San Andreas Fault Observatory at Depth observatory reveals new insights into the 'earthquake machine'
SAFOD is a major research component of EarthScope, a National Science Foundation-funded program being carried out in collaboration with the US Geological Survey (USGS) to investigate the forces that shape the North American continent and the physical processes controlling earthquakes and volcanic eruptions. The SAFOD project is located near the tiny Central California town of Parkfield on the notorious San Andreas Fault-an 800-mile-long earthquake-prone rift that runs between the Pacific and North American tectonic plates. SAFOD entails the drilling of a three-mile long borehole on the Pacific plate just west of the fault. In August 2005, project leaders achieved their goal of drilling through the active fault zone into the North American plate, reaching a depth of 10,081 feet (1.91 miles). When completed in 2007, the observatory will be equipped with dozens of underground seismological instruments installed in a segment of the fault where small earthquakes are born. "We passed an important milestone in August when we finally drilled through an active section of the San Andreas Fault at depth," said Mark Zoback, the Benjamin M. Page Professor in Earth Sciences at Stanford University. "Although the instrumentation won't be fully in place for another year-and-a-half, SAFOD is already giving us unprecedented access to the fault." One of the more intriguing discoveries in seismology in recent years involves the relationship between earthquakes and non-volcanic tremors-low-frequency seismic signals that emanate deep underground in areas where tectonic plates meet. Earthquakes generate distinct surface signals known as compressional and shear waves (P and S waves) that usually last no more than 30 seconds. But non-volcanic tremors-which can originate more than 20 miles below the surface and last up to 20 minutes-seem to occur weeks or months in advance of actual earthquake events, offering scientists a promising new tool for earthquake prediction. From 2000 to 2003, before SAFOD drilling began, researchers from the University of California-Berkeley recorded 110 non-volcanic tremor events along the San Andreas Fault near Parkfield. Their data showed a remarkable correlation between the frequency of tremor events and the subsequent earthquakes. The scientists discovered that an increase or decrease in the number of tremor events over a certain period of time would usually be matched by a similar increase or decrease in the number of micro-earthquakes (magnitude 2.1 or smaller) several weeks later, suggesting a possible causal relationship. Source: Stanford Department of Geophysics
•Date: 6th Dec 2005 • Region: US • Type: Article •Topic: Emergency planning |
