Mysterious seismic swarm foreshadowed monster Japan earthquake

Big earthquakes sometimes bring big surprises. The magnitude 7.5 earthquake that struck Japan on New Year’s Day, killing more than 160 people, is no exception. Over the past 3 years, tens of thousands of small to moderate earthquakes, most barely noticeable, had rattled the Noto Peninsula, a finger of land that juts 100 kilometers into the Sea of Japan from the west coast of Honshu, Japan’s main island. Such earthquake swarms occur throughout the world, typically in areas of volcanic or geothermal activity. But the swarms almost always taper off and end with a whimper. The Noto Peninsula swarm produced a bang. “I can’t think of another earthquake swarm globally that preceded such a large event,” says Zachary Ross, a geophysicist at the California Institute of Technology. Scientists are now puzzling over the details of that process and how the swarm may have led to the 1 January shock. “There are many questions to be resolved,” says Kyoto University geodesist Takuya Nishimura. Earthquake swarms typically occur when heat from deep underground produces high-pressure fluid—water, gas, or magma—that permeates fault systems, says University of Tokyo seismologist Aitaro Kato. The fluid lubricates faults, resulting in slow slips that produce small earthquakes. Unlike the brief flurry of small quakes that often precedes a major temblor, swarms continue rumbling for days or years without generating an identifiable main shock. The ongoing collision of...
Source: ScienceNOW - Category: Science Source Type: news