Planning Notes: Where the Water Line Crosses the Fault
1Cordero is a growing city of about 120,000 people in a dry valley in the western United States. The city plans a new water line from a reservoir in the eastern hills and a new power line from a solar farm. Both routes must cross the Linares Fault, a long break in Earth's crust that runs north to south through the valley. At a public meeting, city geologist Dr. Paloma Arriaga explained what the rocks near the fault show. Rafferty Osei, a seventh grader on the city's youth council, took these notes.
2The Linares Fault is a boundary between two plates. The land west of the fault sits on one plate, and the land east of it sits on another. Along this kind of boundary, the plates slide past each other sideways. Most of the time the fault is stuck. Then it slips suddenly during an earthquake, often by a few meters at a time, and over long periods the slips add up.
3Geologists have found rock bodies that were cut in two by the fault. For each one, the two halves have the same kind of rock and the same age, and the sandstone halves hold the same kinds of fossil scallop shells. Yet the halves now lie far apart along the fault. Table 1 lists four of them. In every case, the half on the west side of the fault lies to the north of its match on the east side. GPS stations near the fault show that the motion is still going on today.
4Far to the south, the fault ends at a long, narrow gulf of the ocean. A peninsula lies on the west side of the gulf, and the mainland lies on the east side. Down the middle of the gulf, short ridges on the seafloor are spreading apart, and lava cools there into new basalt. Rocks on the two coasts of the gulf are listed in Table 2.
5Dr. Arriaga's team used a computer map to slide the peninsula back toward the southeast. When it had moved about 180 km, the edge of its continental shelf fit snugly against the mainland's shelf, and the matching rocks on the two coasts lined up.