A new study by University of Southern California researchers have determined how rock-melting, deep underground forces seem to be the cause of tremors that occur along a portion of the San Andreas Fault that is well-known for earthquakes. The study gives a new explanation for the causes of these tremors.

Earthquake Physics Research

The new research stems from the latest scientific field known as earthquake physics. It investigates the physics and dynamics of temblors from the perspective of looking from the bottom upward, instead of the opposite. It focuses on fluids, friction, and underground rocks.

According to USC Dana and David Dornsife College of Letters, Arts & Sciences Earth sciences assistant professor Sylvain Barbot, the majority of the seismic movements that occur in the state come from the upper ten miles of our planet's crust. However, some of the tremors occurring below San Andreas originate from a much deeper area.

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The New Study

The new study is published in the journal of Science Advances. According to Barbot, scientists still mostly do not know the mechanisms and causes of these tremors. In their new study, the researchers demonstrate how a deep portion of the San Andreas Fault often breaks up and melts its host rocks, which produces the mysterious seismicity.

The importance of this study is in its advancement of our understanding of the mechanisms and predictions of the likely occurrences of earthquakes, including the underground forces which trigger them.

Such an understanding helps agencies make better emergency preparedness programs, policies, and even building codes, especially in areas with lots of earthquakes. The study's findings are also helpful in applications in engineering like hydraulic fracturing, in which rock temperatures quickly change.

The Study Site

The researchers selected Parkfield for the study because it is among the world's most monitored earthquake epicenters. It is being sliced by the San Andreas Fault, with regular significant earthquakes with a 6 level magnitude.

According to the USGS, these quakes occurred specifically in the following years: in 1857 and 1881 in the 19th century; and five years in the 20th century, particularly in 1901, in 1922, in 1934, jumping to 1966, and finally in 2004.

There are also small temblors deeper down which occur at a regular interval of a few months.

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Methodology

The researchers conducted experiments in rocks and made mathematical models, as well as simulations on deep fault activity dynamics at a period of three centuries to look at the behavior and sizes of ruptures.

Study results

The study found that after a significant earthquake, the converging tectonic plates beneath the boundary of the fault undergo a kind of settling, gliding past one another with a slow slipping which does not significantly disturb the ground above.

However, there comes gradual friction of the quartz and granite chunks comprising the bedrock of the crust, which produces heat. When this heat increases and reaches levels of 650º F, it makes the rocks more fluid, which makes them slide and create even more friction - a positive feedback loop. As they slip against one another more quickly, this triggers a quake.

Barbot says that they are attempting to explain every kind of motion these forces make, not only with regards to tremors and earthquakes along the San Andreas Fault but with geological forces in general.

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