Dynamic deformations of shallow sediments in the Valley of Mexico, Part II: Single-station estimates
Por:
Singh S.K., Santoyo M., Bodin P., Gomberg J.
Publicada:
1 ene 1997
Resumen:
We develop simple relations to estimate dynamic displacement gradients (and hence the strains and rotations) during earthquakes in the lake-bed zone of the Valley of Mexico, where the presence of low-velocity, high-water content clays in the uppermost layers cause dramatic amplification of seismic waves and large strains. The study uses results from a companion article (Bodin et al., 1997) in which the data from an array at Roma, a lake-bed site, were analyzed to obtain displacement gradients. In this article, we find that the deformations at other lake-bed sites may differ from those at Roma by a factor of 2 to 3. More accurate estimates of the dominant components of the deformation at an individual instrumented lake-bed site may be obtained from the maximum horizontal velocity and displacement, ?max and umax, at the surface. The maximum surface strain ?max is related to ?max by ?max = ?max/C, with C ? 0.6 km/sec. From the analysis of data from sites equipped with surface and borehole sensors, we find that the vertical gradient of peak horizontal displacement (?umax/?z) computed from sensors at 0 and 30 m equals (umax)z = 0/?z, ?z = 30 m, within a factor of 1.5. This is the largest gradient component, and the latter simple relation permits its estimation from surface records alone. The observed profiles of umax versus depth suggest a larger gradient in some depth range of 10 to 20 m, in agreement with synthetic calculations presented in Bodin et al. (1997). From the free-field recordings of the 19 September 1985 Michoacan earthquake, we estimate a maximum surface strain, ?max, between 0.05% and 0.11%, and a lower bound for the peak vertical gradient (?umax/?z) between 0.3% and 1.3%. This implies that (1) the extensive failure of water pipe joints during the Michoacan earthquake in the valley occurred at axial strains of about 0.1%, not 0.38% as previously reported, and (2) the clays of the valley behave almost linearly even at shear strain of about 1%, in agreement with laboratory tests. The available data in the valley can be used to predict deformations during future earthquakes using self-similar earthquake scaling.
Filiaciones:
Singh S.K.:
Instituto de Geofísica, Instituto de Ingeniería, UNAM, 04510 México, D.F., Mexico
Santoyo M.:
Ctr. Nac. de Prevencion de Disastres, Delfin Madrigal 665, 04650 México, D.F., Mexico
Bodin P.:
Ctr. for Earthquake Res. and Info., University of Memphis, Memphis, TN 38152, United States
Gomberg J.:
Ctr. for Earthquake Res. and Info., University of Memphis, Memphis, TN 38152, United States
U.S. Geological Survey, Ctr. for Earthquake Res. and Info., University of Memphis, Memphis, TN 38152, United States
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