Fluid Influenced Faulting in the Long Valley Volcanic Region

Dennise Templeton and Douglas Dreger

Introduction

We aim to better understand how an evolving hydrothermal system in an active volcanic region directly influences local earthquake production. Our ultimate interest is in the underlying source mechanism of these fluid influenced earthquakes and the factors that affect it. Towards that goal, we first determined the extent of fluid influenced faulting in the Long Valley volcanic region by computing moment tensor solutions from regional broadband data.

The Long Valley volcanic region, located in eastern California within the Sierra Nevada frontal fault system, includes the well-known active magmatic systems under Long Valley caldera and the Mono/Inyo Craters. For over two decades, the Long Valley caldera has been the center of unrest in the region exhibiting periods of increased seismicity, ground deformation, localized increases in concentrations of volcanic gases and subsurface magma movement (Langbein et al., 1995 ; Sorey et al., 1998 ). The Mono/Inyo Craters are a series of craters extending north from the caldera and are thought to have been created from a series of dikes over the past 40,000 years (Bursik and Sieh, 1989). To the south of the caldera is the Sierra Nevada mountain block, where there have been equivocal indications of magma existence even though there is a lack of evidence for recent volcanic or geothermal activity (Hough et al., 2000 ; Peppin et al., 1989).

In this study, we focused on a 100 km wide circular area centered at Long Valley caldera which includes the Mono/Inyo Craters and the seismically active Sierra Nevada block and comprehensively searched for events with coseismic volume changes.

Method and Results

Full moment tensor inversions solve for the complete moment tensor and can be decomposed into double-couple (DC), compensated-linear-vector-dipole (CLVD), and isotropic (i.e. volumetric) components. The presence of a significant volumetric component could indicate that fluids were involved in the source process of an earthquake. Deviatoric solutions a priori constrain the volumetric component to be zero. In this active geothermal and volcanic region, we did not wish to make this assumption. Therefore, we solved for both the deviatoric and full moment tensor solutions using three-component Berkeley Digital Seismic Network data at regional distances.

We studied 130 events with magnitudes greater than 3.5 since 1993. Seven stations were chosen for these inversions that provided the best azimuthal coverage and data quality. In practice however a solution would have a subset of these seven stations in its inversion depending on station availability and data quality issues. Green's functions were computed using the SoCal velocity model which is appropriate for the eastern California and Sierra Nevada regions (Dreger and Helmberger, 1993). Both data and Green's functions were bandpass filtered between 0.02 to 0.05 Hz using a causal Butterworth filter.

We used a nested F test to determine the probability that the additional volumetric component in the full moment tensor solution represented a true aspect of the source mechanism rather than being simply an added non-physical parameter in the inversion. We identified significant volumetric components if the improvement in fit to the data was at the 99 percent significance level between the deviatoric and full moment tensor solution.

Of the 130 earthquakes we originally identified, we were able to compute solutions for 83 events. The ensuing moment tensor catalog allowed us to determine the prevalence of events characterized by isotropic components. From these results we identified 17 earthquakes that had significant volumetric components as well as good station coverage and data quality (Figure 21.1). These events all had between four and six stations in their inversions.

This investigation showed that fluid influenced earthquakes are fairly unique in the Long Valley volcanic region. The majority of these events were located within the Long Valley caldera or near the rim of the caldera. The remaining seven events were located in the seismically active Sierra Nevada block south of the caldera.

Figure 21.1: Location of events occurring between 1993-2003 with significant volumetric components labeled as YY.MMDD. Background seismicity is plotted as small gray dots. Faults, caldera rim, and resurgent dome plotted as solid gray lines.
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Discussion

All events with significant volumetric components in the Long Valley caldera are located in the south moat or near the adjacent caldera rim. Seven of these events occurred in or near the location or time period of the November 1997 earthquake swarm. During and after this swarm there was also extensive independent evidence of magma migration from EDM, GPS, strainmeters, tiltmeters, and volcanic gas discharge rates. This magma migration could have affected the surface hydrothermal system in such a way as to cause these fluid influenced faulting events. The remaining two events were located near the epicenter of a $M_{L}$6.1 May 1980 quake that had a large non-double couple component (Julian and Sipkin, 1985). It was never determined if this event had a volumetric component which would have conclusively determined if fluids were involved. Long Valley caldera has an active geothermal system and thus it is not unexpected to find events with large isotropic components in this area (Lachenbruch et al., 1976).

It was surprising that earthquakes did not occur in or near the vicinity of the Mono/Inyo Craters. There is extensive evidence for the existence of magma under this volcanic chain and it is the expected location of the next volcanic eruption from this system (Hill et al., 1985 ; Bursik and Sieh, 1989). Perhaps this points to differences in the hydrothermal system or heat flux found in this area and with that found in the Long Valley caldera.

The most interesting result of this study was the number of fluid influenced earthquakes within the Sierra Nevada block when there has been no conclusive evidence of recent geothermal or magmatic activity in the area. However, there has been indirect evidence reported of magma bodies south of the caldera from pre-S phases and S-wave shadowing studies (Peppin et al., 1989). Hough et al. (2000) also identified several lines of equivocal evidence of magma or magmatic fluid involvement during an August 1998 earthquake sequence near the Hilton Creek fault. Earthquake activity in the Sierra Nevada block has always been assumed to be tectonic due to the absence of recent volcanism and present day geothermal features. However two major earthquakes occurring between 1978 and 1980 had large non-double couple components (Julian and Sipkin, 1985). Again, it was never determined if either of these events had large volumetric components.

Perhaps tectonic adjustment in the Sierra Nevada block could be providing a convenient conduit for hydrothermal fluids to migrate from their origin in the caldera to the mountain block via the north striking faults found in the area. In later studies we plan to explore the connection between fluid influenced faulting, the shallow hydrothermal system, and the deeper magmatic system.

Acknowledgements

We appreciate support for this project by NSF through contract EAR-0087147.

References

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