Holocene paleoseismicity in the fold and thrust belt of the Hikurangi subduction zone, eastern North Island, New Zealand

Abstract

Along the 500 km long eastern coastline of the North Island, New Zealand there are, at most, seven distinct Holocene marine terraces aged 7 ka B.P. or less. The highest of the terraces is 27 m above present-day mean sea level. The coastal region is subdivided into fourteen distinct subregions based on radiocarbon ages of marine deposits overlying wave-cut shore platforms and geographic distribution of similarly-aged terraces. Holocene marine terraces are the result of uplift associated with large earthquakes (co-seismic deformation). This conclusion is based on characteristic stepped terrace morphology, clustering of ages of terrace deposits within subrogions, and the occurrence of co-seismic uplift in historic time. Differential uplift across structures and distinct age variations at subregion boundaries are also characteristic. Recurrence intervals of uplift in any one terrace sequence vary from ∼ 0.4 ka to ∼ 2.0 ka, and individual amounts of uplift vary from ∼ 1.0 m to ∼ 4.0 m. In the past ∼ 2.5 ka ages from all terraces within subrogions indicate at least 21 paleoseismic events affecting coastal areas of eastern North Island in that period. These events cluster in time, and in separate parts of eastern North Island several earthquakes occurred ∼ 0.3, 0.6,1.0,1.5, 2.1 and 2.3 ka B.P. Our data strongly support the concept of segmentation of deformation along the subduction margin. The likely cause of coastal uplift is movement on steep reverse faults (local structures rather than the subduction thrust) that propagate from or near the subduction thrust some 20–25 km below the region. Earthquakes of moment magnitude 7.3–8.0 are estimated to be associated with these fault movements.