It doesn't happen very often that a seismologist actually gets to observe a seismic wave in nature. Sure, we all sit in front of computer screens and look at the digital representation of the wiggles a seismometer produces. And indeed, the seismometer's mass swings with the rhythm of the wave. But these seismograms are far from the real thing. The blogger actually saw a seismic wave 20 years ago today, when the Loma Prieta Earthquake shook the Bay Area. I remember that it was a balmy afternoon. Everybody was excited because the A's and the Giants had lined up in Candlestick Park (as it was then known) for the third game of the 1989 World Series. I was in the car, picking my son up from after-school activities and dropping my daughter off for soccer practice. We were parked in her school's parking lot when the car suddenly began to rumble and then sway. I thought my son was jumping up and down in the back seat, eager to get home and watch the game on TV. But when I looked in the rear view mirror, I saw him sitting there quietly, staring awestruck out the window. When I looked in the same direction, I saw the asphalt in the parking lot swell and heave as though a giant gopher were digging through the earth at lightning speed. The wave in the asphalt was rapidly moving in our direction; it swayed the car up and down and within a few seconds - it was gone. I think the wave's crest was a few inches high, but everything went so fast that my recollection is somewhat blurred.
|Shaking intensity map for the 1989 Loma Prieta earthquake. Courtesy of CISN. (Click to view larger image.)|
That was a seismic wave, I exuberantly told my son. It probably was a once in a lifetime event to really see one coming and going, I beamed at him. But he was not at all impressed, and asked coolly, why I had turned the car radio off. I knew I hadn't, but indeed, there was no sound. That moment I realized that something big must have happened. My car radio was still on, but the radio station had gone off the air. My thoughts began to race: If I can actually see a seismic wave from an earthquake in the parking lot, then the shaking must have been really severe. Was the rest of my family safe? Was my house ok? We grabbed the daughter and drove home, where we found everybody shaken and stirred, but safe and sound. Then, ever so slowly, the news about the destruction at various locations in the Bay Area began to trickle in. The rest, of course, is history. The quake had a magnitude of 6.9, a total of 63 people were killed, and more than 3700 were injured. Its hypocenter lay along the San Andreas Fault in the Santa Cruz Mountains near the Loma Prieta summit, 11 miles beneath the surface. The Cypress structure of the 880 freeway in Oakland had collapsed, parts of the Marina District were burning, one section of the upper deck of the Bay Bridge had fallen onto the lower deck, and there was widespread damage in Watsonville and Santa Cruz.
These are the terrible facts, but in a way are just statistics to the blogger. What he remembers vividly is the wave in the parking lot. I am sure other people must have had similar experiences on that fateful October afternoon 20 years ago. Please tell the blogger what you remember about the largest quake in the Bay Area since 1906. Email us at by going to "contact" at the top of the page or by writing to "firstname.lastname@example.org". (hra046)
It sounds utterly trivial, but there is nothing better than being prepared. What is true for everyday life is even more important for surviving in extreme situations. Think about what you would do in case of a car accident, a flooded basement, or a fire ravaging your neighborhood. While many such situations can only be planned for theoretically, there is a lot you can actually do to prepare for an earthquake and its aftermath. And in this Thursday's "Great California Shakeout," you will have a chance to test your skills and evaluate your preparations.
|Image courtesy of www.shakeout.org|
Everybody who went to school in California will remember earthquake drills, when the students dove under their desks and were taught to "Drop, Cover, and Hold on." Last November, these drills were carried a major step further as several hundred thousand people in Southern California participated in what was then the biggest earthquake drill ever in the country. Based on a realistic but hypothetical scenario of a magnitude 7.8 quake rattling the Greater Los Angeles Area, emergency planners, response coordinators, and many citizens practiced what to do during and after a destructive temblor (see blog, November 10, 2008).
This Thursday, on October 15 at 10:15 am, we will go even further than last year. More than six million people - roughly a fifth of California's population - have so far signed up to participate in the Great California Shakeout. The purpose of this first statewide exercise is to practice how to protect yourself during earthquakes, and how to get prepared at work, school, and home. Several government institutions, like the Federal Emergency Management Agency (FEMA), the United States Geological Survey (USGS), and the National Science Foundation (NSF), got together with state and private partners to promote earthquake preparedness. At exactly 10:15 am, students and workers in schools, offices, and companies will act as if a major earthquake were to rattle their buildings. They will take cover, turn off gas lines, go through checklists and - most importantly - evaluate afterward how effective their emergency plans were. If you want to participate and get general information on earthquake preparedness, go to http://www.shakeout.org (hra045).
|From the CGS map of California Faults and Ruptures. Yellow square shows the location of the recent earthquakes.|
While people all over the world - and many Californians - are watching in horror the unfolding of the most recent earthquake disasters in Samoa and on the Indonesian island of Sumatra, a very unusual sequence of temblors is happening right at our doorstep. The Owens Valley east of the crest of the Sierra Nevada was shaken last week by three moderate sized earthquakes within 48 hours, and many smaller ones. The strongest shock had a magnitude of 5.2 and occurred Friday (October 2) in the afternoon. The other two had magnitudes of 5.0 and 4.9. Their epicenters were located on the eastern shore of Owens Lake, halfway between the small towns of Keeler and Olancha. All three earthquakes were felt as strong shocks in the nearby towns of Lone Pine, Big Pine and Bishop. Some shaking was reported as far away as Las Vegas, San Diego and even in some locations here in the Bay Area.
Earthquakes are by no means rare in Owens Valley, the long depression east of the granite range of the Sierra Nevada mountains, which, geologically speaking, is the spine of California. In fact, the strongest temblor there competes with the Great San Francisco Earthquake of 1906 for the title of "strongest earthquake" in our state in historic times. It happened on March 26, 1872 and was felt by - among others - John Muir, the 19th century nature lover par excellence. Here is how he described what he felt in his cabin somewhere in what is now Yosemite National Park:
At half-past two o'clock of a moonlit morning in March, I was awakened by a tremendous earthquake, and though I had never before enjoyed a storm of this sort, the strange thrilling motion could not be mistaken, and I ran out of my cabin, both glad and frightened, shouting, "A noble earthquake! A noble earthquake!" feeling sure I was going to learn something.
What Muir calls a noble earthquake was rather deadly for others. In Lone Pine alone, 52 of the 59 houses that existed in this hamlet almost 140 years ago were destroyed, and 27 people died in the rubble. In 1872, no seismometers existed in California - the first one was set-up by the Berkeley Seismo Lab's predecessor in 1887 - so nobody could determine a magnitude or any other scientific measure of the strength of the Owens Valley quake. But given the destruction near the epicenter and the fact that the earthquake caused the Earth's surface to rupture over a stretch of at least 100 miles, seismologist estimate its magnitude at 8.0 or more. The Owens Valley quake is therefore comparable in strength and energy release to the 1906 Earthquake in San Francisco.
The remnants of this rupture can still be seen today as a fault scarp just north of of Lone Pine, a few hundred yards east of Highway 395. The offset, or slip, is still clearly visible even after almost 140 years, having moved four feet vertically and about 18 feet horizontally. The rupture surface of last week's quakes seem to have been on the same fault line as the quake from 1872. The earthquake triplet was also dominated by horizontal strike slip. Looking back from the fault scarp towards the west, one gets a majestic view of Mount Whitney, the highest point not only in the Sierra Nevada, but also in the contiguous 48 states (hra044).
The strongest earthquake so far this year created a deadly tsunami in the South Pacific. At least 100 people were reported killed by the waves in the Samoa islands and in Tonga. The quake, which was located 11 miles below the seafloor in the triangle between Fiji, Samoa and Tonga, had a magnitude of 8.0 according the United States Geological Survey. The Pacific Tsunami Warning Center in Hawaii calculated its magnitude to be even higher, at 8.3, and initially issued a tsunami advisory for Hawaii and the US West Coast, which was later withdrawn.
The hardest hit areas were the independent island nation of Samoa and the US territory American Samoa as well as the Kingdom of Tonga. The superintendent of the National Park of American Samoa, Mike Reynolds, was quoted as having seen at least four separate waves, each about 15 to 20 feet high, rushing between half a mile and a mile inland. A five foot wave was said to have swept from the harbor at least 300 feet into Pago-Pago, the capital city of American Samoa. According to eyewitnesses, the coastal village of Sau Sau was completely destroyed by the waves. Some news reports even mentioned a thirty foot tall wave.
Most tsunamis are generated by sudden vertical movements of the seafloor. If certain types of earthquakes, thrust and normal fault quakes, occur at shallow depth below the seafloor, they can generate extremely rapid movement of the sea bottom. As a rule of thumb, the stronger the quake, the larger the area of displacement and hence the stronger the tsunami.
California's typical style of earthquake, the strike slip, rarely causes a tsunami, because the slip associated with this kind of quake does not have a significant vertical component. Earthquakes in subduction zones, however, are mostly of the two tsunamigenic types. The earthquake which caused the large tsunami in the Indian Ocean on Boxing Day in 2004 occurred in a subduction zone west of the Indonesian island of Sumatra. There the Indian Ocean plate dives under fragments of the Eurasian Plate, creating the Sumatra Trench. Tuesday's quake in the South Pacific happened in a location with a similar tectonic layout. It occurred at the top of the Tonga Trench, where the Pacific Plate dives under the Fiji Plate, a fragment of the large Australian Plate. This plate collision occurs with a speed of about 1.5 inches per year, which makes the area one of the most earthquake-prone zones on our planet.
Completely independent of the quake in the South Pacific, another strong temblor hit the city of Padang on the Indonesian island of Sumatra about 17 hours later. At least 75 people were killed in the earthquake, which had a magnitude of 7.6. The quake triggered several landslides in Western Sumatra, but did not cause a tsunami wave. Its epicenter lay south of the zone of the deadly quake that triggered the Indian Ocean Tsunami almost five years ago (hra043).
When you read the headline and notice the words "Love Waves", please don't think the Seismo Blogger is diverging into the X-rated territory of the web. Instead, he is delivering on the promise made two weeks ago, to explain more about surface waves (see blog July 15, 2009). These waves, which in contrast to P- and S-Waves do not travel through the interior of the Earth, race along its surface instead. They also come in two flavors which differ in at least two aspects: the particle motion they generate and the speed with which they circle the globe.
One type of surface wave was first mathematically described by John William Strutt, a young British physicist. He later not only followed his father into nobility as the third Baron Rayleigh, but also became one of the most prominent researchers of his time (1842-1919) and was honored for his work with the Nobel prize in 1904. At first glance, the Rayleigh waves look like the surface waves in the water (see blog July 15, 2009), but when observing carefully, one will notice that their respective particle motions are different. In a water wave, each particle makes a circular motion in the direction of the propagation of the wave. In a Rayleigh wave, the particles make an elliptical movement against the propagation direction. Hence, their motion is retrograde (see Figure 1). A very nice animation of the difference in particle motion between a water and a Rayleigh wave can be found here.
The second type of surface wave was discovered in 1911 by another Englishman, Augustus Edward Hough Love. Although not quite as famous as Lord Rayleigh, Love nevertheless held the position of Professor for Natural Philosophy at Oxford University for 41 years. Love found that the particles in the waves named after him do not move in a rotating fashion at all. Instead, they jerk back and forth perpendicular to the direction of wave propagation (see Figure 2). They are therefore similar to an S-wave (see blog September 10, 2008).
|Lord Rayleigh||A.E.H. Love|
The speed with which both types of waves circle the globe is truly mind boggling. Love waves race around the Earth at almost 10,000 miles per hour. Their relatives, the Rayleigh waves, lag behind slightly, but still speed at about 7800 miles an hour. It seems that only the International Space Station is faster. As of this writing, the 13 astronauts aboard ISS plough through their orbit at 16,218 miles per hour (hra042).