When the ground shakes in a earthquake a lot of factors go into determining how things will turn out. What the strength of the quake was, how deep underground the event was that caused the quake, how long the quake lasted, and what mechanism produced the quake are some. Clearly if the quake is strong enough there won't be much that doesn't get damaged, and if the buildings are poorly built then things will likewise go badly. But another item that goes into the damage equation is what the ground is made of near the surface of the area hit by an earthquake.
When you build a house you want the foundation to be solid so that the walls will be sturdy and be able to hold up the roof. Likewise, when you build a house in an area prone to earthquakes you want the house's solid foundation to also be built on a solid foundation. When an earthquake occurs an area of solid rock will shake, but soft or loose soils can effectively amplify the shaking of the ground and cause more damage to occur, as was the case in the Marina District of San Francisco during the 1989 Loma Prieta earthquake. Think about which will shakes more violently when moved, a bowl of Jell-O, or a bowl of rocks?
In extreme cases, when the soil is mostly sand, the earthquake can cause liquefaction which turns the ground into quicksand.
You can get a feel for what the ground is made of by looking at the speed with which shock waves, in this case known as shear waves, pass through an area. Generally speaking the looser, or less dense, the soil the slower the waves will travel. Seismic waves travel fine through solid rock -- they just sort of go straight on through -- but they have trouble making it through something like sand or loose soils. That makes sense, right?
Up until now all of the experiments that I have been on have always wanted it to be quiet while the instruments were recording. This one was different. A seismologist at the University of Nevada at Reno, John Louie, developed a technique where the noise source for an active source experiment is all of the normal noise that we humans generate. An active source experiment is one where you make noise to record rather than wait around for an earthquake to make noise for you. Some of the best sources of manmade noise are traffic (the bigger the vehicles the better), aircraft, and fluids flowing through underground pipelines. You can still set off explosions to generate noise for an active source experiment, but what a hassle. If you are trying to record signals in and around noisy places like interstate highways or downtowns then you need to make the explosions a little bit bigger so that the shock waves can be heard by the instruments over all of the other noise. But you usually can't make them too large, or you run the risk of breaking things like water pipes and glass windows. Then there are all of those bureaucrats, and all of their red tape and permits. It's bad enough just getting permission to lay an instrument out on the ground next to a road, much less trying to get permission to dig a large hole and fill it full of explosives.
This experiment was designed to measure the velocities of the shear waves to a depth of about 100 meters along two lines. One line ran down the middle of east and south Los Angeles, California, along the San Gabriel River and its bike path, all of the way from the mountains down to where the river met the sea at Seal Beach. The length of the line that was surveyed was about 60 kilometers long. The idea was to make a profile that showed what the ground below the surface was made of, and then to see if that profile matched what was thought to be below the surface according to various other sources such as the geologic maps of the area.
This experiment was sponsored by the National Earthquake Hazards Reduction Program, U.S. Geological Survey, and the Lawrence Livermore Laboratory, U.S. Department of Energy.
As Johnny Carson may have started off a joke, "You know you are getting close to Los Angeles when..." Below is not an approaching dust storm. It's just some of the Los Angeles Basin smog leaking out. I drove from Flagstaff, Arizona to Los Angeles. When I saw the scene below I knew I was getting close.
We set up shop in one room of a Best Western hotel in Los Alamitos, California. I arrived a day ahead of most of the people to help get things put together and organized.
The maid is going to kill us.
Clearly this group watched a little too much CNN.
DIRECT LINKS TO PAGES:
URBAN SEISMOLOGY IN THE NAUGHTIES
FROM THE MOUNTAINS
TO THE PRAIRIES
TO THE OCEANS, WHITE WITH FOAM
MISSED FINISHING BY THAT MUCH
SIN CITY, BABY!!
THE ELECTRICITY BILL FROM HELL
DAM THE CASINOS...FULL SPEED AHEAD!
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