Clues in a Fault Trench: Forecasting the Next Big Earthquake

A Field Trip to the Coldwater Fault

1The geology club at Ridgeview Middle School spent a Saturday collecting rocks and fossil leaves in the hills above town. In the afternoon, club advisor Ms. Lourdes Kowalczyk led the students to a trench, a long ditch about 3 m deep, that a university team had dug across the Coldwater Fault. A fault is a crack in Earth's crust where blocks of rock slip past each other. Most of the time the Coldwater Fault is stuck, and strain builds up slowly in the rock. When the rock finally slips, a large earthquake shakes the region.

2Dr. Anders Pellegrino showed club member Ezra Mbatha how to read the trench wall. Layers of silt and gravel are laid down flat, one on top of another. After a large earthquake, the layers that were at the surface are broken and shifted along the fault, and jumbled soil slumps off the small cliff the earthquake made. Later, new flat layers bury the break. Bits of charcoal and plant fossils in each layer let the team date it, so they could list the large earthquakes on this part of the fault (Table 1). The town's written records do not go back far enough to include any of them.

3Ridgeview sits 7 km from the fault. The county is testing an earthquake early warning system. Sensors near the fault detect an earthquake's first, faster waves, and about 5 seconds after the earthquake starts, the system sends alerts to phones, schools and trains. The strong shaking that does most of the damage travels outward from the fault at about 3.5 km per second. An alert cannot arrive anywhere before the system sends it.

4A university lab also tested three model buildings on a shake table, a platform that shakes back and forth like the ground in an earthquake. The models were the same size and were shaken in the same way. One had no added parts, one had steel cross-braces in its walls, and one sat on base isolators, thick rubber pads that let the ground move under the building while the building moves less (Table 3).