The map shows all recorded seismic events that have occurred in southern California in the years from 1983 through 1997, with each earthquake represented by a single pixel (many overlap). The color of these pixels varies with the depth of each earthquake's hypocenter, as given by the scale at lower left. Plotted for reference on the background are the surface traces of the major faults in the area (shown as black lines -- the most prominent being the San Andreas fault, which runs from the lower right corner to the upper left corner of the map) and the major highways of the area (shown in tan).

Keep in mind that there are significant errors in some of these depths, and that this can result in misleading patterns. The most obvious example of this is the apparent "flatness" of earthquake depths around the edges of the map, which correspond to the edges of the seismic instrument network used to record this data. Earthquakes outside or on the edges of this network are poorly located, and the depth of the hypocenter is typically "fixed" by the data processors at a depth characteristic for an "average" earthquake in southern California: 6.0 kilometers. (This is done to facilitate the determination of the time and epicenter as calculated by a computer algorithm.) Thus, many of the earthquakes around the edges of this image appear to have occurred at a depth of 6.0 km, when in fact, they may have been deeper or more shallow. Also, the maximum depth of earthquakes shown here is probably too deep for earthquakes in this area -- these anomalous depths are due to over-simplified velocity models of the crust and upper mantle, and a variety of other factors.

Despite these problems, there are many significant patterns to be found in this image. Note the predominance of deep (below 10 km; green to blue) earthquakes in the San Gorgonio Pass area, along certain sections of the San Jacinto fault zone, and, to a lesser degree, in the Los Angeles area. In constrast, note how the Mojave and the Eastern California Shear Zone are characterized by shallow (under 6 km; red to orange-yellow) earthquakes. Upon close inspection, you can deduce the dip of the fault involved in the 1983 Coalinga earthquake from the group of hypocenters at upper left, east of the San Andreas fault. It is even possible, though somewhat difficult (you need to know exactly where to look), to determine the dip of the fault that produced the 1994 Northridge earthquake.

This plot is also available on a relief-map background (198 K), without the fault lines.