The worst damage in Friday's disaster in Japan was not caused by the shaking of the seismic waves themselves, but by the tsunami. Geologic research on sand layers along the coast of northeastern Honshu has shown that the low lying areas in the prefectures of Miyagi and Fukushima have been inundated by huge tidal waves every thousand years or so. Before Friday, the last such tsunami hit the area in 869 AD. It was caused by the Jogan earthquake, which ruptured roughly the same offshore area as Friday's quake. According to historic documents, more than a thousand people perished when the tsunami washed ashore in the plains of Sendai, and the area which is now occupied by the Fukushima Daiichi nuclear plant.
Although Japan has one of the most sophisticated tsunami warning networks in the world, the coastal region around Sendai is just too close to the quake's epicenter to allow a timely warning. Even though the wave heights were forecast correctly, at more than 30 feet, the arrival of the warning was not early enough for the many inhabitants of the area to take action and flee to high ground. For the rest of Japan's Pacific coast, however, the tsunami warning was very effective.
This is also true for the warning for the whole ocean region, which was issued by the Pacific Tsunami Warning Center (PTWC) in Hawaii. Its scientists issued the first bulletin only nine minutes after the quake. It was not very specific, but stated that the earthquake was strong enough to be able to cause a tsunami. About 15 minutes later, the computers at PTWC had run the first tsunami model for the entire Pacific and the center issued a more detailed warning. It included the arrival times of the tidal wave at coastal towns in many countries and the expected wave heights. The model was updated as more data arrived at PTWC.
A tsunami travels across an ocean at about the speed of a jetliner. Thus, the wave hit the harbor town of Petropavlosk on Russia's Kamchatka Peninsula in about two hours. Five hours later, the wave arrived in Hawaii, causing minor flooding in Hilo. At around 8 am PST on Saturday morning, the tsunami reached California, causing considerable damage in the harbors of Crescent City and Santa Cruz. (See this video by a local TV station.) Finally, thirteen hours after the earthquake, the wave was registered in New Zealand. Traveling at an average speed of 495 miles per hour, it took 21 hours for the tsunami waves to reach the southern Pacific coastal region of Chile, which was devastated by an earthquake in February 2010. That event had a magnitude of 8.8 and was comparable in size to Friday's quake off the coast of Honshu (see blog March 1, 2010).
The PTWC was established in 1949 after Hawaii suffered major damage from a tsunami caused by an earthquake in Alaska. At first, it issued warnings only for Hawaii, Alaska and the US West Coast. After the giant Chile earthquake of 1960, an intergovernmental agreement extended the PTWC's responsibilities to the entire Pacific basin. During its early years PTWC relied only on seismic measurements. Later, data from tidal gauges began to be used, and after the Indian Ocean tsunami in 2004, many deep sea observatories were added. These sensors are connected by cable to buoys at the ocean's surface, from which data are sent by satellite links to the center's main building near Honolulu. PTWC is operated by NOAA. (hra062)
|USGS map showing location of Japan's magnitude 8.9 earthquake and aftershocks.|
The earthquake that devastated some parts of the Japanese island of Honshu on Friday was the strongest quake ever measured in Japan. The National Earthquake Information Center (NEIC) of the USGS in Golden, CO, determined its magnitude as 8.9. NEIC scientists routinely use recordings from seismometers from all over the world to compute the strength of a quake. The Japanese Meteorological Agency, which is responsible for earthquake monitoring and tsunami warning in Japan, determined the magnitude as 8.8. Their scientists computed the value from regional seismic networks.
Even though the magnitude of Friday's quake was very large and some coastal areas in northeastern Honshu's Miyagi prefecture were devastated, the quake was by no means the worst natural disaster to hit Japan. On September 1, 1923, large parts of Tokyo and Yokohama were destroyed by the "Great Kanto Earthquake" and subsequent fires, which raged for days. More than 100,000 people lost their lives and almost 400,000 buildings were destroyed. More than 5,500 people died on January 16, 1995 in southern Honshu when the region around Kobe was hit by a quake with a magnitude of 6.9. The damage to Kobe's infrastructure was severe, as the quake toppled elevated freeways and submerged docks in the busy harbor.
On a global scale as well, Friday's quake off the coast of northern Honshu was one of the strongest temblors ever measured with seismometers. When using the USGS magnitude of 8.9, it ranks fifth on the list of most severe quakes in the last century, topped only by the 9.5 quake in Chile in 1960, the 9.2 quake in Alaska in 1964, the 9.1 Indian Ocean quake on Boxing Day 2004, and the 9.0 quake on the Russian Kamchatka Peninsula in 1952.
The seismic waves from Friday's quake were registered all over the world. It took about 12 minutes for the first seismic waves to reach the US West Coast and be recorded by the Berkeley Digital Seismic Network here in Northern California. The seismogram shown here was captured at station BKS, which is located in a tunnel just above the University Botanical Gardens.
On the other hand, it took many hours for the tsunami to cross the Pacific. Around 8 am (PST) the first effects were measured on the West Coast. In Crescent City the Tsunami reached a height of 7 feet, and along the Monterey coast the wave was three feet high, while hardly anything could be measured in the region around the Golden Gate. There the arrival of the tsunami coincided with low tide. (hra061)
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.)|