Science / Medicine : HOW IT WORKS : A Little Bit of Fun to Understand Quake Action

The Northern California earthquake of Oct. 17, 1989, called the Loma Prieta quake after a nearby mountain peak, resulted from slippage of the giant Pacific and North American tectonic plates, which are normally held stationary by friction. The hypocenter of the quake, the point within the Earth where movement began, was on the San Andreas Fault at a depth of 11 miles.

The break expanded rapidly along the fault plane, moving at about 7,000 m.p.h. Slippage extended for about 15 miles in both directions (northwest and southeast) along the fault and upward to within 4 miles of the surface. The Pacific plate moved northwest about 5 feet and upward about 3 feet with respect to the North American plate.

Because the main fault rupture, or break, did not continue all the way to the surface, the upper layers of the Earth were bent rather than broken. The area of deep cuts in the ground, known as fissures, near the north end of the rupture were presumably a result of this bending.

In the paper model, a series of cuts forming a “stepped” pattern that scientists call an echelon is placed directly above the fault rupture. These cuts allow motion on the fault below and illustrate how surface layers might be deformed by underground fault movement. The actual pattern of bending and fissures was more complicated, however.

Instructions 1. Paste this page onto a piece of heavy typing paper, or photocopy it so that it will be on heavier paper, making the model sturdier. 2. Cut out the three pieces of the model. 3. Fold the two large pieces into box shapes. 4. Glue the tabs of the two boxes and assemble. 5. Glue the surface layer to the top of the two boxes, being careful that the area of the fissures remains free of glue. 6. To simulate earthquake displacement, shift the “Pacific plate” box upward and to the north and notice how the fissures buckle.