Passage II

Two scientists discuss the possibility of accurate short-term earthquake prediction (accurately predicting a quake’s strength and location within hours or days).

Scientist 1

Short-term earthquake predictions can be made at the present time by detecting events that occur shortly before a quake. Rocks increase in volume under the great pressures often generated in fault zones. At the same time, numerous cracks open in the rocks. The volume increase can be detected by instruments installed near an active fault. Groundwater is often forced away from the fault by the expansion, raising water levels in nearby wells before a quake. The opening of cracks causes the speed of one type of earthquake wave to decrease compared to the speed that those waves had through the rocks before they cracked. Radon gas (a gas found in rocks in many locations) is released by the cracking, and the released gas can be detected by instruments.

Most quakes are preceded by foreshocks. Foreshocks are minor quake vibrations that occur minutes, hours, or days before the large quake. Monitoring seismographs (instruments used to detect vibrations along faults) for foreshocks can help predict quakes.

Scientist 2

Short-term earthquake predictions cannot be made at the present time with any accuracy. Long-term predictions (within 10 to 100 years) are much more accurate than short-term predictions. Many of the physical changes that occur in rocks under pressure vary according to the rock type or the variety of rock types that are present in a fault zone. Often, the expected volume increase happens long before the quake occurs, or it happens only moments before a quake. Changes in water levels of wells may be caused by factors unrelated to changes in rocks. The detection of radon is a process that takes weeks to conduct and obtain results. Many fault zones have almost constant vibration going on, and foreshocks are almost impossible to separate from these vibrations. One major barrier to short-term prediction is that we cannot directly study or sample the region 10 km or more below the surface, where quakes occur.