What electrical measurements can say about changes in fault systems

Area of Science:

• Geophysics
• Earth Science
• Seismology

Background:

• Earthquake zones in the upper crust exhibit higher electrical conductivity than surrounding rocks.
• Magnetotelluric (MT) measurements provide insights into the lower crustal zone, especially for fault zones near coastlines.
• Oceanic electric currents can leak into the mantle, with the lower crust typically acting as a resistive barrier.

Purpose of the Study:

• To investigate the electrical properties of fault zones and their relationship to crustal strength.
• To understand the role of lower crustal conductivity and fluid pressure in earthquake processes.
• To analyze changes in telluric signals across the San Andreas Fault over time.

Main Methods:

• Utilizing electrical geophysical measurements, including Magnetotelluric (MT) surveys.
• Analyzing long-period electric currents originating from the ocean.
• Conducting telluric measurements across the San Andreas Fault over an extended period (1979-1990).

Main Results:

• The Loma Prieta earthquake zone showed significant ocean electric current leakage, indicating higher conductivity in the underlying lower crust.
• Telluric signals across the San Andreas Fault exhibited changes starting in 1985, suggesting improved connectivity of lower crustal fluids.
• Increased conductivity in the lower crust may be linked to pre-existing, poorly connected water, which could influence fluid pressure in the upper crust.

Conclusions:

• The lower crust beneath fault zones may possess higher conductivity due to interconnected fluids, not necessarily recent water infiltration.
• Changes in lower crustal fluid connectivity could lead to fluid flow into the upper crust.
• This fluid migration may alter the mechanical strength of upper crustal fault zones, impacting earthquake behavior.