Research helps predict earthquake aftershocks

A Victoria University student has developed a model that will help predict the expected occurrence of aftershocks after a large earthquake has occurred.

As part of her work for her PhD thesis, Annemarie Christophersen developed a model which statistically predicts the numbers of aftershocks of different sizes and in different time intervals following a large shallow earthquake.

As an example, her thesis determined the chance of a magnitude 7.5 earthquake in New Zealand being followed by an aftershock of more than magnitude 6 within seven days was 38 percent. The probability increased to 42 percent within 14 days and to 46 percent within 30 days.

The model was derived from a global database of earthquake sequences and has small but significant differences for different geological settings which means it can be used anywhere in the world.

The research had to overcome the diversity of aftershock sequences: sometimes many aftershocks occur, sometimes few. Christophersen's model successfully deals with this diversity and was shown to accurately match the actual occurrence of large aftershocks of various sizes.

Her work is expected to have two immediate applications: forecasting for insurance purposes the expected occurrence of aftershocks which might cause damage and advising emergency managers on the likely further occurrence of aftershocks after a large earthquake has occurred.

The idea for Christophersen's thesis came from David Middleton the general manager of the New Zealand Earthquake Commission and her project was supported by the Commission.

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