April 16, 2016

expert reaction to earthquakes in Japan and Ecuador

Two large earthquakes have struck in Japan and Ecuador, with dozens and hundreds dead, respectively.

Dr Ilan Kelman, University College London and co-director of the NGO Risk RED (Risk Reduction Education for Disasters) which has done work on seismic safety education, said:

“The Japan and Ecuador earthquakes show how far we have come, and how far we have to go, regarding seismic safety. The comparatively low death tolls show that we can construct buildings to withstand large, shallow tremors. For me, the number of fatalities is still too high, since we know what to do to stop buildings collapsing. The challenge of disasters remains using the knowledge we have already to save even more lives.”

Prof. David Rothery, Professor of Planetary Geosciences at The Open University:

“Last night’s (23:58 GMT) earthquake in Ecuador was magnitude 7.8, which means that that shaking at its underground source was about 6 times stronger than in the magnitude 7.0 earthquake in southern Japan just over a day before. The total energy involved was probably about 20 times greater.

“This one was caused by the floor of the Pacific Ocean (the Nazca plate) being subducted below South America). The rupture was began deeper underground (about 20km) than in the recent Japan quakes, which would have lessened the shaking experienced at the ground surface. The greater damage to buildings and the probable greater loss of life in Ecuador may reflect poorer adherence to seismic building codes in the construction of buildings and bridges. There have been seven magnitude 7 or greater earthquakes within 250 km of this one since 1900.

“The epicentre of the Ecuador earthquake was onshore.  If it had been offshore, the strongest shaking would have affected a smaller area on land so the direct damage would have been less, but there would have been the potential to displace the ocean water strongly enough to cause a tsunami powerful enough to cause damage on both local and more distant coastlines.

“There is no causal relationship between the earthquakes in Ecuador and Japan. About 20 magnitude 7 earthquakes occur somewhere on the globe every year.”

Prof. David Rothery, Professor of Planetary Geosciences at The Open University:

“The magnitude 7.0 earthquake that struck Kyushu at 1625 UTC (GMT) yesterday (15 Apr) was about 30 times more powerful than its magnitude 6.2 predecessor at 12:27 UTC on 14 Apr. Both sources were equally shallow (about 10km) and so the shaking at the ground surface was much stronger and more damaging  during the second event.

“It is unusual but not unprecedented for a larger and more damaging earthquake to follow what was taken to be ‘the main event’.   On 9 March 2011  an magnitude 7.2 earthquake in northern Japan was followed two days later by a magnitude 9.0 earthquake that caused a devastating tsunami. Fortunately this time the epicentres have been below land rather than under the sea, and no tsunamis have been triggered.

“On Kyushu there will probably continue to be aftershocks large enough to be felt for several days, but hopefully nothing more powerful than had already happened.  The nearby volcano Mount Aso had a small eruption shortly after the 15 April earthquake. This almost certainly resulted from stirring up of magma and gas already present and nearly ready to erupt in the normal course of events. The earthquakes themselves do not generate fresh magma. The melting of rock is a slow process, related to events deep in the crust and the upper mantle, and is not caused by sudden slippage such as happens in an earthquake.”