HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) References Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by MITRONOVAS, W. Articles by SEEBER, L. Search for Related Content GeoRef GeoRef Citation

Earthquake locations and seismic wave propagation in the upper 250 km of the Tonga island arc

LAMONT GEOLOGICAL OBSERVATORY COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964
EARTH SCIENCES LABORATORIES, ESSA LAMONT GEOLOGICAL OBSERVATORY COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964

Abstract

This paper reports results of a series of short-term seismic observations made on nearly all the major island groups of the Tonga island arc. These data extend the mapping of anomalous propagation of seismic waves from earthquakes beneath the island arc reported by Oliver and Isacks (1967), and significantly increase the number of accurately located earthquakes beneath the Tonga Islands.

The data of this paper, combined with those of Sykes et al (1969), indicate that the inclined seismic zone of the Tonga island arc is very thin even at shallow depths. Most of the hypocenters are confined to a slab of about 25 km thickness. This thickness probably still reflects the precision of locations and may only be a measure of the upper limit of the thickness of the seismic zone. The inclined seismic zone, which has a dip of about 45° at intermediate depths in the mantle, appears to decrease in dip or bend over at depths less than about 50 km and appears to intersect the surface at or near the axis of the trench. The wedge-shaped region above the inclined seismic zone appears to be relatively aseismic, although some minor seismic activity has been located at shallow depths beneath the line of active volcanoes.

P and S phases with anomalously large amplitudes and frequencies from deep earthquakes (> 400 km) were observed at all sites occupied in Tonga, including sites on two volcanoes. These observations and observations from shallower events include ray paths that emerge nearly vertically beneath the volcanoes. Thus, any region of high attenuation (low Q) below the volcanoes, such as magma chambers, must be limited in extent. These data, in conjunction with regional observations of S_n, indicate that a broad zone of low Q material in the uppermost mantle exists west of the volcanoes on the concave side of the arc. However, the P and S phases recorded on the volcanic ridge exhibit an emergent and drawn-out character that contrasts with the sharp and impulsive character of phases recorded on the non-volcanic ridge. These observations are interpreted to indicate considerable heterogeneity beneath the volcanoes.

This article has been cited by other articles:

				G. CRUZ and M. WYSS Large earthquakes, mean sea level, and tsunamis along the Pacific Coast of Mexico and Central America Bulletin of the Seismological Society of America, April 1, 1983; 73(2): 553 - 570. [Abstract] [PDF]

				M. BARAZANGI and B. L. ISACKS A comparison of the spatial distribution of mantle earthquakes determined from data produced by local and by teleseismic networks for the Japan and Aleutian arcs Bulletin of the Seismological Society of America, December 1, 1979; 69(6): 1763 - 1770. [Abstract] [PDF]

				AAAS-Newcomb Cleveland Prize To Be Awarded for a Report or Article Published in Science Science, March 24, 1978; 199(4335): 1333 - 1333. [PDF]

				D. A. Walker and D. A. WALKER High-Frequency Pn Phases Observed in the Pacific at Great Distances Science, July 15, 1977; 197(4300): 257 - 259. [Abstract] [PDF]

				J. W. DEWEY and S. T. ALGERMISSEN Seismicity of the Middle America arc-trench system near Managua, Nicaragua Bulletin of the Seismological Society of America, August 1, 1974; 64(4): 1033 - 1048. [Abstract] [PDF]

				M. P. Hochstein, J. C. Schofield, and G. G. Shor Jr Tonga-Kermadec-Lau Geological Society, London, Special Publications, January 1, 1974; 4(1): 417 - 423. [Abstract] [PDF]

				J. A. GROW and A. QAMAR Seismic-wave attenuation beneath the central Aleutian Arc Bulletin of the Seismological Society of America, December 1, 1973; 63(6-1): 2155 - 2166. [Abstract] [PDF]

				P. MOLNAR, K. JACOB, and L. R. SYKES Microearthquake activity in Eastern Nevada and death valley California before and after the nuclear explosion Benham Bulletin of the Seismological Society of America, December 1, 1969; 59(6): 2177 - 2184. [Abstract] [PDF]

				L. R. SYKES, B. L. ISACKS, and J. OLIVER Spatial distribution of deep and shallow earthquakes of small magnitudes in the Fiji-Tonga region Bulletin of the Seismological Society of America, June 1, 1969; 59(3): 1093 - 1113. [Abstract] [PDF]