Updated January 17th, 2010
|Almost ten earthquakes with magntitudes of 6.0 and larger occured in the greater region of the Mendocino Triple Junction along California's North Coast during the last 30 years. The offshore epicenter of Saturday's quake is marked in red.|
The magnitude 6.5 earthquake, which shook large portions of Northern California and Southern Oregon during the late afternoon of 9 January 2010, was the strongest temblor to hit our state in more than six years. The last strong earthquake occured shortly before Christmas in 2003, when a magnitude 6.6 quake rattled the area around Hearst Castle, the Paso Robles Region and the coastal towns of San Luis Obispo County. Two people were killed in the San Simeon Earthquake, as the 2003 temblor is now known. It caused widespread structural damage in Paso Robles, where several buildings collapsed. In contrast, only a few people were injured in Saturday's quake. The damage was largely confined to coastal cities in Humboldt County. A report by the Redwood Coast Tsunami Workgroup describes damage to residential and commercial buildings, as well as to roads and other infrastructure totaling almost $30 million.
While the North Coast escaped major damage this time, the region between Cape Mendocino and the Oregon border sure had its share of deadly quakes in the last half century. One person was killed on December 21, 1954, when a magnitude 6.5 temblor struck a few miles East of Arcata. On November 8, 1980 a 7.2 quake occured off the coast of Humboldt County. Two people were injured, when portions of on overpass of Highway 101 collapsed on the train tracks below. A whole series of strong quakes, ranging in magntiude between 7.2 and 6.5 rattled the area in late April 1992. Ninetyfive people were injured and the shaking caused major damage and several landslides in and near the towns of Ferndale, Honeydew, Petrolia, Rio Dell, and Scotia. The strongest of these Petrolia Earthquakes, as the series is now commnonly known, generated a tsunami, with wave heights of up to four feet in Crescent City. Hours later, this tsunami was even detected in Hawaii, where it had a run-up of less than a foot. The last strong earthquake in this area occured on June 15, 2005, when a magnitude 7.2 temblor rattled the sea floor roughly 100 miles offshore. Although the quake was felt in Humboldt County, no damage occured on land.
Taken all of these larger earthquake and the thousands of smaller ones together, the region around Cape Mendocino has a very clear distiction among seismologists. When measured by the release of seismic energy, it is in fact the most active region of California, beating by far the active faults of the Bay Area and the seismically notorious Los Angeles Basin and its surrounding mountains. Read why the Mendocino Triple Junction makes the northern Coast the seismicity capital of our state in one of our earlier blogs. For an in-depth seismological analysis of Saturday's quake, see the front page of the BSL websites. (hra048)
After a hiatus of almost three months, the blogger was shaken back into action this morning by the strongest temblor to hit the Bay Area in nearly a year. At 10:09 am PST a magnitude 4.1 earthquake occured on the Calaveras Fault with its epicenter roughly six miles eastnortheast of Milpitas. Its hypocenter - the starting point of any earthquake - lay at a depth of approximately seven miles beneath the surface.
In today's interconnected world, it is very easy even for lay people to get immediate information about each tremor one feels. No, I am not talking about the emotional outbursts which hit the Twitters and Facebooks of the cyberworld ("Wow, how did I feel it shake!") after each reasonably sized shaker. I mean the reliable information, which a dedicated cadre of Bay Area earth scientists puts together with the help of mostly automated computer algorithms and which are posted on the web, as soon as they are available. My first stop while cybershopping for information about recent Bay Area earthquakes is always the map of the latest epicenters . It is published jointly by the United States Geological Survey (USGS) in Menlo Park and the Berkeley Seismological Laboratory, which also hosts this blog. Color coded and sorted by size, you will easily find the latest temblors. The map is updated immediately when a new earthquake occurs and carries all quakes, which occured during the week before the latest event. Click on one of the squares and you will be directed to much more detailed information about this specific quake and its location within the tectonic framework of the Bay Area.
|Did you feel it? Map for Jan/7/2010 Milpitas Earthquake. (Click to view larger image.)|
But the advances of web communication do not only benefit the public while looking for earthquake information. You can also be part of the advancement of seismology and help the researchers, by reporting on what you have felt during the shaking of an earthquake. The USGS has set-up a website under the headline "Did you feel it?", where you can access a questionaire. You will be asked a dozen or so questions about how and where you felt it, how strong you experienced the shaking and if you detected any damage in your building or in structures around you. It does not take more than five minutes, to fill out the online form. The information is handled confidentially, although you may enter your name and communication parameters, if you think, that your earthquake observations warrant more attention by the researchers.
You will ask, how such responses can advance science. Because neither the Earth nor its earthquake faults are homogeneous and simple, seismology has to deal with lots of variables. No two earthquakes are the same and the shaking induced by seismic waves can vary consideraby even over a short distance. The topography, the type of soil and the structure of the underlying base rock determine, how strong a specific location rattles during a temblor. As scientists cannot place a seismometer on every square yard of the Bay Area and because such variations cannot be computed theoretically, the researchers need observations from the field - and that means from you, the person who has felt the quake. A lot of people contribute to the website already. In the first hour after today's quake, more than 15000 had submitted their observations, including the blogger. But because a wider response is always welcome, hit the keyboard the next time you feel a quake and look for "Did you feel it?" (hra047).
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).