It is difficult to remain standing. Damage and injuries occur, but rarely fatal. Occurs about once or twice per year.
Large furniture may topple and cause injuries, but no major damage to buildings. Occurs about times per year. Things fall off the shelves. Does not usually cause damage or injuries. Hanging ceiling lights swing strongly, and unstable objects may topple. Occurs about twice per month somewhere in Japan. Not felt by many people. Usually occurs several times per day somewhere in Japan. For the city was gone. The tragedy prompted countless acts of heroism.
Thomas Ryan, a year-old U. Samuel Robinson, the Canadian skipper of the Empress of Australia , took hundreds of refugees aboard, organized a fire brigade that kept the ship from being incinerated by advancing flames, then steered the crippled vessel to safety in the outer harbor.
Then there was Taki Yonemura, chief engineer of the government wireless station in Iwaki, a small town miles northeast of Tokyo. Hours after the earthquake, Yonemura picked up a faint signal from a naval station near Yokohama, relaying word of the catastrophe.
For the next three days, Yonemura sent a stream of reports that alerted the world to the unfolding tragedy. The wave of good feeling between the two countries would soon dissipate, however, in mutual accusations.
The earthquake also exposed the darker side of humanity. Within hours of the catastrophe, rumors spread that Korean immigrants were poisoning wells and using the breakdown of authority to plot the overthrow of the Japanese government.
Japan had occupied Korea in , annexed it five years later and ruled the territory with an iron grip. Roving bands of Japanese prowled the ruins of Yokohama and Tokyo, setting up makeshift roadblocks and massacring Koreans across the earthquake zone. According to some estimates, the death toll was as high as 6, But around kilometers miles down, the system abruptly changes. The dance of seismic waves around this boundary suggests the rocks below are much denser than those above—the beginning of the lower mantle.
In this layer, the earthen-hued mineral bridgmanite dominates, and the earthquake-generating transformations of olivine that can occur above no longer happen. So if a quake did strike in this layer of the planet, something else must have triggered it.
One possibility is the transformation of a different mineral within the sinking slab, such as the sepia-toned mineral enstatite. But Kiser and his colleagues also spotted another possible trigger in the movements of the slab.
The tiny aftershocks following the magnitude 7. The team suggests the large quake could have caused part of the mangled slab to settle slightly—"we're talking very, very slightly," Kiser says.
That small shift might have been enough to concentrate stresses at the base of the slab as it plunged into the denser lower mantle rocks. One way such an increase in stress could lead to the deep quake is by slightly deforming the rocks, which can generate heat and weaken them. The process could have kicked off a feedback loop, causing the rock to deform faster and faster as it grows hotter and weaker until the blocks rapidly shift in an earthquake.
The building heat might have even generated melt that acted as a lubricant for the slip, Billen says. Further analysis and modeling of the sinking slab's structures and the positions of aftershocks from the magnitude 7. Zhang, for one, "wasn't surprised" that an aftershock may strike within the lower mantle. In his past work, Zhang and his colleagues saw hints that the magnitude 7. But a lower mantle quake would overturn long-standing thinking on our planet's inner workings—and not everyone is convinced of the new study's claims.
In some cases, the methods used to amplify the signals of a quake like this can "produce false alarms," Yingcai Zheng , a seismologist at the University of Houston, writes via email. A false alarm could occur, for example, if waves from a different earthquake bounced off Earth's internal structures and then were picked up by the seismic array, the team notes in the study.
But John Vidale , a seismologist at the University of Southern California, says that the seismic signals appear to come from a true quake at least as deep as the study authors suggest. Additional confirmation of the event could come from searching for another type of earthquake wave , known as a shear or S-wave. The new study identifies the quakes using pressure waves, or P-waves, which rapidly travel through the ground like a slinky pulled forward and back.
S-waves travel slower and rock the ground side to side or up and down. If the S and P waves arrive at the expected time based on the position where the team believes this ultradeep quake occurred, Vidale says, "that kind of nails it. However, Vidale points out that even if the depth can be confirmed, the boundary of the lower mantle at this location is still an open question. Japan is located along the subduction zone and many earthquakes happen every year.
Earthquake research in Japan becomes one of the big national work involving several branches of government. In Japan, earthquake was believed to be induced by huge "catfish" underground in the past days. Today, earthquake is understood to occur when the crust is ruptured along the fault.
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