Spatial seismicity variations and asperities in the New Hebrides Seismic Zone

Abstract

The term “asperity” has been used in many recent studies to describe sections of faults with higher strength than the surrounding zones. The level of background seismicity expected in such zones is unclear. They may appear as quiet zones because of high strength or active zones because of high stress. In this work the problem of seismicity levels in asperities is addressed by identifying regions of anomalously high or low seismicity and comparing the locations of these regions with the locations of asperities as determined from other observations. The regions of anomalously high or low seismicity are identified using a quantitative technique which relies on spatial rate comparisons done with the z test for a difference between two means. This technique allows the determination of the significance of spatial rate differences and the uniqueness of zones of outstanding seismicity. Four active zones, which make up 10%, and eight quiet zones, which make up 16% of the length of the New Hebrides seismic zone, were recognized using this technique. Asperities in the New Hebrides seismic zone were identified using characteristics which include large event initiation and stopping, clusters of aftershocks and moderate events, foreshock activity, and earthquake stress drops. Eight regions which showed two or more of these characteristics were identified. They make up 34% of the length of the seismic zone. The overlap between the asperities and the active and quiet regions provides a test of possible relationships between these zones. Thirty four percent of the asperity area is active, as opposed to the 10% expected, and 6% of the asperity area is quiet, as opposed to the 16% expected. This suggests that in the asperities in the New Hebrides seismic zone, stress concentration overcomes increased strength and that phenomena other than increased strength can cause seismic quiescence. Nearly all of the asperities in the New Hebrides seismic zone show high background seismicity or high stress drops, which suggests that likely zones of mainshock initiation can be identified using background seismicity.