Quake activity along the San Andreas fault is picking up

Lassen Peak has the distinction of being the only volcano in the Cascades other than Mount St. Helens to erupt during the 20th century. On May 22, 1915, an explosive eruption at Lassen Peak devastated nearby areas and rained volcanic ash as far away as 200 miles (320 km) to the east.[5] This explosion was the most powerful in a 1914"17 series of eruptions that were the last to occur in the Cascades before the 1980 eruption of Mount St. Helens in Washington. Lassen Volcanic National Park was created in Shasta County, California to preserve the devastated area and nearby volcanic geothermal features.

Unlike most lava domes, Lassen is topped by craters. A series of these craters exist around Lassen's summit, although two are now covered by solidified lava and sulfur deposits. Lassen is the largest of a group of more than 30 volcanoes that have erupted over the past 300,000 years in the Lassen Volcanic Center.
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Lassen is the southernmost in the chain of 18 large volcanic peaks that run from southwestern British Columbia to northern California. The peaks formed in the past 35 million years as the Juan De Fuca plate and the tiny Gorda plate to its south have been pulled under the overriding North American plate. As the oceanic crust in the Juan de Fuca plate melts under the pressure, it creates pools of lava that drive up the Cascade Range and power periodic eruptions.

Mysterious tremors detected on San Andreas Fault
Jul 9 02:01 PM US/Eastern
By ALICIA CHANG
AP Science Writer

LOS ANGELES (AP) - Scientists have detected a spike in underground rumblings on a section of California's San Andreas Fault that produced a magnitude-7.8 earthquake in 1857.

What these mysterious vibrations say about future earthquakes is far from certain. But some think the deep tremors suggest underground stress may be building up faster than expected and may indicate an increased risk of a major temblor.

Researchers at the University of California, Berkeley, monitored seismic activity on the fault's central section between July 2001 and February 2009 and recorded more than 2,000 tremors. The tremors lasted mere minutes to nearly half an hour.

Unlike earthquakes, tremors occur deeper below the surface and the shaking lasts longer.

During the study period, two strong earthquakes hit"a magnitude-6.5 in 2003 and a magnitude-6.0 a year later. Scientists noticed the frequency of the tremors doubled after the 2003 quake and jumped six-fold after 2004.

Tremor episodes persist today. Though the frequency of tremors have declined since 2004, scientists are still concerned because they are still at a level that is twice as high as before the 2003 quake.

The team also recorded unusually strong rumblings days before the 2004 temblor.

Results of the research appear in Friday's issue of the journal Science. The work was funded by the U.S. Geological Survey and National Science Foundation.

"The fact that the tremors haven't gone down means the time to the next earthquake may come sooner," said Berkeley seismologist and lead researcher Robert Nadeau.

Nadeau first discovered tremors deep in the San Andreas Fault in 2005. Before that, the phenomenon was thought only to occur in Earth's subduction zones, where one tectonic plate dives beneath another.

USGS seismologist Susan Hough found the latest observations intriguing, but said it's too soon to know what they mean.

"We don't have enough data to know what the fault is doing in the long term," said Hough, who had no part in the research.
http://www.breitbart.com/article.php?id=D99B30O81&show_article=1

The Cascades volcanic arc, running from Vancouver in British Columbia down through Washington and Oregon and into northern California, was formed by the convergence of three tectonic plates with the North American plate and their subduction beneath it. As the plates collided and ground against one another, turning rock to molten lava, massive areas of land on the American west coast were wedged upwards, creating spectacular ridges and mountain ranges with about 4,000 volcanic vents, including 18 major volcanoes.

The ongoing subduction beneath the North American plate also accounts for the frequent earthquakes in the region. Of the 10 most dangerous volcanoes in the US, seven are in the Cascades (nine are in the top 12). The region accommodates a population of 10m, all of whom can conceivably be affected by volcanic activity.

Could be harmonic tremors, that could be bad news and it also could merely mean that energy is being redistributed along several pathways.

Harmonics can keep going for days and then go quiet before a major strain.

Friday, July 10, 2009


CHINA

Hundreds Injured In Earthquake


A magnitude-6.0 earthquake rocked southwestern China on Thursday evening, injuring at least 336 people and collapsing 10,000 houses, state news media said.

The temblor, centered in Yunnan province's Yao'an county, damaged an additional 30,000 homes, the New China News Agency said.

Thirty people suffered severe injuries, the news service reported.

Yunnan is a quake-prone, mountainous region bordering Sichuan province, where a magnitude-7.9 quake last year left almost 90,000 people dead or missing.