Teensy little quake

[hrj]

The last several quakes I've felt have been centered very locally, so I was guessing that this one was maybe around a 3 -- just enough to make the bookshelves creak slightly. But evidently it was a 5-something down in the South Bay. The anecdotal commentary so far suggests the local effects were really variable. Perhaps some odd propagation patterns?

Comments

[patsmor.livejournal.com]

I'll be interested to get into my office tomorrow -- a friend who lives about 1 mile from the office and about 4 miles from me in almost straight line to our campus had much jolting and rocking and things falling off her shelves. Since we have lots of open bookshelves and no rules about objects on top of cabinets, etc., I'm wondering if anything will have fallen off.

[patsmor.livejournal.com]

Forgot to mention that Shingle Springs/Rescue/El Dorado Hills feels almost all these things because of an extension/branch of the fault line that runs right through the area. Sure enough, faint, but they got it!

[albionwood.livejournal.com]

Soil conditions and building/foundation construction are the main factors on the effects of a quake. The energy propagates differently through soil than through rock, and different kinds of buildings shake differently. We always see big differences in damage around the Bay area, because we have complex geology and a lot of different kinds of buildings.

When the Pretty Big One hit, I was on the top floor of a 5-story building in Emeryville. But the foundation consists of pile driven to bedrock, so the building reacted as if it were about 12 stories tall. The freeway collapse occurred mostly because of poor soil conditions; same for the Marina district, which was largely built on hydraulic fill (functionally equivalent to Jello).

[albionwood.livejournal.com]

Forgot to mention - there's a way to tell whether you are feeling a small quake centered nearby, or a larger quake with a distant epicenter. The first shock waves (P- and S-waves) emitted from a quake travel in a straight line; they have a sort of rapid, "buzzy" feel, very sharp. After that come the Rayleigh waves, which travel more slowly around the surface of the Earth - so the more distant a quake is, the longer the time lag before you feel them. If the epicenter is nearby, you won't feel any time lag at all, and probably won't feel the difference between the two sets of waves - just one period of shaking. If you feel an early jolt followed by gradually increasing shaking, it's likely the epicenter is distant and the quake is large. (If you're too far away you won't feel those early waves at all, only the later surface waves.)

In the aforementioned Loma Prieta quake, the initial jolt felt much like a small quake on the nearby Hayward Fault; but when the rocking motion started up a few seconds later, I knew it was a big quake.

[patsmor.livejournal.com]

Thanks for that description -- it was very helpful.

What constitutes "distant?" 15 miles? 150?

[albionwood.livejournal.com]

Hmm, good question. It has to be far enough that the difference in travel time for the waves is noticeable. Loma Prieta was, what, about 70 miles from Emeryville? I doubt you could generally feel the difference at distances of less than 30 miles or so. But this quote from a Chron article pretty well describes what I'm talking about:
"First there was some up-and-down shaking, and then it went side-to-side, but it was nothing like '89,"
and he was in SJ, about 25 miles from the epicenter.

[patsmor.livejournal.com]

That's good to know, and it explains what I felt -- first a sharp jolty motion, and then a side-to-side, almostly hammock-y. I'm in Mountain View now, and the calculator on the USGS service says that put me about 18 miles from the epicenter (31 km).

[hrj.livejournal.com]

Whereas I didn't feel any jolt -- just a subtly growing creaking/shaking with a second pulse of that just as it was dying down. (I was amused to have that last observation validated by other people describing a secondary "pulse" right at the end.) I have the advantage of not being on landfill (just barely -- the filled area is the west side of the railroad tracks, which is only about 4 blocks away).

[patsmor.livejournal.com]

I felt the last "pulse" before the end, too. It was what made me wonder if I should get out of bed and do something more active about my situation ;-)

I know my neighbors' upstairs had something fall -- I heard it go over.

Of course, all the folks at work who lived nearer the epicenter had much more energetic stories to tell today.