High-frequencyPandSvelocity anomalies in the upper mantle beneath Asia from inversion of worldwide traveltime data

Abstract

[1] A model of seismic P and S anomalies in the upper mantle beneath Asia (in limits of 35°E-140°E, 12°N-57°N) was constructed based on the tomographic inversion of traveltime data from the revised ISC catalog for the years 1964-2004. The inversions were performed independently in 32 overlapping circular windows that cover the entire study area. The free inversion parameters in each window were defined individually depending on the available data based on synthetic modeling. Such adaptive tuning of parameters enables more optimal usage of the input data in areas with inhomogeneous ray coverage compared to global inversions. This approach resolves high-frequency patterns but is less sensitive to large anomalies with sizes comparable to the window diameter. Thus, this approach is somewhat similar to high-frequency filtration of the velocity distribution. The resolution capacity of the model was tested using checkerboard tests with various pattern sizes. To assess the role of random noise, independent test inversions of two data subsets (with odd and even numbers of events) were performed. Clear reconstructions of known structures, such as subducting plates beneath the Japan and Ryukyu arcs whose locations and shapes have been constrained by other studies, further indicate the reliability of the model. The 3-D models of P and S anomalies presented in horizontal and vertical sections show complex interactions of the lithospheric segments beneath the Alpine-Himalayan orogenic belts. Particular attention is focused on the collisional areas of Iran, Pamir-Hindukush, Tien-Shan, and Burma. The digital version of the 3-D P and S models is available at http://www.ivan-art.com/science/REGIONAL. Citation: Koulakov, I. (2011), High-frequency P and S velocity anomalies in the upper mantle beneath Asia from inversion of worldwide traveltime data,

Geometry of the Pamir‐Hindu Kush intermediate‐depth earthquake zone from local seismic data

“…During the latter process, thickened lithosphere might become gravitationally unstable [Leech, 2001] and delaminate as an intact sheet [Bird, 1979] or viscously drip into the underlying asthenosphere as a Rayleigh-Taylor instability [Houseman et al, 1981]. All these processes have been suggested to occur in the Indo-Eurasian collision zone [e.g., Tilmann et al, 2003;Nábelek et al, 2009;Koulakov, 2011]. It is not easy to distinguish between them, because mantle images from teleseismic waves are often too fuzzy and inferences from surface observations, like uplift history and magmatism, are non-unique.…”

“…Roecker [1982] observed anomalously low seismic wave speeds co-located with intermediate-depth seismicity in the Hindu Kush with local earthquake tomography and interpreted these findings with the presence of continental crustal material at depth. Such a presumably small-scale anomaly would likely not be resolved by the regional or global tomographic models of Koulakov and Sobolev [2006], Koulakov [2011] and Negredo et al [2007], which only image the deeper, largerscale structures. Presence of continental crust at mantle depths (65-110 km) is also supported by crustal xenoliths erupted at 11 Ma in the south-eastern Pamir [Ducea et al, 2003;Hacker, 2005;Gordon et al, 2012].…”

“…It is not easy to distinguish between them, because mantle images from teleseismic waves are often too fuzzy and inferences from surface observations, like uplift history and magmatism, are non-unique. Based on the visual appearance of a drop-like high seismic velocity anomaly that reaches all the way into the mantle transition zone beneath the Hindu Kush, Koulakov [2011] suggested a lithospheric drip currently occurring there. It is not clear whether a Rayleigh-Taylor instability, which is a ductile process, would be capable of creating any seismicity at all.…”

“…However, comparison of these models to nature is difficult because of a lack of observations, not least because of the generally aseismic nature of orogenic mantle deformation, with the apparent exception of the Pamir-Hindu Kush. A variety of models attempting to explain the Pamir-Hindu Kush zone's peculiar geometry (along-strike overturning of dip) and dynamics have been proposed, but fundamental questions, e.g., whether it is related to subduction [e.g., Billington et al, 1977] or exists due to a mantle drip [Koulakov, 2011], whether continental [Roecker, 1982;Burtman and Molnar, 1993] or oceanic material [Chatelain et al, 1980;Pegler and Das, 1998] hosts these earthquakes, or whether Pamir and Hindu Kush form one single [Billington et al, 1977;Pegler and Das, 1998;Pavlis and Das, 2000] or two separate structures [Chatelain et al, 1980;Burtman and Molnar, 1993;Fan et al, 1994] outlined by seismic activity, have not been conclusively solved.…”

Segmentation of the Izu-Bonin and Mariana slabs based on the analysis of the Benioff seismicity distribution and regional tomography results

“…In later works, this algorithm was modified and used to study the upper mantle structure beneath some regions (see, for example, Koulakov et al, 2009a;Koulakov, 2011). This tomographic inversion algorithm is based on the linearised approach which implies that all calculations are performed on the basis of rays constructed in one-dimensional model AK135 (Kennett at al., 1995).…”