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Regional attenuation in Northern California; a comparison of five 1D Q methods

Sean R. Ford, Douglas S. Dreger, Kevin Mayeda, William R. Walter, Luca Malagnini and William S. Phillips
Regional attenuation in Northern California; a comparison of five 1D Q methods
Bulletin of the Seismological Society of America (August 2008) 98 (4): 2033-2046

Abstract

The determination of regional attenuation (Q (super -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 nature of 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 (Q (sub Lg) ), and its power-law dependence on the frequency of the form Q (sub 0) (super feta ) 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. All methods return similar power-law parameters, though the range of the joint 95% confidence regions is large (Q (sub 0) = 85+ or -40; eta = 0.65+ or -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, we advise the use of several methods to calculate the model. We also test the sensitivity of each method to changes in geometrical spreading, the 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 (sub 0) and eta , 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 likely due to its fixed coda-measurement window. Because 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 eta is strongly affected by changes in the method parameterization. When presenting results for a given method we suggest calculating Q (sub 0) (super feta ) for multiple parameterizations using some a priori distribution.


ISSN: 0037-1106
EISSN: 1943-3573
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 98
Serial Issue: 4
Title: Regional attenuation in Northern California; a comparison of five 1D Q methods
Affiliation: University of California, Berkeley, Berkeley, CA, United States
Pages: 2033-2046
Published: 200808
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 41
Accession Number: 2008-116861
Categories: Environmental geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
Secondary Affiliation: Lawrence Livermore National Laboratory, USA, United StatesIstituto Nazionale di Geofisica e Vulcanologia, ITA, ItalyLos Alamos National Laboratory, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200844
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