Two crustal cross sections through the eastern margin of the Tibetan Plateau are jointly determined from deep seismic sounding. The E–W trending line AA’ passes through the western Sichuan plateau (including the Songpan‐Garze terrane and the Longmenshan fault belt) and ends in the Sichuan basin (a part of the Yangtze craton). Line BB’ has a trend of NNE and crosses the Songpan‐Garze terrane. Two‐dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data. Our seismic velocity models indicate that the western Sichuan plateau and the Sichuan basin have crustal thicknesses of 62 and 43 km, average crustal P wave velocities of 6.27 and 6.45 km/s and lower crustal (Vp > 6.5 km/s) thicknesses of 27 and 15 km, respectively. Density models constructed from the seismic velocity models are consistent with observed Bouguer gravity anomalies. We infer that collision between the Tibetan Plateau and the Yangtze craton has caused thickening of the lower crust and uplift of the western Sichuan plateau. We detect a low‐velocity layer in the upper crust of the western Sichuan plateau but observe no equivalence in the Sichuan basin; west dipping thrusts may detach into this low‐velocity layer. The seismic phase PmP in the western Sichuan plateau has low amplitude, suggesting high attenuation in the lower crust (Qp of 100–300). We suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western Sichuan plateau.

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Abstract.The precursory swarm, three mainshocks (M 7.2,6.7, 7.2), and aftershocks of the Songpan earthquakes have been reanalyzed using both local and teleseismic data. The three mainshocks of this sequence occurred on the Huya fault over a 7-day period.Relocations of the aftershocks using local arrival times show that three fault strands were activated during this sequence.Each mainshock occurred on a separate strand, each one south of the strand activated in the previous mainshock, and the aftershock zones of each mainshock appear to abut rather than overlap.Fault plane solutions determined by matching teleseismic P waveforms at World-Wide Standard Seismograph Network stations with synthetic seismograms are consistent with the observed aftershock zones 19The first a& third mainshocks (Mo = 1.3xl0 and 8.4xl0 N m, respectively) showed almost identical senses of motion, a combination of reverse and left-lateral strike-slip motion, on parallel strands, striking Nl5°W, that were separated by a large rightstepping en yghelon offset. The second mainshock (Mo = 4.0xl0 N m), occurred in this offset on a fault at a steep angle (-125°) to the other two strands and showed almost pure reverse motion. Differences in the orientations of the slip vectors of the three mainshocks show that the first mainshock increased the normal and shear stresses on the fault segment that moved in the second mainshock and that the second mainshock decreased the normal stress on the fault segment activated by the third mainshock. These changes in normal stresses may have given rise to the longer time between the first and second events ( 5 days) as compared with the time between the second and third events (30 hours).A precursory swarm that preceded the Songpan sequence by 3 years occurred in a volume that surrounded the northernmost part of the planar aftershock zone.The time between the start of the swarm and the mainshocks and the magnitude of the largest event in the swarm are similar to those seen for precursory swarms in Soviet Central

A flow‐through sampler was deployed to record the seasonal variability of the atmospheric concentrations of semivolatile organic compounds (SOCs) at a remote research station located close to Nam Co Lake on the Tibetan plateau. Between October 2006 and February 2008, fifteen consecutive one month‐long samples, with air volumes ranging from 4,500 to 16,000 m3, were taken and analyzed for polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and organochlorine pesticides (OCPs). Separate analysis of three polyurethane plugs in series in combination with frontal chromatographic theory allows for the correction of the break‐through observed for the most volatile SOCs. The concentrations of Σ56PCB in air range from 0.10 to 2.6 pg·m−3 and are among the lowest values ever reported. Levels of OCPs at Nam Co are generally also very low, particularly during wintertime. The concentrations of hexachlorocyclohexanes (HCHs), endosulfans, and various dichlorodiphenyltrichloroethane (DDT) related substances display a distinct seasonal variability consistent with the monsoon. Back‐trajectory analysis reveals that higher OCP levels during summer correlate with air mass origin south of the Himalayas. A high α/γ‐HCH ratio and a non‐racemic composition of α‐HCH during July/August suggest that evaporation from Nam Co Lake contributes to the relatively high concentrations of α‐HCH (averaging ca. 91 pg·m−3) recorded in the summertime atmosphere.

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