Microscopic Evolution of Laboratory Volcanic Hybrid Earthquakes

Area of Science:

• Geophysics
• Rock Mechanics
• Acoustic Phonon Emission

Background:

• Understanding fluid-microscopic defect interactions is crucial for cracking processes but experimentally challenging.
• Acoustic phonon events (high frequency/HF, low frequency/LF, and hybrid) offer insights into system states.
• Hybrid events are empirically used for volcanic eruption prediction, but their physical origin is unclear.

Purpose of the Study:

• To elucidate the physical origin of hybrid acoustic events in rock mechanics.
• To investigate the role of fluid pressure and secondary instabilities in microcracking.
• To link microcracking acoustic signatures to larger-scale phenomena like hybrid earthquakes.

Main Methods:

• Re-examination of acoustic phonon emission records from rock mechanics experiments (wet and dry conditions).
• Analysis of frequency spectrum of acoustic events from individual microcracking events.
• Correlation of acoustic event characteristics with fluid pressure dynamics during fast equilibration.

Main Results:

• A secondary instability, marked by a transition from HF to LF acoustic signals, occurs during system fast equilibration.
• This instability is linked to a sudden increase in fluid pressure within the process zone.
• The fluid pressure increase drives a secondary instability, generating LF-like acoustic signals, consistent with hybrid events.

Conclusions:

• The study reveals a physical mechanism for hybrid acoustic events: fluid pressure-induced secondary instability during microcracking.
• This mechanism explains the observed transition from HF onsets to LF ringing in hybrid events.
• Findings provide a clearer physical basis for using hybrid events in volcanic and seismic hazard assessment.