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Evidence for a large strike-slip component during the 1960 Chilean earthquake

Kanamori, Hiroo and Rivera, Luis and Lambotte, Sophie (2019) Evidence for a large strike-slip component during the 1960 Chilean earthquake. Geophysical Journal International, 218 (1). pp. 1-32. ISSN 0956-540X. doi:10.1093/gji/ggz113.

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The strainmeter record observed at Isabella (ISA), California, for the 1960 Chilean earthquake (M_w = 9.5) is one of the most important historical records in seismology because it was one of the three records that provided the opportunity for the first definitive observations of free oscillations of the Earth. Because of the orientation of the strainmeter rod with respect to the back azimuth to Chile, the ISA strainmeter is relatively insensitive to G (Love) waves and higher order (order ≥ 6) toroidal modes, yet long-period G waves and toroidal modes were recorded with large amplitude on this record. This observation cannot be explained with the conventional low-angle thrust mechanism typical of great subduction-zone earthquakes and requires an oblique mechanism with half strike-slip and half thrust. The strain record at Ogdenburg, New Jersey, the Press–Ewing seismograms at Berkeley, California, and the ultra-long period displacement record at Pasadena, California, also support the oblique mechanism. We tested the performance of the ISA strainmeter using other events including the 1964 Alaskan earthquake and found no instrumental problems. Thus, the ISA observation of large G/R and toroidal/spheroidal ratios most likely reflects the real characteristics of the 1960 Chilean earthquake, rather than an observational artefact. The interpretation of the large strike-slip component is not unique, but it may represent release of the strike-slip strain that has accumulated along the plate boundary as a result of oblique convergence at the Nazca–South American plate boundary. The slip direction of the 2010 Chilean (Maule) earthquake ( M_w = 8.8) is rotated by about 10° clockwise from the plate convergence direction suggesting that right-lateral strain comparable to that of an M_w = 8.3 earthquake remained unreleased and accumulates near the plate boundary. One possible scenario is that the strike-slip strain accumulated over several great earthquakes like the 2010 Maule earthquake was released during the 1960 Chilean earthquake. If this is the case, we cannot always expect a similar behaviour for all the great earthquakes occurring in the same subduction zone and such variability needs to be considered in long-term hazard assessment of subduction-zone earthquakes.

Item Type:Article
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Kanamori, Hiroo0000-0001-8219-9428
Rivera, Luis0000-0002-0726-5445
Additional Information:© The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2019 February 28. Received 2019 February 5; in original form 2018 July 11. Published: 01 March 2019. We thank Stewart Smith who provided us with the detailed backup materials used for his 1966 study which played a key role for understanding the instrument characteristics of the Isabella strain record. Frank Press provided us with key information on the Isabella strain record used for the 1960 Chilean earthquake study. Duncan Agnew provided us with the strainmeter record of the 2010 Maule, Chile, earthquake recorded at the Piñon Flat Observatory. Guy Masters made available to us hand-digitized data of the Isabella strain seismograms for the 1960 Chilean and the 1964 Alaskan earthquakes archived at the Scripps Institution of Oceanography. Walter Zürn read a preliminary version of the manuscript and provided useful feedback to improve it. We appreciate the effort of Emile Okal and an anonymous reviewer who carefully read the manuscript and raised important questions on some of the key issues. Comments from Shingo Watada were helpful for clarifying some key points. This work would not have been possible without help from colleagues who sent us, upon our request, the copies of the seismograms of the Chilean earthquake. Following is a partial list. Peggy Hellweg: Berkeley Seismographic Stations, University of California, Berkeley. Shingo Watada: Earthquake Research Institute, the University of Tokyo. Brian Ferris: Geonet, New Zealand. Nobuo Hamada, Noriko Kamaya, and Jumpei Shimizu: Japan Meteorological Agency. Jim Mori: Kyoto University. Norihito Umino, Toru Matsuzawa, Tomotsugu Demachi, and Satoshi Hirahara: Tohoku University. The Data Management System of the Incorporated Research Institutions for Seismology ( was used to access the seismic data from the Global Seismic Network and Federation of Digital Seismic Network stations. We used Syngine web service at the Data Management Center of Incorporate Research Institutions for Seismology (Syngine, IRISURL, last accessed 2017 July 20) for computing some of the synthetic seismograms. The Green's functions are pre-computed with the AxiSEM (Nissen-Meyer et al.2014) for several 1-D reference models.
Group:Seismological Laboratory
Subject Keywords:Earthquake dynamics; Earthquake source observation; Surface waves and free oscillations; Dynamics and mechanics of faulting; Subduction zone processes
Issue or Number:1
Record Number:CaltechAUTHORS:20190501-094821280
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Official Citation:Hiroo Kanamori, Luis Rivera, Sophie Lambotte, Evidence for a large strike-slip component during the 1960 Chilean earthquake, Geophysical Journal International, Volume 218, Issue 1, July 2019, Pages 1–32,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95129
Deposited By: Tony Diaz
Deposited On:01 May 2019 18:57
Last Modified:16 Nov 2021 17:10

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