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The morphology of Obsidian Butte and its surrounding flow indicates that they were produced by a single volcanic eruption with extrusion from a single, central vents. The obsidian domes, spines, and breccia mounds that ring the flow are interpreted to be pressure features developed at the outer margin of the flows. Obsidian Butte itself is topographically higher than the flow because of a late pulse of magma that pushed up a central, viscous mass of partly crystallized rhyolite. The Obsidian Butte volcanic rocks were extruded subaerially but were covered by Lake Cahuilla and modified by wave action soon thereafter.
The Coso Volcanic Field is one of the most seismically active regions in the United States, producing dozens of tremors in the M1 and M2 range each week. Tremors in the M3 range occur at a rate of 2-6 per month and M4 quakes occur two-three times each year. Recent activity in the M5 range happened in 1996 and 1998 when tremors of M5.3, M5.1, M5.2, and M5.0 occurred with a day of each other. These tremors were actually recorded along the eastern side of the Coso Volcanic Field, 15 miles (24 km). September 30, 2009 to October 6, 2009 there have been 429 earthquakes ranging from 0.1 up to a 5.2. Some days have activity just about one every minute. On October 2, 2009 there were three earthquakes (5.2, 4.7, and a 4.9) all with in one hour of each other.
EARTHQUAKES IN THE STABLE CONTINENTAL REGION Most of North America east of the Rocky Mountains has infrequent earthquakes. Here and there earthquakes are more numerous, for example in the New Madrid seismic zone centered on southeastern Missouri, in the Charlevoix-Kamouraska seismic zone of eastern Quebec, in New England, in the New York - Philadelphia - Wilmington urban corridor, and elsewhere. However, most of the enormous region from the Rockies to the Atlantic can go years without an earthquake large enough to be felt, and several U.S. states have never reported a damaging earthquake. The earthquakes that do occur strike anywhere at irregular intervals.
Earthquakes east of the Rocky Mountains, although less frequent than in the West, are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).