Averaged Stress Directions Research Articles

Estimating the mechanical anisotropy of the Iranian lithosphere using the wavelet coherence method

Abstract We calculated anisotropic wavelet coherence between Bouguer anomaly and topography in order to map the anisotropy of the effective elastic thickness of the Iranian lithosphere (T e ). An orthotropic elastic plate model is used for inverting the anisotropic wavelet coherence to compute the mechanical anisotropy through the weak axis of the T e . Anisotropy of the T e -weak axis and the strength of the anisotropic parameter, namely the anisotropic coherence effect over the study area are estimated by restricting the rotated Morlet wavelet (fan wavelet) geometry over an azimuthal range of 90°. Large-scale T e variations have been shown to be associated with phenomena, such as mountain belts, subduction zones, craton boundaries, fault zones, and seismogenic regions. Although the correlation between the major tectonic features of the Iranian lithosphere and the distribution of the T e -weak axis is not general or precise, in some regions, such as the Central Iran Blocks, and the Alborz, Kopeh Dagh, Zagros, and Makran orogenic belts, the weak axis has a uniform or slowly varying pattern which changes over their boundaries. A perpendicular alignment between seismic anisotropy measurements in Iran and the T e -weak directions suggests a lithospheric origin for anisotropy. The correlation between averaged stress directions and the weak axis of the T e in Iran indicates that the present day stress field and the fossil strain are still related. Correlation between these factors suggests vertically coherent deformation of the lithosphere in Iran resulting from the multiply convergent orogenic processes. The complex mechanical anisotropy pattern of the Iranian lithosphere results from the interaction of many pre-existent structures which dominantly control the mechanical anisotropy of the lithosphere.

Stress orientations of Taiwan Chelungpu‐Fault Drilling Project (TCDP) hole‐A as observed from geophysical logs

The Taiwan Chelungpu‐fault Drilling Project (TCDP) drilled a 2‐km‐deep research borehole to investigate the structure and mechanics of the Chelungpu Fault that ruptured in the 1999 Mw 7.6 Chi‐Chi earthquake. Geophysical logs of the TCDP were carried out over depths of 500–1900 m, including Dipole Sonic Imager (DSI) logs and Formation Micro Imager (FMI) logs in order to identify bedding planes, fractures and shear zones. From the continuous core obtained from the borehole, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chinshui Shale, which extends from 1013 to 1300 meters depth. Stress‐induced borehole breakouts were observed over nearly the entire length of the wellbore. These data show an overall stress direction (∼N115°E) that is essentially parallel to the regional stress field and parallel to the convergence direction of the Philippine Sea plate with respect to the Eurasian plate. Variability in the average stress direction is seen at various depths. In particular there is a major stress orientation anomaly in the vicinity of the Chelungpu fault. Abrupt stress rotations at depths of 1000 m and 1310 m are close to the Chinshui Shale's upper and lower boundaries, suggesting the possibility that bedding plane slip occurred during the Chi‐Chi earthquake.

Stress orientation, pore pressure and least principal stress in the Norwegian sector of the North Sea

We have compiled data on stress orientation, pore pressure and least principal stress from over 300 wells in the Norwegian sector of the North Sea. Well-defined regional variations are observed in all three parameters. Incorporation of precise stress orientation data from drilling-induced tensile borehole wall fractures shows that the orientation of maximum horizontal stress is approximately E–W between 60°N and 62°N but tends to be NNW–SSE south of 58°N, similar to the average stress direction seen throughout Great Britain and continental NW Europe. We believe this rotation is due to the superposition of plate-driving stresses with those associated with lithospheric flexure caused by deglaciation. Regional variations of the magnitude of the least principal stress and pore pressure also appear to support the hypothesis that the stress field in this region has been strongly affected by deglaciation.

Stress inversion of earthquake focal mechanism solutions from onshore and offshore Norway

A comprehensive compilation of 112 earthquake focal mechanism solutions in Norway and adjacent areas has been completed, including 7 previously unpublished solutions for recent earthquakes determined as part of the NEONOR (Neotectonics in Norway) project. Using the method of Gephart & Forsyth (1984), the 97 solutions on the Norwegian mainland and margin have been inverted with respect to primary stress directions, which on a regional basis and in the best possible way could satisfy the individual solutions. In doing this the Norwegian mainland and margin areas were divided into six zones containing between 5 and 34 earthquake focal mechanism solutions. Two additional zones containing only in situ measurements are also defined, and horizontal stress directions evaluated. The results show a tendency for oblique-slip reverse faulting offshore, for oblique-slip normal faulting onshore and for average stress directions which are in overall compliance with a NWSE oriented regional stress field assumed to be dominated by the mid-Atlantic ridge-push force. Local variations do exist, however, in particular for more shallow earthquakes in mid-Norway coastal areas, where the dominating compressive stress directions are NESW, implying coast-perpendicular extension. While this may be related to different kinds of local effects, it is also possible that deglaciation flexure can explain the observed stress directions in this region, since this is also where we find the maximum postglacial uplift gradients.

Active stress map of Italy

We present a new map of the present‐day stress field in Italy obtained from all the available data. The map reports 200 horizontal stress directions inferred from 109 borehole breakout data, 44 centroid moment tensor solutions, 34 other focal mechanisms, most of which are from polarity distributions, seven stress inversions of microearthquake data, two averages of T and P axes of earthquake focal mechanisms in zones of diffuse seismic activity, and four fault slip data. The integration of breakout data, which yield horizontal stress directions, with fault plane solutions, which reflect the stress regime, allows us to obtain an improved map of the present‐day stress in Italy. This stress field map can be used for a better comprehension of active tectonic processes, for seismic hazard assessment, and to foresee the behavior of faults recognized with other methods. Stress directions obtained from different data, although relative to different depth intervals (e.g., 0–7 km for breakouts and 0–20 km for most of the earthquakes) and to different tectonic units, are consistent. Since many regions in Italy are characterized by an extensional stress regime, we report the minimum horizontal stress (Shmin) orientations. The map shows that an extensional regime affects most of the Apenninic belt. Conversely, a compressional (or transpressional) regime characterizes the eastern Alps, the eastern side of the northern Apennines, and the southern Tyrrhenian to northern Sicily zone. An abrupt change in stress directions marks the transition between northern and southern Apennines, suggesting that the two arcs are characterized by a different tectonic setting and recent evolution. In this paper we report all the data analyzed to date, with their geographic coordinates and average stress directions, and we describe the main stress provinces in Italy in the framework of the tectonic evolution of the region.

Coseismic temporal changes of slip direction: The effect of absolute stress on dynamic rupture

AbstractWe investigate the dynamics of rupture at low-stress level. We show that one main difference between the dynamics of high- and low-stress events is the amount of coseismic temporal rake rotation occurring at given points on the fault. Curved striations on exposed fault surfaces and earthquake dislocation models derived from ground-motion inversion indicate that the slip direction may change with time at a point on the fault during dynamic rupture. We use a 3D boundary integral method to model temporal rake variations during dynamic rupture propagation assuming a slip-weakening friction law and isotropic friction. The points at which the slip rotates most are characterized by an initial shear stress direction substantially different from the average stress direction over the fault plane. We show that for a given value of stress drop, the level of initial shear stress (i.e., the fractional stress drop) determines the amount of rotation in slip direction. We infer that seismic events that show evidence of temporal rake rotations are characterized by a low initial shear-stress level with spatially variable direction on the fault (possibly due to changes in fault surface geometry) and an almost complete stress drop.Our models motivate a new interpretation of curved and cross-cutting striations and put new constraints on their analysis. The initial rake is in general collinear with the initial stress at the hypocentral zone, supporting the assumptions made in stress-tensor inversion from first-motion analysis. At other points on the fault, especially away from the hypocenter, the initial slip rake may not be collinear with the initial shear stress, contradicting a common assumption of structural geology. On the other hand, the later part of slip in our models is systematically more aligned with the average stress direction than the early slip. Our modeling suggests that the length of the straight part of curved striations is usually an upper bound of the slip-weakening distance if this parameter is uniform over the fault plane, and the direction of the late part of slip of curved striations should have more weight in the estimate of initial stress direction.

Modern tectonic stress field in the Mediterranean region: evidence for variation in stress directions at different scales

SUMMARY A compilation of more than one thousand stress indicators (which include in situ stress measurements, focal mechanisms, microtectonic and other geological data) allowed us to reconstruct the modern stress field in the Mediterranean region and the surrounding area. Average stress directions at different scales have been reconstructed by means of a linear interpolation method. This method takes into account the distribution, scale and quality of stress data. The results of the interpolation at plate scale, allow us to recognize slightly deformed regions such as the northwestern European platform, where average maximum horizontal stress direction is oriented roughly NNW-SSE, subparallel to absolute and relative plate velocity directions. Other regions such as the Caucasus, Alps and Pyrenees, where recent tectonic deformation and seismicity are present, display important variations of stress directions. The reconstruction of the average stress directions at different scales within the French Alps pointed out that the average stress field pattern may vary from one scale to another. Nevertheless, variations of stress directions at a given scale are consistent with the kinematics of faults of the same scale.