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1.1 Overview

Leveling measures the height difference between two points along a horizontal line of sight. Summing the relative height differences along a leveling line yields the elevation of those BMs with respect to the height of the first BM. Repeated measurements of height enables measurement of deformation of the Earth's surface with time. Such deformation can occur gradually, such as by land subsidence due to groundwater or oil withdrawal, or by sudden geologic events such as earthquakes. The vertical deformation caused by an earthquake is calculated by subtracting the elevation of points measured before the earthquake from the elevation of the point measured after the event.

Table 1. Preseismic and postseismic leveling surveys analyzed in this study.

While the earthquake deformation signal is ideally calculated by subtracting surveys made immediately before and immediately after the earthquake, this is rarely possible in practice, and thus non-tectonic effects must be quantified and removed. In the Los Angeles basin, ground subsidence caused by groundwater, natural gas, and oil withdrawal is a continuing source of surface deformation that must be removed from the earthquake elevation changes. We determine the rate of non-earthquake effects by comparing at least three surveys of the leveling route made at different times before the earthquake.

After the Northridge earthquake, between June and August 1994 the National Geodetic Survey, as part of this study, conducted high-precision leveling surveys along 16 leveling routes in the Los Angeles area (Table 1 and Figure 2). To infer the vertical deformation associated with the 1994 Northridge earthquake, surveys of the routes made between 1971 and 1989 were subtracted from the 1994 measurements. The 1994 leveling surveys are referred to as the postseismic data set and the surveys made before the Northridge earthquake referred to as the preseismic data set (Figure 3). Elevations measured at BMs in both pre- and postseismic surveys form the basis of the coseismic data set .

• Figure 2. Leveling routes measured as part of this study by the National Geodetic Survey in the San Fernando Valley, Simi Valley and Los Angeles (Survey L24568 parts 1 to 16). Benchmarks are shown as small black dots and each route is labeled 1 to 16 corresponding to Table 1.
• Figure 3. The coseismic leveling data set. Each route is annotated with the years of surveys used to study the possibility of subsidence.

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