INTRODUCTION

Geologists can map the rocks exposed at the Earth's surface as well as locate where movements along faults have juxtaposed rocks of different composition and history. However, the sources of earthquakes occur at depth, not at the surface. Most of the Earth's mineral deposits and oil and gas are not exposed at the surface. Geologists cannot directly sample hard, consolidated rocks in areas covered by water or by young sand, silt, and gravel (such as, for example, the San Francisco Bay) except by drilling, which is usually very expensive and depth-limited. However, several different methods of geophysics (literally, the physics of the Earth) allow us to "see" into the Earth and give geologists an indirect and relatively inexpensive "picture" of the rocks beneath our feet. The different geophysical methods measure changes in certain physical properties of rocks. For example, the seismic refraction method measures the velocities of energy waves created by an explosion or an earthquake in order to infer the kinds of rocks or geologic structures in that area. Electrical methods measure how easily electric currents or electromagnetic waves travel through rocks. Two of the many techniques used by geophysicists, gravity and magnetism, are highlighted in this guide. These data are shown by white contour lines (see box 1.1 on contours) superimposed on a satellite image of the San Francisco Bay area. Hachures, the barbs on some of the contours, point towards areas characterized by relatively lower gravity or magnetic values. As an alternative to presenting these data as numbered contour lines, we also show the gravity and magnetic maps in color in order to help the reader visualize the gravity and magnetic fields (see color figures at the end of the pamphlet). Geophysicists often use both styles of presentation to study the gravity and magnetic fields.

About the LANDSAT image

This LANDSAT Satellite image that forms the background for the gravity and magnetic maps shows a 50,000-square-kilometer area of central California, extending from Lake Berryessa in the upper right of the poster (north) to Monterey in the lower left (south), and also extending from about 30 kilometers out into the Pacific Ocean on the west to Stockton on the east. The image shows the San Francisco Bay area as it looks from an altitude of about 900 kilometers above the Earth. The satellite image was taken using part of the visible part of the electromagnetic spectrum and also part the infrared part of the spectrum, which is not visible to the human eye. The poster uses false-color imaging to enhance the land forms and surface features. The colors of the features, though brilliant, are not those that we normally see. How many different colors do you observe? By looking at the distribution of colors and your knowledge of Bay area geography, can you guess what the colors mean? The color key is as follows:

Deep red--ground covered by heavy vegetation (for example, the Santa Cruz Mountains near the coast north of Santa Cruz

Light red--cultivated crops (rectangular areas most abundant in the San Joaquin and Sacramento Valleys, on the right side of the image)

Shades of green--grass-covered areas (east of the San Francisco Bay) and marshy areas (northwest shore of San Pablo Bay)

Turquoise blue--urbanized areas (note the obvious street grids. Red flecks are mostly parks and greenways)

Royal blue--bodies of water (most prominently, San Francisco Bay and the Pacific Ocean)

The bright yellow circles superimposed on the satellite image represent the epicenters of more than 12,000 earthquakes of magnitude 2 or larger that occurred in the San Francisco Bay area between Jan. 1, 1972 and Dec. 31, 1989. An epicenter is the point at the Earth's surface directly above the focus, or actual location, of the earthquake within the Earth's crust. The diameters of the circles depict the magnitude of the earthquakes-from the smallest (one millimeter), representing earthquakes of magnitude 2-3, to the largest, an eight-millimeter-diameter circle in the Santa Cruz Mountains, representing the 7.1 magnitude Loma Prieta earthquake of Oct. 17, 1989. Note that many epicenters lie along mapped Bay area fault lines.