Problem of the Week

Walls of Water: Tsunamis The loud blaring of the natural disaster warning horns awakens you. You stumble in the dark to find the lights. As you quickly get into your clothes, you recognize the sound of this particular horn as the one that tells you a tsunami is on its way. Getting out of the house as quickly as you can, you join your neighbors in the streets. You all move inland as quickly as possible, heading for the safety of higher ground. In minutes, you reach the point beyond which the authorities believe you will be safe, and the waiting begins. You wait in the night for the passing of a wave that, without the warning system, could have resulted in death. If you live in coastal areas around the world, where seismic activity occurs on the ocean floor, this might be a familiar scenario.

As we have learned in this chapter, seismic events, like earthquakes, occur in places where the earth’s crust is brittle and faulted. When an earthquake takes place on the ocean floor, and there is a vertical displacement of the ocean water above it a tsunami may be born. Tsunamis are actually a series of waves that are set off from the seismic event, like the ripples that occur when a rock is thrown into a pond. To gain a good visual idea of how this happens, visit the PBS Website – "Savage Earth: Predicting Tsunamis." Watch the animation of the birth of a tsunami at a subduction zone. (http://www.pbs.org/wnet/savageearth/ animations/tsunami/index.html )

In the open ocean, the tsunami waves may be little more than a meter high, yet they travel at incredible speeds, up to 700 km per hour. As the waves approach the coastal area, an unusual receding of the water at the shoreline often precedes them. Following this, the first wave nears the shore, growing higher and higher as it enters shallow water; often up to 45 feet tall. The wave does not break, as a typical ocean wave, but remains a wall of speeding water, plowing over everything in its path.

According to statistics posted by the Federal Emergency Management Agency (FEMA) since 1945, tsunamis have killed more people than earthquakes. So, the question becomes, how do we warn people about an approaching tsunami? As in the scenario at the beginning of the feature, citizens in countries around the Pacific Ocean, such as Japan receive warning by horns. We have a warning system here in the United States too – the Tsunami Warning Center in Palme, Alaska. The seismometer, a series of tsunami detectors on the ocean floor, and buoys on the surface are key elements in the warning game. As the tsunami passes, signals are sent to warning centers and citizens are warned. Experts say that giving people even 5 minutes warning may move them far enough inland to save their lives.

The Problem: Assume: Only earthquake waves can be detected as a warning of a possible tsunami approach. There will be no electronically generated warnings from other countries.

The situation: An earthquake occurs 70 km ESE of Nikolski, Alaska (52.7N, 167.9W). This earthquake has produced mass vertical movements of the type that produces tsunamis. There are no seafloor or buoy detectors between the epicenter and Nikolski. There are detectors between the epicenter and the other cities involved in this problem. P-waves travel at 6 km/sec. The distance from the epicenter to Kodiak, Alaska is 1,238 km. The distance from the epicenter to coastal Tokyo, Japan is 4,342 km. The distance from the epicenter to coastal San Francisco, California is 3,841 km.

Determine: How long will it take the first p-wave to reach Nikolski, Alaska? Kodiak, Alaska? Tokyo, Japan? San Francisco, California? Then, following the earthquake: How long will it take a tsunami, traveling the average speed of 700km/hour, to reach: Nikolski, Alaska? Kodiak, Alaska? Tokyo, Japan? San Francisco, California? In which cases will there be enough, or not enough, time to warn citizens of these coastal communities of the approaching tsunami? Discuss what would be the best way to warn citizens living in coastal areas for each city.