Surface deformation due to a strike‐slip fault in an elastic gravitational layer overlying a viscoelastic gravitational half‐space

Abstract

Abstract. We extend a technique previously used to model surface displacements resulting from thrust faulting in an elastic-gravitational layer over a viscoelastic-gravitational half-space to the case of strike-slip faulting. The method involves the calculation of the Green's functions for a strike-slip point source contained in an elastic-gravitational layer over a viscoelastic-gravitational half-space. The correspondence principle of linear viscoelasticity is applied to introduce time dependence. The resulting Green' s functions are then integrated over the source region to obtain the near-field displacements. Several sample calculations are presented involving 90 ø and 30 ø dipping faults and ruptures completely and partially through the elastic layer. We also illustrate the time dependent deformation due to a buried fault. Results show that the use of a viscoelastic half-space underlying an elastic layer introduces a long wavelength component into the deformation field [Cohen and Kramer, 1984], even in cases of non vertical strike-slip fault and inclusion of gravitational effect, that cannot be modeled by purely elastic techniques. Calculations have shown that vertical postseismic displacement is insignificant and that the horizontal movement is about the same magnitude as the coseismic strike-slip displacement. The inclusion of gravity affects the horizontal displacement due to vertical strike-slip faulting in far field and the vertical displacement for dipping strike-slip faulting in near-field. The computed results have been fit to the Global Positioning System measurements of the Landers earthquake taken shortly after the main shock, assuming a relaxation time of the order of days. This relaxation time is considerably shorter than times of the order of years to decades found in previous studies. The major differences between this detailed three-dimensional and simplified two-dimensional model are the decay of magnitude in displacement field and the distinct displacement pattern in the regions beyond the fault tip. The displacement field due to the cyclic earthquakes was constructed by considering the finite fault length and inclusion of gravity. It is found that the displacement field is dominated by the plate motion in the case of short recurrence time. On the other hand, a "looping" and migrating pattern in the displacement field is found in the case of very long recurrence time, which is not seen in those 2D simplified models.

Effects of Three-dimensional Inhomogeneous Viscoelastic Structures on Postseismic Surface Deformations Associated with the Great 1946 Nankaido Earthquake

“…However, for viscoelastic case, the gravitational effect must be included for deformations which involve long periods of time (tens to hundreds of relaxation time). This is because gravity affects both the magnitude and pattern at long periods of time for a thrust-type slip and a strike slip (e.g., RUNDLE, 1982;MA and KUSZNIR, 1994;YU et al, 1996). However, since deformations only for the shorter periods (several times of relaxation time) following the Nankaido earthquake are modeled here, gravitational effects on viscoelastic deformations would be negligible, as are justified by previous studies (e.g., RUNDLE, 1982;IWASAKI, 1985;FERNÁ NDEZ et al, 1996).…”

A snapshot (2003–2005) of the 3D postseismic deformation for the 1999, Mw=7.4 İzmit earthquake in the Marmara Region, Turkey, by first results of joint gravity and GPS monitoring

“…Two different characteristics of the northern and southern part of the fault zone around the rupture of the 1999 earthquakes may be related to the elastic and viscoelastic response of the layers in the crust as a function of their thickness. When the thickness of the viscoelastic layer is greater than the thickness of the overlying elastic layer, the relaxation time of the layer can be longer than the thick elastic layer/thin viscoelastic layer model Jackson, 1977a, 1977b;Savage and Prescott, 1978;Yang and Toksöz, 1981;Yu et al, 1996;Hearn et al, 2002;Hearn, 2003). This may account for the dominant postseismic motions, which are related to the long relaxation time in the northern part of the fault zone as the result of the thin elastic layer in the crust.…”

“…Transient postseismic deformations can continue from immediately after the earthquake time to several tens of years as a function of time dependent stress relaxation (Thatcher, 1983;Shen et al, 1994;Yu et al, 1996;Kenner and Segall, 2000;Hudnut et al, 2002;Ergintav et al, 2002;Owen et al, 2002). While the early phase of the time dependent postseismic motions reflects mainly the material properties of the fault zone and continued slip at depth, longer term deformation has been explained by viscoelastic materials distributed in the crust and upper mantle (Pollitz, 1997;Hearn et al, 2002;Hearn, 2003).…”

Lithospheric instability beneath the Transverse Ranges of California

“…The estimates are, however, quite variable, ranging from 3x1015 Pa s [Ivins, 1996] to 6x1016 Pa s [Yu et al, 1996] to 4x1019 Pa s [Turcotte et al, 1984]. Integrated lithospheric viscosity is difficult to measure, however, though it is generally thought to be much greater than sublithospheric viscosity, of which estimates are of the order of 4x102ø…”