The Oceanside 1:100,000-scale quadrangle lies between 33° and 33°30' N. latitude and 117° and 118° W. longitude. It underlies a rapidly urbanizing part of southern California. The area is tectonically and seismically active and is dissected by four major, northwest trending, oblique right slip, Pacific/North American Plate boundary fault zones. They include the Elsinore Fault Zone in the northeastern corner of the quadrangle, the Newport-Inglewood Fault Zone in the center of the quadrangle (origin of the 1933, M=6.3, Long Beach earthquake), the Coronado Bank Fault in the near offshore region and the San Diego Trough Fault Zone in the southwestern corner of the quadrangle (origin of the 1986, ML=5.3, Oceanside earthquake). Landslides are abundant in the western and offshore parts of the quadrangle. Also, seismic hazards are numerous throughout the area. A tsunami hazard exists along the coastal margin.

The quadrangle is underlain by a thick sequence of forearc and forearc-basin Jurassic and Cretaceous (mostly low grade greenschist facies but partly unmetamorphosed) andesitic flows, sedimentary and volcaniclastic breccias and marine metasedimentary rocks that have been intruded in their older part by the southern California batholith. The batholith is Cretaceous in age and in part coeval with the forearc and forearc-basin rocks. The batholithic rocks are mostly tonalite and granodiorite with less common gabbro, diorite, monzogranite and granite. Pegmatite dikes are common in these intrusive rocks. The western part of the quadrangle is underlain by a relatively thick (>1000m) succession of Upper Cretaceous, Tertiary and Quaternary sedimentary and volcanic rocks that unconformably overlie the older plutonic and forearc basement rock sequence. These rocks consist chiefly of beds of marine, paralic, and nonmarine claystone, siltstone, sandstone and conglomerate and minor flows consisting mostly of Neogene basalt. Many cycles of uplift, erosion, subsidence and deposition since the Late Mesozoic have created the complexity of the existing stratigraphic and structural settings.

Fractured and deeply weathered bedrock associated with K/T boundary subareal extremes mantles much of the interior highlands and particularly the steep slopes adjacent to and southwest of the Elsinore Fault Zone.