By Monty Torres
Big earthquakes don't happen in Fresno. We've all heard and taken comfort from this conventional wisdom for quite some time now, but what you may not have heard is just how close a truly massive earthquake risk is to the central valley, and the kind of damage it could cause here.
It started September 29th, 2009. A massive magnitude 8 earthquake rocked the Samoa Islands region of the south western Pacific Ocean, and that was just the beginning.
“Usually it doesn't happen,” admits Fresno State seismologist and chair of the Earth and Environmental Sciences Department, Doctor Stephen Lewis. “Usually there will be a magnitude 7.5 or 8 earthquake on the Tonga system then it will be years to decades before there 's another one.”
But within just days,...7.6, 7.6, 7.8, in Indonesia, Vanuatu, the Santa Cruz Islands. What followed was a series of eleven major quakes ranging from 6.4 to 8.0 in magnitude, all in the space of just 11 days, rumbling up and down the western edge of the Pacific plate along the ring of fire.
“It's the volcanic activity and eruptions that give us the ring of fire name,” explains Lewis.
The same ring of fire, the same Pacific plate, where California sits on it's eastern edge, the San Andreas fault. But is there a connection?
Do all those massive earthquakes 5,000 miles away tell us anything about what may soon happen here, in the central valley, in Fresno?
Lewis and geologist Dr. John Wakabayashi, also of Fresno state, have an answer, and the answer is no.
“There's pretty much no connection between what would go on there and here in terms of triggering on this end of the pacific plate.,” Wakabayashi asserts.
Lewis agrees. “The stresses that are changed by earthquakes on the western side of the plate don't have any measurable effects on the stresses on the San Andreas fault or other associated faults in California.”
But their next answer is much less reassuring, for the San Andreas fault, they say, needs no additional stresses to explode in another massive tremor.
“If it happened tomorrow it wouldn't surprise anyone,” says Lewis.
“The likelihood is high. Will it be a 6. 8? Will it be a 7.5? That's a little bit harder to say,” adds Wakabayashi.
Strong enough and close enough, both agree, to topple chimneys and structurally damage some older homes in Fresno, but thanks to our distance little else.
“The saving grace for Fresno and the central valley is that there are no local seismic sources, no local active faults,” but Dr. Lewis says, that does not mean Fresno is immune.
“Do you want to hear the bad news? It's a story that isn't told too often.”
The story of a geological structure capable of producing the most powerful type of temblor on the planet.
“It would be pretty big,” Lewis cautions.
The same kind that triggered the 2004 Indonesian tsunami –a megathrust quake.
“A magnitude 9 earthquake here would cause ground shaking down in Fresno about as strong as San Francisco experienced during the 1906 earthquake,” contends Lewis.
A 7.8 magnitude quake that all but destroyed the city of San Francisco, killing an estimated three thousand and leaving up to three-hundred-thousand more homeless.
So we could have a 1906 episode in Fresno? “Caused by this fault system that is conceptually far, far away,” Lewis responds, “yeah.”
Where?
Here, says Lewis, 300 miles northwest of Fresno, stretching 700 miles from the northern coast of California all the way to Canada, along the Cascadia Subduction Zone, where two overlapping tectonic plates lie poised offshore ready to break free in a tremendous release of energy at any time.
“This fault system last erupted in a magnitude 9 plus earthquake in the year 1700,” Lewis warns.
While other scientists may dispute how much damage a major quake would cause in Fresno, what no one disputes is that the Cascadia Subduction Zone is fully capable of magnitude 9 earthquakes and greater, and that giant quakes occur about every 3-hundred to 3-hundred 50 years on the Cascadia.
The last one happened 3-hundred and nine years ago.
