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A temporary 37‐station seismic array was operated in southern Tibet from June 2004 to August 2005 close to Xigaze along a traverse from Tangra Yum Co rift in the west to Yadong‐Gulu rift in the east. This lateral array of the international Hi‐CLIMB project recorded a total of 885 local earthquakes during the 14‐month deployment. Hypocenter locations of these events were obtained using HYPOINVERSE2000, and about half of them were relocated by the joint hypocenter determination method. Lateral variations in crustal seismic P and S wave velocities beneath the array were obtained simultaneously as part of the station corrections during the relocation procedure. While earthquakes were scattered in the region, more than 250 earthquakes were clustered within a small area at about 50 km north of the Indus‐Yalu Suture and west of the Pumqu‐Xianza rift. Crustal and uppermost mantle earthquakes are concentrated beneath the Himalayan crest. Projection of all earthquakes to a north–south profile shows that the subducting Indian plate passes the Indus‐Yalu Suture below the Lhasa Terrane. While earthquakes are concentrated in the upper crust and upper mantle beneath the Himalayas, seismicity is mostly restricted to the upper crust to the north, which is consistent with a ductile middle/lower crust beneath southern Tibet. About 20 deep crustal and uppermost mantle events (at depths of 50–70 km) were observed to be associated with the subducting Indian lithosphere. Our relocation results of these local earthquakes in southern Tibet support the popular “jelly‐sandwich” model for the rheology of continental lithosphere in which the strong seismogenic layers of upper curst and uppermost mantle are separated by a weak lower crust.

Introduction Tectonic SettingThe United States portion of the long-lived part of the North American continent consists of Precambrian cratons, including Archean cratons such as the Wyoming craton, and the Proterozoic Interior Platform (Bleeker, 2003;Hoffman, 1988). Since cratonic accretion, the Cenozoic-Mesozoic Rocky Mountain Cordillera and Paleozoic Appalachian Mountains formed on the west and east sides of the craton (Figure 1), respectively. Extending further out from the relatively stable core of the continent and to the east of the Appalachian range, the Atlantic Coastal Plain province in the east is a Paleozoic passive plate margin (Bally et al., 1989). The western edge of North America has a more complex tectonic history. Its seismically and tectonically active continental margin, Mesozoic-Cenozoic orogenies, and arc volcanism are controlled by the interaction between oceanic Pacific, Kula and Farallon plates and the North American plate (Atwater, 1989). Currently, the Juan de Fuca plate, a remnant of the Farallon plate, is subducting beneath North America at the Cascadia subduction zone (Atwater, 1970), causing arc volcanism, earthquakes, and episodic tremor and slip (Rogers & Dragert, 2003). The further inland location of the Laramide orogeny and eastward migration of magmatism during the Mesozoic can be explained by the contemporaneous flattening of the Farallon slab, which was likely caused by the increase in subduction rate and slab buoyancy (Engebretson et al., 1984(Engebretson et al., , 1985Molnar & Atwater, 1978). The Tertiary extensional system of the Basin and Range is likely due to the steepening of the Farallon Plate (Coney & Reynolds, 1977;Davis, 1980). Compared to tectonically active western North America, there is minimal tectonic activity and topographic variation in the central and eastern United States. However, variations in crustal and mantle structure do exist and have been the

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In this paper, high quality seismic data in 1964~2003 from the ISC (International Seismological Centre) catalog are used to present a relatively more complete and definite geometric structure of the intermediate seismic zone in the Pamir‐Hindu Kush region. Dip directions and the largest depth of the Hindu Kush intermediate seismic zone (H zone) and the Pamir intermediate seismic zone (P zone) change along the strike, so they can be further divided into five segments: HW (west segment of the H zone), HE (east segment of the H zone), PSW (southwest segment of the P zone), PM (middle segment of the P zone), and PNE (northeast segment of the P zone). There is a seismic gap in the H zone at about 170~190 km depth, beneath which the seismic zone dips nearly vertically. Meanwhile, the seismic zone beneath the gap forms an inverted V‐shape zone, the two branches of which are nearly perpendicular with each other. The west branch within the HW segment does not have double seismic layers. The east branch within the HE segment is deeper and has double seismic layers. Focal mechanism solutions of the intermediate earthquakes have regular patterns. In the Hindu Kush region, most T axes are nearly vertical while most P axes are nearly horizontal and perpendicular to the strike of the seismic zone. In the Pamir region, from southwest to northeast, horizontal along‐strike axis changes from P to B and then T; P axes gradually change to be perpendicular to the strike. At last, we discuss the possible geodynamic explanations for the seismicity in this region.

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We employed a double-difference algorithm (hypoDD) to relocate earthquakes within the region bounded by 66°E−78°E and 32°N−42°N in the period of 1964−2003 reported by the International Seismological Center (ISC). The improved hypocentral locations delineate a double-layered Wadati-Benioff zone in the eastern Hindu Kush intermediate seismic belt. Based on this feature and other evidences, we propose that the intermediate-depth earthquakes beneath the Pamir-Hindu Kush region may occur in two collided subduction zones with opposite dip directions.

Mapping the spatial distribution of water in the mantle transition zone (MTZ, 410‐ to 660‐km depth) may be approached by combining thermodynamic and experimental mineral physics data with regional studies of seismic velocity and seismic discontinuity structure. HyMaTZ (Hydrous Mantle Transition Zone) is a Python program with graphical user interface, which calculates and displays seismic velocities for different scenarios of hydration in the MTZ for comparison to global or regional seismic‐velocity models. The influence of water is applied through a regression to experimental data on how H2O influences the thermoelastic properties of (Mg,Fe)2SiO4 polymorphs: olivine, wadsleyite, and ringwoodite. Adiabatic temperature profiles are internally consistent with dry phase proportion models; however, modeling hydration in HyMaTZ affects only velocities and not phase proportions or discontinuity structure. For wadsleyite, adding 1.65 wt% H2O or increasing the iron content by 7 mol% leads to roughly equivalent reductions in VS as raising the temperature by 160 K with a pyrolite model in the upper part of the MTZ. The eastern U.S. low‐velocity anomaly, which has been interpreted as the result of dehydration of the Farallon slab in the top of the lower mantle, is consistent with hydration of wadsleyite to about 20% of its water storage capacity in the upper MTZ. Velocity gradients with depth in absolute shear velocity models are steeper in all seismic models than all mineralogical models, suggesting that the seismic velocity gradients should be lowered or varied with depth and/or an alternative compositional model is required.