Last week was spring break here in Madison, WI. It’s been a while since I took part in the annual mass exodus, but I decided to take advantage of the time off, pack up, and get out of Dodge for a few days. My destination was chilly, drizzly Seattle for an in-person crash course about the study of earthquakes and their associated dangers in the Puget Sound area. I also went out to meet some of the people—both professional scientists and volunteer citizens—who are helping this effort along.
Spring Break 2011! Downtown Seattle as seen from West Seattle across Elliott Bay.
The timing of my trip during the week after Japan’s subduction-related magnitude 9.0 Tohuku earthquake and the resulting tsunami, and not long after a shallow crustal temblor hit Christchurch, NZ, was purely coincidental. It did, however, provide an engaging, albeit tragic, backdrop for discussing earthquake hazards in the area and individual motivations for contributing to the study of these hazards. The susceptibility of the Puget Sound to both of these types of earthquakes, as well as to large deep earthquakes such as the 2001 Nisqually quake, and the parallels to these recent events is not lost on many in the region.
I arranged the trip with the help of the staff at the Pacific Northwest Seismic Network, headquartered in the Department of Earth and Space Sciences at the University of Washington in Seattle. For two days, I toured the area from the UW campus down to Olympia and back up to North Seattle. My guide was Doug Gibbons, one of PNSN’s very knowledgeable technicians and an outreach specialist, who let me to pick his brain the whole time while he navigated through traffic on I-5 and many other labyrinthine routes that connect towns and neighborhoods on the Sound’s east side.
Along the way, Doug pointed out many of the local geologic features—some, like the drumlins and floodplains, that are relics of past glaciations, and others resulting from the Northwest’s particular tectonic setting—that impart the region with much of its distinct character, as well as its distinct seismic hazards. We also stopped off to visit a number of volunteers (the subject of a longer piece on which I’m currently working) that PNSN has enlisted to broaden their post-quake data collection capacity.
|Display monitors and seismographs at PNSN headquarters.|
My visit was brief but chock-full of fascinating nuggets and interesting characters. While I plan to add more posts related to this trip, I thought I’d limit this entry and just share some low-budget video I shot at PNSN’s lab. Low budget, but awesome. In it, Doug demonstrates and explains some of the potential threats to Seattle’s buildings and infrastructure from earthquake-related ground shaking using a “toy” shake table.
The back-and-forth motion of the shake table is powered by a motor that drives a belt and flywheel assembly, which is connected to the table's platform via a small rod. The rate at which the platform oscillates can be regulated with a control knob for the motor to simulate seismic waves of different frequency.
In the first clip, Doug initially shows the effect of ground/soil type on shaking of two equivalent buildings. The building at left has a rigid support, simulating bedrock or at least more solidly compacted soil, whereas the building at right is supported on foam padding, simulating the sort of less consolidated glacial till on which much of the low-lying Seattle area is constructed. He then swaps out the first table for a second, which simulates the behavior of buildings of varying height built on similar ground.
(You might need to turn up the volume a bit. )
1) Higher frequency waves and ground shaking don’t necessarily mean greater damage. Lower frequency (i.e, longer period) waves may cause more damage, as shown in both simulations. Furthermore, each building has its own resonance frequency at which shaking stabilizes and less structural damage occurs.
2) Injuries can occur regardless of how vigorously a building is shaking, or how much structural damage a building sustains. This is because bookshelves and other unsecured objects, which shake independently of the building will still be “flying everywhere.”
In the second clip, he briefly discusses modifications and construction techniques that are used to dampen shaking and reduce structural damage. These include adding weight, including cross braces or trusses between floors, and enlarging building footprints.
PNSN Earthquake Demo - part 2 from Timothy Oleson on Vimeo.
From what I gather, the shake table demo is usually presented to groups of children. I’m not sure what it says about me that I really wanted to see the demo for myself. Honestly, though, I think it is just as useful a tool for explaining the fundamental hazards for adults. Regardless, Doug was a good sport and obliged me, and I thank him for it.
|The popcorn demo.|
If the amount of energy released by a magnitude 1.0 earthquake, which typically produces imperceptible ground shaking, is represented by a single kernel, then a magnitude 2.0 is represented by 10 kernels, a 3.0 by 100 kernels and so on. When he really starts blowing young minds, Doug says, is when he starts talking about large earthquakes in terms of bucketfuls and then roomfuls of kernels.
Finally, if you’re interested in seeing still another cool earthquake demo, head to Matt Kuchta’s blog, Research at a Snail’s Pace, to check out his “Earthquake Machine.” He demonstrates the locking and slipping behavior of faults with a brick, a string, and some sandpaper…and, he records the data! (Matt, incidentally, is a fellow GeoBadger who I know from my former incarnation as a geology grad student. He is also a prolific blogger and a frequent provider of simple, yet awesome, demos on all sorts of science. I recommend dropping in from time to time to see what contraptions he’s come up with.)