Motion Documented at the San Andreas Fault

Scientists from the Earthscope Plate Boundary Observatory, the University of Hawaii. The University of Washington and the Scripps Institution of Oceanography at the University of Southern California in San Diego have been analyzing data from the San Andreas Fault. Their goal was to differentiate broad regional tectonic motion from shorter–scale motions of the local crust.

Using an array of GPS instruments to detect motion at the fault, they implemented a comprehensive statistical technique. The scientists studied the release of stress to estimate the impact of a major quake on the nearby region. Southern California has historically been hit by a major earthquake every century or so.

Sam Howell, doctoral candidate in geophysics from the University of Hawaii and the study’s lead author says,

While the San Andreas GPS data has been publicly available for more than a decade, the vertical component of the measurements had largely been ignored in tectonic investigations because of difficulties in interpreting the noisy data. Using this technique, we were able to break down the noisy signals to isolate a simple vertical motion pattern that curiously straddled the San Andreas fault.

The recent study shows that the movements are consistent and coherent. They noted that every year, along a 124-mile stretch of the fault, GPS measurements have shown a .08 shift per year.

This shift or uplift is caused by the grinding of the Pacific and North American tectonic plates. This causes the Las Angeles basis to sink while at the same time a part of San Bernardino County is rising at the same rate.

These shifts minimize the stress. However, large sections of the fault haven’t moved significantly in 150 years and some sections haven’t moved for three centuries. The longer the time period between earthquakes, the more powerful and dangerous the next one. As each year passes, the odd of the “big one” hitting increases.

Howell says,

It’s pretty much impossible to say when the next one will happen.

The last major quake to hit California was the great San Francisco earthquake in 1906, which was a destructive 7.8. This happened when the fault’s northern section slipped. The southern section of the fault is being monitored closely.

Thomas Jordan, director of the Southern California Earthquake Center says,

The springs on the San Andreas system have been wound very tight. And the southern San Andreas fault in particular looks like it’s locked, loaded and ready to go.