Comment on “Slowness-azimuth corrections of teleseismic events for IMS primary arrays in China” by Hao and Zheng
Tóm tắt
Knowledge about backazimuth and slowness deviations at seismic arrays can be used as a tool to study subsurface lateral heterogeneity and improve the ability to locate events. Recently, Hao and Zheng (J Seismol 13:437–448, 2009) estimated the backazimuth and slowness deviations for teleseismic P waves recorded by the HILR array and the LZDM array using f–k analysis. They attributed the significant deviations at the LZDM array to dipping structures beneath the array. However, another possible factor, namely the altitude variations of array elements, was not taken into consideration during the slowness estimation process. For the LZDM array, the maximum altitude difference is ~15% of the array aperture and not negligible. In this study, we made some numerical experiments to investigate the difference between the estimated and theoretical slowness vectors when ignoring the altitude difference. The results reveal that remarkable artificial slowness shift is produced. Assuming a P-wave velocity of 5.4 km/s immediately beneath the array, the magnitude of slowness shift increases from 1.4 to 2.2 s/° when the theoretical slowness decreases from 16 to 4 s/°. For a 10° emergence angle, the backazimuth deviation reaches nearly 40°, and the relative slowness deviation can be greater than 60%. It is also shown that ignoring the altitude difference gives rise to a northeastward slowness shift, opposite to the southwestward shift proposed by Hao and Zheng, suggesting that they have heavily underestimated the slowness residuals at the LZDM array. Note that the elevation of one of the array stations is much lower than others. Avoiding the use of this station, the elevation variation range of array stations decreases by nearly one half, and the artificial backazimuth and slowness deviations decrease by more than one half.
Tài liệu tham khảo
Aki K, Richards PG (1980) Quantitative seismology. Freeman, San Francisco
Bokelmann GHR (1995) Azimuth and slowness deviations from the GERESS regional array. Bull Seismol Soc Am 85(5):1456–1463
Bondar I, North RG, Beall G (1999) Teleseismic slowness-azimuth station corrections for the international monitoring system seismic network. Bull Seismol Soc Am 89(4):989–1003
Cansi Y, Plantet JL, Massinon B (1993) Earthquake location applied to a mini-array-k-spectrum versus correlation method. Geophys Res Lett 20:1819–1822
Hao C, Zheng Z (2009) Slowness-azimuth corrections of teleseismic events for IMS primary arrays in China. J Seismol 13:437–448. doi:10.1007/s10950-008-9137-8
Koch K, Kradolfer U (1997) Investigation of azimuth residuals observed at stations of the GSETT-3 Alpha network. Bull Seismol Soc Am 87(6):1576–1597
Li SL, Zhang XK, Zhang CK, Zhao JR, Cheng SX (2002) A preliminary study on the crustal velocity structure of Maqin-Lanzhou-Jingbian by means of deep seismic sounding profile. Chin J Geophys 45(2):210–217 (in Chinese)
Liu M, Mooney WD, Li S, Okaya N, Detweiler S (2006) Crustal structure of the northeastern margin of the Tibetan plateau from the Songpan-Ganzi terrane to the Ordos basin. Tectonophysics 420:253–266
Niazi M (1966) Corrections to apparent azimuths and travel-time gradients for a dipping Mohorovicic discontinuity. Bull Seismol Soc Am 56(2):491–509
Rost S, Thomas C (2002) Array seismology: methods and applications. Rev Geophys 40:1008
Schweitzer J (2001) Slowness corrections - one way to improve IDC products. Pure Appl Geophys 158:375–396
Tibuleac IM, Stroujkova A (2009) Calibrating the Chiang Mai seismic array (CMAR) for improved event location. Seismol Res Lett 80(4):579–590. doi:10.1785/gssrl.80.4.579
Xu J, Zhao Y, Su X, Lv T (2006) The site survey for Dajian mountain seismic array in Lanzhou. Seismol Geomagnet Observ Res 27(Suppl):51–57