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Title: Regional Body-Wave Attenuation Using a Coda Source Normalization Method: Application to MEDNET Records of Earthquakes in Italy

Abstract

We develop a new methodology to determine apparent attenuation for the regional seismic phases Pn, Pg, Sn, and Lg using coda-derived source spectra. The local-to-regional coda methodology (Mayeda, 1993; Mayeda and Walter, 1996; Mayeda et al., 2003) is a very stable way to obtain source spectra from sparse networks using as few as one station, even if direct waves are clipped. We develop a two-step process to isolate the frequency-dependent Q. First, we correct the observed direct wave amplitudes for an assumed geometrical spreading. Next, an apparent Q, combining path and site attenuation, is determined from the difference between the spreading-corrected amplitude and the independently determined source spectra derived from the coda methodology. We apply the technique to 50 earthquakes with magnitudes greater than 4.0 in central Italy as recorded by MEDNET broadband stations around the Mediterranean at local-to-regional distances. This is an ideal test region due to its high attenuation, complex propagation, and availability of many moderate sized earthquakes. We find that a power law attenuation of the form Q(f) = Q{sub 0}f{sup Y} fit all the phases quite well over the 0.5 to 8 Hz band. At most stations, the measured apparent Q values are quite repeatable frommore » event to event. Finding the attenuation function in this manner guarantees a close match between inferred source spectra from direct waves and coda techniques. This is important if coda and direct wave amplitudes are to produce consistent seismic results.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
913548
Report Number(s):
UCRL-JRNL-227999
Journal ID: ISSN 0094-8276; GPRLAJ; TRN: US200802%%867
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geophysical Research Letters, vol. 34, n/a, May 25, 2007, L10308; Journal Volume: 34
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; AMPLITUDES; ATTENUATION; AVAILABILITY; EARTHQUAKES; ITALY; SPECTRA

Citation Formats

Walter, W R, Mayeda, K, Malagnini, L, and Scognamiglio, L. Regional Body-Wave Attenuation Using a Coda Source Normalization Method: Application to MEDNET Records of Earthquakes in Italy. United States: N. p., 2007. Web. doi:10.1029/2007GL029990.
Walter, W R, Mayeda, K, Malagnini, L, & Scognamiglio, L. Regional Body-Wave Attenuation Using a Coda Source Normalization Method: Application to MEDNET Records of Earthquakes in Italy. United States. doi:10.1029/2007GL029990.
Walter, W R, Mayeda, K, Malagnini, L, and Scognamiglio, L. Thu . "Regional Body-Wave Attenuation Using a Coda Source Normalization Method: Application to MEDNET Records of Earthquakes in Italy". United States. doi:10.1029/2007GL029990. https://www.osti.gov/servlets/purl/913548.
@article{osti_913548,
title = {Regional Body-Wave Attenuation Using a Coda Source Normalization Method: Application to MEDNET Records of Earthquakes in Italy},
author = {Walter, W R and Mayeda, K and Malagnini, L and Scognamiglio, L},
abstractNote = {We develop a new methodology to determine apparent attenuation for the regional seismic phases Pn, Pg, Sn, and Lg using coda-derived source spectra. The local-to-regional coda methodology (Mayeda, 1993; Mayeda and Walter, 1996; Mayeda et al., 2003) is a very stable way to obtain source spectra from sparse networks using as few as one station, even if direct waves are clipped. We develop a two-step process to isolate the frequency-dependent Q. First, we correct the observed direct wave amplitudes for an assumed geometrical spreading. Next, an apparent Q, combining path and site attenuation, is determined from the difference between the spreading-corrected amplitude and the independently determined source spectra derived from the coda methodology. We apply the technique to 50 earthquakes with magnitudes greater than 4.0 in central Italy as recorded by MEDNET broadband stations around the Mediterranean at local-to-regional distances. This is an ideal test region due to its high attenuation, complex propagation, and availability of many moderate sized earthquakes. We find that a power law attenuation of the form Q(f) = Q{sub 0}f{sup Y} fit all the phases quite well over the 0.5 to 8 Hz band. At most stations, the measured apparent Q values are quite repeatable from event to event. Finding the attenuation function in this manner guarantees a close match between inferred source spectra from direct waves and coda techniques. This is important if coda and direct wave amplitudes are to produce consistent seismic results.},
doi = {10.1029/2007GL029990},
journal = {Geophysical Research Letters, vol. 34, n/a, May 25, 2007, L10308},
number = ,
volume = 34,
place = {United States},
year = {Thu Feb 01 00:00:00 EST 2007},
month = {Thu Feb 01 00:00:00 EST 2007}
}
  • A 1-D coda method was proposed by Mayeda et al. (2003) in order to obtain stable seismic source moment-rate spectra using narrowband coda envelope measurements. That study took advantage of the averaging nature of coda waves to derive stable amplitude measurements taking into account all propagation, site, and Sto-coda transfer function effects. Recently this methodology was applied to micro earthquake data sets from three sub-regions of northern Italy (i.e., western Alps, northern Apennines and eastern Alps). Since the study regions were small, ranging between local-to-near-regional distances, the simple 1-D path assumptions used in the coda method worked very well. Themore » lateral complexity of this region would suggest, however, that a 2-D path correction might provide even better results if the datasets were combined, especially when paths traverse larger distances and complicated regions. The structural heterogeneity of northern Italy makes the region ideal to test the extent to which coda variance can be reduced further by using a 2-D Q tomography technique. The approach we use has been developed by Phillips et al. (2005) and is an extension of previous amplitude ratio techniques to remove source effects from the inversion. The method requires some assumptions such as isotropic source radiation which is generally true for coda waves. Our results are compared against direct Swave inversions for 1/Q and results from both share very similar attenuation features that coincide with known geologic structures. We compare our results with those derived from direct waves as well as some recent results from northern California obtained by Mayeda et al. (2005) which tested the same tomographic methodology applied in this study to invert for 1/Q. We find that 2-D coda path corrections for this region significantly improve upon the 1-D corrections, in contrast to California where only a marginal improvement was observed. We attribute this difference to stronger lateral variations in Q for northern Italy relative to California.« less
  • Partial melting at the grain boundaries of mantle rock is a source of attenuation often invoked in the interpretation of teleseismic data. However, related experimental studies are extremely difficult and hence are scarce. We have used a convenient metallurgical system to model a rock sample with a layer of fluid at grain boundaries: the analog of the rock samples is a cylinder of fine-grained, pure aluminum (diameter is 6 cm; height is 6 cm), and the analog of the fluid is provided by a thin layer of aluminum-gallium eutectic at the grain boundaries. The metal gallium, liquid at room temperature,more » diffuses extremely rapidly into the grain boundaries of aluminum where it forms a soft eutectic that melts at 28[degrees]C. The total amount of gallium in the sample represents about 0.2 wt%. Ultrasonic (1 MHz) pulses were transmitted through the sample by a transducer, and the full waveform of the reflected and scattered signals, including the coda, were recorded. Analyses of the decay parameter of the coda envelope yield values of Q[sup [minus]1]. Preliminary results show that the thin film of aluminum-gallium eutectic at the grain boundaries noticeably increases attenuation. For pure aluminum at room temperature, Q[sup [minus]1] = 0.095 [times] 10[sup [minus]3]; after invasion of gallium into the grain boundaries, the value measured at 26[degrees]C (in solid state) was Q[sup [minus]1] = 0.62 [times] 10[sup [minus]3]; at 28[degrees]C, when the autectic became liquid, Q[sup [minus]1] = 1.01 [times] 10[sup [minus]3], an order of magnitude higher than for pure aluminum. 26 refs., 6 figs.« less
  • Scattering by heterogeneities is an important factor controlling shapes oF seismograms. Analyzing horizontal component seismograms in the band 1.0-8 Hz of local earthquakes in western Japan, the author found that coda energy concentration just after the S wave arrival is more than expected for any multiple isotropic scattering model in uniform infinite medium. This behavior was seen even for deep earthquakes and became stronger with increase of frequency. To try to interpret this observation, nonisotropy is introduced into the multiple scattering model. The observed envelopes are compared with the model in order to estimate the amount of nonisotropy in scattering.more » Coda wave envelopes are synthesized using the Monte Carlo method for the nonisotropic scattering media. Two types of angular dependent scattering are considered as models of the nonisotropy with relation to the perturbation of seismic wave velocity in the lithosphere: perturbed media with exponential and Gaussian autocorrelation functions. It is found that forward scattering is dominant and the strength of sideways scattering is at least 10 times larger than that of backward scattering. The amount of the nonisotropic scattering means that correlation distance is estimated to be larger than several hundred meters in the case of Gaussian random media. 31 refs., 17 figs.« less
  • During the past few years there have been numerous reports of changes in coda wave attenuation occurring before major earthquakes. These observations are important because they may provide insight into stress-related structural changes taking place in the focal region prior to the occurrence of large earthquakes. The results of these studies led us to suspect that temporal changes in coda wave attenuation might also accompany volcanic eruptions. By measuring power decay envelopes for earthquakes at Mount St. Helens recorded before, during, and after an eruption that took place during September 3--6, 1981, we found that coda Q/sup -1/ for frequenciesmore » between 6 and 30 Hz was 20--30% higher before the eruption than after. The change is attributed to an increase in the density of open microcracks in the rock associated with inflation of the volcano prior to the eruption. Q/sup -1/ was found to be only weakly dependent on frequency and displayed a slight peak near 10 Hz. The weak frequency dependence is attributed to the dominance of intrinsic attenuation over scattering attenuation, since it is generally accepted that intrinsic attenuation is constant with frequency, whereas scattering attenuation decreases strongly at higher frequencies. The weak frequency dependence of Q/sup -1/ at Mount St. Helens contrasts with results reported for studies in nonvolcanic regions. The peak in Q/sup -1/ near 10 Hz at Mount St. Helens is attributed to the scale length of heterogeneity responsible for generating backscattered waves. Results for nonvolcanic regions have shown this peak to occur near 0.5 Hz. Thus a smaller scale length of heterogeneity is required to explain the 10-Hz peak at Mount St. Helens. copyright American Geophysical Union 1988« less
  • A simple integral expression for the asymptotic normalization coefficient of a bound-state wave function is presented. Correspondingly, a method for correcting wave functions computed by expansion in oscillator wave functions is described. (auth)