Shake, rattle and roll

By: 
Stacey Powells
Times Staff

Some theories on why the earth keeps on moving in the Mojave

Thousands of earthquakes have occurred in the Mojave Desert near the China Lake Naval Base. The California Geological Survey (CGS) has sent geologists and seismologists into the Ridgecrest area to do what they do best; jump onto the earthquake bandwagon, digging and measuring, and hopefully coming up with answers as to why the recent uptake on quakes.
CGS geologist Dr. Jay Patton said that seismologists and geologists are just a tad concerned because there have been so many quakes in such a short time span, but are also excited at the prospect of being able to find out exactly what has been going on with the Earth’s crust in that region. Additionally, Patton has a hobby of creating reports and summaries about earthquakes, writing them at an undergraduate level. “I want people to learn about the seismic-tectonic settings for where these quakes are happening,” said Patton. Smaller quakes in the M2.0-3.5 magnitude have been happening for weeks but then the 6.4 occurred on July 4, followed by the M7.1 the next day, which Patton says was probably “triggered” by the M6.4.
Patton interpreted the 6.4 to be on a northeast striking fault plane, a left-lateral strike-slip earthquake fault which triggered the M7.1, which is interpreted to be a right-lateral strike-slip on the northwest trending fault.
“The 6.4 was on a fault parallel to the Garlock Fault, a major left-lateral strike-slip fault, conjugate to the Pacific-North America plate boundary which most people associate with the San Andreas Fault,” he said.
Patton said about 15-20 percent of the relative plate motion goes up the eastside of the Sierra Nevada mountains. From the San Andreas Fault, which feeds into the Eastern California Sheer Zone, there were two large historical quakes. The 1992 Landers earthquake and the 1999 Hector Mines earthquake. Further to the north in the Owens Valley there was the 7.9 in 1872.
“That feeds up into the Walker Lane which is a series of strike-slip faults on the eastside and then it goes through Lake Tahoe and the Reno area and then back into the Cascades,” he said.
Patton said there is active research right now to figure out how that Pacific-North America relative plate motion feeds back out towards the subduction zone.
“We are still learning about that,” he said.
“These earthquakes are probably on two completely different fault systems so I would call them two separate earthquakes,” continued Patton. “Something interesting about these earthquakes are that they are in a region that may have elevated stress following the 1872 earthquake as well as elevated stress following the 1992 and 1999 quakes.”
Patton said those earlier earthquakes may have triggered these most recent quakes, but that is just a hypothesis. Patton said another interesting thing to think about is the Blackwater Fault, which is between the 1872 Owens Valley rupture and the recent earthquakes.
“That fault has probably experienced an increase in stress so it’s possible that this sequence along with the other quakes may lead to a triggered quake on the Blackwater Fault,” he said. “That may happen next week or next month or decades from now, just like the 6.4 and 7.1 happened 20 years after the Hector Mine earthquake in 1999.”
But again, Patton said it’s all hypothetical.
“When these earthquakes happened, the surrounding crust deformed so there are increases and decreases in stress along the faults, but those stresses are very small compared to the overall stress that is stored or released during an earthquake,” Patton said. “In order for an earthquake to be triggered by another earthquake, that fault would need to be already really loaded and just about ready to go.”
Per the U.S. Geological Survey, thousands of quakes have been registered for the last few weeks near Ridgecrest. “It’s going like gangbusters and will be a while before those people will be able to get a good night sleep,” he said.
Jeremy Lancaster, a supervising engineering geologist from the Department of Conservation’s California Geological Survey, explained on a July 6 Facebook post when and exactly where the series of earthquakes near Ridgecrest happened over the past few days.

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