Digging Deep at Plate Boundary

Sunday, May 27, 2012 Vessel drills deep in 3/11 quake area Kyodo SENDAI — A deep-sea drilling vessel has collected rock samples from 820 meters below the seabed near the epicenter of the Great East Japan Earthquake, the Japan Agency for Marine-Earth Science and Technology said. The maritime research agency said Friday the samples could be from an area forming part of the fault that triggered the magnitude 9.0 earthquake, and that analyzing them could help predict future major quakes off the Pacific coast. An international research team led by Kyoto University conducted the drilling aboard the Chikyu deep-sea exploration vessel in an area 200 km from the Oshika Peninsula in Miyagi Prefecture, the agency said. The samples are the first taken from below the seabed at the boundary of two plates in the Japan Trench, which lies off the Tohoku region, it said.

The Chiba Story (liquefaction) 2

Liquefaction at its most serious looks like Kobe after the Great Hanshin Earthquake: solid ground loses its solidity, and buildings topple. Now, Chiba prefecture has been afflicted by soil liquefaction.

What does it look like in Chiba?

It sounds cartoonish, but it is horribly serious.

For instance, you can only fill a glass half full. If it's on the table in your house, neither table nor glass is standing up straight because your house is leaning at a crazy angle. If you walk around outside in the affected areas, you might feel suddenly tall because heavy utility poles have sunk to a fraction of their former height. Or maybe you will feel suddenly dwarfed by odd mushroomlike structures sprouting from the streets, because manholes and their hollow concrete cylinders have floated up.

What do you do with a house that didn't fall down, but that won't let you sleep in it because you keep rolling out of bed? Let's hope there are smart engineers out there who can figure out a way to fix Chiba and the damage caused by soil liquefaction.

The Chiba Story (liquefaction)

Liquefaction! Do you picture the earth beneath your feet turning to water? Here's what happens, with a little help from Dr. Hazen, the first geologist to explain the phenomenon, back in the 20s.

When the soil is sandy and not well compacted, it's texture is grainy. If pressure is applied from above (for instance, a building is constructed on top of it) the soil can bear the pressure. Any water left in the sandy soil tends to be squeezed out. It moves from high pressure (under the building) to low pressure (anywhere not under the building).

But--if water is suddenly and repeatedly injected among the sand grains--instead of bearing the load, the grains roll around like well oiled ball bearings. The repeated shaking of unstabilized soil by earthquakes is one way water finds its way in. The sandy grains--separated from each other and the friction that holds them together--flow in a liquidy way.

This happened in San Francisco's Marina district in the 80s, in Alaska, in Kobe Port in the late 90s, and in New Zealand earlier this year. Now it is happening in Japan's Chiba prefecture.

Earthquake! (2)

Here is a link to the explanation of the Japanese earthquake intensity scale. This scale describes the earthquake in terms of human perceptions.

http://en.wikipedia.org/wiki/Japan_Meteorological_Agency_seismic_intensity_scale

Earthquake!

Here is a link to the record of the recent earthquakes that are devastating Japan. Please note that the times are NOT Japan time.

http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/quakes_all.html