Friday, December 22, 2006

Unsettled Basement

Residents of east-central Oklahoma were shaken by two small earthquakes yesterday. While we usually associate earthquakes with the Pacific Rim, they are not uncommon across the "stable" continental platforms, generally occuring in the area of old sutures or past rifts.

Yesterday's tremors originated along the Humboldt Fault, which runs along the east edge of the Nehama Ridge. The latter is a crumpled uplift of the deep, Precambrian granite, which runs from Omaha, Nebraska, to Oklahoma City. The Ridge, itself, resulted from the formation of the Mid-Continent Rift, an opening in the North American plate that occured 1.1 billion years ago. This healed rift lies about 50 miles west of the Nehama Ridge; as it opened, the rift filled with volcanic magma and deformed the basement rocks to either side. Had the rifting process continued, proto-North America would have split into two continents.

Earthquakes are fairly common along the Nehama Ridge but most are "microquakes," too weak to be felt. The strongest recorded Nehama quake occured in 1867, in eastern Kansas; that magnitude 5.2 earthquake damaged buildings. Yesterdays tremors only rattled nerves.