Quasi-static Motions near the San Andreas Fault Zone

123

A method is formulated for the exact calculation of Green functions for finite quasi‐static sources in an elastic layer over a linear viscoelastic half space. The displacements due to the same sources in an elastic layer over an elastic half space are first computed, and then the well‐known correspondence principle of linear viscoelasticity is applied. The most difficult part of the problem, the inversion into the time domain, is accomplished by means of a novel technique. As a first example of the method, the growth of the error in an approximate Green function for a strike slip source is evaluated. Second, the viscoelastic uplifts due to an expanding sphere are calculated for two half‐space rheologies. Third, the time dependent uplifts due to point dip slip sources are computed. Finally, the dip slip Green functions are integrated over a finite rectangular fault surface, and the coseismic and postseismic uplifts are compared.

Section: Elastic Green Functionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Viscoelastic crustal deformation by finite quasi‐static sources

Rundle¹

1978

123

“…Triangles denote individual stations, and dark lines enclose regions within which uniform strain fields have bccn fit to the angle change data. Principal active right-lateral faults arc shown, and circled dots locate epicenters ooe significant earthquakes Barker, 1976],. where it has bccn either assumed that the 1941/ , 54 straining represented normal secular deformation or concluded on the basis of less-than-complete analysis that deformation was uniform during the entire 1941-1967 interval Whitten's [1960].…”

mentioning

confidence: 99%

Horizontal crustal deformation from historic geodetic measurements in southern California

Thatcher¹

1979

Data from horizontal control surveys are used to determine the regional deformation pattern across the southern San Andreas fault system and the history of movements since ∼1930. The interseismic straining is spread over a wide zone, with no sharp concentration across major faults and with significant deformation extending out to distances on the order of 100 km from the San Andreas fault. The maximum right‐lateral shear straining parallels the trends of the northwest striking faults of the region and tensor rates of from 0.1 to 0.2×10−6/yr are observed. A slight decrease in strain rate away from the San Andreas is indicated by the data, but shallow seismic slip on this fault alone is incapable of releasing all accumulated strains. The substantial residual deformation must then be accommodated by other means, and the range of possibilities includes (1) permanent inelastic deformation over a broad region, (2) massive pre‐ and/or post‐seismic slippage on the San Andreas beneath the brittle seismic zone, and (3) earthquake faulting on several subparallel active strands of the San Andreas system. Although available data are insufficient to exclude any of these possibilities, the diffuse seismicity and numerous active faults of the region suggest the third mechanism is the predominant one. Aseismic fluctuations in the rate of deformation have been inferred during the 1959–1974 southern California uplift, and long‐lived, wideranging postseismic effects have been observed following several major earthquakes and are particularly well‐documented for the 1940 El Centro shock (M = 7.1). Such effects are large and unexpected, complicate determination of the pattern of secular deformation, and are difficult to separate from suspected earthquake precursors. Determination of the rate of relative plate motion across southern California is made difficult by both the great breadth of the zone of secular straining and the observed irregularities in deformation rate.

“…We now apply these models to the northern 'locked' segment of the San Andreas fault and try a numerical estimate of some of the quantities involved in the model. We use throughout a = 10 km for the maximum fault depth [see, e.g., Barker, 1976] and /x = 3 x 10 • dyne/cm 2 for the shear modulus. As stated above the models are intended to apply to the fault region.…”

Section: Ah(x3 T) = Vto(x3 --Z)mentioning

confidence: 99%

On the recurrence time of great earthquakes on a long transform fault

Bonafede¹,

Boschi²,

Dragoni³

1982

A Maxwell viscoelastic model is proposed for the lithosphere in the proximity of a long, vertical strike slip fault. In this framework, formulas for the recurrence time of great earthquakes are derived by imposing boundary conditions of (1) uniform strain rate in the fault region or (2) steady, aseismic sliding at depth. In both cases a minimum effective viscosity of the fault region is found in order that a critical stress for earthquake occurrence can be reached; at higher viscosities the recurrence time approaches a minimum, limiting value. Application of the model to the northern ‘locked’ segment of the San Andreas fault would indicate a recurrence time significantly longer than the often quoted 100‐year period. These results differ considerably from those obtained from similar models employing boundary conditions of assigned stress at large distance from the fault.