Tephra Stratigraphy of Maar Craters: Implications for Ash Fall Hazard and Risk Assessment in the Auckland Region

Maar craters are a common feature of the Auckland Volcanic Field, often forming freshwater lakes once eruptive activity has abated. A c. 61m core recovered from paleolake sediments that fill Onepoto Basin, a maar crater in North Shore City. Auckland reveals a long, high-resolution record of ash fall from both distal and proximal sources. A detailed chronological framework of the core has allowed volcanic ash fall hazard in the Auckland region to be assessed with greater reliability than has been possible in the past. Drilling of the c. 61m core was completed during November 2001. Detailed logging and analysis of the core identified preservation of 78 distinct ash fall events spanning the time interval 9.5-144ka. The 78 ash horizons represent five individual volcanic sources, including the proximal Auckland Volcanic Field, and distal sources Okataina and Taupo Volcanic Centres. Taranaki volcano, and Tongariro Volcanic Centre. Detailed chronology of the ash fall events is obtained from correlation of ash fall layers the core with events of known age. combined with new radiometric ages from this study. This new information has enabled the frequency of ash fall events from local and distal sources to be determined. Combining ash fall frequencies in the Onepoto core with those proposed in other studies results in a frequency of 1 event per 500 years in the Auckland region (for the period of well-dated events).