November 2nd, 2011 • 5:13 PM
Japan: Major Plate Movement, No Earthquake?

by Meghan Rouillard

Asuka Saito reported that Japanese media have covered this event, which hasn’t been covered by English media so far, nor did it register with USGS. Since, as one Japanese scientist warns, the implications are for potentially strong seismic events as a result of it, I decided it would be worth mentioning here.

So, back to the title... how can major plate movement result in a “zero magnitude” earthquake? The “slow-slip” phenomenon is what it’s called. The following is Asuka’s rough translation of a Japanese article from the Mainichi newspaper into English:

<Slow-slip> A plate slip at Boso Penninsula may trigger earthquakes

The Japanese National Research Institute for Earth Science and Disaster Prevention (NIED) reported on Oct 31st that the Philippine Sea Plate and an edge of a continental plate are slowly sliding, and warned of earthquakes in nearby regions.
This slow slip has been observed for the past 30 years, and slow slidings has been observed once in 6 years on average.
This time, however, it has been only 4 years and two months since the last sliding which happened in Aug 2007.
This is the shortest time elapsed between each slow sliding. The 3/11 earthquake might has to do with this irregularity, and keep in mind, in 2007 slow sliding caused an earthquake in Boso peninsula.
According to NIED, the maximum movement of this recent sliding happened over the five day period between Oct 26th to 30th, sliding 6 cm at 20 km deep under the ocean.
A professor at Tsukuba University in Japan commented "this is not a precursor, but when the periodicity of each slow sliding gets shorter and shorter, we should comprehend it as an increased potential for giant earthquakes."

While a “regular” earthquake would have tremors which would be felt and detected, a “slow earthquake” could be preceded by these earthquake-like “slow-slip” events which don’t even need to register as quakes, because the movement occurs over a longer period of time. Several slow earthquake events around the world appear to have triggered major, damaging seismic earthquakes in the shallower crust (e.g., 2001 Nisqually, 1995 Antofagasta). We cited the case of the Nisqually earthquake in our feature on Cascadia Subduction Zone earthquakes, which can be found here.

But the slow earthquakes themselves can be dangerous. These events are reportedly capable of causing enormous tsunamis, which can be extra dangerous, because they aren’t as easy to feel. Or the shaking will be less than would typically correspond to a massive tsunami, as a deadly Japanese case is mentioned here:

“In 1896, the Sanriku earthquake, which occurred along the same intersection of continental plates that ruptured in the most recent earthquake in Japan, created a tsunami of astonishing might — one that took many by surprise.
Waves as high as 124 feet (38 meters) swept ashore, killing as many as 27,000 people and injuring as many as 9,000 However, Kanamori told OurAmazingPlanet, contemporary accounts of the earthquake describe only a gradual, slow shaking, so there was no concerted effort to flee.
"I was curious about it why it was so slow," Kanamori said. Data revealed the earthquake was a 7.2 magnitude, a surprisingly small number for such a huge tsunami.”

While plate movements per se, contrary to the views of seismologists, are likely not the final cause of quakes, which could be linked to cosmic and solar phenomena if not something on an even large scale, this recent event in Japan is something we should keep ours eyes on.

A last thought on this: in poking around for more on slow earthquakes, I came across this 2009 study on typhoons as triggers of them. From that article:

"Scientists have made the surprising finding that typhoons trigger slow earthquakes, at least in eastern Taiwan. Slow earthquakes are non-violent fault slippage events that take hours or days instead of a few brutal seconds to minutes to release their potent energy. The researchers discuss their data in a study published the June 11, issue of Nature... How does the low pressure trigger the slow quakes? The typhoon reduces atmospheric pressure on land in this region, but does not affect conditions at the ocean bottom, because water moves into the area and equalizes pressure. The reduction in pressure above one side of an obliquely dipping fault tends to unclamp it. "This fault experiences more or less constant strain and stress buildup," said Linde. "If it's close to failure, the small perturbation due to the low pressure of the typhoon can push it over the failure limit; if there is no typhoon, stress will continue to accumulate until it fails without the need for a trigger."

This is very interesting, at least! We do know that Asia has been experiencing an especially severe typhoon season this year, which makes this study all the more intriguing now. It looks like all earthquakes may not be created equal.

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