In this paper, we study a series of L-torsion invariants from the viewpoint of the mapping class group of a surface. We establish some vanishing theorems for them. Moreover we explicitly calculate the first two invariants and compare them with hyperbolic volumes. 1. Magnus representation Let Σg,1 be a compact oriented smooth surface of genus g with a boundary ∂Σg,1 ∼= S . In this paper, we alwa...