The potential for damage to structures in Chaco Culture National Historical Park resulting from earthquakes, landslides, industrial blasting, road building and vehicular traffic has been investigated. The Historical Park, located in northwestern New Mexico, contains over 2,000 known archeological sites. The structures of interest, many of them multistory and a few containing over 200 rooms, date from the 11th and 12th centuries. Most of the remaining walls are 1.5 to 3.0 m in height, but a number exceed 5.0 m. A 2.0 mm/sec particle velocity is recommended as the upper limit for induced motions in the structures resulting from industrial blasting, road building and vehicular traffic. Minimum distances of these activities from the structures are recommended based on field recordings and analysis of the induced vibrations from these sources. Minimum distances of 1.2 km from blasting, 0.5 km from railroad traffic, 45 m from road building and 25 m from vehicular traffic are recommended based on normal blasting practices in the area, conventional rail traffic, usage of road building equipment and normal vehicular traffic patterns. Recommendations are also made for controlling vibrations from one road in the Historical Park considered to be too close to historical structures. Levels of expected ground motion from earthquakes, even for relatively short time periods of interest such as 50 yrs, indicate that possible future earthquake damage to the structures should be considered. The implication is that at least some of the past deterioration of the structures in the Historical Park may have been caused by earthquake ground motion.
During 1990 we collected eight lines (11.5 km) of shallow seismic reflection data across the Bootheel lineament, a discontinuous feature that extends about 135 km in a north-northeast direction through northeastern Arkansas and southeastern Missouri. The profiles image reflectors at depths between about 55 m to 800 m. Gentle folding with wavelengths of about 800 m and amplitudes of 10 m to 25 m is evident on nearly every profile, generally coinciding with the surface traces of the lineament. We interpret our lines to show a complex zone of strike-slip deformation consisting of multiple flower structures, with deformation at least as young as the Eocene/Quaternary unconformity.
Following the 1989 Loma Prieta earthquake, a dense array of seven digitally recorded, three-component seismograph stations was deployed on Robinwood Ridge 7.3 km northwest of the epicenter. The purpose of this array was to investigate the cause of high levels of structural damage and ground cracking observed on the ridge crest. Aftershocks recorded by the array allow a comparison of ground motion up the slope of the ridge from the base to the crest. The data present an extremely complicated pattern of ground motion that demonstrates the importance of the three-dimensionality of the problem. Slowness analysis of P wave trains show initial arrivals propagating away from the source with small angles of incidence and large apparent velocities, consistent with direct arrivals. After 0.5 sec, propagation azimuths become more random and apparent velocities drop, indicating nearly horizontal wave propagation and multiply reflected and diffracted phases within the ridge. Slowness analysis and particle motion diagrams of horizontal components of motion show dramatic variations in ground motion with changes in azimuth of the source and a complicated interaction between body waves and Rayleigh and Love waves. Results suggest that the larger amplitude, more coherent arrivals at the array stations favor a propagation direction parallel to the ridge axis. An amplification factor of from 1.5 to 4.5 is seen for frequencies from 1.0 to 3.0 Hz with wavelengths comparable to the base of the ridge, part of which may be caused by local site effects and part by topographic amplification. In addition, amplifications of up to a factor of 5 are seen at higher frequencies and are attributed to local site effects. These effects are most notable from 4 to 8 Hz on the vertical components, and from 6 to 9 Hz on the horizontal components. The entire Robinwood Ridge area may also have been situated in a region of heightened mainshock ground motion due to source directivity and radiation pattern effects.
Some buildings in Santa Cruz, California, particularly those in the downtown section which is built on a flood plain, were severely damaged by the 17 October 1989 Loma Prieta Earthquake. During an aftershock study conducted in the fortnight following the mainshock, an array of seismographs was deployed across downtown to study the site response of the flood plain. Analyses of the records of eight aftershocks reveal that ground motions of the flood plain are amplified relative to those of crystalline rock sites by factors of 4–8 in the frequency band 2–8 Hz. Both resonance in the whole sedimentary column which overlies the crystalline basement and resonance/broad‐band amplification in the low‐velocity alluvium of the flood plain contribute to this site response.
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