Too soon after the mainshock to distinguish fresh building damage, the April aftershocks did bring additional damage, which we know from the infrastructure impacts. The most damaging aftershock occurred on April 7, 2011, situated nearshore and downdip of the original epicenter. The disposition of the fault rupture, dipping down toward the land, means that many aftershocks tend ed to be deeper with reduced surface shaking. With the exception of the largest aftershock which continued the fault rupture to the south, these initial earthquakes, located in an aftershock cloud around the main fault rupture, did not greatly add to the damage. The number of aftershocks runs into the thousands with 82 shocks of M6. By this time, according to Omori’s Law, daily activity was 0.03 percent of the first day. 0, and then the intervening gaps lengthened exponentially: April 2011, July 2011, December 2012, October 2013, and most recently in February 2021, almost at the 10th anniversary. Within 24 hours, there were two more earthquakes above magnitude 7. With Tohoku, the biggest aftershock had a moment magnitude of 7.7 and occurred within 29 minutes of the mainshock, extending the plate boundary fault rupture south. “Båth’s Law” predicts that the largest aftershock is typically 1.1 to 1.2 magnitude units smaller than the mainshock. In a previous blog looking at aftershocks after the 2011 Christchurch, New Zealand, earthquakes, I outlined the work of Markus Båth. With nothing other than empirical experience, accumulated at diverse times and towns, a rational policy was formulated since the thirteenth century that would impress a twenty-first century behavioral economist.įollowing the Mw9.0 Tohoku Earthquake on March 11, 2011, three out of eight of the largest M7.0+ aftershocks occurred within the first seven days. After a week, the additional risk reduction for staying outside another night is only an eighth of the risk saved through camping out the first night.Īlmost half of the total risk across 100 days after the mainshock occurs in the first week. He also identified that activity falls off rapidly in passing away from the mainshock source. We now know how to calculate the risk-saving.ĭid the earthquake quarantine week save lives? According to Japanese seismologist Fusakichi Omori in 1894, aftershock activity decays proportional to 1/t, which is now known as Omori’s Law. Like the 40 days of quarantine for a ship suspected to be carrying the plague, the waiting time was founded on centuries of astute empirical observation. This week of earthquake quarantine was a tradition passed down the generations in central Italy. On the eighth day they returned to their homes. For a week of aftershocks, they lived away from their buildings. Many of the town’s masonry buildings were left in a precarious state, and the survivors dragged their wood-frame beds out of the ruins into hastily assembled sheds and tents kept in storage for such an emergency. The concept of an “earthquake quarantine” was evident in the Italian town of Pistoia, Tuscany, some 10 miles (16 kilometers) northwest of Florence, when it was hit by a strong earthquake in March 1293. How long should a community wait after an earthquake before they return to their homes or start to rebuild? When is the risk of aftershocks over?Īwareness of aftershocks has carried through the centuries. The Japan Meteorological Agency recently called the M7.1 earthquake on February 13, 2021, an aftershock of the March 11, 2011, Mw9.0 Tohoku event, nearly 10 years after the original mainshock.
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