I am doing a project for my sixth grade science class. The project is on plate tectonics. My maps show that volcanoes and earthquakes occur mostly on plate boundaries. But once in awhile earthquakes occur in the middle of a plate. Can you tell me why? (I know this is not a volcano question.)

rocky Jimmie Mulligan

Dear Jimmie,

Good observation. Two possibilities come to mind. Earthquakes north of India are the result of the collision of India with Asia. The plate boundary used to be a subduction zone, like much of the present-day Pacific Rim. The intervening oceanic plate has all been subducted but the Indian continent continues to push north. Earthquakes in the mid-continent in the U.S. are related to old structures, a failed rift, and the weight of sediment from the Mississippi River. Think of sitting in an old chair that breaks at its weakest point. The weight of all that sand and mud pushes down on the rocks until they break at their weakest point, the old rift.

The guys at Caltech study earthquakes. You can send your question to them at:
polet@seismo.gps.caltech.edu

Steve Mattox, University of North Dakota 

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