Temporal variations in coda duration of local earthquakes in the Wellington region,New Zealand

Temporal variations of coda duration, relative to event magnitude, for local earthquakes near Wellington have been investigated. The region is one of plate convergence and subduction. The data consist of routinely made observations for events from 1978 through 1985 (1552 events), magnitude 1.6 to 5.2, depth 0.5 to 90.0 km. The observed average (over time) correspondence between duration (as measured from the origin time) and magnitude is reasonably well predicted by the single backscattering theory of coda formation for events of magnitude 4 or less; for larger events the observed durations are longer than predicted. This theory predicts that a temporal increase in scattering attenuation will reduce the coda duration relative to magnitude. Thus, any temporal changes in the duration-magnitude relation can be interpreted in terms of changes inQ. However, it is necessary to consider spatial biases since the observed durations are relatively long for shallow events and, for events of all depths, at stations situated in the south-eastern half of the region, usually believed part of the accretionary border. For both these situations other evidence would suggest that the scattering coefficient would be relatively high (lowQ). These observations may be due to a relatively high component of surface wave scattering and the importance of multiple scattering in the later part of the coda in regions of lowQ as suggested by finite element studies of coda formation. Despite the lack of any significant earthquakes during the 1978–1985 period there is nevertheless a significant temporal change in duration observed in the Wellington region: a change from relatively long to relatively short codas occurred in mid-1981. This change correlates well with changes in the rate of activity,b-value, radon emission, and ground tilt as derived from lake levels. It is not yet clear how all these parameters are related physically, but an episode of aseismic slip, or creep, along the plate interface below the region may have been the cause.