02 Feb 2010
For
the second time this month, a large quake happened at the
southeastern corner of the Juan de Fuca plate, where the
Mendicino Rift, the southern edge of the plate meets the
rift that to the south forms the San Andreas Fault and to
the north forms the Juan de Fuca Subduction Zone.
It is the Subduction Zone (Yellow arrows) that is ripe
for the "250 year period earthquake" that in 1700 ripped,
sending a devastating tsunami across the Pacific, and
striking Japan, causing thousands of deaths... it also
caused major problems in the Pacific North West, causing
some native villages to be suddenly more than a mile inland
in some places -- or more than a mile out to sea in others.
More than 50 years later, when the first Europeans made
contact with them, they were still talking about it...
its also more than 60 years 'overdue'.
The zone is very similar to the one in Thailand that
spawned the tsunami there, and to the one in Chile that was
so strong (9.7+) that the sensors on Mt St Helens detected
it... and then again when it made its way around the globe
to pass by again.
Steve Walter, a seismologist at the USGS in Menlo Park,
said Thursday's quake was not an aftershock because it
occurred on a different fault, but "we could speculate it
might have been triggered by the one last month."
When one area of the fault slips, it causes pressure on
the other portions... eventually, one of them at either end
of the fault line will cause a cascading 'rip' along the
fault line, which will be felt as a 9+ quake that will shake
for several minutes
More maps and info:
AP Story about the quake:
2nd large quake in month hits off N.
Calif coast
by Associated Press
Posted on
February 4, 2010 at 9:36 PM
Orig.
story:
http://www.nwcn.com/news/2nd-large-quake-in-month-hits-off-N-Calif-coast-83613707.html
SAN FRANCISCO - Residents of Northern California's
Humboldt County were rocked by a magnitude-5.9
earthquake Thursday, but officials said there were no
immediate reports of major injury or damage from the
second large temblor to hit the area within a month.
The U.S. Geological Survey reported the quake struck
at 12:20 p.m. about 35 miles northwest of the community
of Petrolia and nearly 50 miles west of Eureka. The
shaking was felt within a 150-mile radius, as far north
as southern Oregon and as far south as Sonoma County,
according to the USGS Web site.
Local officials and residents reported feeling a
rolling sensation that caused items to fall from walls
and shelves. Many said the movement didn't feel nearly
as severe as the magnitude-6.5 quake that struck the
same region Jan. 9 and caused more than $40 million in
damage and one serious injury -- an elderly woman who
fell and broke her hip.
Eureka Fire Chief Eric Smith said crews were checking
on structures that were damaged in the previous
earthquake to make sure they're secure.
Phil Smith-Hanes, the county spokesman, said its
Office of Emergency Services had not received any
reports of major damage or injuries. "At first blush, it
doesn't look like it's a big deal," he said.
Humboldt County Sheriff's Office spokeswoman Brenda
Godsey, who was in a restaurant at the time, said the
quake lasted 25 to 30 seconds, with none of the violent
jolts felt last month.
"Everybody was very calm and just sort of waited it
out," she said, adding that sheriff's dispatchers
received no significant calls.
At the Petrolia General Store, a box of cereal and
some sardine cans tumbled from the shelves, but the
windows remained intact.
"Glass bottles were rattling, and you could see the
lights swinging back and forth," said Amie Barr, 30, a
store employee. "But it didn't feel as strong (as the
Jan. 9 quake)."
She added, "We have them all the time, so we're
pretty used to it here."
Steve Walter, a seismologist at the USGS in Menlo
Park, said Thursday's quake was not an aftershock
because it occurred on a different fault, but "we could
speculate it might have been triggered by the one last
month."
The temblor -- initially reported by the USGS as
magnitude-6.0 but later revised to magnitude-5.9 -- did
not trigger any tsunami warnings.
The USGS said it was studying Thursday's quake and
expected to be able to determine in a few days if it was
an aftershock to the stronger quake from last month.
Walter said the far-northern coast of California is
the most seismically active area in the state, but the
potential for damage and injuries there is smaller
because it's less populated.
"They get more earthquakes and much bigger quakes up
there," he said. "They have had more in the past, and
they will have more in the future than any other part of
California."
