Major earthquake series strikes Kyushu IslandStarting on Thursday, a series of several large earthqu

Major earthquake series strikes Kyushu IslandStarting on Thursday, a series of several large earthquakes struck the southernmost main Japanese Island of Kyushu, centered near the city of Kumamoto, a city with an estimated population of about 750,000.The first 2 quakes had reported moment magnitudes of 6.0 and 6.2. The moment magnitude scale is a direct measurement of the energy released by an earthquake, but that doesn’t translate directly to shaking/damage felt by humans. Instead, a scale called the Mercalli scale measures the shaking people at the surface will actually feel, taking into account the local geology and the distance to the earthquake hypocenter. The first 2 quakes registered VIII on that scale for the city of Kumamoto, implying the possibility of heavy damage. These two quakes reportedly killed at least nine people and caused tens of thousands of people to evacuate their homes.At 1:25 a.m. local time overnight into Saturday, a much larger quake, with a moment magnitude of 7.0, struck the same area. The power released by this quake was much larger and the Mercalli scale registered a magnitude IX event for the city – a vastly more dangerous event. This level implies substantial damage – major loss of structures and possible collapse of multiple buildings.As I’m typing this, the sun is just about to come up over Japan, so the first real estimates of damage will be available as this post appears on our page. The USGS uses an automated system based on the Mercalli magnitude estimates and the population density to model the potential damage from an earthquake and this one has the potential to have hundreds to thousands of casualties, with losses in the hundreds of millions of US dollars.The entire Japanese archipelago is seismically active as it is produced by the subduction of oceanic plates beneath Eurasia. The 2011 Tohoku earthquake occurred on the subduction zone itself; that type of fault can produce true great earthquakes and major tsunami waves. Today’s earthquakes occurred close to a different type of fault.Subduction zones are large thrust faults, known as megathrust faults. Although those faults form when two plates move together (converge), plates don’t usually move directly at each other at nice, right angles. More commonly, there will be some angle between the motion directions of the two plates – they will converge obliquely. The motion that isn’t directly down the trench commonly creates a second fault, a strike-slip fault, in the middle of the volcanic islands above the subduction zone.This strike-slip fault in Japan is called the Median Tectonic Line. It is a large system, running from Honshu south through Kyushu and taking up a “twisting” motion between Eurasia and the subducting Philippine Sea plate.By using seismic instruments near an earthquake, scientists can map out the kind of fault that ruptured during an earthquake and this quake series occurred on faults that are close to strike-slip, with a little bit of up and down motion. These quakes are consistent with motion on the Median Tectonic Line system. They may have occurred on different segments or smaller faults associated with the major fault; that will be determined by researchers in the coming days as more local data becomes available.Although the power of these quakes is less than the power of a megathrust quake, they have the potential to do as much damage as the shaking from the much larger quake. Megathrust faults are offshore, out to sea and at some distance from inhabited areas. They release more energy, but they are farther from populated centers and the shaking intensity drops with increasing distance from the fault. Strike-slip faults like the MTL can run directly through populated areas and their ruptures tend to be shallower, so they will dump all their energy directly into populated areas. These quakes will impact a smaller area than the 2011 megathrust event and there is limited chance of them setting off tsunami waves, but the damage can still be extensive.Finally, the first two quakes can be considered “Foreshocks” of the larger quake. About 40% of large earthquakes have smaller earthquakes nearby that represent foreshocks; the stress built up on the fault starts breaking the rocks in smaller ruptures before the full fault fails. However, it’s impossible to know whether any single quake will turn out to be a foreshock of a larger event; only about 10% of moderate sized earthquakes actually turn out to be foreshocks of larger events.It is possible that the early quakes and evacuations associated with them could have saved lives when the main shock hit. However, the two earlier quakes also likely damaged a number of buildings in this area and weakened structures; our ability to estimate how much cumulative damage there will be from a series of multiple earthquake is quite limited and so it’s also possible that the early quakes weakened structures and made them more vulnerable to the following large shock.-JBBImage credit: USGShttp://on.doi.gov/1SQF0nxReferences:http://www.cnn.com/2016/04/15/asia/japan-earthquake/http://earthquake.usgs.gov/learn/topics/mercalli.phphttp://scedc.caltech.edu/Module/shockmod.htmlhttp://on.doi.gov/1SHzEe0http://wapo.st/1Yy9cbehttp://bit.ly/1XAcTgjhttp://bit.ly/1Yyrx86 -- source link

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