Analysis of local seismicity, crustal and upper mantle structure in Central Asia using data recorded by a seismological network in the Pamir and Tien Shan

The Pamir Mountains form the northernmost promontory of India-Eurasia collision. Cenozoic shortening formed thick crust and a high plateau comparable to Tibet, albeit accommodated over ~50% less meridional orogenic width. Ongoing convergence of about 20 mm/yr causes substantial crustal deformation and seismicity currently concentrated along the Main Pamir Thrust (MPT) that frequently hosts large thrust earthquakes. The Pamir-Hindu Kush mountain range is also unique in featuring frequent mantle earthquakes reaching hypocentral depths of 300 km and more. These mantle earthquakes form a Wadati-Benioff-type zone, which dips steeply northwards beneath the Hindu Kush, but is inclined towards the south in the Pamir. Different models involving subduction of either one contorted or two opposing slabs have been proposed. However, the nature of this seismicity as well as the type of lithosphere involved - oceanic or continental - remains elusive. In the framework of the multidisciplinary project TIPAGE (Tien Shan Pamir Geodynamic Programme), a temporary seismic network consisting of 40 stations was operating in the Pamir and Tien Shan mountains between 2008-2010. During the first year of the deployment, 24 broadband stations were set up in a 350 km long north-south profile with approximately 15 km station spacing. Additional 16 stations were distributed throughout the Pamir. In summer 2009, the network was reconfigured to evenly cover the entire study area. These new data were retrieved in summer 2010. We detected and located local seismic events in an automatic procedure producing an event catalogue that is complete down to magnitudes of around 2.5. A subset of events has been manually relocated with a joint hypocenter determination technique to check performance of the automatic procedure and for preliminary interpretation. The obtained event distribution shows a sub-vertically dipping, continuous zone of seismicity extending to depths of about 300km beneath the Hindu Kush. In the eastern Pamir, seismicity is limited to the uppermost 15 km near the MPT and to a narrow, southward-dipping region between 90 and 180 km depth further south. No seismic events are found between 15 and 90 km depth. Based on our new data we will present a preliminary 1D velocity model for the region as basis for improved hypocentral locations and as a starting model for a future tomography. We used teleseismic events and receiver function analysis to determine crustal thickness and upper mantle structure . Along the profile, crustal thickness varies between 60 km beneath the southern Tien Shan and 80km beneath the southern Pamir. In migrated cross-sections, a prominent southward-dipping structure is visible in the mantle south of the MPT coinciding with Pamir mantle earthquakes. Based on crustal reverberations and Moho conversions we determined gross crustal Vp/Vs ratios, which show a sudden southward decrease from intermediate values (~ 1.73) in Kyrgyzstan to very low values ranging from 1.64 to 1.67 under the Pamir plateau, indicative of an overall felsic crust.