Vein structures, like ripple marks, are formed by short-wavelength shear waves

摘要

Vein structure, a distinctive structure in deep-sea hemipelagic clayey and siliceous mudstones at convergent plate boundaries, consists of closely spaced mud-filled veins forming an array usually parallel to the bedding plane. This structure has been regarded as a seismite that formed during earthquake shaking by resonance of fractures. Our detailed field observations and shaking model experiments verified that both basic (first-stage) and advanced (developed-stage) vein structures can be explained by a systematic theory. Essentially, a vein structure forms in an array of fractures as a result of shearing, not by push waves but by shear waves of very short wavelength, by a mechanism very similar to that which forms ripple marks. We found the height of a vein array (H) to be systematically related to vein spacing (S ) such that H = 5S, suggesting that the shear strength of the sediment is proportional to the thickness of the standing wave part of a sediment layer. Shear or oscillatory flow occurs in the top part of a sediment layer, under which standing waves with wavelengths on the order of millimeters to centimeters lead to vein structure formation. Vein structure can develop not only in response to earthquake shaking but also by propagation of shear waves along a large fault or by other kinds of shears associated with density or debris flows, landsliding, or faulting.