Temporal variations in coda attenuation associated with the 2008 Wenchuan(M_(W)7.9)Earthquake in SW,China

In this study,waveform data obtained from Western Sichuan Seismic Array(WSSA)in China was utilized to reveal the temporal variations in coda attenuation around the eastern Tibet Plateau and Western Sichuan.Based on the single-scattering model,coda attenuation factor QC^(-1) is calculated in narrower overlapping frequency bands at 1.0-24 Hz by measuring the coda decay rates for local earthquakes before and after the Wenchuan mainshock.The temporal variations in coda attenuation are investigated within three periods.The periodⅠlasted from January 2007 to the end of 2007.The periodⅡlasted from January 2008 till Wenchuan earthquake.The periodⅢlasted from the mainshock to the end of 2008.The resulted temporal variations demonstrate an increase in average QC^(-1) by approximately 35%-45%in the vicinity of Longmenshan and 30%-35%in Sichuan Basin after the Wenchuan earthquake in lower frequency bands within 1.25-8 Hz.On the contrary,the average QC^(-1) is shown to decrease by approximately10%-18%in southern segment of Longmenshan,15%-38%in the Chuan-Dian block,and 10-12%in the South China block.These results are confirmed by a statistical t-test at 99.9%confidence level No statistically significant change in QC^(-1)(<10%)is found in the Songpan-Ganzi block after the mainshock.Temporal variations of coda attenuation differ significantly in individual blocks after the mainshock.The coda attenuation is proposed to be a beacon to tectonic static stress changes associated with the Wenchuan earthquake.

A new seismic daylight imaging method for determining the structure of lithospheric discontinuity

The fine-scale structures of lithosphere discontinuities contain important information on the dynamics of lithosphere formation, development, transformation, and destruction. In this paper, a new seismic daylight imaging method is developed to explore the small-scale structures of lithosphere discontinuities. This method makes use of the P-wave first arrival and coda in the 0.5–4 Hz high frequency band of teleseismic events, and reaches a resolution of 2 km for lithosphere discontinuities. This method rests on the basic principle that the autocorrelation of the vertically incident transmission response below the seismic station is equivalent to the reflection response with the source and station both on the free surface. The transmission responses include the first-arrival P-waves below the station traversing the discontinuities to reach the free surface, and the multiple reflections between the free surface and the discontinuities. In this study, the normal incidence requirement of the method is further extended to include dip incidence illumination, which expands its applicability. The accuracy and feasibility of the seismic daylight imaging (SDI) theory are verified by synthesized theoretical seismograms, and the factors affecting the imaging results are discussed. The data processing steps and the interpretation criteria for the method are also given. The fine-scale lithosphere structure of two permanent stations at the eastern North China Craton is determined by the method described here, as well as instantaneous frequency. Clear discontinuities are found in the lithospheric mantle at 52 and 75 km below the two stations, respectively. Seismic daylight imaging and the receiver function reveal a more consistent lithosphere structure beneath the MBWA permanent station of the West Australia Craton, with the unmistakable presence of the lithosphere discontinuities.High-frequency SDI can be used to detect the fine-scale lithospheric structures. As its waveform is more complex, and hence appropriate reference to existing seismological information, such as from tomographic velocity inversion and the receiver function, is recommended.