Search

In this study, we used the Pn tomography method to obtain detailed velocity and anisotropy structures of the uppermost mantle beneath Sakhalin–Kuril–Kamchatka region for improving the understanding of plate subduction, arc–arc collision, and volcanism. We found low Pn velocities beneath volcanoes and areas characterized by pronounced tectonic activity and high Pn velocities with strong anisotropy in the subducting plate. Low Pn velocity anomalies beneath southern Sakhalin connected the low‐velocity anomalies in the mantle and crust, indicating the ascent of fluid or melt, and may provide a magmatic source for the rear‐arc Rishiri volcano. In the absence of plate subduction, low‐velocity anomalies and north–south Pn anisotropy manifested beneath northern Kamchatka, revealing the lateral propagation of mantle flow beneath this northern region. We suggest that the eastern boundary of the slab window at the Kamchatka–Aleutian junction is likely to be located near the Komandorsky Islands. Plain Language Summary: The Northwest Pacific region is prone to recurrent earthquakes and volcanic activities, attributable to plate subduction and multiple plate interactions. This study, focuses on elucidating the uppermost mantle structure beneath the Sakhalin–Kuril–Kamchatka region using seismic tomography. Beneath the island chain featuring volcanoes and areas with a lot of tectonic activity, seismic waves travel slower. This phenomenon implies the possible presence of magma or hot materials beneath these regions. In southern Sakhalin, a linkage is established through low Pn velocities connecting areas of low seismic wave velocity in the deeper mantle and crust. This linkage suggests a conduit for fluids or molten materials, potentially feeding for the Rishiri volcano situated beyond the volcanic arc. Strong low velocities, coupled with north–south Pn fast directions are identified beneath northern Kamchatka, indicating the northward propagation of mantle flow. Our imaging analysis suggests that the eastern edge of the slab window (the region where the plate is absent) at the Kamchatka–Aleutian junction is likely proximate to the Komandorsky Islands. This study provides new seismological insights into the plate collision and subduction processes as well as the deep structures in the Sakhalin–Kuril–Kamchatka region. Key Points: Low Pn velocities clarify the upward path of the mantle‐derived thermal material beneath Sakhalin and its influence on surface volcanismStrong low Pn velocities and N–S anisotropy beneath northern Kamchatka signify the lateral propagation of mantle flow directed northwardThe eastern boundary of the slab window at the Kamchatka–Aleutian junction may be located near the Komandorsky Islands [ABSTRACT FROM AUTHOR]

An attempt has been made to reveal the upper mantle velocity structure of the eastern Tibetan Plateau using 628 teleseismic events recorded from 2003 to 2009 at 95 stations. A total of 8 532 P-wave arrival time residuals were inverted by using the FMTOMO (fast marching tomography) software package. Tomographic results show upper mantle velocity heterogeneity in many aspects. In the southern part visible high velocity anomaly is denoted as the Indian lithosphere. This part seems to be affected by slab tearing at 94°E longitude as it is located on the eastern Himalayan syntaxis (EHS). The high velocity zone down to 500 km depth in the northern part could be the Asian lithosphere. At the central part some high velocity anomalies can be identified as detached patches of the lithosphere, surrounded by low velocity anomalies. These anomalies are the potential to create thermal convection and trigger plateau uplift or plateau growth. Sudden velocity change occurs on both sides of patches where low velocity anomaly is visible in between patches and Bangong-Nujiang suture even in between Songpan-Ganzi terrain and Asian Plate. In both cases intense low velocity zone spread down to 500 km. The depth range of low velocity anomalies in between two plates observed from 200 to ∼500 km. Hence the low velocity anomalies detected in our results may reflect either the hot asthenosphere upwelling or the mantle wedge due to the presence of the cold lithosphere. [ABSTRACT FROM AUTHOR]

Abstract: To investigate upper crust anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and azimuthal anisotropy, and it was applied to Pg travel time data around the epicenter of the 14 April 2010 Ms=7.1 Yushu earthquake. A high-resolution two-dimensional (horizontal) seismic velocity and azimuthal anisotropy model was obtained using this simple tomography method including both station and event depth corrections. The results show clear anisotropy in the upper crust and it is as important as the velocity variation in explaining the travel time residuals. The most striking result is a high velocity anomaly with large anisotropy at the epicenter on the Yushu–Garze fault. The main rupture originated within this high velocity anomaly and propagated southeastward into a low velocity anomaly with small anisotropy at Yushu. The azimuthal anisotropy shows fast velocity direction along Yushu–Garze fault and larger anisotropy exists in the epicenter than surrounding regions. These results demonstrate a clear example that lateral variation and anisotropy in seismic structures of the upper crust controlled the origination (stress accumulation) and rupture propagation of the 2010 Yushu earthquake and distribution of aftershocks as well. The simple 2D Pg-wave travel time tomography method presented here introduces a new approach utilizing abundant aftershocks data for investigating the rupturing process of a major earthquake and possible fracture distribution around seismogenic zone. [Copyright &y& Elsevier]

High-resolution 3-D P and S wave velocity (Vp and Vs) models of the crust and uppermost mantle beneath the northeastern (NE) Tibetan Plateau and the surrounding areas were obtained by applying an improved double-difference seismic tomography method on abundant body wave travel-time data of local and regional earthquakes collected from 1985 to 2018. The improved method not only increases constraints from observed Moho depth information but also incorporates the later-arriving Pg and Sg phases to improve the ray coverage in the middle-lower crust and uppermost mantle. The results show significant low-velocity anomalies in the crust beneath the NE Tibetan Plateau and high-velocity features beneath the surrounding stable Alxa and Ordos blocks. The areas near the boundaries of the cratonic blocks show high velocity in the upper crust and uppermost mantle beneath the Hexi Corridor, and high velocity in the upper crust beneath the Liupan Shan belt. Both of those regions have significant low-velocity anomalies in the middle-lower crust similar to the Tibetan Plateau, which suggests they probably were parts of the Alxa and Ordos blocks respectively, and were cut off from the two cratonic blocks during the lateral expansion of the Tibetan Plateau because of middle-lower crustal ductile deformation and asthenospheric flow. We propose a new tectonic model controlled by middle-lower crustal ductile deformation and asthenospheric flow to illuminate the tectonic evolution and growth mechanism of the NE Tibetan Plateau. • A high-resolution 3-D crustal and uppermost mantle velocity model beneath the NE Tibetan Plateau was determined. • Compared with the stable Alxa and Ordos blocks, obvious low-velocity features are detected in the crust of the NE Tibet. • A new tectonic growth model controlled by middle-lower crustal ductile deformation and asthenospheric flow was proposed. [ABSTRACT FROM AUTHOR]

Abstract: We examine the Pn and Sn travel time data with long and short epicentral distance ranges separately and obtain 2.5-dimensional tomographic models for the uppermost mantle velocity and the Vp/Vs ratio beneath the Sichuan–Yunnan and adjacent regions. The tomographic models show that the average Vp and Vs of the upper layer are approximately 8.10km/s and 4.60km/s and those of the deeper layer are 8.19km/s and 4.65km/s, respectively; the average velocity ratio is 1.76 under the study area. A comparison of the Pn and Sn velocity distribution reveals that the high- and low-velocity anomalies are consistent. High velocity and low Vp/Vs ratios exist in the Sichuan Basin and low velocity and high ratios distribute in the Myanmar–Yunnan, Songpan–Ganzi, and Hainan regions, respectively. Our models provide more evidence for the subduction of the Indian plate beneath the Myanmar–Yunnan region and the existence of the hot material upwelling beneath the Hainan region. In our velocity model, there is a significant difference between the southern and the northern parts of the Yunnan region (i.e. north and south of 26° N); we infer that these regions are controlled by different dynamic processes. The velocity distributions at different depths are mainly similar in most of the regions of the study area, and the differences between the upper layer and the deeper layer beneath the Yunnan region indicate that the heat flow from the upper mantle has complex characteristics. [Copyright &y& Elsevier]

Abstract: Analyses of global seismic waveform data for the 2008 devastating Wenchuan Earthquake have indicated where the rupture started and how it expanded. Here we reveal that the rupture process of this earthquake was strongly controlled by structural heterogeneities of the upper crust. We made a tomographic inversion of 200,000 local travel time data, according to which the rupture was confined in a presumably brittle layer of high P- and S-wave velocity anomalies within the upper ~15 km depths. The rupture propagated northeastward through the southwest (SW) segment of the fault and then jumped to the northeast (NE) segment with little slip at the intervening, presumably ductile zone characterized by low P and S velocities with high Poisson''s ratio. The rupture initiated near the southwestern end of the SW segment within a body of high Poisson''s ratio, which extends down into the lower crust, suggesting upward migration of deep-seated aqueous fluid to trigger the earthquake by reducing the mechanical strength of the brittle layer. [Copyright &y& Elsevier]