Non-double-couple microearthquakes at Long Valley caldera, California, provide evidence for hydraulic fracturing

Journal of Volcanology and Geothermal Research
By: , and 

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Abstract

Most of 26 small (0.4≲M≲3.1) microearthquakes at Long Valley caldera in mid-1997, analyzed using data from a dense temporary network of 69 digital three-component seismometers, have significantly non-double-couple focal mechanisms, inconsistent with simple shear faulting. We determined their mechanisms by inverting P- and S-wave polarities and amplitude ratios using linear-programming methods, and tracing rays through a three-dimensional Earth model derived using tomography. More than 80% of the mechanisms have positive (volume increase) isotropic components and most have compensated linear-vector dipole components with outward-directed major dipoles. The simplest interpretation of these mechanisms is combined shear and extensional faulting with a volume-compensating process, such as rapid flow of water, steam, or CO2 into opening tensile cracks. Source orientations of earthquakes in the south moat suggest extensional faulting on ESE-striking subvertical planes, an orientation consistent with planes defined by earthquake hypocenters. The focal mechanisms show that clearly defined hypocentral planes in different locations result from different source processes. One such plane in the eastern south moat is consistent with extensional faulting, while one near Casa Diablo Hot Springs reflects en echelon right-lateral shear faulting. Source orientations at Mammoth Mountain vary systematically with location, indicating that the volcano influences the local stress field. Events in a ‘spasmodic burst’ at Mammoth Mountain have practically identical mechanisms that indicate nearly pure compensated tensile failure and high fluid mobility. Five earthquakes had mechanisms involving small volume decreases, but these may not be significant. No mechanisms have volumetric moment fractions larger than that of a force dipole, but the reason for this fact is unknown.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Non-double-couple microearthquakes at Long Valley caldera, California, provide evidence for hydraulic fracturing
Series title Journal of Volcanology and Geothermal Research
DOI 10.1016/S0377-0273(03)00420-7
Volume 132
Issue 1
Year Published 2004
Language English
Contributing office(s) Volcano Hazards Program
Description 27 p.
First page 45
Last page 71
Country United States
State California
Other Geospatial Long Valley caldera