In a compacted basin

natural hydraulic fracturing is one of the key factors of the pore fluid expulsion of uncompacted belt.An uncompacted belt can be described as an abnormal pore fluid pressure belt surrounded by “shell”.The kinetics of the natural fracturing of the “shell” in continous sedimentation(loading)is different from that in succesive erosion(unloading).In loading condition

the shear fracturing of the bottom shell of the uncompacted belt would appear earlier than that of the top.In unloading condition

the “shell” would crack only when the load pressure decrease resulted from erosion is greater than the minimal load pressure required in vertical tensile crack of the “shell”

and the crack usually happens on the top shell.In this view

the expulsion of the pore fluid of the uncompacted belt can be described as a intermittent change process of expelling up and down.