Lateral variations of seismic-wave attenuation in the upper mantle above the inclined earthquake zone of the Tonga Island Arc: Deep anomaly in the upper mantle

摘要: Large variations of the predominant frequencies and amplitudes seismic phases Pn Sn occur for hundreds paths that cross much region between Tonga island arc Fiji islands. The data clearly delineate a high attenuation in uppermost mantle behind arc. attenuation, zone about 300 km wide, is ridge (the frontal arc) Lau third arc). alignment active volcanoes on seems to mark eastern boundary this high-attenuation zone. If low or Q, correlates with strength, our observations indicate considerably weaker beneath basin, gap substantial thinning lithosphere exists This excellent agreement hypothesis basin product crustal extension. A detailed study P S waves traverse wedge above inclined intermediate deep earthquakes shows material anomalously present throughout prism depths 150 along That is, imply an anomalous increase asthenosphere near zone; Q may be as small 50 less than 20 waves. Regions also exist several other arcs position similar These zones seem compatible models which mobilized by descending lithospheric slab.