|Enriquillo Plaintain Garden fault landform SSW of Port-au-Prince (Image from Google Maps)|
One of the reasons, that the massive relief effort after Tuesday's devastating earthquake in Haiti is only very slowly getting into high gear, is the fact, that many roads are impassable and numerous bridges are destroyed. First responders and rescue teams use satellite images to assess the destruction and accessibility of various neighborhoods in Port-au-Prince and in the outlying towns and villages. In addition, the view from space also helps Earth scientists to gain insight into the tectonic movements, which caused the earth crust to break so violently on Hispaniola.
As described in Wednesday's blog, the focus of Tuesday's magntiude 7 temblor lay at a depth of 8 miles on the Enriquillo-Plaintain Garden fault, which cuts through the southern section of Hispaniola. It runs along an almost straight East-West trending line through the mountains southwest of Port-au-Prince. The fault scarp is clearly visible in the satellite picture (Figure 1), which was taken last year from an imaging satellite. If you zoom in on it, you can see that the scarp is actually a linear valley with a stream running through it.
|Figure 2: Three-dimensional elevation model of the region around Port au Prince. The mountains are not snow-capped, rather the color is an indication of the elevation (green low, white high). (Image from Jet Propulsion Laboratory)|
This scarp is also clearly visible on the maps which Nasa created from the data gathered by the Space Shuttle Radar Topography mission. During several flights of the Space Shuttle an automatic radar camera took pictures of the Earth's surface underneath the Shuttle's orbit. For each spot, two pictures were taken from slightly different angles. Various computer codes can be used to combine the two images. Figure 2 shows a three dimensional map of the region around Port-au-Prince under an oblique view from the northwest. The scarp in the mountains is the prominent feature in the center of the image (white arrow). Figure 3 shows a different combination of the two radar images from Haiti. They were merged into an anaglyph. If you happen to have a pair of those crazy 3-D glasses handy, (red for left eye, cyan for the right), you will also clearly see the fault.
In the meantime, Gavin Hayes from the USGS's National Earthquake Information Center in Boulder, CO, has used dozens of seismic recordings to compute the actual movement along the Enriquillo-Plaintain Garden fault. According to his calculations, the fault ruptured exactly underneath the visible scarp over a length of approximately 25 miles. It broke in a left-lateral movement, as indicated by the arrows in figure 3. Within roughly 15 seconds, the northern block had shifted by almost 9 feet to the left, while the southern block had moved a similar distance to the right, resulting in a total fault slip of about 18 feet. Such values are typical for a magnitude 7 quake. During the Bay Area's 1989 Loma Prieta shaker the San Andreas Fault ruptured over a length of approximately 22 miles. (hra050)
|Figure 3: Radar anaglyph image of the Port-au-Prince area. Red arrows indicated the direction of plate movement.(Image from Jet Propulsion Laboratory)|
|Figure 1: Map showing the tectonics of the Caribbean Region (Click to view larger image.)|
The devastating earthquake, which shook the Caribbean island of Hispaniola and a large area of the Greater Antilles on Tuesday afternoon, may have cost tens of thousands of lifes. Haiti's capital Port-au-Prince was particularly hard hit, because the distance from the focus of the shallow quake to the city center was less than 12 miles. Tuesday's quake had a magnitude of 7.0, which makes it comparable in size to the the last big temblor in the Bay Area, the Loma Prieta quake of 1989 (see blog 16 October 2009).
The Caribbean region, often sold by tourism promoters as "tropical paradise", is no stranger to natural disasters. Four hurricanes made landfall in Haiti alone during the 2008 season, causing widespread destruction. In addition, earthquakes and volcanic eruptions are almost daily occurences, because most Caribbean islands lie close to a major plate boundary. In this region, the giant westward drifting North American Plate encounters the much smaller Caribbean Plate, resulting in a complex pattern of interaction. In the eastern region this plate collision leads to the formation of a subduction zone, which in turn gives rise to the gentle arc of the Lesser Antilles. As in many other subduction zones most of these islands are of volcanic origin. During the last 15 years the almost continuous eruption of the Soufriere Hills volcano has destroyed more than half of the island of Montserrat. A gigantic explosion of Mont Pelée volcano on Martinique in 1902 cost more than 65000 lives.
|Figure 2: Faults on the Island of Hispaniola|
The northern part of the plate boundary lies close to the islands of Puerto Rico, Hispaniola und Jamaica. There the plates rub against each other horizontally, very similar to the plate movement which governs California. But while the relative motion along the major faults in our state is right-lateral, North America slides westwards with respect to the Caribbean, resulting in a left-lateral movement along the fault (see black arrows in figure 1).
While this fault line is clearly defined by a deep sea trench near Puerto Rico, the boundary zone is much wider under Hispaniola. The larger Septentrional Fault runs along the north coast of the island, while a secondary fault system, the so called "Enriquillo-Plaintain Garden fault" cuts through the southern part (see figure 2). Tuesday's quake was the first big temblor to occur on this fault since 1860. It is also likely that the very large temblor, which devastated the Jamaican city of Port Royal (now Kingston) in 1692, occured on a western extension of this fault system.
One of the reasons for the wide spread damage of Tuesday's quake are the poor construction practices in Haiti, the western hemisphere's poorest country. Even the Presidential Palace, the Parliament Building and the headquarters of the UN peace keepers in Port-au-Prince were destroyed, and so were many schools and hospitals in several towns in the western part of Haiti. (hra049)
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)