In order to explain damage and observed ground motions in Mexico City during the 1985 Michoacán earthquake, simultaneous consideration must be given to source, path, and site conditions. This is clear from teleseismic records and local vertical displacements. Incident waves had an important part of energy in the frequency band of 0.3-1 Hz. Damage distribution and observed motion in the lake bed zone cannot be satisfactorily explained using one-dimensional theory. The effects of lateral irregularities are required. To assess its effects we describe the stratigraphic setting of the valley and discuss some features of damage distribution with results for one- and two-dimensional wave propagation models. These are useful to establish on quantitative basis the importance of lateral heterogeneity.

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Abstract.The origin of Upheaval Dome, in Canyonlands National Park of southeastern Utah, has been a topic of controversy among geologists and planetary scientists. The structure has long been thought to have been created by salt diapirism from the underlying Paradox Formation. Recent studies have suggested that impact could have formed the dome. To test the various hypotheses, we acquired, processed, and interpreted seismic reflection data within and adjacent to the structure. Both conventionally stacked and prestack-migrated images show <100 m relief in the Paradox Formation, contrary to salt diapirism hypotheses.

Imaging of subsalt sediments is a challenge for traditional migration methods such as Kirchhoff and one-way wave-equation migration. Consequently, the more accurate two-way method of reverse-time migration (RTM) is preferred for subsalt imaging, but its use can be limited by high computation cost. To overcome this problem, a 3D target-oriented reverse-time datuming (RTD) method is presented, which can generate redatumed data economically in target areas beneath complex structures such as salt domes. The redatumed data in the target area then can be migrated inexpensively using a traditional migration method. If the target area is much smaller than the acquisition area, computation costs are reduced significantly by the use of a novel bottom-up strategy to calculate the extrapolated Green’s functions. Target-oriented RTD is tested on 2D and 3D SEG/EAGE synthetic data sets and a 3D field data set from the Gulf of Mexico. Results show that target-oriented RTD combined with standard migration can image sediments beneath complex structures accurately with much less calculation effort than full volume RTM. The requirement is that the area over the target zone is smaller than that of the acquisition survey.