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California earthquakes: why only shallow focus?
Science (New York, N.Y.)
|June 26, 1970
Summary
Laboratory experiments show that increasing temperatures cause frictional sliding in granite and gabbro to shift from stick-slip to stable sliding. This temperature-dependent behavior may explain why earthquakes disappear at shallow depths in California.
Area of Science:
- Geophysics
- Rock Mechanics
- Seismology
Background:
- Frictional sliding on geological faults is a key process in earthquake generation.
- Understanding the physical conditions that govern fault behavior is crucial for seismic hazard assessment.
Purpose of the Study:
- To investigate the effect of temperature on frictional sliding behavior in granite and gabbro under simulated crustal pressures.
- To explore the implications of temperature-dependent frictional properties for earthquake occurrence at varying depths.
Main Methods:
- Laboratory experiments were conducted on sawcut and natural fault samples of granite and gabbro.
- Frictional sliding tests were performed at pressures ranging from 1 to 5 kilobars.
- Temperature was systematically increased from 200 to 500 degrees Celsius during sliding.
Main Results:
- Frictional sliding in both granite and gabbro exhibited a marked temperature dependence.
- At pressures of 1-5 kilobars, a transition from stick-slip to stable sliding was observed as temperature increased.
- This transition occurred within the 200-500 degrees Celsius range.
Conclusions:
- Elevated temperatures significantly alter the frictional characteristics of crustal rocks.
- Increased temperature with depth could lead to the cessation of earthquake activity observed at shallow levels.
- The findings provide a potential mechanism for the abrupt disappearance of earthquakes in certain geological settings, such as California.
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