The orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and R = (σ2–σ1)/(σ3–σ1), a measure of relative stress magnitudes, under the assumption of uniform stress in the source region. This analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation. In the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions; the method allows for errors in orientations of both the fault planes and slip directions. We have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model; thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind. By using a grid search of stress models rather than a linearization scheme, we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses. The method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population. The technique has been applied to 76 events from the San Fernando earthquake sequence for which we have found best fit stresses (plunge and azimuth): σ1, = 7,187; σ2 = 27,281; σ3 = 62,84; and R = 0.65. The average misfit between this stress model and all the data is about 8°, and all but eight of the aftershocks have misfits of less than 20°. These values are considerably less than the uncertainty of the focal mechanism determinations; therefore significant stress inhomogeneities are not required by the data. Our analysis does not support the suggestion of a change in stresses during the aftershock sequence, as proposed by others on the basis of an apparent change in focal mechanisms.

An improved method for determining the regional stress tensor using earthquake focal mechanism data: Application to the San Fernando Earthquake Sequence

We summarize evidence for Quaternary and active faulting collected in the field during three Sino-French expeditions to southeastern Tibet (1980–1982). Detailed mapping of Quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the Himalayas in the last 2 ± 0.5 m.y. The maximum horizontal principal stress in south Tibet appears to be only the intermediate principal stress σ2, σ1 being vertical. South of the “chord” joining the eastern and western syntaxes of the Himalayan arc, extensional strains are principally localized within seven regularly spaced rift zones, three of which have been studied in some detail. The extension direction is determined to be N96° E ± 7° mainly from statistical averaging of strikes of newly formed normal faults. Throw rates on normal faults are evaluated for different time spans (2 ± 0.5 m.y., 60 ± 40 kyr, and 10 ± 2 kyr B.P.), using structural and topographic reliefs, as well as synglacial and postglacial vertical offsets. The rate of Quaternary extension is about 1% m.y.−1 along an 1100-km-long ESE traverse across south Tibet. This corresponds to a “spreading” rate of 1 ± 0.6 cm/yr. This rate and the divergent horizontal projections of slip vectors of earthquakes along the Himalayan front constrain the rate at which rigid India underthrusts southern Tibet to be 2 ± 1 cm/yr. Although most of the normal faults appear to be independent of, and nearly orthogonal to, the ≃E-W Mesozoic-Tertiary tectonic fabric, the Yadong-Gulu rift appears to be guided for over 130 km by the older, oblique (≃NE-SW) Nyainqentanglha range and fault zones along it. This reactivated zone is the most prominent left-lateral strike-slip fault system in SE Tibet. Excepting this zone, and the vicinity of the SE extremity of the Karakorum fault, Quaternary strike-slip faulting is rare in south Tibet, i.e., south of the chord between the syntaxes of the Himalayan arc. North of the chord, the tectonic style is different. There minor conjugate strike-slip faulting is widespread and appears to control Quaternary normal faulting, which is more diffuse and subdued than in south Tibet. Along the chord, the presence of a major zone of active right-lateral, en echelon strike-slip faults (Karakorum-Jiali fault zone) probably reflects the greater facility of eastward extrusion in north central Tibet, in response to the northward push of India. The eastern Himalayan syntaxis may be an obstacle to such extrusion movements south of the chord.

Quaternary extension in southern Tibet: Field observations and tectonic implications

Kinematic analysis of fault-slip data

Criteria for the sense of movement on fault surfaces in brittle rocks

Field observations collected during the Sino-French Himalaya-Tibet cooperation project (1980–1982) show that right-lateral faulting between southern and northern Tibet, roughly along the “chord” joining the eastern and western syntaxes of the Himalayan arc, is a major component of the active tectonics north of the Himalayas Just north of the well developed rifts of southern Tibet, we studied three right-lateral, 100–200 km long, strike-slip faults forming an en echelon array along that chord Evidence for earthquake surface breaks was found on each of these faults For the M = 8, Beng Co (or Damxung) strike-slip earthquake of November 18, 1951, rupture parameters could be constrained to be ū = 8 ± 2 m and L = 90 km Offset glacial and postglacial morphological markers (105 years and 104 years BP) imply displacement rates of the order of 1–2 cm/yr on the three faults The en echelon fault zone appears to extend westward ≈2000 km across the plateau to the Karakorum fault and is thus referred to as the Karakorum-Jiali fault zone (KJFZ) Fast rates of slip on it and on faults along the northern rim of Tibet appear to contrast with the slower rates of deformations in the interior of the plateau Our estimates of slip rates on the strike-slip and normal faults of southeastern Tibet suggest that eastward extrusion of North Tibet between the right-lateral KJFZ and the left-lateral Altyn Tagh and Kun Lun faults absorbs at least 30% of the present convergence between India and Asia We propose an instantaneous velocity model of the active tectonics of Asia compatible with such faulting rates and geometries Evidence for Tertiary right-lateral strike-slip movements in southern Tibet and around the eastern syntaxis of the Himalayas is also found in the structural record Offsets of various large-scale geological features imply that these movements have been large

A. Cisternas

Papers

The inverse problem in microtectonics and the separation of tectonic phases