It was the middle of Sunday night in the South Pacific nation of Papua New Guinea. One of its islands, New Britain, is known for its striking natural beauty, its active volcanoes and its powerful earthquakes. But only rarely is the island hit by two strong earthquakes within 30 minutes of each other, as happened on Sunday. So far, no major damage has been reported, even though the two quakes had magnitudes of 6.9 and 7.3, respectively. Their foci were both more than 40 miles beneath the surface, in an area where the Solomon Sea Plate is subducted under the Bismarck Plate. For each of these quakes, the first waves traveled halfway around the world within 20 minutes, but the vibrations of the first masked many of the arrivals from the second. That made it rather difficult for seismologists to determine the exact location and magnitude of the second earthquake.
|A seismogram of Sunday's quake, recorded at an earthquake station in California. (Click to view larger image.)|
But here in California, the seismograms of the double earthquake whammy had their own kind of beauty - at least in the eyes of us narrow-minded seismologists. Here is why: A year ago, the blogger described the two types of surface waves generated by strong earthquakes (see blog July 27, 2009). In addition to having different names - one type is called Love waves and the others are Rayleigh waves - the two kinds of wave travel with substantially different speeds, and each of them makes the ground vibrate in its own characteristic way. The Love waves shake the ground only horizontally, perpendicular to the direction of wave propagation. Rayleigh waves make the ground particles move elliptically in a plane that lies parallel to the arrow of propagation.
In most seismograms, the two types of surface waves are mixed and cannot be easily distinguished from one another. But because New Britain lies almost directly west of California when measured on a great circle, the difference in speed and particle motion of these two types of waves clearly showed on Sunday's seismograms, like the one shown here from an earthquake station in the Sierra foothills of Fresno County. It shows the ground motion in three orthogonal directions: The green trace displays the movement up and down, the red trace shows the North-South movement in the horizontal plane and the blue trace gives the East-West movement.
As the waves moved through our state from west to east, the Love waves only showed on the red traces, representing the North-South movement of the ground, which in this case is perpendicular to the direction of wave propagation. In contrast, the Rayleigh waves showed most strongly on the vertical and East-West traces, because of their elliptical motion. This pattern repeats for the waves of the second quake.
In both cases, the Love waves arrived first, as they travel with greater velocity. They traversed our Golden State with a speed of almost 11,000 miles per hour. The Rayleigh waves lagged behind, because it took them almost 44 minutes to race across the Pacific Ocean at a speed of "only" 9000 miles per hour. (hra060)
Figure 1: The giant bulge on Mount St. Helens, about a week before the eruption (Photo: USGS)
Earthquakes are a regular occurrence under active volcanoes. They can number a thousand or more per day. Over the years, researchers have learned to use the number, the location and the types of earthquakes within a volcanic edifice to predict the immediate behavior of the fire mountain they are monitoring. In most cases, these temblors are a consequence of the thermal and mechanical stresses caused by the movement of magma under a volcano. In one notorious case, however, an earthquake led to a volcanic eruption of cataclysmic proportions. It happened 30 years ago today under a fire mountain in the state of Washington, which had lain in a volcanic slumber for almost 125 years
|Figure 2: The earthquake which led to the eruption of Mount St. Helens was recorded at a seismic station in Capitol Peak, WA. (Photo: USGS)|
Before March 1980, there was only one way to tell that Mount St. Helens was a volcano. Its glacier-covered conical shape resembled those of other famous fire mountains, like Shasta, Mt. Rainier or Fujiyama. But in the early spring three decades ago, Mount St. Helens began to rumble. Seismologists registered an ever increasing number of small earthquakes, fumaroles began to vent, and minor eruptions shot ash and steam out of its crater. The most ominous sign that something big was brewing under the mountain developed on its north side. Within four weeks, this flank bulged out with hitherto unknown speed (see Figure 1). Like rapidly rising bread dough, the north slope of Mount St. Helens grew and grew, sometimes by ten feet a day.
Then, on May 18 at 8:32 a.m. an earthquake of magnitude 5.1 rattled the mountain (see Figure 2). What would have had only minor consequences under normal circumstances led to a chain of events in which 57 people died and thousands of square miles of pristine land ended up devastated. The quake occurred about a mile under the volcano and its rattling was strong enough to shake loose the unstable bulge on the volcano's north side. The bulge began to collapse and slip down the mountain, thereby producing the largest historically recorded landslide-debris avalanche. Almost one cubic mile of rocks raced down the flank with speeds of up to 150 miles per hour, devastating everything in a 24 square mile area north of the volcano.
But it got even worse. Until the bulge began to slide, its weight had kept the magma under Mount St. Helens at bay. However, once this lid was off, the pressurized magma violently made its way to the surface, thereby blowing away the summit of Mount St. Helens. The rest is history: The mountain is now 1300 feet shorter than it was before the blast, and 540 million tons of volcanic ash covered a 22,000 square mile area in eleven states.
Today, there are still many earthquake swarms under Mount St. Helens, but there is no bulge and the mountain appears to pose no imminent threat. And the wasteland of gray volcanic ash from thirty years ago is now a thriving ecosystem, reconquered by Nature. (hra059)
Ever been to Death Valley and seen the fish? No, the blogger is not joking - there are fish in one of the driest and hottest places on Earth. Even if you don't believe it, these fish do indeed exist. They live in a unique, aquifer-fed geothermal body of water called Devil's Hole. It is part of the Death Valley National Park and it is aptly named, as the surroundings are nothing but grey, treeless, hot desert. The fish belong to a species of pupfish, Cyprinodon diabolis. Because they number only a few hundred, they are on the federal endangered species list. And recently, they got a major scare when their unique habitat was shaken by the strong waves generated by the magnitude 7.2 Baja California earthquake of April 4 (see blog April 5, 2010).
Well, we actually don't know if the fish were really scared, but what was captured by an automatic video camera seems to be scary enough. Researchers from the University of Arizona in Tucson installed the underwater video system to study the largely unknown spawning behavior of the pupfish. Normally the waters of Devil's Hole are as sedate as you can get. But when the earthquake hit, the poor little fish experienced a tsunami in the desert. Sediments were stirred up, and the water sloshed back and forth. To view the video, click here.
It is by no means unusual for a small body of water to be wildly shaken by earthquake waves. Seismologists even have a word for it. They call these waves "seiches" (pronounced saysh) after the word for sloshing used in the French-speaking part of Switzerland. It was there, at Lake Geneva, that the Swiss researcher François-Alphonse Forel discovered these waves after an earthquake in 1890. Many lakes and lagoons have experienced such seiches. Some researchers even found traces of such waves at the shores of Lake Tahoe - 30 feet above today's waterline. This seismically-induced sloshing of water is by no means restricted to lakes, bays or Death Valley's Devil's Hole. The water in almost any swimming pool can be stirred by earthquake waves. How bad this can be was captured by a security video camera at one of the hotels in Mexicali, less than 50 miles from the epicenter of the Baja California earthquake. It caught the seiches in the hotel's pool induced by the earthquake's ground shaking. Be patient when watching the video, because nothing happens for the first 30 seconds -- but then, all hell breaks loose (hra058).
|Video of seiche in hotel pool. (Click the image to play in .wmv format. Click here to play in .MOV (Quicktime) format.)|
|Figure 1: Ernst von Rebeur-Paschwitz|
Sometimes it is easy for historians of science to exactly pinpoint the beginnings of a new field of research. The birth date of modern genetics, for instance, is considered to be April 25, 1953, the day when James Watson and Francis Crick published their paper about the structure of the DNA molecule in the British journal "Nature." The nuclear age began on December 17, 1938 in Berlin when the German chemists Otto Hahn and Fritz Straßmann for the first time split the nucleus of a Uranium atom. But when did the field of seismology start? Was it in 132 AD, when the Chinese scientist Zhang Hêng invented the seismocope? Or was it with the publication of the "Lawson-Report," comprehensively describing the effects of the Great San Francisco Earthquake of 1906?
|Figure 2: The first recording of a teleseism.|
Well, sorry historians, stop searching for the cradle of seismology. Our field is a science which developed slowly over several centuries in many places with contributions by quite a few fine researchers. Nevertheless, in the history of seismology several days clearly stand out. One of those happened 121 years ago today.
The scene is the Astrophysical Observatory on Telegraph Hill in the Prussian City of Potsdam, near Berlin. There a German astronomer, Ernst von Rebeur-Paschwitz (Figure 1), had set-up a horizontal pendulum. He wanted to precisely measure the changes of the gravitational attraction on the Earth caused by the movement of other planets. But during the afternoon of April 17, 1889, at 5:21 pm to be precise, he saw his pendulum swing in an extremely strong, but still rather regular movement (Figure 2). At first, Rebeur-Paschwitz had no idea what had caused his sensitive instrument to swing so wildly. That puzzle was resolved a few months later, when he read a note in "Nature" about an unusually strong earthquake in Japan. It had occurred several hours before the wild swings of his pendulum. It was then when Rebeur-Paschwitz realized that his instrument had caught the seismic waves that were generated by the earthquake in Japan located more than 5,500 miles away from Potsdam.
Although the shaking of local earthquakes had been recorded several times before, nobody had ever registered the waves from a far away earthquake. The first recording in Potsdam of such teleseisms on April 17, 1889 marks the birth of the study of the structure of the Earth by means of seismic waves (see blog January 14, 2009). Unfortunately, Rebeur-Paschwitz did not live long enough to see the fruits of his detection. He died of tuberculosis a few years after his historic recording. (hra057)
|The epicenter of Sunday's quake was located roughly 100 miles east of the Straits of Gibraltar. However its focus lay 400 miles below the ancient city of Granada (Click to view larger image.)|
Most of us seismologists think we know it all. Just look at this year's large earthquakes: Within a few hours after the devastating Haiti Earthquake in January, the fault on which it occurred was identified (see blog January 13, 2010); after the giant temblor in Chile in late February, it took only minutes to issue a tsunami warning (see blog March 1, 2010); and within 24 hours, scientists found ruptures in the Earth's surface caused by the strong quake which shook northern Baja California and the southern part of our state on April 4th (see blog April 9, 2010). This blog is full of entries describing all the things we know about earthquakes - but here is a real puzzler from Europe, which nobody has been able to explain so far.
Last Sunday night, shortly after midnight, the Earth rumbled under the Alhambra, the old Moorish citadel overlooking the ancient Spanish town of Granada. Seismic sensors all over Europe reacted to this 6.3 magnitude quake, but few people felt it and no damage was reported from anywhere. The computer programs in the data centers of the European seismic networks and at USGS's Earthquake Information Center in Golden, Colorado, quickly and automatically calculated the exact location of the quake: Its hypocenter lay just 8 miles southeast of Granada's city center - but at a depth of almost 400 miles.
|This map shows all earthquakes east of the Strait of Gibraltar, which have been recorded in the last 20 years. Their colors depict the depth of the hypocenters. The depest temblors recorded before Sunday's quake (red star) was the green cluster with depths not exceeding 100 miles.|
When seismologists got to their offices on Monday morning, they scratched their heads: "400 miles? Something must be wrong!" The deepest quakes in Europe occur under the Tyrrhenian Sea just north of the Italian island of Sicily. There, the African plate subducts under the European plate, similar to the movement of the Gorda plate beneath North America north of California's Cape Mendocino. But even in Italy, the quakes' foci do not reach deeper than 300 miles. In addition, there is no known subduction zone within 1000 miles of Granada - and all deep earthquakes known to researchers occur within the confines of these zones. Hence the question arose: Did some measurement go awry which led to locating the Granada quake wrongly to such super depth?
The answer is that the computations were all correct and consistent. All data centers came to the same conclusion. There was no doubt that the temblor occurred 400 miles below Granada. However, when researchers had time to search the archives, they found, that this quake wasn't so unusual after all. Since seismic recordings began, three very similar earthquakes have occurred in exactly the same location, in 1990, in 1973 and the strongest one (M=7.0) on March 29, 1954. European scientists even wrote three scientific papers describing these extremely deep events, but they left the question unanswered, why and how they occured. Sunday's quake only adds to the mystery. (hra056)