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Large intermediate-depth earthquakes and the subduction process

Astiz, Luciana and Lay, Thorne and Kanamori, Hiroo (1988) Large intermediate-depth earthquakes and the subduction process. Physics of the Earth and Planetary Interiors, 53 (1-2). pp. 80-166. ISSN 0031-9201. http://resolver.caltech.edu/CaltechAUTHORS:20141216-101506128

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Abstract

This study provides an overview of intermediate-depth earthquake phenomena, placing emphasis on the larger, tectonically significant events, and exploring the relation of intermediate-depth earthquakes to shallower seismicity. Especially, we examine whether intermediate-depth events reflect the state of interplate coupling at subduction zones, and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes. Historic record of large intraplate earthquakes (m_B ≥ 7.0) in this century shows that the New Hebrides and Tonga subduction zones have the largest number of large intraplate events. Regions associated with bends in the subducted lithosphere also have many large events (e.g. Altiplano and New Ireland). We compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with M > 6 and depth between 40 and 200 km. The final catalog includes 335 events with 47 new focal mechanisms, and is probably complete for earthquakes with m_B ≥ 6.5. For events with M ≥ 6.5, nearly 48% of the events had no aftershocks and only 15% of the events had more than five aftershocks within one week of the mainshock. Events with more than ten aftershocks are located in regions associated with bends in the subducted slab. Focal mechanism solutions for intermediate-depth earthquakes with M > 6.8 can be grouped into four categories: (1) Normal-fault events (44%), and (2) reverse-fault events (33%), both with a strike nearly parallel to the trench axis. (3) Normal or reverse-fault events with a strike significantly oblique to the trench axis (10%), and (4) tear-faulting events (13%). The focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down-dip extensional stress prevails in a gently dipping plate. In contrast, along decoupled subduction zones great normal-fault earthquakes occur at shallow depths (e.g., the 1977 Sumbawa earthquake in Indonesia). Type 2 events, with strike subparallel to the subduction zone, and most of them with a near vertical tension axis, occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km. The increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and, consequently, the change in focal mechanism from type 1 to type 2 events. Events of type 3 occur where the trench axis bends sharply causing horizontal (parallel to the trench strike) extensional or compressional intraplate stress. Type 4 are hinge-faulting events. For strongly coupled zones we observed temporal changes of intermediate-depth earthquake activity associated with the occurrence of a large underthrusting event. After the occurrence of a large underthrusting event, the stress axis orientation of intermediate-depth earthquakes changes from down-dip tensional to down-dip compressional (e.g., 1960 Chile, 1974 Peru, 1982 Tonga and 1952 Kamchatka earthquakes), or the number of large intermediate events decreases for a few years (e.g., 1964 Alaska and 1985 Valparaiso earthquakes). We conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate-depth seismicity, spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate-depth events.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/0031-9201(88)90138-0 DOIArticle
http://www.sciencedirect.com/science/article/pii/0031920188901380PublisherArticle
Additional Information:© 1988 Elsevier Science Publishers B.V. Received September 22, 1987; accepted October 21, 1987. We thank Bob F. Svendsen for his computer expertise. This research was supported by U.S. Geological Survey Grants 14-08-0001-G1170 and 14-08-002-G1277 (H.K.) and NSF Grant EAR-8451715 (T.L.) and a Shell Faculty Career Initiation Grant to T.L. Contribution No. 4504, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California.
Funders:
Funding AgencyGrant Number
USGS14-08-0001-G1170
USGS14-08-002-G1277
NSFEAR-8451715
Shell Companies FoundationUNSPECIFIED
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Caltech Division of Geological and Planetary Sciences4504
Record Number:CaltechAUTHORS:20141216-101506128
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20141216-101506128
Official Citation:Luciana Astiz, Thorne Lay, Hiroo Kanamori, Large intermediate-depth earthquakes and the subduction process, Physics of the Earth and Planetary Interiors, Volume 53, Issues 1–2, December 1988, Pages 80-166, ISSN 0031-9201, http://dx.doi.org/10.1016/0031-9201(88)90138-0. (http://www.sciencedirect.com/science/article/pii/0031920188901380)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:52856
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:16 Dec 2014 18:44
Last Modified:16 Dec 2014 18:44

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