Active relay ramps and normal fault propagation on Kilauea Volcano, Hawaii

Individual segments of the Koae Fault System, Hawaii, show four patterns of structures around fault tips, and these are inferred to represent four evolutionary stages of fault growth. These are: (1) monoclinal bending occurs, probably above a steeply dipping fault at depth, (2) cracks develop in the hinges of the monocline, (3) throw starts to develop on the cracks when they reach a width of about 3 m, which is probably when they link downwards to the normal fault, and (4) rollover and related cracks develops in the hanging wall as throw increases. Widths of monocline- and fault-related cracks obey a power-law distribution, with a 3 m upper cut-off, beyond which the monocline- and fault-related cracks develop a throw and become faults. Relay ramps are common within the highly segmented Koae and Hilina active normal fault systems. Three distinct geometries of relay ramps can be identified at Kilauea Volcano, and these are inferred to represent the following three evolutionary stages of relay ramps. (1) Where the bounding faults understep, the relay ramps have a gentle dip, and a set of en échelon cracks may cut across the relay ramp; these cracks suggest that the two understepping faults connect into a single fault at depth. (2) The dip of the relay ramp increases as the faults overstep. Connecting faults start to cut across the relay ramp. (3) When the relay ramp is breached by the connecting fault, a single, irregular fault is produced. Cracks or small breaching faults across a relay ramp suggest the bounding faults are connected at depth, and suggest that the bounding faults may both slip during an earthquake event.