Open-File Report 96-517
Immediately after the Northridge earthquake, the vast majority of structures were assessed for earthquake effects solely by visual inspection. Thus subtle or hidden damage may have escaped detection. The settlement or uplift of the structures deduced from the movement of bench marks (BMs) offers a new tool to identify potentially weakened structures. A similar approach was followed after the M=7 Cape Mendocino earthquake in northernmost California [Stein, et al., 1996]. BM disturbance revealed damage to a major bridge crossing the Eel River, as well as road and rail embankment failures caused by landslides and liquefaction. The structures identified by anomalous BMs in the Los Angeles area may thus be in a weakened state, making them vulnerable to shaking during future earthquakes. Thus we strongly recommend that all 40 sites of anomalous BMs be examined by structural engineers for follow-up evaluation. Driving directions and a preliminary inspection of each BM is included in Appendix I.
In addition, we suggest that a higher percentage of geodetic monuments be emplaced and measured precisely on critical transportation arteries, structures, and lifelines. This will ensure faster and more accurate post-earthquake damage assessments in the future. Traditionally, BMs are placed on frontage roads and less traveled road and rail routes to ensure the ease of access by surveyors and engineers. BMs have been located in bridge abutments and approaches when convenient for surveyors. Instead we argue that the need to identify a weakened structure supersedes the ease of access, and thus freeways and overpasses should be monumented so that rapid post-earthquake evaluation is possible. To be most effective, BMs should be placed on both sides of each bridge or overpass, and also within 500 m of the ends of the bridge to provide a stable reference site for comparison to the bridge.
U.S. Department of the Interior |
U.S. Geological Survey
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