Open-File Report 96-517
Each bench mark in the coseismic data set was classified by the type of structure upon which it is located (Table 2). The five classifications are: bridges, bridge piers and abutments (class B), culverts and retaining walls (class S), electrical towers (class T), building foundations (class F), catchment basins (class C), and all others, such as BMs set in roads, curbs, bedrock, sediment or engineered fill (class N), (see Tables 2 and 3).
The deformation observed at 997 of the coseismic leveling BMs could be explained exclusively by the permanent coseismic deformation of the Northridge earthquakes. Residuals in excess of 3 cm or more were observed at 40 BMs, and these BMs suggest that shaking effects contributed to their movement. The few GPS monuments showing residuals greater than 3 cm are discussed in detail by Hudnut et al., (1996), few of the GPS stations are placed in engineered structures or fill. Possible causes of the disturbance to leveling BMs include ground shaking [Wald & Heaton, 1994], surface cracking and faulting [Hecker, et al., 1995], landslides [Harp & Jibson, 1995], settlement and liquefaction [Holzer, et al., 1996], or damage to structures where BMs are situated [Buckle, 1994; Caltrans, 1994]. Culverts and retaining walls displayed the largest percentage of damaged BMs (Table 3). Only one BM near an electricity tower had a residual of 3 cm or greater. Although not on the tower itself, the BM was within 30 m of the structure. Each of the BMs that had significant residuals is listed in Table 3 and driving directions and access instructions are given in Appendix I.
The majority of the disturbed, or anomalously displaced, BMs are concentrated in the Granada Hills, Santa Clarita Valley and along Route 118 between Fillmore and the junction of Route 118 with Interstate 5 (Map 1). These are areas where the peak ground velocity was calculated to be greatest, between 80 to 100 cm/second [Wald, et al., 1996] (Figure 11). The majority of landslides occurred in these foothill areas. In contrast, only three BMs south of downtown Los Angeles had large residuals. The coseismic displacements caused by the Northridge earthquake were quite small at these distances. One BM at El Segundo (No. 18, Map 1), where the peak ground velocity was about 20 cm/second, is located in sandy soil and may have shifted as a result of ground shaking and settlement. BMs 19 and 20 (DY2538 and DY2535, see map) are located in Long Beach; both are situated on or near concrete catchment basins which may have been damaged in the earthquake.
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U.S. Geological Survey
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