Regional Attenuation in Northern California: A Comparison of Five 1D Q Methods

Abstract

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) sourcepair/receiver-pair (SPRP), and (5) coda-source normalization (CS). The methods are used to measure Q of the regional phase, Lg (Q Lg ), and its power-law dependence on frequency of the form Q 0 f η 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 (Q 0 = 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, Q 0 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 Q 0 f η for multiple parameterizations using some a priori distribution.

Attenuation Tomography of the Yellow Sea/Korean Peninsula from Coda-source normalized and direct Lg Amplitudes

“…Therefore, spreading transitions from a spherical to a cylindrical type at approximately 100 km. FORD et al (2008) found that results differed only slightly when r 0 is between 60 and 120 km. For brevity we will refer to this method as AMP (for AMPlitude).…”

“…Q models taken from the literature can vary greatly within the same region. In previous work (FORD et al, 2008), we compared five different 1-D methods to measure Q Lg and attempted to assess the error associated with the results. The assessment showed the possible influence of lateral variations in attenuation.…”

Ground-Motion Observations of the 2014 South Napa Earthquake

“…The site V S30 values are taken from the Next Generation Attenuation-West 2 (NGA-West 2) site database if they are available; otherwise, we take the closest value from the gridded V S30 map of Wills et al (2000; and Kalkan (2015) GMPE, however, uses Q 0 as a frequencyindependent input independent variable that can be changed for each realization of the GMPE; for the case of the South Napa earthquake, Q 0 50. This is motivated partially to fit the data and also to be within the range suggested by Ford et al (2008) for the San Francisco Bay area (Erol Kalkan and Vladimir Graizer, personal comm., 2014). The Chiou and Youngs (2014) GMPE has the option of implementing the effect of directivity, specific for the fault geometry and the station distance and location, using the direct-point parameter (DPP) described in chapter 5 of Spudich et al (2013).…”

CodaQin Different Tectonic Areas, Influence of Processing Parameters

“…Examples are Ford et al (2008) and Negi et al (2015). Although using coda waves, the method is independent of the scattering model used for coda waves and the results are therefore not directly comparable with coda Q.…”

“…For Lg coda Q studies, Mitchell et al (2015) pointed out the importance of using the same procedures and parameters and claim to have done so in their many studies. For California, Ford et al (2008) used five different methods, none of which was the CWD method, to determine Lg coda Q and found significant differences due to the varying methods, but also to the choice of parameters. Hellweg et al (1995), in a study of California coda Q, also points out the importance of using the same parameters, in particular lapse time and length of analysis window, when comparing coda Q.…”