That is the indication from research released this year that showed a relationship between two major distant earthquakes and California’s San Andreas fault.
The research project was conducted by US Geological Survey geophysicists, David Shelly and David Hill with Georgia Institute of Technology earth scientists, assistant Professor Zhigang Peng, and graduate student, Chastity Aiken.
In the research paper, published in Nature Geoscience magazine in May this year, the scientists proved that distant earthquakes had an impact on the San Andreas fault in central California.
Distant earthquakes related to fault slip
The study showed that earthquakes, such as the M9.1 earthquake in Sumatra that triggered tsunami in 2004, and the M8.8 earthquake in Chile in 2010, had caused parts of the San Andreas fault deep underground, to suddenly slip, setting off small tremors.
“There is still no evidence that large quakes in one part of the world can set off large quakes right away on different faults thousands of miles away,” said USGS scientist, Dr David Shelly who is a member of the USGS Volcano Science Center at Menlo Park in California.
He said the research did show that “big earthquakes are triggering an acceleration of the fault that can last for hours or days”.
Decade of records shows earthquake impacts
“You are triggering something that lasts longer than the seismic waves that are coming through. Over time, that can increase the stress on neighboring patches on the fault, which can generate tremors."
The research on the San Andreas fault near Parkfield (in Monterey County), was looking at the impact of major earthquakes from 2001 to 2010. For the past 10 years, the USGS had installed sensitive scientific equipment deep in the fault to get measurements before, during, and after earthquakes occur.
The researchers found that 17 earthquakes that had occurred around the world in the past decade, had caused the fault to suddenly slip in its deep reaches, between 10 and 20 miles underground.
Triggered fault detectable
The M9.0 quake that rocked Japan in March this year, was not part of the study, but its seismic waves did cause minor tremors on the San Andreas, said Dr Shelly.
The study abstract states that, “the passage of radiating seismic waves generates transient stresses in the Earth’s crust that can trigger slip on faults far away from the original earthquake source.”
This triggered fault slip is detectable in the form of both earthquakes and seismic tremor. The significance of the triggered events remains controversial because they are often delayed, occurring long after the triggering stress has passed.
The study observed tremor on the San Andreas fault that was initiated by passing seismic waves, but moved along the fault at a much slower rate than the radiating seismic waves.
Slow slip in creep event
The scientists concluded that the ‘migrating tremor records triggered slow slip of the San Andreas fault as a propagating creep event’
They found that the triggered tremor and fault creep could be initiated by distant earthquakes as small as magnitude 5.4 and could persist for several days after the seismic waves had passed.
The researchers also noted that creep events in other locations could sometimes trigger earthquakes. Although this study was focused on triggered tremor, not triggered earthquakes, the scientists suggested that prolonged triggered creep episodes could be relevant for both phenomena.
The triggered creep episodes could also provide a physical explanation for the time delay commonly observed between passing seismic waves and distantly generated earthquakes.
Reference:
David R. Shelly, Zhigang Peng, David P. Hill & Chastity Aiken, (2011) ‘Triggered creep as a possible mechanism for delayed dynamic triggering of tremor and earthquakes’, Nature Geoscience 4, pp 384 - 388.
See also; Earthquake Hotspots Flare on Pacific Rim
