One Month’s Warning

Residents of Gisborne were given one month’s warning of the destructive earthquake which occurred just before noon on March 5th 1966.

Foreshocks commenced on February 2nd 1966, but were too small to be felt. However, the two 4th magnitude earthquakes 28 minutes apart during the evening of February 5th were certainly felt. The first event at magnitude 4.1 struck at 8.19 p.m. and was followed by a magnitude 4.0 shake at 8.47.

According to the New Zealand Department of Scientific and Industrial Research Bulletin 194 “Gisborne Earthquake New Zealand March 1966” there were 50 foreshocks before the magnitude 6.2 quake on March 5th. At the time, the two earthquakes and their 48 smaller counterparts were not recognised as foreshocks, and it is likely that they would not be recognised as such today, although they would be noted as unusual.

One of the vexing issues facing seismologists is that they still cannot recognise a foreshock, until the main earthquake has occurred nearby and a study is carried out. The typical foreshock doesn’t seem to exhibit any characteristic which marks it as a precursor to a larger event. In all respects it is simply an earthquake. Examination of recognised historical foreshocks is being carried out in the hope that a previously unrecognised aspect which identifies them as precursors can be found.

Nowadays, with the benefit of increased knowledge and surveillance, earthquake pairs, swarms and clusters are more readily identified, and can be investigated as unusual activity. However, there are still no absolutes in seismology. A series of earthquakes that is unusual in one location may be normal within the context of activity at another location.

Back in 1966, there were only 18 seismographs in operation in New Zealand at the time of the Gisborne earthquake. They gathered data on smoked glass or paper, and were read manually. There was no nationwide data network allowing information to be gathered and analysed at a central location within minutes of an earthquake event.

Even so, the scientists of the 1960s were very active in placing the foundations for our current knowledge and identifying areas for further study. In preparing their 1969 report for the DSIR, they had obtained information on the Gisborne earthquake from many overseas sources including Scandanavia, “at distances where phases transmitted through the core [of the Earth] are of especially large amplitude for New Zealand earthquakes.” [DSIR Bull.194, p 8].

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