Category
Earthquakes in the Midwest
Posted on Tuesday, April 22, 2008 @ 10:29 am by Dave Schumaker
In response to last week’s M5.2 earthquake in Illinois, the University of Cincinnati has published an interview with Professor Attila Kilinc, a geologist in the school’s Department of Geology. The interview tackles questions relating to the risk and history of earthquakes in the Midwest. It also tackles the very important question of whether or not California will fall into the ocean.
Q: How common are earthquakes in the Midwest and was the severity of this tremor a first for this area”
A: Between 1776 and the present, 170 earthquakes have been charted in Ohio of magnitude 2.0 or greater. There have been at least 150 below magnitude 2.0, which averages out to approximately 1½ earthquakes a year. This latest was not a first, severity-wise: Several others measured inbetween 5.3 and 5.4; in 1980, for example, an earthquake in Sharpsburg, Ky., measured 5.2.
[Via Eurekalert]
102 Years Ago…
Posted on Friday, April 18, 2008 @ 12:03 am by Dave Schumaker
One hundred and two years ago today, at 5:12am, the earth around the San Francisco Bay Area started to violently shake thanks to a ~M7.8 earthquake. A minute and a half later, buildings lay in ruins, while north of the city in Marin and Tomales Bay, fence posts laying across the “newly discovered” San Andreas fault (which didn’t have a name at that time) were offset by nearly 30 feet. The mayhem was just beginning however, as fires from knocked over furnaces and stoves began to rage out of control and completely destroy much of the city.
Despite the tragic loss of life in San Francisco (up to 3,000 people by some estimates), some good did result from the catastrophe. It marked the dawn of earthquake science and seismology. The Lawson Report pegged the cause of the 1906 earthquake on the San Andreas fault. You can actually read the Lawson Report (published in 1910) online here: Volume I, Part I; Volume I, Part II; and Volume II.
The preliminary report (published May 31, 1906) details the initial recollection of the San Andreas Fault.
One of the remarkable features of the Coast Ranges of California is a line of peculiar geomorphic expression which extends obliquely across the entire width of the mountainous belt from Mendocino County to Riverside County. The peculiarity of the surface features along this line lies in the fact that they are not due, as nearly all the other features of the mountains are, to atmospheric and stream erosion of the uplifted mass which constitutes the mountains, but have been formed by a dislocation of the earth’s crust, or rather a series of such dislocations, in time past, with a differential movement of the parts on either side of the plane of rupture. In general this line follows a system of long narrow valleys, or where it passes through wide valleys it lies close to the base of the confining hills, and these have a very straight trend; in some places, however, it passes over mountain ridges, usually, at the divide separating the ends of two valleys; it even in some cases goes over a spur or shoulder of a mountain. Along this line are very commonly found abrupt changes in the normal slope of the valley sides giving rise to what are technically known as scarps. These scarps have the appearance of low precipitous walls which have been usually softened and rounded somewhat by the action of the weather. Small basins or ponds, many having no outlet, and some containing saline water, are of fairly frequent occurrence and they usually lie at the base of the small scarps. Trough-like depressions also occur bounded on both sides by scarps. [...]
I love living in San Francisco, but it baffles my mind that one day this will happen again. And somehow, my choice of living here means I accept that fact and am okay with it. Crazy.
[Via USGS]
New Forecast for California Earthquakes
Posted on Monday, April 14, 2008 @ 12:22 pm by Dave Schumaker
The USGS released the results of a new study on the probabilities for earthquakes throughout the state of California. Using new modeling techniques, they have found that the possibility of a large earthquake hitting the state of California to be greater than originally thought.
One notable aspect of the study is that it’s the first time that earthquake hazards can be meaningfully compared between Northern and Southern California.
California has more than a 99% chance of having a magnitude 6.7 or larger earthquake within the next 30 years, according scientists using a new model to determine the probability of big quakes.
The likelihood of a major quake of magnitude 7.5 or greater in the next 30 years is 46%-and such a quake is most likely to occur in the southern half of the state.
[...]
The consensus of the scientific community on forecasting California earthquakes allows for meaningful comparisons of earthquake probabilities in Los Angeles and the San Francisco Bay Area, as well as comparisons among several large faults.
The probability of a magnitude 6.7 or larger earthquake over the next 30 years striking the greater Los Angeles area is 67%, and in the San Francisco Bay Area it is 63%, similar to previous Bay Area estimates. For the entire California region, the fault with the highest probability of generating at least one magnitude 6.7 quake or larger is the southern San Andreas (59% in the next 30 years).
For northern California, the most likely source of such earthquakes is the Hayward-Rodgers Creek Fault (31% in the next 30 years). Such quakes can be deadly, as shown by the 1989 magnitude 6.9 Loma Prieta and the 1994 magnitude 6.7 Northridge earthquakes.
The latest episode of the USGS Corecast talks about this study and features an interview with USGS geophysicist Tom Parsons.
[Via USGS]
Earthquake Swarm off Oregon Coast
Posted on Monday, April 14, 2008 @ 8:44 am by Dave Schumaker
I may potentially have my computer issues resolved, we shall see. Regardless, I’m back for now! In the past week, there has been a flurry of earthquake activity off the coast of Oregon. While earthquakes in this part of the world are normal, the type and pattern of the earthquakes are puzzling scientists.
The earthquakes are located about 150 nautical miles southwest of Newport, Ore., in a basin between two subsurface “faulted” geologic features rising out of the deep abyssal sediments. The hill closest to the swarm location appears to be on a curved structure edging out in a northwestern direction from the Blanco Transform Fault toward the Juan de Fuca ridge, Dziak said.
Analysis of seismic “decay” rates, which look at the decreasing intensity of the tremors as they radiate outward, suggest that the earthquakes are not the usual sequence of a primary event followed by a series of aftershocks, Dziak said.
“Some process going on down there is sustaining a high stress rate in the crust,” he pointed out.
An article from the Associated Press says that the pattern is similar to those associated with volcanic events. The problem is, there are no known volcanoes in the area.
Scientists don’t know exactly what the earthquakes mean, but they could be the result of molten rock rumbling away from the recognized earthquake faults off Oregon, said Robert Dziak, a geophysicist for the National Oceanic and Atmospheric Administration and Oregon State University.
There have been more than 600 quakes over the past 10 days in a basin 150 miles southwest of Newport. The biggest was magnitude 5.4, and two others were more than magnitude 5.0, OSU reported.
On the hydrophones, the quakes sound like low thunder and are unlike anything scientists have heard in 17 years of listening, Dziak said. Some of the quakes have also been detected by earthquake instruments on land.
[Via ScienceDaily]
Possible Cascadia and San Andreas fault link
Posted on Thursday, April 3, 2008 @ 2:11 pm by Dave Schumaker
A new study published in the Bulletin of the Seismological Society of America shows a possible link to seismic activity in the southern Cascadia Subduction zone and the northern San Andreas fault.
Seismic activity in southern Cascadia has a recurrence interval of approximately 220 years, while the northern San Andreas has a recurrence interval of approximately 200 to 240 years. Researchers at Oregon State University identified 15 turbidites in northern California associated with activity on the San Andreas fault. Of these 15, 13 of them occur at virtually the same time as earthquakes in the Cascadia subduction zone.
In a parallel study, they found that during the same period, 13 of these 15 San Andreas earthquakes occurred at almost the same time as earthquakes along the southern Cascadia Subduction Zone, which stretches from northern Vancouver Island to northern California. The marine and land paleoseismic record suggest a recurrence rate of approximately 220 years for the southern Cascadia fault, which is substantially shorter than the 600-year cycle suggested by previous research for full ruptures in Cascadia.
The Cascadia earthquakes also preceded the San Andreas earthquakes by an average of 25 to 45 years. “It’s either an amazing coincidence or one fault triggered the other,” said Goldfinger. The generally larger size of the Cascadia earthquakes, and the timing evidence suggests Cascadia may trigger the San Andreas Two seismic events on the San Andreas were apparently not associated with Cascadia, including the 1906 earthquake which followed the previous Cascadia earthquake by approximately 200 years.