Why is a mega-quake of strength 20 impossible

Japan quake contradicted theories

The Tohoku earthquake, which struck Japan on March 11, 2011, was not only one of the strongest ever, it also provided as much seismological data as no other strong earthquake before it. Even the first evaluations of this data, now published in three articles in “Science”, refute established assumptions about such so-called megathrust earthquakes. Neither the size of the offset zone, nor the places of origin of the seismic waves, nor the amount of energy accumulated before the quake met previous expectations. The data also reveal that a zone immediately south of the earthquake area could well have the potential for another strong earthquake.

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When the Tohoku-Oki earthquake shook the northeast coast of Japan on March 11, 2011, this had consequences that are still felt today - not only in Japan, but also worldwide. The strong magnitude 9 earthquake and the tsunami it triggered devastated large parts of the Japanese coast and killed thousands of people. The Fukushima Daiichi nuclear power plant is still on the verge of a worst-case scenario. But as tragic as the consequences for the country and the people are, the Tohoku-Oki earthquake also offers the chance to be better equipped at least for possible further severe earthquakes of this kind in the future.

“This event is now the best-documented strong earthquake ever,” explains Mark Simons, Professor of Geophysics at the Caltech Seismological Laboratory. The wealth of data that was collected before, during and after the quake now provides valuable information about what happens in a so-called megathrust earthquake. These earthquakes, which are among the most severe of all, occur when the plates that are pushed one on top of the other in a subduction zone get stuck and the tension is suddenly released. Along the affected faults, here the border between the Pacific Plate and the Okhotsk Plate, on which northern Japan lies, the subsurface of both edges is strongly offset from one another.

Offset zone surprisingly compact

Three research teams have now evaluated the extensive data on this earthquake and, in some cases, gained surprising insights. For the seismologist Simons, one of the most interesting findings from the data analysis was the spatial compactness of the event: Due to the sudden voltage discharge, the fault moved over a length of 250 kilometers - that's only about half of what one would have previously expected for a tremor of this magnitude. The area in which the greatest misalignment occurred, at 30 meters or more, only includes 50 to 100 kilometers of the fault.

“It's something we haven't seen before,” says Simon. "I'm sure this has happened in the past as well, but the technology has only developed so far in the last ten to 15 years that we can now measure this offset much more accurately using GPS and other methods."

High frequency waves elsewhere than expected

Another significant finding is the fact that the various seismic waves of the quake emanated from different areas of the fault. "The high-frequency waves of the Tohoku earthquake were formed much closer to the coast, away from the offset area in which the low-frequency waves originated," explains Simon's colleague Jean Paul Ampuero. There are two reasons for this, according to the researchers: On the one hand, the greatest stresses and thus a potential source for high-frequency waves are found at the edge of the fault zone, not in its center, where the rock was the first to crack.

Simons compares this with the forces that act when a sheet of paper is torn: "The greatest tension does not arise where the paper has just torn, but rather where it is about to tear," says Simon. "We previously thought that the high-frequency energy was causing the fault to slip, but that doesn't fit our models of this event." Another factor is how the fault reacts to these tensions: it appears in the case of the Tohoku - Quite as if only the deeper parts of the plate boundary reacted to the load by generating high-frequency waves.

Magnitude was considered impossible in this place

The researchers were also surprised by the fact that such a strong earthquake actually occurred at this point: “The level of stresses associated with this strong offset is almost five to ten times higher than what we normally experience see a megathrust earthquake, ”said the geophysicist Hiroo Kanamori, who happened to be in Japan at the time of the earthquake. “Until now, it was assumed that the rock near the Japan Trench could not build up such a large elastic tension.” It is still unclear why this was the case in the case of the Tohoku earthquake.

According to Kanamori, either the subducted seabed or the overlying plate could have unusual structures, such as towering ridges, which encourage the two plates to hook together and thus build up more tension than usual. "Because of this local thickening - whatever the exact reason - the Pacific Plate and Okhotsk Plate were connected for a long time, probably 500 to 1,000 years before they gave way in this Magnitude 9.0 event," Kanamori said. "In the future, detailed geophysical studies of the seabed structures could clarify which mechanisms of local reinforcement played a role in this area."

Is the southern part of the fault also endangered?

Overall, the scientists sum up, very little was known about the area of ​​the later epicenter in the run-up to the quake, as there was only limited historical data here. "But instead of saying that probably no strong earthquake would occur here, we should have said that we just don't know," explains Simons. The same now applies to an area of ​​the fault immediately south of the current offset zone, where the plate edges are tilted against each other in a very similar way.

“It is important to note that we are by no means predicting an earthquake there. But we don't have any data on this area, so attention should be focused on it - especially considering its proximity to Tokyo, ”the seismologist emphasizes. In his opinion, the new Japanese seabed observation system, which could provide more information about this region in the future, is of great importance. (Science, 2011; DOI: 10.1126 / science.1206731)

(California Institute of Technology, May 20, 2011 - NPO)

May 20, 2011