Azimuthal and regional variations of coda waves in New Zealand
DOI:
https://doi.org/10.5459/bnzsee.32.3.170-179Abstract
We present the results of using seismic waveform data to show that the elliptical nature of isoseismal distributions in New Zealand is related to regional structural trends. The data also suggest that there are regional and azimuthal variations in the attenuation of coda waves, which may need to be considered in ground motion attenuation relations.
We stacked over 20,000 waveforms from the New Zealand seismographic network. The data were filtered, normalized and stacked. Noisy or clipped records were down-weighted or removed. We also treated dense networks as a single station and generated a single stack for these networks. Stacks of shallow earthquake sources are presented by region and azimuth.
Variations in coda length throughout New Zealand suggest regions of high scattering. Strong azimuthal dependence in the coda is observed for non-volcanic zone stations. NE-SW waveform stacks, which follow the strike of the subduction zone, contain significantly longer codas than those with NW-SE raypaths. Long coda trains are also observed in the volcanic and geothermal zones yet there is little or no apparent azimuthal variation. These coda are particularly strong throughout the records which explains the difficulty analysts have had in picking S waves.
References
Aki, K. (1969), Analysis of the seismic coda of local earthquakes as scattered waves, J. Geophys. Res., 74, 615-631.
Aki, K. and Chouet, B., (1975), Origin of coda waves: source, attenuation, and scattering effects, J. Geophys. Res,. 80, 3322-3342. DOI: https://doi.org/10.1029/JB080i023p03322
Bonner, J.L., Herrin, E.T., and Goforth, T.T., (1996), Azimuthal variation of Rg energy from quarry blasts in central Texas, Seismol. Res. Lett., 67, no. 4, 43-56.
Downes, G.L., (1995), Atlas of isoseismal maps of New Zealand earthquakes, Institute of Geological & Nuclear Sciences Monograph 11, IGNS, Lower Hutt, New Zealand.
Dowrick, DJ., (1991), A revision of attenuation relationships for modified Mercalli intensity in New Zealand earthquakes, Bull. NZ Nat. Soc. for Earthquake Engineering, 24, no. 3, 210-224.
Kozuch, M.J., Smith, E. and Vere-Jones, D., (1996), Modeling New Zealand isoseisms with site-specific attenuation (abstract), Eos, Trans. Am. Geophys. Union, 77, no. 46, F510.
Kozuch, M.J. and Chadwick, M., (1997), Isoseismal distributions for New Zealand earthquakes verified with azimuthal and regional variations in coda waves (abstract), Eos, Trans. Am. Geophys. Union, 78, no. 46, F434.
Kvamme, L.B. and Havskov, J., (1989), Q in southern Norway, Bull. Seismol. Soc. Amer., 79, 1575-1588.
Mayeda, K., Koyanagi, S., Hoshiba, M., Aki, K. and Zeng, Y., (1992), A comparative study of scattering, intrinsic, and coda Q-1 for Hawaii, Long Valley, and Central California between 1.5 and 15.0 Hz, J. Geophys. Res.,97, 6643-6659. DOI: https://doi.org/10.1029/91JB03094
O'Doherty, K.B., Bean, C.J. and McCloskey, J., (1997), Coda wave imaging of the Long Valley caldera using a spatial stacking technique, Geophys. Res. Lett., 24, 1547-1550.
Pulli, J.J., (1984), Attenuation of coda waves in New England, Bull. Seismol. Soc. Amer., 74, 1149-1166.
Sato, H., (1977), Energy propagation including scattering effects: single isotropic scattering, J. Phys. Earth, 25, 27-41.
Smith, W.D., (1978), Spatial distribution of felt intensities for New Zealand earthquakes, NZ J. Geol. Geophys., 21, 293-311.
Smith, W.D., (1995), A development in the modelling of far-field intensities for New Zealand earthquakes, Bull. N.Z. Nat. Soc. for Earthquake Engineering, 28, 196-217.
Vere-Jones, D., Pang, J. and Smith, E.G., (1995), Statistical modelling of felt intensity data for three New Zealand earthquakes, Report to the NZ Earthquake Commission. Institute of Stats. and Operations Res., Victoria University of Wellington, 41.
