By Dan Oancea

January 27, 1700 – Miho village, eastern Japan:

"The water also went into the pine trees of Ego. The receding water went out very fast, like a big river. It came in about seven times before 10 a.m. of that day and gradually lost its power… Because the way the tide came in was so unusual, and was in fact unheard of, I advised the villagers to escape to Miho Shrine…It is said that when an earthquake happens, something like large swells result, but there was no earthquake in either the village or nearby."

This is the first written evidence of a tsunami event that could be linked to a devastating earthquake that occurred on the Pacific coast of America – the large 1700 Cascadia Subduction Zone Earthquake.

North American geological evidence describes the event as a sudden rush of ocean water (tsunami) which carried and deposited a thin layer of sand on top of coastal peat bogs and Indian dwellings. Not just that but the majestic coastal rain forests also preserved a record deep in their stacked tree rings.

It is more difficult to document an ancient earthquake when analyzing a pile of sedimentary rocks that were not located on the coast at the time of the seismic event. A good site to have a look at some ‘weird’ paleoseismic structures preserved in sedimentary rocks is the site of the University of North Carolina. The structures are mostly dikes and pipes resulted from the forced injection of a mixture of unconsolidated sediments (sands) and water that cut through different layers of rocks.

Under favorable circumstances an earthquake could cause a layer of water saturated soil to lose its consistency and strength – i.e. liquefaction. The result in the case of a confined unconsolidated layer would be a sand blow resulting in dike structures similar to those documented by paleoseismologists. Modern buildings or human made structures situated on top of one of these layers in a seismically active region - they call for disaster.

Q: How do you document an old earthquake?

A: Happily trench, peel and slice - look at what the USGS geos are doing.