A Study of Long-Range Seismic Profile Data in Western China

• Main Author: Xie, Jiakang
• Format: Report
• Language: English
• Subjects: AMPLITUDE; ATTENUATION; CENTRAL ASIA; COMPARISON; DATA ACQUISITION; DATA PROCESSING; EARTH MANTLE; EARTHQUAKES; FREQUENCY DEPENDENCE; GST(GEOMETRICAL SPREADING TERM); HIGH FREQUENCY; INDEPTH II; INDEPTH III; KIRGYZIA; KNET(KHYRGHISTAN NETWORK); LONG RANGE(DISTANCE); MANTLE HEAD WAVES; MANTLE LID; MANTLE TURNING WAVES; MEASUREMENT; NUCLEAR EXPLOSIONS; Nuclear Explosions and Devices(non-military); PASSCAL EXPERIMENTS; PEB62D; PN AMPLITUDE; PN ATTENUATION; PN Q; PN SPECTRA; PROFILES; Radiofrequency Wave Propagation; SANG-MAQI; SANG-TUNL; SCATTERING; SEISMIC DATA; SEISMIC SPECTRA; SEISMIC VELOCITY; SEISMIC WAVES; SEISMOLOGICAL STATIONS; Seismology; SPECTRA; TABLES(DATA); TIBET; TIBETAN PLATEAU; TRAVEL TIME; UNDERGROUND EXPLOSIONS; UPPER MANTLE. TARIM BASIN; VARIATIONS; WAVE PROPAGATION; WNDO-TUNL; WUDH12468

Pn Spectra are collected from three PASSCAL seismic experiments in the Tibetan Plateau (TP) over four path groups. The first and second path groups run southward from the eastern Tarim Basin to stations in north and south central Tibet. The third and fourth path groups form two NNE-oriented profiles in the eastern TP. Events recorded by the first 2 path groups also are recorded by the Khyrghistan network (KNET) to the west. A comparison of Pn spectra averaged over both path groups and KNET reveal that mantle lid under the TB attenuates P waves more severely than central Asia, particularly at high frequencies ( 1 Hz). Apparent Q(sub 0) and eta (Pn Q at 1 Hz and its frequency dependence, respectively) are estimated with a simplified geometrical spreading. Over path group 1 that heavily samples northern TB, Q(sub 0) and eta are estimated to be 183 +/- 33 and 0.3 +/- 0.1, respectively. Over path groups 2 and 3 that sample either a mixture of northern and southern Tibet or eastern Tibet, the estimated Q(sub 0) and eta are between approx. 250 and 270 and 0.0 and 0.1, respectively. Over the fourth path group that samples the easternmost TB the respective estimates are 374 +/- 51 and 0.3 +/- 0.1. A comparison of Pn attenuations in continental regions suggest that they are similar in the easternmost TB, Central Asia, Scandinavia, and the Canadian shield. Northern Tibet has the highest attenuation at all frequencies. Pn attenuation under southern Tibet is similar to the shield regions at low frequencies (/= 1Hz), and similar to northern Tibet at higher frequencies (up to 4 Hz). It appears that a southward increase in the lid temperature causes increases in both Pn velocity and low-frequency Pn Q, but causes no change in high-frequency Pn Q. This phenomenon may be best explained by a propagation mechanism in which Pn traverses the lid as a single, deep refraction at low frequencies, and as multiply bounced and scattered (whispering-gallery) rays at high frequencies. The original document contains color images.