Tectonic Shear Stress Research Articles

Seismotectonics of the easternmost transverse ranges, California: Relevance for seismic potential of the southern San Andreas Fault

Earthquake locations, depths and focal mechanisms from the Southern California Regional Network (1977–1985) are used to identify the orientation and sense of slip of active subsurface faults in the Easternmost Transverse Ranges (ETR). The ETR are separated from the Salton Trough province by the southernmost strands of the San Andreas fault (SAF). Much of the seismicity in the ETR is concentrated well northeast of the SAF at relatively shallow depths under the Little San Bernardino Mountains. Many of these earthquakes reflect slip on steeply dipping, left‐lateral faults striking northeast to east, at relatively high angles to the adjacent SAF. Focal mechanisms in the ETR show predominantly strike‐slip, normal, or oblique‐normal faulting, and share common near‐horizontal T axes striking WNW. P axes range from near vertical to near horizontal and strike mostly NNE. In contrast, reverse and strike‐slip focal mechanisms that exhibit persistent north trending, near‐horizontal P axes characterize the San Gorgonio Pass area immediately to the west. These different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the SAF. This boundary, which in the Coachella Valley may be the Mission Creek fault, is also distinguished by abrupt changes in (1) rate and depth of seismic activity; (2) topography; (3) Quaternary vertical deformation; (4) strikes and dips of major branches of the SAF; and (5) seismic velocities in the crust and upper mantle. The preponderance of secondary normal faulting in the ETR versus secondary reverse faulting in the San Gorgonio Pass region suggests that fault‐normal stress is much less across the SAF adjacent to the ETR. If a friction law where strength is proportional to normal stress applies to the SAF, then a smaller tectonic shear stress would be required tor slip in large earthquakes along the Salton Trough segment. In this case, the southernmost SAF may have shorter repeat times, smaller average displacements, and lower moment release rates than earthquakes that rupture through the major restraining bend in San Gorgonio Pass.

Stressing of locked patches along a creeping fault

An analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere. The model consists of strike-slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress. The fault zone has an inhomogeneous strength distribution both depth-wise and along strike. It is modeled as being composed of locked patches and freely slipping parts treated as cracks. The solution is given with the use of the “line-spring” model which analyzes the problem by thickness-averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary. As a boundary condition, thickness-averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section. The analysis indicates that the slip distribution along creeping parts of the fault, as well as the stress distribution along locked patches, depends strongly on the geometry of these zones. The model is used to examine stress concentrations associated with a slip-deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region. It is also used to simulate slip and stressing processes associated with the creeping portion of the San Andreas fault in central California, between the presently locked zones of the great 1906 and 1857 ruptures, and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic Parkfield earthquakes. Near-fault creep and broadscale displacement data along the fault since the 1966 Parkfield earthquake and inferences from seismicity distributions are used. Limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the Parkfield locked patch, but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone. An effective remote stressing rate of order 0.3 × 10−6 × shear modulus/yr is inferred, together with a less well constrained 30 to 40 km lithospheric thickness, for consistency with the displacement data. Results enable estimates of stress accumulation along the locked 1857 rupture zone and the build-up of fracture energy release capability (of order 107 J/m2 in 150 yr) at its lower margin.

Modeling of the surface static displacements and fault plane slip for the 1979 Imperial Valley earthquake

Abstract Synthesis of geodetic and seismological results for the 1979 Imperial Valley earthquake is approached using three-dimensional finite element modeling techniques. The displacements and stresses are calculated elastically throughout the modeled region. The vertical elastic structure in the model is derived from compressional and shear wave velocities as used in the seismic data analysis (Fuis et al., 1981) combined with a sediment density profile. Two strategies for applying initial conditions are followed in this modeling. In the first strategy, a sample seismological estimate for fault plane slip is used to predict the resultant surface motions. We show that the geodetic strain results over distances of tens of kilometer from the fault (Snay et al., 1982) are basically consistent with the model seismic fault displacements. Geodetic results from within a few kilometers of the fault trace (Mason et al., 1981) seem to require more slip at shallow depths than appears at seismic time scales. This is consistent with the occurrence of aftercreep at shallow depths in less well-consolidated material, which would bring surface displacements into line with maximum slip at depth, but not greatly affect the net moment. In the second strategy, we consider stresses on the fault plane, rather than displacements, as model variables. To constrain this part of our numerical modeling, we assume that the fault driving stress is governed by ambient tectonic stress and an opposing Coulomb friction derived from experiment. The coseismic stress drop from point to point on the failed fault is given by the difference between the tectonic shear stress and the frictional stress. After arriving at such a uniform model which adequately represents the Snay et al. results, we further modify a small region near the seismic “asperity” to make the fault plane motions qualitatively and quantitatively resemble the model of coseismic motions used in the first strategy. The observed offset on the fault trace (Sharp et al., 1982) is approximated in this final stress-driven model by removing the driving stress on the southern third of the fault. Thus, the principal features of the coseismic slip pattern are explained by a stress-driven fault model in which: (a) a spatially unresolved asperity is found equivalent to a stress drop of 18 MPa averaged over an area of 15 km2, and (b) driving stress is essentially absent on the fault segment overlapping the 1940 earthquake rupture zone.

Nonlinear model of tectonic release from underground explosions

Reversal of teleseismic Rayleigh wave polarity has been observed for some underground explosions in eastern Kazakh. Furthermore, the reversed-polarity Rayleigh waves are observed to be time-delayed by several seconds. We analyze two-dimensional, nonlinear numerical simulations of underground explosions to examine the hypothesis that these phase reversals and apparent time delays result from the action of tectonic prestress on the explosion-created nonlinear volume (the shatter zone model of tectonic release). We conclude that tectonic shear stress exceeding about 10 MPa (100 bars) is sufficient to reverse the Rayleigh wave polarity and cause an apparent time delay of several seconds, even in the absence of triggered strain release on faults. Shear stresses of this magnitude are plausible at several hundred meters depth, in that they do not exceed strength bounds for a fractured rock mass, as estimated from Byerlee's law. The nonlinear model predictions imply that if tectonic release is modeled elastically as the relaxation of the deviatoric part of the prestress into a spherical cavity, the appropriate cavity radius is approximately 80 per cent of the explosion elastic radius. The apparent time delays result from interference between the explosion and tectonic source functions, and can be quantitatively related to the presence of overshoot in the isotropic part of the explosion moment rate tensor. It is therefore conceivable that careful observations of Rayleigh wave phase for the anomalous events (relative to nearby unreversed events) could help constrain the amount of overshoot in the explosion source function, even though the overshoot occurs on a time scale of a few tenths of a second and has a negligible effect on Rayleigh wave spectral amplitudes.

Heat flow and energetics of the San Andreas Fault Zone

Approximately 100 heat flow measurements in the San Andreas fault zone indicate (1) there is no evidence for local factional heating of the main fault trace at any latitude over a 1000‐km length from Cape Mendocino to San Bernardino, (2) average heat flow is high (∼2 HFU, ∼80 mW m−2) throughout the 550‐km segment of the Coast Ranges that encloses the San Andreas fault zone in central California; this broad anomaly falls off rapidly toward the Great Valley to the east, and over a 200‐km distance toward the Mendocino Triple Junction to the northwest. As others have pointed out, a local conductive heat flow anomaly would be detectable unless the frictional resistance allocated to heat production on the main trace were ≲100 bars. Frictional work allocated to surface energy of new fractures is probably unimportant, and hydrologic convection is not likely to invalidate the conduction assumption, since the heat discharge by thermal springs near the fault is negligible. Explanations for the low dynamic friction fall into two intergradational classes: those in which the fault is weak all of the time and those in which it is weak only during earthquakes (possibly just large ones). The first class includes faults containing anomalously weak gouge materials and faults containing materials with normal frictional properties under near‐lithostatic steady state fluid pressures. In the second class, weakening is caused by the event (for example, a thermally induced increase in fluid pressure, dehydration of clay minerals, or acoustic fluidization). In this class, unlike the first, the average strength and ambient tectonic shear stress may be large, ∼1 kbar, but the stress allocated to elastic radiation (the apparent stress) must be of similar magnitude, an apparent contradiction with seismic estimates. Unless seismic radiation is underestimated for large earthquakes, it is difficult to justify average tectonic stresses on the main trace of the San Andreas fault in excess of ∼200 bars. The development of the broad Coast Range heat flow anomaly southward from Cape Mendocino suggests that heat flow increases by a factor of 2 within 4 m.y. after the passage of the Mendocino Triple Junction. This passage leaves the San Andreas transform fault zone in its wake; the depth of the anomalous sources cannot be much greater than the depth of the seismogenic layer. Some of the anomalous heat may be supplied by conduction from the warmer mantle that must occur south of the Mendocino transform (where there is no subducting slab), and some might be supplied by shear heating in the fault zone. With no contribution from shear heating, extreme mantle upwelling would be required, and asthenosphere conditions should exist today at depths of only ∼20 km in the northernmost Coast Ranges. If there is an appreciable contribution from shear heating, the heat flow constraint implies that the seismogenic layer is partially decoupled at its base and that the basal traction is in the sense that resists right lateral motion on the fault(s). As a result of these basal tractions, the average shearing stress in the seismogenic layer would increase with distance from the main fault, and the seismogenic layer would offer substantial resistance to plate motion even though resistance on the main fault might be negligible. These speculative models have testable consequences.

Water flow paths around a dislocation on an earthquake fault

The Nur-Booker theory of water flow paths around an edge dislocation on an earthquake fault is developed further. It is assumed that water exists predominately in cracks whose planes are normal to the direction of greatest tensile stress (or smallest compressive stress). The permeability is assumed to be anisotropic. The direction of greatest permeability is parallel to the plane of the cracks that are opened up by the greatest tensile stress component. Thus the direction of largest permeability is a function of the stress field. If the anisotropy is very large, water is forced to flow parallel, rather than normal, to the fault plane of the dislocation. If a large tectonic shear stress is present and if the anisotropy of the permeability also is large, water flows away from the fault plane in regions where the dislocation stress field hinders further slip on the fault plane. Water flows toward the regions of the fault plane, where the dislocation stress field aids further slip on the fault plane.

Radiation patterns of S waves from underground nuclear explosions

Use is made of polarization angles of S waves from the Greeley, Boxcar, and Benham events to determine focal mechanisms of earthquakes presumably triggered almost instantaneously by the events. Although other possibilities are not completely excluded, the observed S waves are interpreted as mixed waves of direct S phase and reflected phases (sS, pS) from a composite source of explosion and double-couple force. Assuming a vertical strike slip, the strike of the P-wave nodal plane and the relative strength of the doublecouple source compared with that of the explosion are determined. The seismic moments of triggered earthquakes are calculated as 3.9∼6.3×1023 dyne cm for Boxcar and 1.4∼2.3×1024 dyne cm for Benham. Particularly in the case of Benham, the rise time of about 2 sec together with the seismic moment of 2.3×1024 dyne cm is compatible with the observations of seismic waves. The possibility of estimating the pre-existing tectonic shear stress is suggested in the case of Boxcar. The tectonic stress is estimated at 130 bars from the assumption of stress relaxation due to a cavity created by the nuclear detonation.