Growth of a normal fault by the accumulation of slip over millions of years

Fault growth in the upper crust is mainly achieved by the accumulation of slip during earthquakes. Large faults with cumulative displacements of kilometres experience hundreds of large magnitude earthquakes but the precise manner in which maximum earthquake size evolves over millions of years is uncertain. To bridge the gap between faulting on earthquake (e.g. <100 ka) and geological (e.g. >1 Ma) timescales we examine the Cape Egmont Fault in offshore New Zealand, using good quality seismic-reflection data tied to wells and seabed bathymetry. Displacements on nine horizons, including the seabed, three horizons of 14-225 ka in age and five horizons of 5.3-1.6 Ma in age, were mapped along the ca. 70 km length of the fault. These horizons record maximum displacements ranging from 6 to 2344 m and suggest that this active fault commenced its latest phase of movement about 3.7 Ma ago. Displacement profiles for all horizons and inter-horizon increments are of similar geometry, indicating an approximately constant fault length and fixed maximum displacement position during growth. Fault length was established rapidly and inherited from an underlying Cretaceous structure with fault dimensions in the overlying sequence mainly achieved by up-dip propagation. As the magnitude of earthquakes is proportional to their rupture length, the maximum size of earthquakes on the fault remained constant for at least the last 3.2 Ma, with the distribution of slip along the fault during these events approximately comparable. This contrasts with the maximum displacement rates which varied by an order of magnitude during the last 3.7 Ma (0.18-2.8 mm/yr). Decreases in displacement rates were accompanied by proportional increases in the average earthquake recurrence intervals (ca. 1.3-18 ka), and reflect regional changes in the rates of extension.