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closeDid you feel that earthquake at 5:33 a.m. June 20?
It had a magnitude of 4.4 and was centered south of Nacimiento Lake. It was an aftershock from the deadly 2003 San Simeon Earthquake.
I was surprised to hear it called an aftershock. I thought aftershocks came much sooner after the main event. Sure, the San Simeon quake had a magnitude of 6.5 and was just 10 miles from this latest quake, but it happened five and a half years ago. I guess there’s no seismic statute of limitations.
Chris Walls assured me, “It was most definitely an aftershock of the December 2003 San Simeon quake.”
And he should know. He’s regional engineer for the Plate Boundary Observator, which has installed hundreds of scientific devices to measure Earth-crust movements.
“There will be aftershocks from the (2003) San Simeon event for a couple of decades to come,” he said. “For much larger events like the 1857, magnitude-8 Fort Tejon earthquake, aftershocks may still have occurred 100 years afterwards.”
He said any quake in or around the same zone as the 2003 quake will be considered an aftershock for the foreseeable future.
The Plate Boundary Observatory program is dedicated to gaining a better understanding of the huge sections of the Earth’s crust, known as plates, and how they drift on our planet’s molten interior and how they grind against each other.
The launching of the Plate Boundary program was intertwined with the 2003 quake, as you can see from this timeline:
• Oct. 1, 2003: The National Science Foundation awarded the contract for the plate boundary program to UNAVCO Inc., a nonprofit research organization.
• Dec. 22, 2003: The San Simeon quake churned much of the North County, including Paso Robles, taking the lives of Jennifer Myrick, 20, and Marilyn Frost-Zafuto, 55.
• Jan. 9, 2004: The Plate Boundary Observatory program installed its first Global Positioning System station. It was set up on a ranch in Adelaida roughly midway between San Simeon and Paso Robles.
The GPS station has a tripod three or four feet tall, with steel-pipe legs epoxied into holes drilled in bedrock. It supports a small dome containing the GPS receiver. It is solar-powered and reports its data by radio.
Now, five and a half years later, the program has 1,100 GPS stations, mainly in the Western States and Alaska, including dozens in this county and around Parkfield. The program also has strainmeters, tiltmeters and seismic systems.
We Earthlings stand on shaky ground. The more we learn about it, the better.
Contact Phil Dirkx at phild2008@sbcglobal.net or 238-2372.
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