Rheology of the lithosphere inferred from postseismic uplift following the 1959 Hebgen Lake earthquake

[1] We have modeled the broad postseismic uplift measured by geodetic leveling in the epicentral area of the 1959 Mw = 7.3 Hebgen Lake, Montana earthquake, a normal faulting event in the northern Basin and Range province. To fit the observed uplift we calculate synthetic postseismic deformation using the relaxation response of a gravitational viscoelastic Earth to the earthquake. For a model with an elastic plate overlying a viscoelastic half-space, we find that the elastic thickness is 38 ± 8 km, which is close to the local crustal thickness. The half-space viscosity is estimated at 4 × 1018±0.5 Pa s. The leveling data do not require a viscous lower crust but permit a lower bound viscosity of 1020 Pa s. The observed broad uplift cannot be explained by physically plausible afterslip on and below the coseismic fault. However, local deformation across the coseismic surface rupture requires shallow afterslip reaching the surface. The postseismic deformation induced by the estimated viscoelastic structure decays exponentially with a time constant of ∼15 years. Because of coupling between the elastic layer and the viscoelastic substrate, this relaxation time is significantly longer than the 2 year Maxwell relaxation time of the viscous half-space itself. Our result suggests the importance of postseismic relaxation in interpreting high-precision global positioning system velocities. For example, our model results suggest that postseismic transient velocities from both the 1959 Hebgen Lake and the 1983 Mw = 6.9 Borah Peak earthquakes are currently as large as 1–2 mm/yr.

[1]  Semih Ergintav,et al.  Time-Dependent Distributed Afterslip on and Deep below the İzmit Earthquake Rupture , 2002 .

[2]  J. Jackson.  Strength of the continental lithosphere: Time to abandon the jelly sandwich? , 2002 .

[3]  Kanamori,et al.  Viscoelastic flow in the lower crust after the 1992 landers, california, earthquake , 1998, Science.

[4]  Thatcher,et al.  Migration of fluids beneath yellowstone caldera inferred from satellite radar interferometry , 1998, Science.

[5]  Robert B. Smith,et al.  Seismicity of the Intermountain Seismic Belt , 1991 .

[6]  E. Humphreys,et al.  Crustal velocity structure across the eastern Snake River Plain and the Yellowstone swell , 1998 .

[7]  J. C. Savage,et al.  Deformation from 1973 to 1987 in the epicentral area of the 1959 Hebgen Lake, Montana, earthquake (Ms = 7.5) , 1993 .

[8]  Fred F. Pollitz,et al.  Joint estimation of afterslip rate and postseismic relaxation following the 1989 Loma Prieta earthquake , 1998 .

[9]  S. M. Nakiboglu,et al.  A reevaluation of the isostatic rebound of Lake Bonneville , 1983 .

[10]  B. Evans,et al.  Strength of the lithosphere: Constraints imposed by laboratory experiments , 1995 .

[11]  Robert B. Smith,et al.  The Yellowstone-Snake River Plain Seismic Profiling Experiment: Crustal structure of the Eastern Snake River Plain , 1982 .

[12]  T. Tabei.  CRUSTAL MOVEMENTS IN THE INNER ZONE OF SOUTHWEST JAPAN ASSOCIATED WITH STRESS RELAXATION AFTER MAJOR EARTHQUAKES , 1989 .

[13]  R. Reilinger,et al.  Evidence for postseismic viscoelastic relaxation following the 1959 M = 7.5 Hebgen Lake, Montana, Earthquake , 1986 .

[14]  M. Matsu'ura,et al.  A maximum likelihood approach to nonlinear inversion under constraints , 1987 .

[15]  J. Rundle,et al.  Lithospheric loading by the 1896 Riku‐u Earthquake, northern Japan: Implications for plate flexure and asthenospheric rheology , 1980 .

[16]  B. Bills,et al.  Lake Bonneville - Constraints on lithospheric thickness and upper mantle viscosity from isostatic warping of Bonneville, Provo, and Gilbert stage shorelines , 1987 .

[17]  S. Miyazaki,et al.  Distribution of seismic coupling on the subducting plate boundary in northeastern Japan inferred from GPS observations , 2000 .

[18]  S. R. Holdahl,et al.  Time‐dependent models of vertical deformation for the Yellowstone‐Hebgen Lake Region, 1923–1987 , 1991 .

[19]  B. Bills,et al.  Viscosity estimates for the crust and upper mantle from patterns of lacustrine shoreline deformation in the Eastern Great Basin , 1994 .

[20]  Robert B. Smith,et al.  Dynamic elevation of the Cordillera, western United States , 2000 .

[21]  S. Ward,et al.  Comparison of the 1959 Hebgen Lake, Montana and the 1983 Borah Peak, Idaho, earthquakes from geodetic observations , 1987 .

[22]  Fred F. Pollitz,et al.  Mobility of continental mantle: Evidence from postseismic geodetic observations following the 1992 Landers earthquake , 2000 .

[23]  F. Pollitz,et al.  Mantle Flow Beneath a Continental Strike-Slip Fault: Postseismic Deformation After the 1999 Hector Mine Earthquake , 2001, Science.

[24]  J. Johansson,et al.  Space-Geodetic Constraints on Glacial Isostatic Adjustment in Fennoscandia , 2001, Science.

[25]  T. Iwasaki,et al.  QUASI-STATIC CRUSTAL DEFORMATIONS DUE TO A SURFACE LOAD , 1982 .

[26]  G. Nabel,et al.  Regulation of the proinflammatory effects of Fas ligand (CD95L). , 1998, Science.

[27]  Paul Johnston,et al.  Sea‐level change, glacial rebound and mantle viscosity fornorthern Europe , 1998 .

[28]  Thatcher,et al.  Present-Day deformation across the basin and range province, western united states , 1999, Science.

[29]  Falk Amelung,et al.  Reservoir-induced deformation and continental rheology in vicinity of Lake Mead, Nevada , 2000 .

[30]  P. Molnar,et al.  Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere , 1983 .

[31]  Walter H. F. Smith,et al.  New, improved version of generic mapping tools released , 1998 .

[32]  Y. Okada.  Surface deformation due to shear and tensile faults in a half-space , 1985 .

[33]  Fred F. Pollitz,et al.  Gravitational viscoelastic postseismic relaxation on a layered spherical Earth , 1997 .

[34]  D. Dzurisin,et al.  Mechanisms of crustal uplift and subsidence at the Yellowstone caldera, Wyoming , 1994 .

[35]  R. Stein,et al.  Geometry of the 1954 Fairview Peak‐Dixie Valley earthquake sequence from a joint inversion of leveling and triangulation data , 1996 .