Brad DeLong's Semi-Daily Journal: A Weblog

The aftershock list was also remarkable:

http://quake.wr.usgs.gov/recenteqs/Quakes/quakes.big.htm

MAP 3.6 2004/09/28 10:35:17 35.947N 120.489W 10.2 7 km ( 5 mi) NW of Parkfield, CA
MAP 3.7 2004/09/28 10:33:55 35.817N 120.359W 7.1 11 km ( 7 mi) SE of Parkfield, CA
MAP 3.2 2004/09/28 10:29:41 35.829N 120.369W 9.4 10 km ( 6 mi) SE of Parkfield, CA
MAP 3.3 2004/09/28 10:29:22 35.836N 120.380W 9.0 8 km ( 5 mi) SSE of Parkfield, CA
MAP 3.4 2004/09/28 10:29:15 35.852N 120.401W 5.3 6 km ( 4 mi) SSE of Parkfield, CA
MAP 4.7 2004/09/28 10:24:15 35.806N 120.349W 6.7 13 km ( 8 mi) SE of Parkfield, CA
MAP 4.5 2004/09/28 10:23:49 35.855N 120.396W 5.1 6 km ( 4 mi) SSE of Parkfield, CA
MAP 3.0 2004/09/28 10:22:22 35.910N 120.458W 6.6 3 km ( 2 mi) WNW of Parkfield, CA
MAP 4.1 2004/09/28 10:22:18 35.840N 120.388W 5.6 8 km ( 5 mi) SSE of Parkfield, CA
MAP 5.0 2004/09/28 10:19:07 35.826N 120.453W 2.5 8 km ( 5 mi) SSW of Parkfield, CA
MAP 6.0 2004/09/28 10:15:24 35.815N 120.374W 7.9 11 km ( 7 mi) SSE of Parkfield, CA

Brad, could you interpret that for those of us who live on more stationary land? What does a magnitude 6 earthquake feel like?

Thanks,
Barry from Michigan

"What does a magnitude 6 earthquake feel like?"

http://www.geo.mtu.edu/UPSeis/magnitude.html

"Slight damage to buildings and other structures."

god is fixing the election

It's Parkfield

Bush feels your pain, re-elect him and he will end all earthquakes.

The September 28, 2004, M6 earthquake at Parkfield, California.

 
1.   This is the anticipated Parkfield earthquake, Mw=6.0 on the San Andreas fault. It is the seventh in a series of repeating earthquakes on this stretch of the fault. The previous events were in 1857, 1881, 1901, 1922, 1934, and 1966. It ruptured roughly the same segment of the fault that broke in 1966.

2.   This earthquake occurred at 10:15 AM PDT on September 28, 2004, had a hypocenter of 35 degrees, 49 minutes north, 120 degrees 22 minutes west, and a depth of 8 km or 5 miles. From this point along the San Andreas fault, about 7 miles SW of the town of Parkfield, it ruptured primarily northwest along the fault. The previous two earthquakes ruptured the opposite direction from NW to SE along this section. Strong shaking during this event lasted for about 10 seconds.

3.   Like most earthquakes, the recent earthquake is expected to be followed by numerous aftershocks. Aftershocks are additional earthquakes that occur after the mainshock and in the same geographic area. Usually, aftershocks are smaller than the mainshock, but occasionally an aftershock may be strong enough to be felt widely throughout the area and may cause additional damage.

Typically, the chance of an earthquake comparable to or larger than today's earthquake is 5-10% in the next 7 days. Of the six historical earthquakes of comparable size that have occurred in the Parkfield region, one, in 1857, was followed 9 hours later by a larger earthquake that ruptured the San Andreas fault to the south, and caused widespread damage in southern California. Since that time measurements indicate that insufficient slip has accumulated to allow that event to repeat and so the probability of that event recurring soon is judged unlikely. The other five historical Parkfield earthquakes were not followed by a larger earthquake.

In the unlikely event that a comparable or larger event occurs, such an earthquake would likely rupture the San Andreas fault toward the south, and be felt most strongly in southern California. It is unlikely that a larger earthquake will rupture the San Andreas fault toward the north (toward the SF Bay region) because this portion of the San Andreas fault is slowly creeping and not thought to have accumulated sufficient stress for a larger earthquake.

The likelihood of aftershocks, including the relatively unlikely larger earthquakes described above, is greatest during the first day of aftershocks, and diminishes rapidly with time. Scientists from the USGS are carefully monitoring the instruments in this region for any hints of changes in deformation that might help to update these probabilities.

Note- if you're in California, do fill out the "Did you feel it?" report at the usgs. The more data, the merrier.

As a decades-long Bay Area resident, I've felt a few 3-6 quakes and a 6.9 (below 3 you won't feel them). How they feel depends on depth, duration, what type of ground you're on, where you are in a building, and how far away you are. They could be:

(for my 3-6's)
a single jolt (imagine your office's A/C suddenly starting up: the walls creak once and stop)

a short rumble as if a very heavy truck was driving by- the walls and floors tremble slightly.

a back and forth feeling- like walking on a suspension bridge.

a true shaking with several shakes per second, as if you were in the back of a pickup truck on a dirt road. Sometimes its a fast vibration (as if the road had lots of regular bumps or a rumble strip), sometimes it also includes jolts (like the road also had a few potholes). This can last from 1-30 seconds. The scary part is the waiting: you know that one day, a quake will start this way and then get worse.

If you're on the top of a multi-story building, you feel it sway- I hear that tall buildings in windstorms will do the same.

for Loma Prieta- a 6.9... I was 100 miles away, standing and leaning against a table reading some mail. Suddenly I could feel I was swaying forwards and back- at first I thought I was dizzy, then I saw my lamps and the outside powerlines all swaying too. I could immediately tell that the earthquake was in a SW direction from me. (i.e. Imagine you're on a large floating platform in a perfectly flat lake. Suddenly a boat zooms by, and the waves rocking the platform all obviously come from one direction.)

Ah! Parkfield. We spent our honeymoon on the V-6 Ranch in a one room hunting cabin sited close to a sharp fault scarph. The V-6 is owned by the one of the Varians. Wonder if they are related to Hal Varian just down the hall from you?
It was a great place to view comets. That was where I saw my first and second comets. Nice dark sky which is great for comet and constelation viewing. The Varians have 20,000 ac with cows, deer and wild pigs in abundance. They run the ranch as as recreational stock farm and rent cabins and campsites to fishermen, pig hunters and horsemen .
It was quite a sight to see the whole family out branding, cutting and vaccinating the young cows.
They also own and run the Parkfield motel and the Parkfield bar and grill. We saw the daughter cooking and tending bar in the morning, driving the tractor in the afternoon and herding cows in the evening.
Interestingly it is located near the "Black Hole". Take a look for that feature on the Parkfield topo.
BTW, the earth moved that week but not in the way you are thinking.

Parkfield residents will be unhappy that you've failed to acknowledge their proper name: Parkfield, not Parkview. You can't find it on a lot of California maps, so they may be a bit sensitive. On the other hand, maybe they don't care.

Earthquakes are scary. But what about earthquakes coming from a still-active volcano? Mt. St. Helens just woke up. And since I'm about 70 miles away from it, and over a thousand from Parkdaleville, forgive me if I direct your attention north. Keep your eye on PNW news.

http://tinyurl.com/54y4u

If the four hurricanes are God's punishment on Florida for the election of 2000, what do the earthquakes in CA and the volcano in WA mean?

Stickler: to make sure that the Gropenfuhrer is defeated when he's up for election again?

I didn't feel this one today, which is a testament to the shock absorbers in the highrise I work in in downtown Los Angeles. On the other hand, I totally felt the 6.9 Northridge quake in 1994. It moved the apartment building in North Hollywood that I was living in (about 15 miles away) like it was a toothpick house. I had CD's scattered all over the place. The office I worked in was about 2 miles from the epicenter; if we'd been at work (it happened at 4:30 am on a MLK holiday) we'd have been killed instantly as the A/C units fell from the ceiling and crushed all the desks and filing cabinets. What I'll never forget is the sound: that low, rumbling crescendo that you think is never going to stop.

On the whole, give me earthquakes over hurricanes or torndados.

If the four hurricanes are God's punishment on Florida for the election of 2000, what do the earthquakes in CA and the volcano in WA mean?

Uh, that Cthulu is restless in his eternal sleep?