"We have not seen earthquakes in the Tahoe area deeper than 15 to 18 kilometers [9 to 11 miles]," said study coauthor Kenneth Smith, a research seismology professor at the University of Nevada Seismological Laboratory in Reno.
At the same time, Blewitt's GPS (global positioning system) instruments on top of Slide Mountain showed that the mountain had moved six millimeters [a little less than three-tenths of in inch] horizontally outward from the quakes' epicenter and eight millimeters [a little more than three-tenths of an inch] upward.
The movement was too large to have been caused solely by the earthquakes.
"In that range of magnitude, the earthquakes are so small that even with 1,600 events, you wouldn't expect to see anything more than a millimeter [four-hundredths of an inch] of movement on the surface," Blewitt said.
"It had to be something else that was creating the earthquakes and also causing the mountain to move."
Enter the Magma
Magma is formed when a part of the Earth's lower crust or upper mantle melts. The movement of magma causes volcanic activity. As rock inside the Earth melts, it can rise toward the surface and in some cases erupt in a volcano.
In the Lake Tahoe region, there is two-million-year-old volcanic material on the surface of the Earth. However, scientists don't believe the magma could reach the surface today.
"It is too far down, about 20 miles [32 kilometers] below the surface, and there is no evidence to suggest magma from this event is still moving," Smith said.
Instead, the scientists believe that the magma in this case was injected up into an area of 15 square miles (40 square kilometers). The injection created a magma pocket about one yard (about one meter) tall deep inside the Earth. This caused the Earth's surface to rise.
"It's like taking your fist and putting it under the pillow," study co-author David von Seggern. Von Seggern is the seismic network manager at the University of Nevada Seismological Laboratory in Reno.
"Your fist is several inches wide, but your pillow only seems to rise a little bit. When you inject a volume of material at depth, the surface has to go up a little bit."
The magma injection pushed the crust apart, producing high strains in the rock, which in turn caused earthquakes.
"We're confident that we're dealing with a magma-related type of process," Smith said.
Boundary in Action
Scientists know that the Basin and Range area is encroaching west into the Sierra region. They hope that the magmatic phenomenon will help them better understand how changes in the Earth's lower crust affect the lay of the land.
"There is a lot of debate over how extension really takes place in the Basin and Range—and how the interface between the Sierra and the Basin and Range evolves," Smith said. "We may actually see this boundary in action in the lower crust, and that's an important finding."
The discovery could also help explain geological phenomena elsewhere in the world.
"For all we know, this type of activity could be quite common," Blewitt said. "Because of recent technological developments, we're able to combine the GPS analysis and the seismological analysis and, for the first time, really start looking at the lower crust, which has always been a bit of a mystery."
For more seismology news, scroll down.
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