"Well - excited and scared"

[steuard]

I just had my first real earthquake! Sure, on a couple of occasions I've felt a quick jolt around the house, but never enough to even be completely sure it was actually a quake. No doubts about this one, though. While my first thought was "What did they drop upstairs this time?", I quickly realized that the rumbling and shaking was stronger and longer lasting than the usual bumps from the lab above, and I scrambled under my desk just as the ripples hit. (Thanks to my Fields and Waves class at Harvey Mudd, I know pretty precisely what was going on at each stage of the way.)

It didn't last long, and it didn't seem like anything disastrous had happened, but we were evacuated from the college buildings as a precaution. I'm obviously new at this (and I may have been more flustered than I thought), because I didn't think to grab my bag lunch off of my desk (it was almost noon) and or even to close my door behind me (we've got laptops in here). Fortunately, someone else closed it for me, and they let us back in before I got too hungry. All in all, it made for a more interesting day, and happily it sounds like there weren't any serious injuries anywhere (one news story commented that a couple of people had minor head injuries from taking cover under tables). If you want all the latest California earthquake news, I highly recommend this USGS site.

Comments

[wrenb.livejournal.com]

I'm glad to hear that you and the campus are in good shape! I saw that the epicenter was 15 miles from HMC, so I was curious. When you no longer live in earthquake country you lose perspective, and a 5.4 sounds the same as a 6.4.

[chalgaryn.livejournal.com]

I think of around a 6 as when you start getting noticeable damage, but 15 miles away would take an upper 6 at least to cause real problems (at least in California where we have good earthquake related building codes...this earthquake back east might have knocked down an old brick chimney or something).

[donaithnen.livejournal.com]

Huh, i didn't really notice different phases for the quake except for the trailing off bit, but maybe that's cause i was ~30 miles farther away from it than you. Or perhaps i just didn't know what i was supposed to be paying attention to :)

[chalgaryn.livejournal.com]

Yeah, farther away = longer, rolling sensations; closer = more rolling/shaking combo (with more variety here because the waves were bouncing back at us off the mountains)

[steuard.livejournal.com]

Interesting! That distance dependence fits with wave physics, too: the power transmitted in the P and S waves will spread out in three dimensions (throughout the Earth), so the intensity would decrease like 1/r^2 away from the epicenter. On the other hand, the surface waves only spread out two dimensions (along the Earth's surface), so the intensity would decrease like 1/r. The farther away you are, the smaller 1/r^2 is compared to 1/r, so at a distance you'd mainly feel the rolling sensation of the surface waves.

[steuard.livejournal.com]

Well, I didn't make the connection between what I'd felt and what I learned in class until I was describing my experience to someone else at lunch. (And I suppose it's possible that what I thought were different types of wave motion were in fact just the building's quake-survival features kicking in.)

But the basic physics is that there are three types of waves that propagate out in an earthquake. From fastest to slowest, they are:

1. P waves ("primary"): longitudinal compression waves, much like sound waves.


2. S waves ("secondary"): transverse shearing waves, a bit like waves on a string.


3. Surface waves (e.g. "Rayleigh" waves): vertical waves like swells in the ocean.

The surface waves are always the last to arrive: not only are they slower to begin with, but they have to follow the curvature of the earth's surface while the others can basically take a shortcut straight through the ground.

The sequence of events that I felt began with a general shaking and rumbling, which I thought at first was someone dropping something upstairs. But then, just as I was realizing that this had to be an earthquake, I felt what seemed like an actual "sloshing" of the floor beneath my feet. So while I won't swear to it, it seems natural to guess that that different "sloshing" feeling was the surface wave arriving, a few moments behind the initial shakes.

[akjdg.livejournal.com]

By 'quake-survival features', it sounds like you mean some sort of active mass damping (I don't know what it's called) device. What building might that be? I can't think of one in Claremont large enough to warrant one, save perhaps for a retrofit on Sprague or something.

My experience with 'large' quakes are many in the 20-50 mile distant M~5 quakes around Anchorage, and the loooong, rolling big and far away quakes such as the M7.9 Denali quake of 2004 and the M7.8 quake in 1987 offshore from Yakatat. For me, the Denali quake (D~400mi) was noticed as a sudden onset of vertigo, then realizing it was a quake, then observing about a minute of low frequency rolling. It was immediately obvious that it was a great quake at a great distance.

Oddly, I recall the 1987 quake as being a very sharp event (it was some 600 miles distant) but I was young and impressionable...

Glad you're ok, and you and your lunch were safely reunited!

[steuard.livejournal.com]

By "quake-survival features", I was referring (perhaps entirely incorrectly) to the notion I'd picked up somewhere that many buildings in California these days are somehow partially decoupled from motions of the ground (due to clever design of the foundation-ground interface allowing some relative motion, perhaps?). So I wondered whether the broader rolling feeling I got toward the end of the shaking might have been the building settling into some resonant frequency of such a system after a lag. I'll readily admit that the premise could be way off, though, and I still think the surface wave theory is more likely.

[akjdg.livejournal.com]

I'm not really up to speed on 'active' seismic design, but the two general approaches I'm aware of are:

(1) actively controlled mass that is actuated against the sensed building motion to largely negate the building sway and keep it somewhat still. Typically employed in high rises.

(2) Ball bearing isolation structure between the foundation and building. Building is on rollers so as the ground moves laterally beneath, the building stays relatively still. The ball bearings are in little gravity wells so the building generally stays where it belongs. This might be employed to retrofit an old building (say a masonry building) that is valuable but not readily retrofitted by other means.

In terms of passive design, it is basically a matter of making sure the building's rigidity and flexibility are designed to survive an earthquake and yet be comfortably stiff under normal design loads.

I think your experience is more likely the surface waves.