Vertical deformation associated with the 1987 Edgecumbe earthquake, New Zealand, from lower order levelling

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Abstract
Levelling data from all available surveys in the vicinity of the Edgecumbe Fault Monitoring Pattern (FMP), 3 km south of the town of Edgecumbe in the Bay of Plenty, have been analysed for coseismic and postseismic vertical deformation associated with the 1987 March 2 Edgecumbe earthquake. A study has also been made of second and third order levelling along the Edgecumbe-Awakeri road and railway, crossing the main fault 1. 5 km southeast of the town, and a comparison made with the McCracken Road FMP results. Coseismic deformation resulting in total vertical displacements of 1. 4–1. 8 m across the fault was relatively complex in the Edgecumbe FMP area. This was because of a bifurcation of the surface trace, although differential settlement of alluvial sediments may have also contributed. Postseismic deformation continued in the same sense in both areas, eventually attaining 3% of the coseismic component. Although some further fault displacement and anomalies occurred, most of the movement took the form of downward tilting toward the epicentre. Most of the postseismic deformation occurred within a month of the earthquake, decaying with time to reach a very low rate six months later.