University of California Berkeley

The measurement of regional attenuation Q-1 can produce method dependent results. The discrepancies among methods are due to differing parameterizations (e.g., geometrical spreading rates), employed datasets (e.g., choice of path lengths and sources), and methodologies themselves (e.g., measurement in the frequency or time domain). We apply the coda normalization (CN), two-station (TS), reverse two-station (RTS), source-pair/receiver-pair (SPRP), and the new coda-source normalization (CS) methods to measure Q of the regional phase, Lg (QLg), and its power-law dependence on frequency of the form Q0f^η with controlled parameterization in the well-studied region of northern California using a high-quality dataset from the Berkeley Digital Seismic Network. We test the sensitivity of each method to changes in geometrical spreading, Lg frequency bandwidth, the distance range of data, and the Lg measurement window. For a given method, there are significant differences in the power-law parameters, Q0 and η, due to perturbations in the parameterization when evaluated using a conservative pairwise comparison. The CN method is affected most by changes in the distance range, which is most probably due to its fixed coda measurement window. Since, the CS method is best used to calculate the total path attenuation, it is very sensitive to the geometrical spreading assumption. The TS method is most sensitive to the frequency bandwidth, which may be due to its incomplete extraction of the site term. The RTS method is insensitive to parameterization choice, whereas the SPRP method as implemented here in the time-domain for a single path has great error in the power-law model parameters and η is greatly affected by changes in the method parameterization. When presenting results for a given method it is best to calculate Q0fη for multiple parameterizations using some a priori distribution. We also investigate the difference in power-law Q calculated among the methods by considering only an approximately homogeneous subset of our data. All methods return similar power-law parameters, though the 95% confidence region is large. We adapt the CS method to calculate QLg tomography in northern California. Preliminary results show that by correcting for the source, tomography with the CS method may produce better resolved attenuation structure.

Almost 50,000 long period (LP) events were recorded at Mt. Etna from November 2003 to May 2006. We analysed these events, as well as very long period (VLP) events which were associated with some of them. During some intervals the spectral and wavefield features of LP events remained steady, with significant changes occurring between the intervals. Based on the times of the changes, we distinguish five different sub-periods. In particular, during sub-period III (June–November 2005) the wavefield at the stations nearest to the summit area was composed of P waves. Locations for 150 LP events occurring in sub-period III, determined using radial semblance, changed, at the same time as the events' spectral features changed. It was during this sub-period that many of the LP events were associated with VLP events. Based on similarity of the waveforms, we distinguished two families of VLP events, with gradually evolving waveforms. The two families are located in slightly different places, but near the sources of the LP events. The change between the families occurred at the same time as the spectra of the LP events changed. Finally the source of the VLP events was investigated by performing complete waveform moment tensor inversion of stacks of the two families. Synthetic Green's functions for the full moment tensor were calculated for a homogeneous halfspace. For both families, the source region with the highest variance reduction lies approximately beneath the active craters, at 500 m below the altitudes of the stations. The solutions for both families are very similar with sources that are between 60 and 70% isotropic. Attempts to determine deviatoric moment tensors produced consistently poorer fits. The remaining energy is poorly constrained and is likely to be noise. In conclusion, these results highlight changes in the LP and VLP events at Mt. Etna over time, and the causal relationship between them.

The determination of regional attenuation Q^-1 can depend upon the analysis method employed. The discrepancies between methods are due to differing parameterizations (e.g., geometrical spreading rates), employed datasets (e.g., choice of path lengths and sources), and the methodologies themselves (e.g., measurement in the frequency or time domain). Here we apply five different attenuation methodologies to a Northern California dataset. The methods are: (1) coda normalization (CN), (2) two-station (TS), (3) reverse two-station (RTS), (4) source-pair/receiver-pair (SPRP), and (5) coda-source normalization (CS). The methods are used to measure Q of the regional phase, Lg (QLg), and its power-law dependence on frequency of the form Q0fη with controlled parameterization in the well-studied region of Northern California using a high-quality dataset from the Berkeley Digital Seismic Network. We investigate the difference in power-law Q calculated among the methods by focusing on the San Francisco Bay Area, where knowledge of attenuation is an important part of seismic hazard mitigation. This approximately homogeneous subset of our data lies in a small region along the Franciscan block. All methods return similar power-law parameters, though the range of the joint 95% confidence regions is large (Q0 = 85 ± 40; η = 0.65 ± 0.35). The RTS and TS methods differ the most from the other methods and from each other. This may be due to the removal of the site term in the RTS method, which is shown to be significant in the San Francisco Bay Area. In order to completely understand the range of power-law Q in a region, it is advisable to use several methods to calculate the model. We also test the sensitivity of each method to changes in geometrical spreading, Lg frequency bandwidth, the distance range of data, and the Lg measurement window. For a given method, there are significant differences in the power-law parameters, Q0 and η, due to perturbations in the parameterization when evaluated using a conservative pairwise comparison. The CN method is affected most by changes in the distance range, which is most probably due to its fixed coda measurement window. Since, the CS method is best used to calculate the total path attenuation, it is very sensitive to the geometrical spreading assumption. The TS method is most sensitive to the frequency bandwidth, which may be due to its incomplete extraction of the site term. The RTS method is insensitive to parameterization choice, whereas the SPRP method as implemented here in the time-domain for a single path has great error in the power-law model parameters and η is strongly affected by changes in the method parameterization. When presenting results for a given method it is best to calculate Q0f^η for multiple parameterizations using some a priori distribution.