andes. The maximum horizontal stress (SHmax) is uniformly oriented in the E‐W direction throughout western South America. Averages of the SHmax direction at grid points spaced 2.5° were taken as representing the “regional” field. The E‐W direction of this regional field is not affected by the change of strike of the Andean chain nor by the contact of a flat subducted slab underneath. The regional SHmax direction is oriented 15° more clockwise than the direction of the Nazca plate convergence. The difference between the regional SHmax orientations and the absolute plate motion may be only about 6°. This may indicate that contact forces with the Nazca plate may not be the only major contributor to the intraplate stresses in western South America. The eastern limit of this Andean stress province seems to coincide with aseismic regions in the Upper Amazon basin and in the Parana basin. In the central Amazonian region, seismicity and stress data suggest a seismic province with N‐S compressional stresses. The origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle Amazon basin. In northeastern Brazil, seismicity is characterized by upper crustal strike‐slip earthquakes bordering the Potiguar marginal basin. A model is proposed for this region in which the stress field is the result of a superposition of regional E‐W compressional stresses and local extensional stresses (oriented perpendicular to the continental margin) possibly related to density contrasts and sediment loading in the continental shelf.

Stress observations and driving force models for the South American Plate

Stress observations are compared to predictions of driving force models for the South American plate. Stress observations have the potential to give detailed information about the forces driving the plates, in that sets of forces with the same net torque can produce quite different stress patterns. For a two‐dimensional (vertical section) model of a plate there are three conditions for dynamical equilibrium: the net forces and the net torque about any point must all be zero. These conditions are used to estimate the magnitudes of forces as a function of age, with ridge push and slab pull as active forces, and plate drag and slab resistance as passive forces. The results are consistent with earlier published models, and indicate that the primary balance for an oceanic plate is between slab pull and the resisting drag forces. We generalize the model for the possible forces driving the South American plate and compare with observed stress measurements and kinematics. The South American plate has a small slab pull component, amounting to about one‐fifth the ridge push force, with the other components reasonably isolated geographically, Thus, the plate balance is largely between ridge push and plate drag. This gives an opportunity of resolving the effects of trench suction or collision at the subduction zones. Most remarkable is the nearly uniform E‐W orientation of the regional stress field, SHmax directions, which is extensional in the high Andes; a second feature is the N‐S orientations in the Amazon basin. The two‐dimensional, horizontal stress patterns are compared for successive models of combinations of driving forces. Models with ridge push, slab pull at the Scotia and Caribbean arcs, and trench suction near the west coast, balanced by plate drag, produce similar stress patterns to those observed. The magnitude of a possible trench suction force is estimated using a model with an eddy between the Nazca slab and the overlying South American plate.

The recent crustal stress field in central Europe: Trajectories and finite element modeling

The recent crustal stress field of Central Europe and especially of the adjoining areas to the east is presented in terms of the directions of maximum horizontal stress (SHmax). The analysis is based on fault plane solutions, in situ stress measurements, geologic fault slip determinations, and repeated precise geodetic triangulations. A bending of the direction of SHmax from the well‐known NW‐SE direction in the western part of the study area to directions of NE‐SW to E‐W in the eastern part is shown. First results on the recent crustal stress field of the study area were presented by Grünthal and Stromeyer (1986), who substantiated this tendency of bending in the central and eastern parts of the study area by few observations only. Therefore one aim of this paper was to compile observations on the areas with few data points. Generally, these additional data confirm the previously established pattern; in some areas, especially in the Pannonian basin, the stress features became more complicated compared with those solely based on a few data points. The second part of the paper presents steady state elastic finite element model calculations to provide some possible explanations of the observed stress orientations as a result of plate driving forces. Simulation of the North Atlantic seafloor spreading and the northward directed motion of the African plate by appropriate boundary loads produces a pattern of SHmax directions for the western part of the Eurasian plate which is compatible with the broad‐scale observed stress directions. Subregional anomalies such as the fanlike stress pattern perpendicular to the arc of the Western Alps or the radial directions around the Pannonian basin can be explained only when additional stress producing features are introduced overmodulating the regional field. Rigorous introduction of physically constrained model parameters for all these features was not feasible to date. Therefore the preliminary empirical model calculations presented in this paper are attempts to discuss which features have the most influence on the stress field. They are, in a regional scale, the North Atlantic seafloor spreading, the northward motion of the African and the Arabian plate, and obviously, subregionally, increased stiffness of the Apulian promontary as well as of the Bohemian massif.

Regional patterns of tectonic stress in Europe

Nearly 1500 stress orientation determinations are now available for Europe. The data come from earthquake focal mechanisms, overcoring measurements, well bore breakouts, hydraulic fracturing measurements, and young fault slip studies and sample the stress field from the surface to seismogenic depths. Three distinct regional patterns of maximum compressive horizontal stress (SHmax) orientation can be defined from these data: a consistent NW to NNW SHmax stress orientation in western Europe; a WNW‐ESE SHmax orientation in Scandinavia, similar to western Europe but with a larger variability of SHmax orientations; and a consistent E‐W SHmax orientation and N‐S extension in the Aegean Sea and western Anatolia. The different stress fields can be attributed to plate‐driving forces acting on the boundaries of the Eurasian plate, locally modified by lithospheric properties in different regions. On average, the orientation of maximum stress in western Europe is subparallel to the direction of relative plate motion between Africa and Europe and is rotated 17° clockwise from the direction of absolute plate motion. The uniformly oriented stress field in western Europe coincides with thin to medium lithospheric thickness (approximately 50–90 km) and high heat flow values (>80 m W/m2). In western Europe a predominance of strike‐slip focal mechanisms implies that the intermediate principal stress is vertical. The more irregular horizontal stress orientations in Scandinavia coincide with thick continental lithosphere (110–170 km) and low heat flow (<50 m W/m2). The cold thick lithosphere in this region may result in lower mean stresses associated with far‐field tectonic forces and allow the stress field to be more easily perturbed by local effects such as déglaciation flexure and topography. The stress field of the Aegean Sea and western Anatolia is consistent with N‐S extension in a back arc setting behind the Hellenic trench subduction zone. The stress field is influenced in places by regional geologic structures, e.g., in the Western Alps, where SHmax directions show a slight tendency toward a radial stress pattern. Not all major geologic structures, however, appear to affect the SHmax orientation, e.g., in the vicinity of the Rhine rift system horizontal stress orientations are continuous.

Tectonic stress field in the Indian subcontinent

A map of maximum horizontal compressive stress orientation in the Indian subcontinent has been prepared using orientations derived from three different stress indicators: borehole elongation breakouts, in situ hydraulic fracturing measurements, and earthquake focal mechanisms. Most part of the subcontinent appears to be characterized by a compressional stress regime (thrust and strikeslip faulting) imposed by plate boundary forces although SHmax orientations do not, in general, show clear correlation with the direction of motion of the Indian plate. Four provinces are recognized on the basis of regionally consistent orientations. These are the midcontinent stress province, the southern shield, the Bengal basin, and the Assam wedge. Their boundaries have been determined taking into consideration regional tectonics and seismicity. Central and northern India, including the Shillong Plateau stretching up to the great Himalaya, Pakistan, and Nepal are included in a broad “midcontinent” stress province characterized by NNE‐ENE oriented SHmax. The mean orientation of SHmax in this province is N23°E, subparallel to the direction of compression expected to arise from the net resistive forces at the Himalayan collision zone, suggesting that it is largely determined by the tectonic collision processes. Much of southern India (Mysore plateau and the high‐grade metamorphic terrain south of the plateau) appears to be part of a second stress province characterized by NW oriented SHmax. These appear close to those of the intraplate stress field prevailing in the central Indian Ocean. A third stress province was recognized in the Bengal basin including parts of West Bengal, Tripura, Manipur, and Mizoram in northeastern India and most of Bangladesh. This province extends eastward from the marginal fault in the western margin of the Bengal basin to the Indo‐Burma subduction zone and is bounded on the north by the E‐W striking Dauki fault. SHmax within the sedimentary pile of the Bengal basin is oriented in E‐W direction, while P axes of earthquakes within the basement and the crust beneath the basin and within the subducted slab of the Indian plate beneath the Indo‐Burman ranges generally trend north ‐ N30°E. SHmax orientations within the sedimentary pile of the basin are parallel to the local (approximately E‐W) direction of the convergence of Indian and Burmese plates, suggesting a casual relation to the resistive forces at the subduction zone in the Indo‐Burma region. Interestingly, the stress field in the basement and the crust beneath the Bengal basin and in the subducted slab is similar to the one prevailing in the midcontinent stress province. Assam wedge stress region occupies the northeastern corner of the Indian plate, including Upper Assam, Arunachal Pradesh, and much of Nagaland. This region subducts beneath a sharply bent continental collision boundary consisting of the northeastern limb of the Himalayan and northern limb of the Indo‐Burman fold belts. As a result, the stress field in this province is depth‐differentiated and most likely responsible for the absence of consistent SHmax directions.

中・西部日本の地殻応力

We compiled data on the direction of the crustal stress measured in-situ and estimated from shallow (depth≤35km) earthquake focal mechanisms in the central and western parts of Honshu. In the central part of Honshu, the stress data were compiled by Tsukahara and Ikeda in 1991. This report summarizes the stress state in a rather large area: the central and western parts of Honshu. The results are as follows. (1) The stress state estimated from the stress direction and the type of faulting in and around the Izu Peninsula are explained in terms of dynamic interaction between the Philippine Sea (PHS) plate and the Asian (ASA) plate. (2) In the central part of the Kanto plain and the Kanto mountains, the minimum horizontal compressive stress (SHmin) is equal to or smaller than the vertical stress (i. e., tensile stress), and the azimuthal direction is roughly east-west. This stress condition cannot be explained by the westward movement of the Pacific (PAC) plate or the northwestward movement of the PHS plate against the ASA plate. (3) In the areas of the northern part of Kanto, Shin'etsu, Gifu, Hokuriku, and Chugoku, the azimuthal direction of the maximum horizontal compressive stress (SHmax) is west-northwest=east-southeast. Although this direction is approximately the same as that of the movement of the PAC plate, the stress state in these areas is not attributed to the plate movement because (a) some provinces with quite different stress directions lie between the PAC plate and these areas, and (b) SHmin in these areas is tensile stress. It seems that the stress state of this area is dominated by a large scale crustal movement of the Japan Sea or Eastern Asia. (4) In the area of Lake Biwa to Osaka, the stress state is slightly different from that in the areas discussed in (3), i. e., (a) the azimuth of the SHmax direction is approximately east-west, and (b) the type of faulting is different. There are strike-slip and reverse faults as opposed to strike-slip faulting alone for the surrounding area. The reason why this area has its own unique characteristics is not yet understood. (5) In the central Shikoku region, the azimuth of the SHmax direction is approximately east-west and the stress state regime is of the strike-slip faulting type. The east to west SHmax azimuth agrees well with the maximum shortening direction obtained from geodetic surveys in the region during these 100 years, which is attributable to the large crustal deformation at the Nankai Earthquake (1946, M=8.1). In contrast, however, the maximum shortening direction after the earthquake has been north to south. Therefore, the east-west SHmaxazimuth will change to north-south in the future.

関東・東海地域の地殻応力

A map of the maximum compressive stress direction has been made for the Kanto-Tokai area from the data of in-situ stress measurements, focal mechanisms of earthquakes, and geological evidences. Data compiled, which amounts to 77 points, includes the following: 10 points of hydraulic fracturing stress measurments, 3 points of overcoring stress measurments, 7 points of Quaternary dike trends, 9 points of Quaternary monogenetic volcano alignments (including flank volcanoes), 41 points of active fault movements and 7 points of focal mechanism solutions of shallow earthquakes (depth <20km, M>6.5). The data points which lie close to one another give a similar stress condition: the stress orientation and stress state classified by fault type such as normal, reverse, and strike-slip faults. We have divided the Kanto-Tokai area into seven provinces on the basis of the data compiled after consideration of the plate interation among the Eurasian, the Philippine Sea and the Pacific plates. Stress condition within each province appears almost uniform. The seven stress provinces and their averaged azimuths for the maximum horizontal compressive stresses SHmax are as follows: southern part of Nagano Prefecture (NA), N85°W; northern part of the Kanto district (KA), N80°W; Shizuoka district (SZ), N40°W; Nishiizu district (NI), N10°E; Higashiizu district (HI), N40°W; Tanzawa district (TN), N-S; and the Miura-central Boso district (MB), N35°W. The azimuths of SHmax in stress provinces NA and KA are both nearly E-W, which is attributed to the westward relative movement of the Pacific plate against the Eurasian plate. The azimuths of N35°-40°W in provinces SZ, HI and MB are considered to be due to the northwestward movement of the Philippine Sea plate relative to the Eurasian plate. The cause for the individual orientation of SHmax as N10°E in province NI is interpreted as being a result of the downward bending of the Philippine Sea plate at the Suruga trough. That is, the extension perpendicular to the bending axis is effected on the surface by the downward bending, which result in the parallel orientation of the SHmax to the trough. In the case of province TN, the stress orientation in the province is not interpreted in terms of downward bending because the subduction has not been expected in the province. This is one of the subjects for future study. In stress province MB, the direction of SHmax agrees well with the horizontal maximum compressive strain accumulated during the period from 1882/1902 to 1973/80. However, the direction does not agree well with that of the strain accumulated during the period from 1924/25 to 1973/80. The difference is one reason for low seismicity in the upper crust in province MB, since current crustal movement does not work efficiently to increase the in-situ differential stress. The difference may be attributed to the long-term after-effect of the 1923 Kanto earthquake.