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Analysis of repeated leveling ob- servations in the vicinity of the San Gabriel fault in Southern California indicate subsidence irnsediately south of the fault relative to points to the north, south, and east. These observa- tions were previously interpreted as reflecting tectonic motions associated with either the "Palmcale Bulge" or with preseismic effects of the San Fernando earthquake. Relative subsidence between 1953 and 1964 reaches approximately 9 cm and extends over a distance of more than 20 km. Subsidence occurs directly above the Saugus aqui- fer and shows a temporal correlation with the history of water level decline within the aqui- fer. The degree of subsidence of individual benchmarks is roughly proportional to the product of aquifer thickness and water level decline at the location of the benchmarks. These observa- tions strongly suggest that movements of the surface near the San Gabriel Fault, previously inferred to be of tectonic origin, actually re- sult from near surface sediment compaction with- in the Saugus basin. Introduction Releveling estimates of crustal movement may reflect tectonic deformation, nontectonic motion, or systematic measurement errors. Proper inter- pretation of releveling measurements entails de- scriminating between these effects. The re- leveling measurements in Southern California which are presented here were initially inter- preted as representing tectonic movements asso- ciated with the "Palmdale Bulge" (Castle et al., 1976). More recently, Strange (1980) has sug-

u.s. geological survey open file report/open-file report

Recent vertical movements of the crust in the Western United States: reduction and analysis of leveling data and its interpretation in light of related seismological information