Open-File Report 96-517
Deformation. The January 17 1994, M=6.7, Northridge earthquake permanently deformed the ground surface in the Los Angeles urban region, increasing the elevation of the ground by as much as 20 inches (50 cm), and changing horizontal positions by up to 8 inches (20 cm). The network of geodetic monuments used by engineers and surveyors was thus distorted and rendered inaccurate. The monuments are used for construction and monitoring of the urban infrastructure, including dams, levees, aqueducts, and storm drains; bridges, buildings, and heavy machinery; road and rail transportation routes, communication conduits and pipelines; and property boundaries. We resurveyed the height of 979 leveling bench marks (BMs) and the positions and heights of 66 Global Positioning System (GPS) monuments; emplaced 252 new BMs where the previous monuments were lost or unrecoverable, and added 128 new GPS monuments along critical highways for rapid damage assessment to the main traffic arteries after future earthquakes. In total, 496 monuments were added to the national network. All surveyed monuments are shown on the map; new coordinates and heights are given in the appendix.
Shaking. The Northridge earthquake also strongly shook the ground, with peak shaking exceeding the acceleration of gravity (>1 g), and with peak ground velocities of 65 inches per second (or 165 cm/sec). Such sustained shaking and high ground velocities damaged structures and caused surface cracking, liquefaction, ground slumping, settlement, and landslides, which further contributed to damage. Because half of the BMs are located on engineered structures, their displacement not only records the permanent change in height caused by the earthquake to the geodetic network, but also any disturbance of the structures caused directly or indirectly by shaking. We developed a geophysical model of the permanent ground deformation that is consistent with the movement of the geodetic monuments. All BMs with vertical displacements that differ from this model by more than 1.2 inches (3 cm) are considered to be anomalous. The 40 such anomalous or disturbed BMs are shown in the map and listed in the report. Those in engineered structures include railroad and highway bridge abutments and spans, tower and building foundations, catch basins, retaining walls, and culverts; the remainder are typically in engineered fill. The anomalous BMs are identified on the map; driving directions and preliminary site inspections to each anomalous BM are included.
Recommendations. Structures associated with anomalous BMs may be in a weakened state, making them vulnerable to shaking during future earthquakes. Because few structures have been assessed for earthquake effects except by visual inspection, subtle or hidden damage suggested by the settlement or uplift of the structures merits re-inspection. We further suggest that more geodetic monuments be placed on critical transportation arteries, structures, and lifelines, to ensure faster and more accurate post-earthquake damage assessments in the future.
This report includes an analysis, a CD-ROM containing the data files in several GIS (Geographic Information System) and illustration formats, a large-scale color map, and an appendix giving driving instructions to the sites of anomalous bench marks.
U.S. Department of the Interior |
U.S. Geological Survey
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