|Map showing many recent earthquakes near Bombay Beach, CA. (Courtesy of USGS)|
Seismologists are - like many other scientists - a curious and a restless bunch. Pronounced patience is usually not one of their prime virtues. However, in their job of earthquake monitoring, they sometimes resemble Vladimir and Estragon, the lead characters in Samuel Beckett's "Waiting for Godot." But instead of expecting a mysterious fellow named Godot who never shows up, seismologists know exactly what to expect: A major temblor will come someday and wreak havoc in earthquake prone areas like California, Japan, Greece, or the Hindu Kush. And many a time this "Waiting for the Big One" is no less demanding than the dialogue of Beckett's characters: We seismologists know that our Godot will come, but we don't know when.
However, with events starting last weekend, earthquake researchers in Southern California are on alert. Forty-two smaller earthquakes near the hamlet of Bombay Beach on the eastern shore of the Salton Sea preceded a magnitude 4.8 event, which rattled the area early Tuesday morning. This moderate earthquake was widely felt in the sparsely populated region at the southern end of our state. Several dozen aftershocks followed, the largest having a magnitude of 3.1.
Why did this earthquake series put our colleagues in the Southland on higher alert? The temblors occured exactly at the southernmost tip of the San Andreas Fault (see blog November 4, 2008). Looking north from there towards Los Angeles, this section of California's major earthquake fault has not ruptured for many decades. This means that a lot of tectonic energy is currently stored in this part of the fault. It contains so much stress, that its possible rupture was the scenario for the biggest earthquake drill ever held in our state. During "The Great Southern California Shake Out" last November, the scenario assumed that the San Andreas Fault would start rupturing near Bombay Beach and would not stop for 120 miles, all the while sending out its destructive waves (see blog November 10, 2008).
Although seismologists operating the Southern California Seismic Network in Pasadena monitor the most recent shocks and aftershocks very carefully, no one can predict what will happen. Are those recent earthquakes starting to loosen up this locked section of the San Andreas fault - and thereby leading to the expected megarupture? Or was it just a cough of the Earth's crust, which will blow away without consequence, like the many sandstorms in the deserts surrounding the Salton Sea? (hra034).
Cannery Row, John Steinbeck, the Aquarium - they all seen synonymous with Monterey. Particularly the Monterey Bay Aquarium, with its dazzling and sometimes dizzying display of sea creatures, is an attraction for locals and tourists alike. The avid blog reader may ask, what does this advertising pitch for the aquarium have to do with seismology. The answer can be found in a sprawling laboratory building a few miles north of Cannery Row in Moss Landing. This is where a sister institution of the aquarium has its home, the Monterey Bay Aquarium Research Institute (MBARI). Without the work and support of their dedicated people, the seismic station MOBB (see two previous blog entries) probably would not exist.
Figure 1: A 3-D map of the Mars project (image courtesy of MBARI)
Founded in 1987, MBARI does much more than biological research in support of the aquarium. Several of their researchers are interested in marine geology, particularly in seismology. This led to a cooperation with BSL on placing seismometers on the ocean floor. The most recent result of the joint effort was the deployment of MOBB on the sea floor in April 2002 by the undersea robot "Ventana." This remotely operated vehicle is part of MBARI’s research fleet. It is carried out to sea by its mothership, "R/V Point Lobos." From a control room in the bow of the ship, a pilot operates the Ventana using a joystick and computer controls.
In the years following the deployment, Ventana regularly returned to MOBB every few months, supplied it with new batteries and collected its data. The recordings were then shipped to BSL and - ex post facto - integrated into our data base.
But MBARI folks had much bigger plans for MOBB. They wanted it to become part of a large ensemble of scientific sensors on the seafloor. These sensors were to be permanently linked to land by a cable, which would provide power and a high speed data link. This idea led to MARS, not the planet, but the Monterey Accelerated Research System. Its heart is a node resting on the seafloor some 23 miles off the coast of Moss Landing. It was installed last November and allows eight different sensors to be connected to it - and hence their data to become available in real-time to researchers on land. The first instrument connected was a sonde which measures the salinity, temperature and pressure of the ocean water. MOBB was added late last month. For what is still to come on the slope of submarine Monterey Canyon, visit the MARS website.(hra033)
|MOBB deployment with ROV (Courtesy of MBARI)|
Seismometers have always been marvels of the latest technology of their era. Consider the instrument invented by the German geophysicist Emil Wiechert more than a hundred years ago: A 2000 pound mass rests on a needle point at the bottom of a pylon - held in place by nothing more than a few springs. With a clever system of mechanical levers, the movement of this big mass relative to the shaking Earth was scratched into soot paper - resulting in a seismogram. Modern seismometers have much smaller masses of only a few ounces. They use electronic circuitry to allow recordings of extremely small Earth movements of a few nanometers - a thousand times less than the diameter of a human hair. These instruments, however delicate, are made for use on land, operable only with careful set-up and adjustment procedures.
Now imagine trying to place such a sensitive, modern seismometer under thousands of feet of water at the bottom of the ocean. Not only must the instrument, an Ocean Bottom Seismometer (OBS), be absolutely water-proof for such depths. It also needs to be able to operate reliably for months at a time without any connection to the outside world. Even the set-up is a challenge: On land BSL's engineers and technicians go out of their way to ensure that the sensor is placed correctly and operates in a stable thermal environment. On the seafloor, an underwater robot guided by commands from a ship above has to do the job (see Fig. 1).
OBS tested in Byerly Vault (Photo by Horst Rademacher)
The OBS off the coast of Monterey, which we recently incorporated into our real-time network (see blog February 28, 2009), was built by Guralp Systems Ltd. in the UK. Its core is a very broadband feedback seismometer with three components, similar to ones used on land (see Fig. 2). But the OBS has many additional features: A stable internal clock provides the time signal, which on land we literally pull out of the air from the signals of those same GPS satellites that help you navigate around town. A motorized gimbaling system makes sure that the sensor is set-up horizontally and stays that way in the mud on the seafloor. On land, the leveling is done by humans, by tuning with finely threaded stands.
And then there is the issue of water resistance. Good diving watches can be taken down to 300 feet; below that, the O-rings are squeezed so tightly that they leak. Diving even deeper, the water pressure might crack the watch's housing. To protect the OBS against the forces of the watery depths, it is placed in a titanium housing, which is rated to withstand the pressure of a 18000 foot deep water column. (hra032)
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)