|Community Internet Intensity Map for October 20 M4.0 Berkeley earthquake Click to view larger image.|
It was early Thursday afternoon, at 2:41 pm to be exact, when the Earth shook under Berkeley. Nothing was damaged, but the jolt was widely felt. At this time of the year, people here in the East Bay are always somewhat on the edge. Almost exactly 22 years ago, on October 17, 1989, the Loma Prieta quake hit and dozens of people died during the collapse of the double-decker Cypress Structure carrying freeway 880. Two years later, on October 19, 1991, the Oakland Hills firestorm claimed many more lives, and several thousand houses burned to oblivion. No wonder then, that long time residents of the area took cover when the latest temblor hit the area, even though it was mild compared to other temblors. Thursday's quake had a magnitude of 4.0 and was located on the Northern Hayward Fault more than 6 miles below the Clark Kerr Campus of UC Berkeley. It was felt over a large area in Northern California, from Santa Rosa in the North, to Sacramento in the East and all the way to Gilroy to the South. Within three hours of the quake, more than 15,000 people had reported their observations. If you felt the quake but have not yet reported, it is not too late. You can do so on the "Did you feel it?" website. Your report is important, because from the the collective observations, seismologists draw conclusions about the different ways the ground in the Bay Area shakes in response to seismic waves.
The quake was the strongest temblor to be felt in the Bay Area since March 1, when a quake with a magnitude of 4.5 struck the area near the Geysers hydrothermal area in Sonoma and Lake counties.
The quake under Berkeley behaved exactly the way seismologists expected. It was a strike slip motion along the Hayward Fault. The area west of the fault, that is, the lowlands of Berkeley, Oakland and Albany moved a few tenths of an inch to the North with respect to the Berkeley Hills. This northward movement is caused by the sliding of the Pacific Plate against the North American Plate. The Hayward Fault is one of the three major earthquake faults in the Bay Area which convey this plate movement. The others are the San Andreas Fault further to the West and the Calaveras Fault in the East. Even with a modest earthquake like Thursday's temblor, aftershocks can be expected. By 5 pm, two aftershocks, one of magnitude 1.8, the other a 2.2, had already occurred. Shortly after 8 pm, a third aftershock with a magnitude of 3.8 again rattled the nerves of the people in the East Bay. (hra064)
Every textbook about Earth Science tells you that the giant lithospheric plates move past each other at a velocity of a few inches per year. The most common metaphor is that their speed compares directly to the rate at which a fingernail grows. That sounds gentle, harmless, and most of all managable. Unfortunately, these numbers are utterly misleading, because they are very long term averages measured over hundreds or even thousands of years. When you look at plate movement in the short term, you recognize a completely different picture. Most of the time, nothing happens at all. The plates are tightly locked against each other, snapping perhaps in a few small earthquakes here or there. This tectonic inactivity gives us, the residents of earthquake zones, a false sense of security. Because suddenly, within a fraction of a second and without any warning, the geologic interlocking may fail and then all hell breaks loose. The mechanical energy, caused by the constant push of the churning viscous mantle below and stored in the plates over hundreds of years, is released. In a few seconds the plates jump past each other by dozens of yards. In many respects, the plate movement is like the fate of a drag racing car. Most of time, the machine sits around idly, but once in a while, its engine is fired up and the dragster races a few hundred yards at lightning speed.
This is what happened last Friday off the east coast of Honshu. When the westward moving Pacific Plate finally unlocked itself from the Eurasian Plate, the result was the giant earthquake. Over the long term the Pacific and Eurasian Plates drift past each at an average rate of about 3.25 inches per year. But on Friday, they raced passed each other by dozens of yards in about two minutes.
Several researchers have already made models of how the interlocking between the plates actually failed, the quake's so called "rupture process". Gavin Hayes from the USGS office in Golden, CO, used the recordings of 60 seismic stations from all over the globe to compute the rupture. According to (his calculations) the rupture area underground was almost 200 miles long and 125 miles wide. Imagine, the entire state of West Virginia slipping up to 20 yards eastwards in less than two minutes.
A group of scientists from the Geoforschungszentrum in Potsdam, Germany, used the recordings of almost 500 sensitive GPS-stations in Japan to model the rupture. Their calculations yielded a 250 mile long zone with a offset of almost 28 yards (see Figure 2). Researchers from Harvard University used several hundred earthquake stations on the US mainland for their calculations and came to similar conclusions. They estimate the rupture to be 240 miles long by 150 miles across. They even simulated how the rupture spread over this area (see their animation).
In the meantime, the massive temblor has been given a name, and the magnitude of the "Great Tohoku Earthquake" has officially been upgraded to 9.0. (hra063)
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)