Mar 31, 2011 08:30 GMT  ·  By
This is a topographic radar image of a section of the San Andreas fault line
   This is a topographic radar image of a section of the San Andreas fault line

A team of investigators working on the US West Coast managed to drill a very deep borehole through the San Andreas fault line, one of the most famous and dangerous in the world. The group was able to determine that a soft spot exists in the fault line, and that it is mostly made of clay.

Researchers were trying to determine the unusual behavior that the fault line exhibits in certain regions of California, one of the most earthquake-prone states in North America. The new discovery sheds more light on the processes at work in underlying the weird behavior.

Interestingly, the San Andreas fault line is entirely locked up at its north and south endings. This translates into elevated earthquake risks for Los Angeles and San Francisco. However, some 200 miles (320 kilometers) northwest of LA, near Parkfield, the situation is quite different.

The fault line here is moving very slowly, but it's moving nonetheless. “This is a special part of the San Andreas fault,” explains scientist David Lockner, who is based at the US Geological Survey (USGS).

“It seems weak enough that it slides slowly and continuously, rather than in a jerky motion.” adds the expert, who is also the lead author of the new study on the fault line's “soft spot.”

As part of the San Andreas Fault Observatory at Depth initiative, the researcher and his team drilled several miles beneath the ground at this location, in a bid to determine the interplay of factors that was causing the gradual slip.

The area was identified as holding the key to solving this mystery because regular earthquakes, of magnitude 1 to 4, occur here regularly. However, larger tremors are almost unheard of. During the research, the drill team reached a depth of 2.7 kilometers (1.7 miles).

At that location, temperatures reach about 115 degrees Celsius (239 degrees Fahrenheit), Our Amazing Planet reports. Samples retrieved from this depth revealed the presence of a magnesium-rich clay mineral called saponite, which appeared to make up most of the material experts found.

Lockner conducted this study as part of a larger, international initiative to get a better understanding of the mechanisms that lead to the appearance of earthquakes. Details of his work appear in the March 23 online issue of the top scientific journal Nature.

“This is one part of a broad effort to understand the properties of the San Andreas system. While we're concentrating on the lab measurements, there are others working on field measurements,” he explains.

This line of study became more important over the past couple of years, given the large number of massive tremors that struck China, Haiti, Chile and now Japan.