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Volcanic Seismic Sources

Douglas Dreger and Hrvoje Tkalcic


Long Valley, CA Seismicity

In the spring of 1997 the Long Valley Caldera began a new phase of unrest that lasted through early 1998. This unrest initiated in May with reinvigored inflation of the resurgent dome observed on Long Valley geodetic networks (Langbein et al., 1998). In June the seismicity rate began to increase markedly, the deformation rate continued to be high and the flux of CO2 release increased between September to December. The seismic unrest culminated on November 22 with a series of four magnitude 4.6-4.9 earthquakes located in the south moat of the caldera. Preliminary moment tensor analysis of these events revealed anomalous seismic radiation. Deviatoric moment tensors have large non-double-couple(NDC) components, and full moment tensor inversions reveal a component of coseismic volumetric expansion in all four events (Figure 13.1). These are the first direct observations of coseismic volumetric expansion in Long Valley seismicity.

The double-couple and full moment tensor inversions have 5 and 7 free parameters respectively. We evaluated whether the full moment tensor solutions provides a significant improvement in fit considering the increase in model parameters using an F-test (e.g. Menke, 1989). The F statistic is formed from the ratio of the two data-model variances. It is possible to compute critical values, Fcrit, for specific levels of confidence and degrees of freedom in the inversion (e.g. Menke, 1989). If observed F exceed Fcrit then the improvement in fit is statistically significant at the prescribed level. In our inversions of complete, three-component, long-period (0.02 to 0.05 Hz) displacement waveforms we have 3000 data points for 5 stations (200 points per component with dt=1s). The F-test assumes that each time point is uncorrelated, which is clearly not valid. For example a number of points are required to define the pulse width of the P-wave. We use a 20s period low pass butterworth filter in processing the data and therefore assume that the data is correlated over a 20s time scale. This reduces the number of uncorrelated data to 150 points. The degrees of freedom are then 145 and 143 for the double-couple and full moment tensor inversions, respectively. The Fcrit for these degrees of freedom and 95% significance level is 1.32. Observed F values for events 1, 2, 3 and 4 are 1.96, 2.06, 3.59, and 2.33 respectively, indicating that the improvement in fit afforded by the full moment tensor parameterization is statistical significant above the 95% level. We note further that even in the extreme case of two uncorrelated points per trace the observed F are statistically significant at the 95% level.

A Jackknife test was performed to investigate the sensitivity of the isotropic components and general orientation of the principle axes of the seismic moment tensors to variations in the input data. This test evaluates whether lateral heterogeneity or whether a single station or groups of stations may be biasing the results. The Jackknife test revealed that the isotropic components for the anomalous events are significantly separated from double-couple reference earthquakes, and that the isotropic components that have been calculated are stable for all combinations of 1-, 2-, 3-, 4- and 5-stations indicating that there is no station specific or propagation path bias.

Figure 13.1 shows that the solutions for the anomalous events have significant isotropic components indicating volumetric expansion at the source. With the exception of EVT1 the deviatoric parts of the moment tensors are primarily CLVDs. The combined isotropic-CLVD solutions and anomalously long source durations apparent in the P-waveforms of these events together with the observation of a compressional strain transient recorded at the Devil's Post Pile borehole strainmeter suggest that fluids were involved in the source process (Dreger et al., 1999). Langbein et al (1998) report that the strain transient and other geodetic data may be modeled as either an inflation source beneath the south moat of the caldera or as right-lateral strike-slip on a west-northwest trending fault, which is consistent with background seismicity. The seismic moment tensor observations and the strain transient suggest that seismicity was triggered by the injection of high pressure fluid or possibly by pressurization of water saturated faults in the south moat by a pressure pulse generated by magmatic heating.


  
Figure 13.1: Map showing the Long Valley, Caldera, resurgent dome, Inyo-Mono craters chain, and earthquakes we investigated. The P-wave radiation patterns for the full moment tensor solutions are plotted. EVT1, EVT2, EVT3 and EVT4 result in anomalous seismic moment tensors with significant isotropic components indicating volumetric expansion. Note that EVT5 and EVT6 are events that result in nearly pure double-couple solutions. The locations of 1980 Mammoth Lakes earthquakes are shown as stars and those with significant non-double-couple (NDC) components are shown as filled stars. The event labeled WC is the 1978 Wheeler Crest earthquake that also had a large non-double-couple component to the seismic moment tensor. The Devil's Postpile dilatational strain meter (POPA) is located by the filed square.
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References

Dreger, D. S., H. Tkalcic, and M. Johnston, Broadband Evidence of Non-Double-Couple Moment Tensors and Volumetric Expansion in Recent Mammoth Lakes, California, Earthquakes, submitted to Science, 1999.

Langbein, J., S. Wilkinson, M. Johnston, J. Feinberg, R. Bilham, The 1997-98 Inflation of Long Valley Caldera and comparison with the 1989-95 episode, EOS, 79, 949, 1998.

Menke, W., Geophysical Data Analysis: Discrete Inverse Theory, Academic Press Inc., pp289, 1989.


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