High-µ Signature in Lavas of Mt. Oku: Implications for Lithospheric and Sub-lithospheric Contributions in the Magmatism of the Cameroon Volcanic Line (West Africa)

Abstract
This study presents geochemical data for mafic lavas from Mt. Oku to investigate the mantle source for Mt. Oku lavas and regional variations in isotopic ratios along the Cameroon Volcanic Line (CVL). The investigated mafic lavas present trace element and isotope signatures akin to ocean island basalts (OIB) and cover the entire range of the CVL. Major and trace element compositions of the studied lavas indicates that they have undergone fractionation of olivine + clinopyroxene + Fe–Ti oxide ± Cr–spinel with insignificant crustal contamination. The lavas were generated by <2% partial melting from a heterogeneous source containing less than 4% garnet. Major and trace elements abundances indicate the presence of hydrous minerals indicative of modal metasomatism.
The highest value of radiogenic 206Pb/204Pb (20.7) so far measured along the CVL is reported in this study. The isotopic features of the lavas suggest that the HIMU signatures are dominantly inherited from the asthenosphere (primary source) and melting of enriched components hosted in the subcontinental lithospheric mantle (SCLM; secondary source).
The isotopic composition of Mt. Oku lavas (87Sr/86Sr = 0.7030–0.7036, 143Nd/144Nd = 0.5127–0.5129, 206Pb/204Pb = 17.9–20.7) suggest that the lavas are the result of contributions from the asthenosphere and the SCLM. Using a combination of Sr–Nd–Pb isotope display on a 3D plot and Sm/Yb, we identify four essential components denoted; A, B, C and D involved in the petrogenesis of Mt. Oku lavas and the approximate melting depth. While A and B are from the asthenosphere with HIMU mantle flavour, C and D owe their origin from the SCLM with EM1 and DMM characteristics, respectively. Although depleted in Sr–Nd isotopes, D is radiogenic in 206Pb/204Pb, indicating the influence of metasomatism.
Considering the geophysical studies on mantle convection and plumes, we have developed a schematic model that explains the petrogenesis of the CVL lavas. The continuous spectrum from relatively depleted to HIMU characteristics of CVL lavas is associated with the progressive change in source components (i) SCLM with ± pockets of enriched metasomes to (ii) SCLM + asthenosphere sources and finally to (iii) typically asthenospheric source material ± pockets of enriched metasomes. Lavas derived from stage (i) exhibit geochemical variability involving DMM–like and EM1–like signatures. The magma generated at Stage (iii) is dominated by the HIMU mantle component and accounts for over 85% of CVL lavas.

Keywords: High–µ, Mt. Oku, Cameroon Volcanic Line, subcontinental lithospheric mantle, sub–lithospheric mantle.

Lithospheric deformation, foundering and underthrusting beneath Tibet

Abstract:
Long-standing debates exist over the timing and mechanism of uplift of the Tibetan Plateau and, more specifically, over the connection between lithospheric evolution and surface expressions of plateau uplift and volcanism. Receiver function data recorded by regional seismic networks of the China Earthquake Administration suggested a different deformation mechanism between the SE and NE margins of the Tibetan Plateau. On the other hand, waveform tomography revealed a T-shaped high wave speed structure beneath the south-central Tibet, which provides seismic evidence for a scenario of the foundering and underthrusting of the Tibetan and Indian lithosphere.

An introduction to Lamb’s problem in seismology

Speaker: Haiming Zhang (School of Earth and Space Sciences, Peking University, P. R. China)

Abstract
Lamb’s problem is a classical problem in seismology. Since Lamb’s classical work in 1904, many mathematicians, mechanics and seismologists have devoted themselves to the study of this problem and developed different solutions. In recent years, new progress has been made in this old problem. At present, the generalized closed form solutions of Lamb’s problem with all kinds of source and receiver positions have been obtained. In this talk, Dr. Zhang will briefly review the research history of Lamb’s problem, and introduce two classical solutions and the latest development of this problem.

A short introduction to the speaker:
Dr. Zhang is now an associated professor of geophysics at the School of Earth and Space Sciences, Peking University, P. R. China. He received Ph.D. in 2004, and has taught in the University of Chinese Academy of Sciences and Peking University successively. His research interests lie in earthquake source dynamics and theoretical seismology.

Surface wave mode coupling – implications for surface wave tomography and for seismology in general

Abstract:
Surface wave tomography (SWT) is a powerful tool for studying the shear-velocity (Vs) structure of the Earth’s crust and upper mantle, typically using seismic surface waves in the period range 50-300 s. The quest for higher resolution Vs models requires using higher frequency (shorter period) as well as higher mode surface wave data, which are more challenging to incorporate in tomographic analyses. Such wave types are more susceptible to complex wave propagation effects arising from laterally heterogeneous Earth structure, which cannot be modelled by the ray-theoretical framework of conventional SWT methods. This talk focuses on one such effect, namely cross-branch mode coupling, or interconversion between surface wave modes. I will present a synthetic study that analyses surface wave mode coupling in detail, through 2-D numerical modelling, and also assesses its impact on multi-mode waveform inversion in two-step path-average SWT. The results of this study show significant mode conversion and, consequently, a breakdown of the path average approximation at periods of 30 s or less, where errors in the inferred group velocities can be as large as 7 per cent. This study serves to inform future generations of SWT methods. In the last part of this talk, I will describe how surface wave mode coupling is relevant for earthquake ground motion prediction, and therefore finds application in seismic hazard assessment.

微動とアクティブ震源を用いた地殻モニタリング

GNSS等の測地データの蓄積により、地震や火山噴火に伴う地表変動が明らかになってきた。しかし地震に伴う地殻応力の変化や、火山噴火に伴うマグマ溜まりの変動は、地殻深部で生じる現象である。つまり地震や火山噴火に伴う動態を正確に理解するには、地殻内部の変動を観測するのが好ましい。本研究では、日本列島全域に設置されている地震計で記録された微動データに対して地震波干渉法等を適用し、地殻深部を伝わる弾性波速度（S波速度）の時空間変化を推定した。モニタリングで得られた弾性波速度変化は、地殻の間隙水圧の変化を反映しており、その変化のパターンは岩石の種類や浸透率に依存することが分かってきた。同様の手法は、海溝付近に設置されている海底地震計にも利用できる。紀伊半島沖のDONETに適用したところ、プレート沈み込みに伴う歪の蓄積と、地震に伴う歪の解放といった断層の動態を捉えることができる可能性が示された。近年は、さらに精度良く（高い時空間解像度で）モニタリングを実施するために、定常モニタリング震源とDASを用いたシステムの開発を行っている。本発表では、これらのモニタリングの結果と課題、将来に向けた取り組みについて紹介する。