Transcript for Computer Animation of Loma Prieta Aftershocks, segment 10 of 12


The final animation shows selected earthquake focal spheres portrayed as three-dimensional beach balls. The Loma Prieta main shock is the larger sphere at the bottom center of the aftershock zone. The colored quadrants of the focal spheres represent compressional first motions. The white quadrants are dilatational. Note that you are looking at the outside of the focal sphere, not the lower hemisphere that is often plotted on maps. The different colored spheres represent different fault types. The green mechanisms are a thrust, the blue mechanisms are normal, and the red and yellow mechanisms are strike slip. The strike slip mechanisms are colored by the slip direction on the plane closest in strike to the San Andreas fault.

The camera first flies beside the fault plane at a depth of ten kilometers. We then fly in a semicircle above ground and retrace the flight path. The focal spheres are two point four kilometers in diameter and centered at the hypocenter of the earthquake. The colored quadrants show the direction of compressional first motion, and the white quadrant shows the place where dilatational first motion left the earthquake source. One of the two nodal planes separating the compressional and dilatational quadrants is the plane on which the shear slip occurred. The red dots are magnitude three or larger aftershocks. Note that the steeply dipping nodal plane of the main shock coincides with the plane of aftershocks and is the main shock slip plane. For the other focal spheres it is often difficult to say on which nodal plane the slip occurred. The variety of orientations of the nodal planes and coloring in different spheres means that a mixture of different slip directions were active. Most aftershocks do not exactly match the focal mechanism of the main shock. However, the most common mechanism is oblique thrust, drawn in green.

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