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Subduction process of a fracture zone and aseismic ridges — the focal mechanism and source characteristics of the New Hebrides earthquake of 1969 January 19 and some related events

Chung, Wai-Ying and Kanamori, Hiroo (1978) Subduction process of a fracture zone and aseismic ridges — the focal mechanism and source characteristics of the New Hebrides earthquake of 1969 January 19 and some related events. Geophysical Journal of the Royal Astronomical Society, 54 (1). pp. 221-240. ISSN 0016-8009. https://resolver.caltech.edu/CaltechAUTHORS:20141111-163604121

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Abstract

The subduction of the D'Entrecasteaux fracture zone-seismic ridge system in the New Hebrides island arc is investigated on the basis of the focal process of the New Hebrides earthquake of 1969 January 19 (m_b= 6.4, h= 107 km), mechanisms of some related events, seismicity and regional tectonics. A notable feature of this island arc is the discontinuity of the New Hebrides Trench in the central New Hebrides where the ridge-fracture zone is subducting and intersecting the arc. The 1969 New Hebrides earthquake occurred along the subducted portion of the fracture zone and is characterized by unusual waveforms with remarkably large excitation of long period waves, rare for earthquakes of comparable mb and depth; the P-wave records show anomalously long duration and complexity indicating that the earthquake was a multiple event with a source duration of at least 30 s. P-wave first motions, seismicity data and synthetic body-wave seismogram indicate that the earthquake represents transverse left-lateral motion on a nearly vertical, E—W trending fault plane with a slip vector subparallel to the down-dip direction of the Benioff zone. The total seismic moment is found to be 5 × 10^(26) dyne cm by matching the amplitudes and waveforms between observed and synthetic surface waves. The location and mechanism of this earthquake suggest that the D'Entrecasteaux fracture zone structurally extends to the east of the trench. This structural boundary at depth seems to be reflected in the spatial distribution of two earthquake swarms which are bounded sharply at the latitude of 15.2°S. At the extension of the ridge-fracture zone, the activity of intermediate depth earthquakes, which are characterized by a very consistent pattern of down-dip extensional mechanism, is much higher and their depths are systematically shallower than in the adjacent regions. These features can be interpreted as a consequence of subduction of a buoyant ridge and the resultant increase in the extensional stress at the intermediate depths of the sinking slab. Fault-plane solutions of 22 earthquakes suggest that the subduction of aseismic ridges in the New Hebrides is characterized by high-angle thrusts. The lithospheres on the two sides of the D'Entrecasteaux fracture zone under the arc subduct more or less independently and generate alternating left-lateral and right-lateral earthquakes along the subducted portion of the fracture zone.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1111/j.1365-246X.1978.tb06764.xDOIArticle
http://gji.oxfordjournals.org/content/54/1/221PublisherArticle
ORCID:
AuthorORCID
Kanamori, Hiroo0000-0001-8219-9428
Additional Information:Copyright © 1978 The Royal Astronomical Society. Received 1977 December 19; in original form 1977 September 12. We are very grateful to Yoshio Fukao who made many valuable suggestions and kindly reviewed the manuscript. Discussions with Clarence Allen on the faulting mechanisms contributed to our interpretation of the data. Seiya Uyeda kindly reviewed the manuscript and made helpful comments on our tectonic model. Thomas Ahrens and Robert Geller are appreciated for reading the manuscript. We thank John Cipar for discussions as well as his suggestions for improvements of the manuscript. Useful discussions with Bernard Minster, James Whitcomb, John Ebel and Kunihiko Shimazaki are also acknowledged. This research was supported by the Division of Earth Sciences, National Science Foundation, NSF Grant EAR76-14262 and NSF Grant EAR77-14675.
Funders:
Funding AgencyGrant Number
NSFEAR76-14262
NSFEAR77-14675
Issue or Number:1
Record Number:CaltechAUTHORS:20141111-163604121
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20141111-163604121
Official Citation:Wai-Ying Chung and Hiroo Kanamori Subduction process of a fracture zone and aseismic ridges — the focal mechanism and source characteristics of the New Hebrides earthquake of 1969 January 19 and some related events Geophys. J. Int. (1978) 54 (1): 221-240 doi:10.1111/j.1365-246X.1978.tb06764.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51617
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:12 Nov 2014 15:48
Last Modified:09 Mar 2020 13:18

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