We use receiver functions calculated for data collected by the INDEPTH‐IV seismic array to image the three‐dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern Tibet. Our results indicate an average crustal thickness of 65 to 70 km in northern Tibet. In addition, we observe a 20 km Moho offset beneath the northern margin of the Kunlun Mountains, a 10 km Moho offset across the Jinsha River Suture and gently northward dipping Moho beneath the Qaidam Basin. A region in the central Qiangtang Terrane with higher than normal crustal Vp/Vs ratio of ∼1.83 can be the result of the Eocene magmatic event. In the Qiangtang Terrane, we observe a significant lithospheric mantle discontinuity beneath the Bangong‐Nujiang Suture at 80 km depth which dips ∼10° to the north, reaching ∼120 km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20 km depression of the 660‐km discontinuity in the mantle transition zone beneath the northern Lhasa Terrane in central Tibet, which suggests this phase transition has been influenced by a dense and/or cold oceanic slab. A modest ∼10 km depression of the 410‐km discontinuity located beneath the northern Qiangtang Terrane may be the result of localized warm upwelling associated with small‐scale convection induced by the penetration of the sinking Indian continental lithosphere into the transition zone beneath the central Tibetan Plateau.

S U M M A R YThe 127 station NorthEast China Extended SeiSmic Array (NECESSArray) provides large quantities of high quality seismic data in northeast China that allow us to resolve lateral variations of Lg Q or crustal attenuation at 1 Hz (Q o ) to 2.0 • or greater. Using the reverse two-station/event method with 11 642 Lg path-amplitudes from 78 crustal earthquakes, we obtain a 2-D tomographic image of Lg Q o with values ranging from ∼50 to 1400. A high degree of detail in the lateral variation of Lg attenuation is revealed in our tomographic image. High Q o regions are found in the Great Xing'an, Lesser Xing'an and Songen-Zhangguangcai Ranges. Low Q o regions are observed in the Songliao, Sanjiang and Erlian Basins. The lowest Q o is found near the Wudalianchi volcanic field and other Quaternary volcanic fields, the southern Songliao Basin, the western edge of the Erlian Basin and the Sanjiang Basin. Low Q o values are measured for paths that cross sedimentary basins with thick, unconsolidated sediments. Most of the high Lg attenuation in the Songliao Basin correlates reasonably well with low crustal Rayleigh wave phase velocity anomalies. The highest attenuating regions also correlate well with regions of Holocene volcanism.Key words: Tomography; Heat flow; Earthquake ground motions; Seismicity and tectonics; Guided waves; Seismic attenuation; Seismic tomography. I N T RO D U C T I O NThe Lg wave is the most prominent high frequency (f > 0.5 Hz) regional phase observed at regional distances. It has a group velocity of about 3.5 km s -1 in northeast China and can be modelled as the sum of higher mode surface waves (Oliver et al. 1955;Knopoff et al. 1973;Herrmann & Kijko 1983) or as many supercritically reflected crustal S waves (e.g. Bouchon 1982). This wave only propagates efficiently over continental paths and can be seen at distances up to 30 • in the Canadian Shield (Hasegawa 1985). The study of Lg wave attenuation has often been used for characterizing crustal structure since the sensitivity of the Lg wave to crustal properties makes it an important tool in better understanding the underlying causes of seismic anomalies in the crust (Ruzaikin et al. 1977;Ni & Barazangi 1983;Xie & Mitchell 1990;McNamara et al. 1996;Xie 2002;Zor et al. 2007;Phillips & Stead 2008;Zhao et al. 2010;Bao et al. 2012). Lg attenuation is also an important factor in determining seismic hazard since the large amplitude of the Lg wave is capable of causing structural damage at regional distances (Campillo et al. 1989).Early efforts to map crustal attenuation relied on the temporal decay rate of the Lg coda (e.g. Singh & Herrmann 1983). In these studies source effects were model dependent and site effects, which are due to the near surface effect of sediments and the focusing and defocusing of the seismic waves, were omitted. Furthermore, efforts at understanding Lg attenuation have been restricted to modelling it as if only geometric spreading and attenuation affect the amplitude, while in reality, the Lg wave train is much more comp...

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