Characteristics of earthquake sequences in the Central Volcanic Region, New Zealand

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Abstract
Earthquake sequences in the Central Volcanic Region (CVR) of New Zealand are part of a continuum of types of earthquake sequence that range from main shock ‐aftershock sequences to swarms. Most sequences, irrespective of the largest event in the sequence and the duration of the sequence, have swarm characteristics; that is, the difference in magnitude between the largest and second largest event is 0.3 magnitude units or less. Smaller sequences typically have a duration of a few hours to a day in length, whereas sequences with large magnitude events may continue for many weeks. The time distributions of earthquakes within the sequences range from simple foreshock or aftershock patterns to more complex patterns involving both foreshocks and aftershocks separated by periods of seismic quiescence. However, it does not follow that a sequence with an aftershock time distribution will have a magnitude distribution that will be representative of a main‐shock sequence. Routine location data indicate the source volume of the smaller sequences is a few tens of cubic kilometres. Larger sequences, such as the swarms that occurred in 1922, 1964–65, and June‐July 1983, and the large main‐shock sequences that have occurred in the CVR, have source volumes of many hundreds or thousands of cubic kilometres. However, many of the earthquakes in smaller sequences have very similar waveforms, suggesting the actual source volume is even less than routine location data would indicate. In one case the best estimate of the source volume is a few hundred cubic metres. The absence of volcanic tremor and low‐frequency (B‐type) earthquakes in the sequences suggests they are not directly related to ongoing volcanism but occur as a result of regional deformation. When data from CVR sequences covering a wide magnitude range are combined, the magnitude of the largest earthquake in a given sequence is observed to be a function of the logarithm of the duration of the sequence in hours and the logarithm of the epicentral area of the sequence in square kilometres, according to the relation MMAX = 2.65 (± 0.25) + 0.42 (± 0.19) log (duration) + 0.39 (± 0.33) log (epicentral area). By applying this formula while a sequence is in progress, using the duration and epicentral area to that time, only an underestimate of the largest earthquake expected in the sequence can be obtained; however, this still may have use in advising local authorities and the general public during periods of earthquake activity.

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