Upper mantle inhomogeneity beneath New Zealand: Seismic evidence

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