On that earthquake | TheUnion.com

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On that earthquake

I learned some interesting things in the earthquake article, like Greenville is 30 miles north of Grass Valley.

I think it is more like 70 to 75 miles north of us. Depending on the ground your house is on, the recent earthquake could have been a lot more exciting if only 30 miles distant.

I also learned that, "The fault lines run underneath the eastern slope of the Sierra Nevada where major geological plates are being slowly pulled apart, causing occasional seismic activity".

To my knowledge all of the lower United States are on the North American tectonic plate — except that small area west of the San Andreas Fault which is on the Pacific Plate.

An example of where two plates are being torn apart is the Mid Atlantic Ridge in the middle of the Atlantic Ocean. Evidence does show that the Great Basin, which begins in California east of the Sierras and extends all across Nevada is bulging upward resulting in expansion and cracking.

The type of faults usually occurring in this situation are thrust faults with vertical movement rather than horizontal movement such as occurs along the San Andreas Fault — a strike slip fault. It was reported that the recent earthquake near Greenville occurred along a strike-slip fault

Reading further I found out that Mt. Lassen is a "dormant" volcano. I believe it is considered "active", geologically speaking, as a volcano has to be quiet for thousands of years before being considered dormant — and even longer to be labeled, "inactive."

It would have been interesting to mention that Mt. Lassen is on the southern end of the Cascade Range and that the earthquake we experienced took place in the Sierra Nevada Range — two distinct and completely different geologic provinces.

Whether an earthquake in one province could cause geologic activity in an adjacent province probably cannot be predicted.

Carole Gibson

Nevada City