Upper mantle inhomogeneity beneath New Zealand: Seismic evidence

Abstract

Seismic wave forms recorded in New Zealand from nearby deep earthquakes show significant frequency differences, which can be correlated with ray paths in the upper mantle. Stations on the east side of the North Island (Gisborne, Wellington, and Tuai) record frequencies that are dominantly 3 cps or greater from deep shocks that are both nearby and up to 10° north toward the Kermadec Islands. Stations on the west side of the North Island (Tarata and Karapiro) record frequencies that are dominantly 2 cps or lower from the same earthquakes. Closer examination of the pattern shows two exceptions. First, shocks southward from 39½°S tend to show high frequencies, not low, at the western stations. Second, shocks on the easterly side of the seismic zone north of New Zealand tend to show low frequencies, not high, at the eastern stations. Evidence is presented to show that these observations can be explained only in terms of inhomogeneities within the upper mantle. Alternative explanations that can be rejected include: depth, distance, or magnitude effects; source mechanism; crustal effects; instrument or station peculiarities. We interpret the results to indicate the existence of a low-Q region within the upper mantle under part of the North Island of New Zealand and the surrounding oceans. Where this region exists, seismic waves traversing it lose a significant fraction of their higher-frequency components. Where the region does not exist, high frequencies can be propagated to relatively large distances. The region appears to lie northward from 39½°S and north-westward from a NE-SW line through the center of the North Island. In depth, we prefer an interpretation in terms of a horizontal layer lying between 75 and 125 km, although the data could be reconciled with a dipping layer lying above the zone of seismicity. The low-Q region appears to be absent along a continuous zone, which is possibly narrow, extending along the eastern side of the North Island and north to 32°S and probably beyond. Secondary evidence from travel-time residuals and isoseismals adds support to this interpretation. Estimates for Q within the low-Q region yield values in the range Q = 40–80.