Archives for: February 2009
The Berkeley Digital Seismic Network (BDSN) has just been extended by one station, which transmits its readings in real time. Given the fact that our network already consists of 31 earthquake stations all over Northern California, from Fresno County in the south to the Oregon border (see blog January 9, 2009), the addition of one new station would not be particularly newsworthy. This extension, however, is definitely unusual. While the all other seismometers are located in vaults, tunnels or old mine adits, the new station is located under 3000 feet of water. The Monterey Ocean Bottom Broadband station (MOBB for short) had been placed on the seafloor in Monterey Canyon approximately 25 miles offshore from Moss Landing (see map 1). Last Friday MOBB was connected to a unique underwater cable, which now transmits the seismic data in real time to our data center in Warren Hall on Campus.
|Map 2: The ocean bottom station MOBB (yellow star) is now part of the real-time
network. Blue squares are existing seismic stations, the red and yellow circles
are bouys operated by NOAA. The open circles are recent earthquakes in the
Greater Bay Area.
Why would anybody in his right mind place a seismometer on the seafloor and thereby risk it being flooded with corrosive salt water, caught in the net of a deep sea fishing trawler or otherwise lost in a submarine landslide? A glance at the map of earthquake faults in California shows the answer. Here is why: In order to locate earthquakes properly, seismologists use a numerical method similar to geodetic triangulation on land. The arrival times of the seismic waves at each station are fed into a computer program, which then calculates the location by taking into account how fast the seismic waves travel through various rock types in the Earth's crust under our state. The program gives the best results when it is fed with readings from seismic stations located all around the epicenter.
But in Northern California, the San Andreas Fault runs very close to the coast. That means there are very few seismic stations west of the fault. The one exception is station FARB, which we set up on the Farallon Islands off the Golden Gate (see map 2). That means most of the earthquakes along the San Andreas Fault are located almost exclusively with the data from stations east of the actual faultline. Even though we correct for this lopsidedness, the calculations would be even more reliable if we could include data from more stations west of the fault - with the addition of MOBB, we achieve this goal.
In our next blogs, we will describe the exciting technical details of MOBB and the cable, which connects it to land. (hra031)
A few weeks ago, the blogger promised to describe how seismic waves can be used to "X-ray" the Earth's interior (see blog January 14, 2009). Well, here is a very recent example from Europe, but let's start with some basic physics. Have you ever observed what happens to a pencil when you stick it into a glass of water? It looks bent (see picture). The pencil doesn't really bend, of course - just pull it out of the water for proof. It looks bent, however, because the velocity of light in water is considerably lower than in air. As a consequence, light rays bend when they hit the boundary layer between the air and the water, hence the crooked writing utensil. Physicists call this phenomenon "refraction".
Seismic waves are also refracted when they hit a boundary layer inside the earth. Like the pencil, they bend at the transition. The boundary between the Earth's crust and the mantle below it is just such a transition. There is a big change in the seismic velocity, or the speed at which seismic waves can travel through the two layers. This "discontinuity" is actually one of the strongest refractors in the hidden world beneath our feet. Seismologist have found a way to compute the depth of this boundary layer by measuring certain onsets in a seismogram.
Recently a group of 60 researchers from all over Europe compiled thousands of such computations and merged them into a map of the European underground. The colors of this map show the depths at which the crust ends and the Earth's mantle begins. The crust is thickest under Scandinavia (dark blue area in the map), where it extends to a depth of almost 40 miles (60 km). This is a consequence of the last ice age, when for more than 100,000 years this area was buried under a pile of ice one mile high. The weight of the ice pressed the crust deeper and deeper into the mantle. The large blue zone covering the right half of the map represents crust that is 25 miles thick. There, beneath Russia and the Caucasus, is a "shield" of rocks that are one-billion years old.
|Thickness of the Earth's crust under Europe (Source: Geophysical Institute of the University of Warsaw and the Geological Institute of the University of Helsinki)|
The section on the left, in red and yellow, shows that the mantle begins a little bit more than 10 miles deep. These are typical values for young oceanic crust under the eastern North Atlantic. Under the green area in Central Europe, the mantle starts at about 15 miles depth, a common value which we also find in many areas under the United States. The mountain ranges of the Alps and the Pyrenees can also be seen as small blue splotches on the map. Under the Mediterranean, the crust is thinner again. This sea was once a large ocean, which got squeezed into its modern elongated shape by the plate tectonic collision of Africa and Europe. The reddish area under the Arctic Sea and near the North Pole shows how powerful the tool of seismic imaging is. Without the analysis of seismic waves, we would not know that the Earth's crust under this nearly inaccesible area is just over five miles thick. (hra030)