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Nelson Historical Society Journal, Volume 6, Issue 3, 2000
Introduction
Introduction
In comparison with other areas of New Zealand. Marlborough has had more than its share of earthquakes, two large ones in 1848 and 1855, and a smaller one in 1966. In geological terms. Marlborough lies in the middle of the deforming area between two great Earth plates: the Australian Plate to the northwest and the Pacific Plate to the southeast. Along the east coast of the North Island the boundary between the two plates lies about 200km from the coast, but nearer the South Island it swings towards the coast and comes inland beneath Marlborough, where it manifests itself as an approximately 150km-wide zone of earthquake fault lines. These cut through the landscape along the Wairau Valley (Wairau Fault), the Awatere Valley (Awatere Fault), the Clarence Valley (Clarence Fault) and along the southwest side of the Seaward Kaikoura Range (Hope and Kekerengu faults). (Fig. 1)
The Pacific Plate is descending like a conveyor belt into the Earth's interior below the Australian Plate at a rate of about 39mm per year, and it is this relative movement that is the driving force of earthquakes in the region. Uplift along the Marlborough faults during many earthquakes over the last 25 million years has helped create the Inland and Seaward Kaikoura Ranges and the high country bordering the northwest side of the Awatere Valley.
Earthquakes that are accompanied by rupturing of the Marlborough faults are large events and have Richter magnitudes greater than 6. The earthquake magnitude scale is not linear, because for each unit of magnitude there is about a 30-fold increase in the amount of energy released. The largest energy release of any earthquake recorded by instruments occurred during the Chilean earthquake of 1960. which had a magnitude of 9.3 with an energy release perhaps equal to that of the entire world arsenal of nuclear bombs.
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Fig.1. Map showing the main earthquake faults in Marlborough that delineate a wide zone of deformation between the Australian Plate to the northwest and the Pacific Place to the southeast. Arrows indicate the direction and amount of relative movement (39mm/yr) of the Pacific Plate. Inset map of the New Zealand region shows the Australian – Pacific plate boundary, delineated in oceanic areas as troughs (Tr) and a rise (Macquarie R.), and in the South Island as the Marlborough faults and Alpine Fault.
The amounts of energy released during the earthquakes that have affected the Marlborough area lie within this range, and it is a sobering thought that the magnitudes estimated for the earthquakes that occurred in 1848 and 1855 were about 7.4 and 8.1 respectively, and that of 1966 had a magnitude of 6.1. Of course, what you feel depends on how close you are to the origin of the earthquake and how deep it is, as the following account relates.