Axis Of Maximum Horizontal Compression Research Articles

The state of stress in the Earth's crust along the northwestern flank of the Pacific seismic focal zone before the Tohoku earthquake of 11 March 2011

This paper presents a detailed reconstruction of the state of stress in the Earth's crust in the Japanese seismic focal zone before the Tohoku earthquake of March 11, 2011. The stress parameters are determined using the method of cataclastic analysis (MCA) of discontinuous displacements. This method integrates some of the main principles of the theory of plastic deformation and a generalization of the results of rock failure experiments. The reconstruction of the state of stress is based on earthquake focal mechanism data for two magnitude ranges, Mw: 1) 3.0–5.5 and 2) 5.0–7.0. The calculations for the first data set are performed at six depths. Stress reconstruction for the second data set is performed from 0 to 50km. The focal mechanism data provides reasonably stable parameters of the stress field for magnitudes in this range. A large amount of data on the focal mechanisms of earthquakes with magnitudes between 3.0 and 5.5 has allowed us to obtain new data on changes in the stress field, which were missed in the previous calculation for the second data set. It is shown that in the upper part of the lithosphere (up to 90km) the axis of maximum compression is generally oriented slightly (25–35°) under the oceanic plate west of the Japan Trench, and it dips subvertically east of the trench. The upper part of the tectonosphere is divided into two geodynamic regimes: subhorizontal compression west of the trench axis and subhorizontal extension east of the trench axis. West of the Japan Trench, the axis of maximum horizontal compression (SHmax) is orthogonal to the trench axis, but east of the Japan Trench, its orientation has changed (~90°). The underthrusting shear stress that acts normal to the horizontal planes has a similar west and west-southwest direction in the upper layers (0–50km). Deviations in the orientation are observed at depths between 70 and 90km at the junction between the Japan Trench and the Nankai Trough and in the crust of the northeastern part of the Tohoku region. Obtained results are summarized in 2 D-pattern, which can be used to explain the loading mechanism in the region of the Tohoku earthquake. The results of the stress reconstruction also allowed us to obtain the new specific features of the field stress: the oceanic trench divides lithosphere into the areas of high effective pressure west of the trench axis and the areas of low effective pressure east of the trench axis. The rupture developed in the area of the maximum stress gradient and it stopped in the north-east and the south-west directions, where there were areas of high effective pressure.

Crustal stress and fault strength in the Canterbury Plains, New Zealand

The Mw=7.1 Darfield (Canterbury) and Mw=6.2 Christchurch earthquakes and related aftershocks in Canterbury, New Zealand have revealed a major hazard in the Canterbury region in the form of the Greendale Fault and a number of associated faults. The strength of these apparently low slip-rate faults may affect the recurrence intervals of subsequent earthquakes. We use P- and S-wave picks from a dataset of aftershocks of the Darfield earthquake to estimate earthquake locations and focal mechanisms. We use S waveforms to determine shear-wave splitting (SWS) parameters and we estimate the azimuth of the axis of maximum horizontal compression (SHmax) from inversions of focal mechanisms. Two 2D methods of clustering the focal mechanisms for stress inversion are used, one to estimate the regional stress field and another to investigate variations in SHmax with distance from the Greendale Fault. A 3D method is also used to investigate variations in SHmax with depth. The tectonic stress field is remarkably uniform and has an average maximum horizontal compressive stress orientation of SHmax=116±18°, forming an angle with the average strike of the Greendale Fault of c. 25°. However, several SHmax estimates along the Greendale Fault from the regional 2D clustering method are sub-parallel to the fault strike (93.6±13.1°, 100.8±11.5° and 100.8±12.6°), indicating that the fault may be frictionally weak, in an Andersonian sense. This variation occurs via an anti-clockwise rotation of SHmax southwards across the Greendale Fault. SWS fast directions (ϕ) generally match nearby SHmax, suggesting stress-aligned micro-cracks, but ϕ estimates at stations Cch3 and MQZ, which are near known and inferred faults, are sub-parallel to these faults and differ greatly from nearby stress orientations, indicating structure-dependent anisotropy. A lack of seismicity in the area prior to the Darfield earthquake precludes detailed analysis of time variations. However, there are two end member scenarios: if the pre-seismic stress orientation near the Greendale Fault was in the same direction as we have measured after the earthquake, then it was mis-oriented for rupture. Alternatively, if the stress rotated from the average regional orientation during the earthquake, then we can use the rotation to determine that an average of c. 40% of the pre-seismic differential stress on the Greendale Fault was released during the Darfield earthquake.

Late Cenozoic tectonics of the Red River Fault Zone, Vietnam, in the light of geomorphic studies

The main episode of Pliocene–Quaternary tectonic activity in the Red River Fault Zone in Vietnam was related to dextral strike-slip motions while uplift of the SW and NE sides of the fault zone was limited. In the Vietnamese segment of this zone, traces of pre-Pliocene mature relief were preserved to a much smaller extent compared to those in Yunnan Province, China. The main factors controlling headward erosion of tributary streams were lithologic contrasts within the solid bedrock. Tectonic-geomorphic studies indicate that the amount of Quaternary dextral offset of the Red River Fault Zone in Vietnam, calculated from offset and deflection of tributary valleys of the Red River, ranges between some 400m and 5.3km, averaging 1.1–2km. The corresponding rates of slip are 0.43–1.1mm/yr for individual displaced segments and 5.5–7.8mm/yr for the inferred cumulative displacement. Fluvial terraces in the medial segment were mainly shaped in the Late Pleistocene and Holocene, except the highest one (40–50m) which was probably formed in Mid-Pleistocene time. Taking into account the amount of dextral displacement of terrace steps and related alluvial fans, the most realistic estimation of Mid-Pleistocene through Holocene slip rate is 0.9–3.9mm/yr. The axis of maximum horizontal compression associated with dextral slip of the fault zone was aligned NNW–SSE to N–S, and the fault motion in the Quaternary resulted mainly from aseismic creep.

Horizontal crustal deformation on the Hikurangi Margin

Abstract The first-order geodetic triangulation of New Zealand, completed between 1925 and 1949by the Department of Lands & Survey, has been reobserved in part between 1976 and 1984 along a series of narrow belts, as a contribution to to the national Earth Deformation Programme of the Royal Society of New Zealand. This has provided a set of data of uniformly high quality, which has been analysed by the method of simultaneous reduction, to yield a continuum model of horizontal deformation for the region between Christchurch and Auckland. This spans the Hikurangi Margin, the southern extension of the Tonga-Kermadec boundary of the subducting Pacific plate that terminates at the Chatham Rise. The continuum model of deformation comprises the 16 terms that specify the horizontal derivatives of the horizontal velocity field of orders 2–4, and their error covariance matrix. Between Hawke Bay and Chatham Rise the pattern of shear-strain rate shows a consistent azimuth of the axis of maximum relative shortening of 110/290°, with a maximum intensity (tensor) at the east coast of Wellington of about (0.20 ± 0.02) μrad/yr, decreasing rapidly westward. This direction is consistent with that that would be predicted for the axis of maximum horizontal compression in the overriding plate for the case of isotropic factional slip between the plates at the obliquely convergent subduction margin. North of Hawke Bay, the azimuth of the axis of maximum relative shortening swings to northeast-southwest. This confirms previous evidence of extension across the Central Volcanic Region, and also of trenchward extension in the East Cape peninsula; this effect may be due to gravity sliding of the superficial, and rapidly uplifted, Tertiary sediments (which carry the geodetic monuments). Analysis of bending in the East Cape peninsula shows axes of maximum bending rate concave to the SSE. In the northern South Island, strong bending, with concave normals to the axes of maximum bending lying in the northwest quadrant, marks the southern extremity of Pacific plate subduction, with its transition to the intense compressional mountain building of the central South Island. This pattern is not incompatible with the role of the Chatham Rise as a westward-thrusting rigid indenter. A fourth-rank invariant of the deformation model shows the inflexion of bending on the Hikurangi Margin to be of S-form.

Fault plane solutions for the 1984 Morgan Hill, California, Earthquake Sequence: Evidence for the state of stress on the Calaveras Fault

Fault plane solutions for 946 aftershocks of the April 24, 1984, Morgan Hill, California, M6.2 earthquake reveal a pattern of complex faulting within a 10‐km‐wide zone surrounding the Calaveras fault. The fault plane solutions fall into three groups: strike‐slip mechanisms located along the Calaveras fault with north‐northwest striking dextral slip planes nearly parallel to the fault, strike‐slip mechanisms located northeast of the Calaveras fault with north striking dextral slip planes, and reverse mechanisms located southwest of the Calaveras fault with northeast or southwest dipping slip planes. The average azimuth of P axes for aftershocks located on the Calaveras fault is N10°±1°E. In contrast, the average azimuths of P axes for aftershocks northeast and southwest of the Calaveras fault are N49°±7°E and N37°±3°E, respectively. By assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field, we are able to infer from the observed average slip directions an orientation for σ1, the axis of maximum horizontal compression, 63°–80° from the local strike of the Calaveras fault. Such a high‐angle orientation of σ1 is incompatible with the classical Andersonian model of strike‐slip faulting but is consistent with an alternate model in which a weak Calaveras fault is driven by plate motion slightly convergent with respect to the San Andreas fault system in central California. If this inferred high‐angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 Morgan Hill main shock, the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions. The orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the Coulomb failure criterion. Areas with predicted increases in the Coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity. The invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 MPa. In addition to explaining a preponderance of the seismic observations of the aftershock sequence, the presence of a predominantly fault‐normal compressive regional stress field accounts for the contemporary development of folds, reverse faults, and topographic relief observed throughout the central California Coast Ranges.

Seismotectonics of northeastern United States and adjacent Canada

Data for local earthquakes recorded by a network of stations in northeastern United States and adjacent Canada were analyzed to study the seismicity, the relationship between earthquakes and known faults, the state of stress, and crustal and upper mantle velocity structure. In addition, portable seismographs were deployed in the field to study aftershocks. As a result, accurate locations for about 364 local earthquakes (2 ≤ mb ≤ 5) and 22 focal mechanism solutions were determined. A comparison of the spatial distribution of these events (1970–1979) with historical earthquakes (1534–1959) reveals that seismic activity in the northeast is relatively stationary in space: those areas that have had little or no seismicity historically are relatively aseismic today, whereas the historically active areas are also active today. The instrumental locations, historical seismicity, and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces. (1) The Adirondack‐western Quebec province is a northwesterly trending zone of seismic activity, about 200 km wide and at least 500 km long, extending from the SE Adirondacks into western Quebec, Canada. Thrust faulting on planes striking NNW to NW appears to predominate, and the inferred axis of maximum horizontal compression is largely uniform and trends WSW, nearly parallel to the calculated absolute plate motion of North America. Little or no seismicity is found where anorthosite outcrops at the surface. Correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep NE or SW gradient in Bouguer anomalies. This zone does not appear to extend southeastward to Boston, as proposed by some workers. (2) The Appalachian province is a northeasterly trending zone of seismic activity extending from northern Virginia to New Brunswick, Canada. Highangle reverse or thrust faulting on N to NE trending planes appears to predominate. The western margin of this province, however, appears to be relatively aseismic. We attribute this relative lack of activity to one or more of the following: the presence of igneous activity postdating rifting of Africa from North America, the occurrence of intense metamorphism during the Acadian orogeny which may have annealed preexisting faults, and the predominance of ductile as opposed to brittle deformation in the geologic past. The inferred axis of maximum horizontal compression along the eastern margin of the Appalachians is rather uniform and trends W‐WNW, almost perpendicular to the magnetic lineations offshore. We suggest that this W‐WNW compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers.

Distribution of Quaternary rhyolite domes of the Coso Range, California: Implications for extent of the geothermal anomaly

Thirty‐eight separate domes and flows of phenocryst‐poor, high‐silica rhyolite of similar major element chemical composition were erupted over the past 1 m.y. from vents arranged in a crudely S‐shaped array atop a granitic horst in the Coso Range, California. Most of the extrusions are probably less than about 0.3 m.y. old. The area is one of Quaternary basaltic volcanism and crustal extension. The central part of the rhyolite field is characterized by high heat flow, low apparent resistivity, and substantial fumarolic activity indicative of an active geothermal system. The immediate source of heat for the surficial geothermal phenomena is probably a silicic magma reservoir that may still contain molten or partially molten material at a depth of at least 8 km beneath the central part of the rhyolite field. Outlying rhyolite extrusions probably reflect the presence of feeder dikes emanating from the reservoir beneath the central region. Azimuths of dikes appear to be parallel to the regional tectonic axis of maximum horizontal compression, analogous to some dike‐fed flank eruptions on basaltic shields and andesitic stratovolcanoes. The areal extent of a magma reservoir and the present total heat content of the silicic magma system at Coso may be less than was previously estimated. However, the area is still considered to be one of significant geothermal potential.