Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Seattle Earthquake yesterday (was: a few minutes ago)



William Beaty wrote:

I wonder if most quakes are actually brief impulses lasting less than a
second, but the signal is "chirped" by the nonlinear medium so it becomes
ripples spread over time. If so, then the quake would last longer for
sites at a greater distance from the origin.

Based on 40 years of living in California, I'm pretty sure there is an
effect of the type you describe. Here are some data for you to crunch;
see if you find a pattern. The results are based on personal
experience, not secondhand accounts (T = estimated period of lowest
well-represented shaking frequency):

Earthquake Richter My distance Subjective impression
(roughly) from epicenter at my location

Tehachapi 6+ ~ 100 km ~ 45 seconds vigorous
ca 1968 shaking; T ~ 1 s

Sylmar 6.5 40-50 km 45-60 seconds vigorous
Feb 1971 shaking; T ~ 1 s

Small quakes 2.5-3.5 ~ 1 km Single impulsive jolts
on Hayward (fault (1-2 s in length);
fault while runs sensation was of
attending beneath massive object
Berkeley football striking the
1978-80 stadium) building, not shaking

San Fernando 6.7 70 km 45-60 seconds vigorous
Jan 1994 shaking; T = 1-2 s

Lavic Lake 7.0 ~ 300 km 20-30 seconds of
Oct 1999 long-period (T ~ 3 s)
gentle roll

The Tehachapi quake struck while we were watching the evening news on
an L.A. television station. We were far enough out of town that the
shaking at our location had concluded about ten seconds before the
newsroom studio started to shake (in effect, we got to observe the
same earthquake twice, live, as the studio shaking was clearly visible
on camera). The surface wave speed (a few km/s) is slow enough that
California is considering installing an electronic alarm system on the
San Andreas fault to give Los Angeles-area schools a few seconds, or
perhaps up to a full minute, advance warning of major shaking, with
the hope that this would be enough time to evacuate classrooms.

The Lavic Lake earthquake was interesting because I immediately
recognized that (1) due to its long period, the epicenter was far away
(my original gut estimate was 250 km) and (2) due to the fact I could
still feel it distinctly from that distance, it was clearly a major
event. When I checked the Southern California Earthquake Data Center
on the web (http://www.scecdc.scec.org/recenteqs/) a few minutes
later, they had already posted a preliminary magnitude estimate from
their automated detection system.