Micropalaeontological evidence for the Holocene earthquake history of the eastern Bay of Plenty, New Zealand, and a new index for determining the land elevation record

• Published in: Quaternary science reviews Vol. 23; no. 14; pp. 1651 - 1667
• Main Authors: Hayward, Bruce W, Cochran, Ursula, Southall, Kate, Wiggins, Emma, Grenfell, Hugh R, Sabaa, Ashwaq, Shane, Phil R, Gehrels, Roland
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2004
• Subjects: Bacillariophyceae; Foraminifera; PSE, New Zealand; PSE, New Zealand, North I., Bay of Plenty
• ISSN: 0277-3791; 1873-457X
• DOI: 10.1016/j.quascirev.2004.01.010

Fossil foraminifera and diatoms are used to identify sudden, probably earthquake-related, elevational changes in three Holocene sedimentary sequences from the high-tidal fringes of Ohiwa Harbour, eastern Bay of Plenty, New Zealand. Modern analogue calibration sets of faunal and floral census data are used to estimate palaeosalinities and palaeotidal elevations that help quantify seismic-related, vertical displacements. Age models for the three vibracored sequences are built on a combination of tephrostratigraphy and radiocarbon dating. A new index, the land elevation record (LER), is introduced to graphically portray earthquake-related vertical land displacements on a time-depth diagram. Also plotted are elements used to calculate LER, such as the indicative depth estimated from microfossils, inferred sediment compaction, and the New Zealand Holocene palaeo-sea-level curve. All three Ohiwa cores, spread over 3 km of coast, contain both freshwater and intertidal sediments. A prominent erosional contact between freshwater peat or soil and overlying intertidal mud, records a major subsidence event in each core of c. 2 m, dated at ca 2600 cal years BP. The deepest core (7.4 m) indicates that this is the only substantial vertical displacement event to have occurred in the last 8 ka. A small subsidence event (ca 0.3–0.7 m) is indicated close to the top of one core, but is not present in the other two sites. This may be the result of a local land subsidence during the poorly known Taneatua Earthquake of 1866. There is no historic human record of earthquake displacements around Ohiwa, but mid-Pleistocene, interglacial, marine sediments have been uplifted 10–60 m in several identified fault blocks. Our study provides conclusive evidence of at least 2 m of earthquake-related, subsidence during the Holocene, with a recurrence time of major earthquakes of ca 5 ka.