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)
|En echelon cracks where Highway 25 crosses the San Andreas Fault (Photo by Horst Rademacher)|
There is a whole lot of shaking going on in Tres Pinos in these days. Well, it is not exactly the earth shattering rumbling associated with big earthqakes. But the 500 or so residents of this tiny hamlet south of Hollister have certainly been taking note. Joe Lucido tells the blogger: "We felt quite a bit of shaking last night...The last couple of weeks have been like nothing we can remember here in San Benito County. The talk is all over town." So what is going on in this section of the state, about one hour's drive south from Silicon Valley?
Tres Pinos is located between Hollister and the Pinnacles. In the immediate vicinity of this town, the Calaveras Fault branches off from the San Andreas Fault. South of the split, the section of the San Andreas is "creeping" all the way to Parkfield. Creeping means that the mechanical stress associated with the movement of the plates is not stored in the rock until it breaks in a big temblor. Instead the energy is more or less continuously released. You can see evidence of this creep in the pavement of Highway 25 south of Tres Pinos. The fault crosses the road in many places. There, the asphalt is broken in a typical, staggered pattern that seismologists call "en echelon cracks" (see picture).
However, not all of the energy is released in fault creep. Sometimes the friction between the two flanks of the fault is too high and some energy accumulates. When the rocks crack under this load, earthquakes happen. These quakes with magnitudes of 3 or 4 are small by worldwide standards, but strong enough to be felt. Even though the movement of the plates - and thereby the transfer of the mechanical stress into the fault - is very steady, friction along the fault is not nearly as constant. This is the reason that small temblors sometimes occur in clusters. If there are enough of them, they begin to rattle the nerves of the folk in Tres Pinos and nearby.
That is exactly what's happening now. Since January 13, there were nine small quakes with magnitudes above 3 and dozens more between 2 and 3. Such clusters are by no means unusual. Since 1970, a total of 328 quakes with magnitudes above 3 happened on this stretch of the fault. As the graph below shows, most of them come in clusters. If you want to stay abreast of the rumbling under Tres Pinos and beyond, look at the realtime earthquake map provided by the USGS. (hra029)
Is your water heater properly strapped to the wall? Do you have additional insurance on your house with the California Earthquake Agency (CEA)? If so, then you can thank a horrible event, which happened 15 years ago today in Southern California.
|Damage from the Northridge earthquake (Photo courtesy of M. Celebi, USGS)|
It was about 4:30 am when Clarence Dean was rudely awakened in his home in Lancaster in northern Los Angeles County. A major earthquake had shaken his house badly. He got up, dressed quickly and then jumped onto his Kawasaki Police 1000 motorcycle. Dean was an LA motorcycle cop, and this Martin Luther King Day would have been his day off. But he knew he would be needed at his Van Nuys police station because of the earthquake. From his home, it took him only a couple of minutes until he reached Highway 14. There he took the westbound direction and sped towards LA, the blue emergency strobe flashing on his bike. But he never made it to his assigned post. The swooping viaduct where Highway 14 meets Interstate 5 had collapsed in the quake. Where two lanes of concrete had been built in a gentle arc high above the ground, there was nothing left. In the darkness, Officer Dean spotted the gap too late and flew 75 feet through the air before crashing, 30 feet below, in a cascade of sparks and screaming metal.
The policeman was one of 72 people who died on that morning in a 6.7 quake, with a hypocenter 10 miles beneath the town of Reseda in the San Fernando Valley. Had it not been a federal holiday, many more people might have died during the early morning commute. Besides the 5/14 interchange, the Santa Monica Freeway (I 10) had collapsed near Cienega Boulevard, making this major artery unusable for weeks. Most of the damage, however, occurred in the town of Northridge, hence the name for this costliest temblor in US history. Homes, apartment buildings and even hospitals collapsed all over the San Fernando Valley. It was later estimated that the quake had caused more than $20 billion worth of damage. As a consequence, many insurers of private homes stopped offering protection against earthquakes in our state.
The same region had been the victim of a similarly damaging temblor 23 years earlier, the San Fernando earthquake of 1971. Seismologists later found out that these two quakes occurred on two different, previously unknown faults. After the Northridge quake, the California Legislature passed several laws in a rare form of bipartisan unity. One gave life to CEA, which now underwrites earthquake insurance statewide. The building code was also amended, requiring each newly installed gas-powered water heater to be strapped to the wall. Too many heaters had fallen on that fateful morning 15 years ago, breaking the gaslines and causing devastating fires. (hra028)
|Picture 1: The first X-ray photograph|
It is often said that seismologists use earthquake waves to illuminate the interior of the Earth like a radiologist uses X-rays to study the insides of the human body. There is indeed some similarity, as both of these branches of physics are of the same age. The German researcher Wilhelm Conrad Röntgen took his first X-ray photograph in 1896 (see picture 1). Roughly at the same time, the first seismographs were installed and earthquake researchers learned to read the "trembling of the rock" (see blog January 9, 2009).
In a strict physical sense, however, major differences exist between X-ray pictures and seismograms. Through the different shades of grey in the X-ray picture one can clearly distinguish between the wedding ring, the bones, and the soft tissue in the hand. This is due to the fact that metal, the calcium structure of the bone, and the soft tissue absorb electromagnetic waves differently. Metals swallow X-rays almost completely, but the calcium in the bones is not quite as opaque as metal, while the soft tissue lets most of the rays pass.
Although seismic waves experience some absorption in the rocks of the Earth, it is by no means the dominant feature in a seismogram. Look at picture 2. It is a recording of the magnitude 7.6 earthquake in eastern Indonesia on January 4, 2009 (see blog January 5, 2009). The blue line represents 40 minutes of ground movement at our seismic station "CMB" in Columbia in Tuolumne County. Starting from the left, it begins as an almost straight line. Then, roughly at 19:57 UTC, the ground starts to wiggle. There is another shaking at minute 20:02 UTC, a bigger one at 20:12, and an even larger one at 20:17, with the biggest one at 20:31. Each of the arrows marks one of these "onsets" or "phases," as seismologists call them.
What are these phases? As the seismic waves travel from the earthquake focus through the earth, they hit major boundary layers in its interior. At each of such "discontinuities" the waves are reflected, refracted or diffracted. Each of these processes alters the path of the waves, which in turn makes them travel longer until they reach CMB. In addition, different kinds of waves travel through the Earth with different velocities. The P-waves (red arrows) are faster than the S-Waves (green arrows) (see also blog September 10, 2008). In one of the next blogs, we will explore, how these phases can be used to "X-ray" the Earth's interior. (hra027)
|Picture 2: Seismogram showing M 7.6 earthquake in West Irian Jaya.|