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

SUMMARY The strainmeter record observed at Isabella (ISA), California, for the 1960 Chilean earthquake (Mw = 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 ( Mw = 8.8) is rotated by about 10° clockwise from the plate convergence direction suggesting that right-lateral strain comparable to that of an Mw = 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.

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Geologic controls on up-dip and along-strike propagation of slip during subduction zone earthquakes from a high-resolution seismic reflection survey across the northern limit of slip during the 2010 Mw 8.8 Maule earthquake, offshore Chile

Geosphere ◽

10.1130/ges02099.1 ◽

2019 ◽

Vol 15 (6) ◽

pp. 1751-1773 ◽

Cited By ~ 3

Author(s):

Anne M. Tréhu ◽

Bridget Hass ◽

Alexander de Moor ◽

Andrei Maksymowicz ◽

Eduardo Contreras-Reyes ◽

...

Keyword(s):

High Resolution ◽

Subduction Zone ◽

Seismic Reflection ◽

Plate Boundary ◽

Accretionary Prism ◽

Structural Heterogeneity ◽

Deformation Front ◽

Maule Earthquake ◽

Subduction Zone Earthquakes ◽

Trench Sediments

Abstract A grid of closely spaced, high-resolution multichannel seismic (MCS) reflection profiles was acquired in May 2012 over the outer accretionary prism up dip from the patch of greatest slip during the 2010 Mw 8.8 Maule earthquake (offshore Chile) to complement a natural-source seismic experiment designed to monitor the post-earthquake response of the outer accretionary prism. We describe the MCS data and discuss the implications for the response of the accretionary prism during the earthquake and for the long-term evolution of the margin. The most notable observation from the seismic reflection survey is a rapid north-to-south shift over a short distance from nearly total frontal accretion of the trench sediments to nearly total underthrusting of undeformed trench sediments that occurs near the northern edge of slip in the 2010 earthquake. Integrating our structural observations with other geological and geophysical observations, we conclude that sediment subduction beneath a shallow décollement is associated with propagation of slip to the trench during great earthquakes in this region. The lack of resolvable compressive deformation in the trench sediment along this segment of the margin indicates that the plate boundary here is very weak, which allowed the outer prism to shift seaward during the earthquake, driven by large slip down dip. The abrupt shift from sediment subduction to frontal accretion indicates a stepdown in the plate boundary fault, similar to the stepovers that commonly arrest slip propagation in strike-slip faults. We do not detect any variation along strike in the thickness or reflective character of the trench sediments adjacent to the change in deformation front structure. This change, however, is correlated with variations in the morphology and structure of the accretionary prism that extend as far as 40 km landward of the deformation front. We speculate that forearc structural heterogeneity is the result of subduction of an anomalously shallow or rough portion of plate that interacted with and deformed the overlying plate and is now deeply buried. This study highlights need for three-dimensional structural images to understand the interaction between geology and slip during subduction zone earthquakes.

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Triggering an Unexpected Earthquake in an Uncoupled Subduction Zone

10.5194/egusphere-egu21-418 ◽

2021 ◽

Author(s):

Kevin P. Furlong ◽

Matthew W. Herman

Keyword(s):

Subduction Zone ◽

Plate Boundary ◽

Strike Slip ◽

Seismic Gap ◽

Plate Interface ◽

Thrust Faulting ◽

Current Thinking ◽

Abstract Submission ◽

Geologic Record ◽

Eastern Edge

In the 1970&#8217;s, the Shumagin Islands region of the Alaska subduction zone was identified as a seismic gap expected to host a future great (Mw >8.0) earthquake. More recent geodetic data indicate this region is poorly coupled, and the geologic record shows little evidence of past large events - leading to current thinking of the &#8220;Shumagin Gap&#8221; as a region of low seismic hazard. From July to October 2020 (with aftershocks continuing through the time of this abstract submission in January), a series of earthquakes occurred in this region, potentially incompatible with this low-coupling interpretation. The initial Mw 7.8 plate interface thrust faulting earthquake on July 21st straddled the eastern edge of the Shumagin Gap, followed by an Mw 7.6 strike-slip earthquake on October 19th within the slab under the eastern side of the Shumagin Gap. Stress modeling indicates that this strike-slip earthquake is in fact favored if the Shumagin Gap has low coupling, whereas a highly coupled Shumagin Gap would inhibit that type and location of earthquake. The initial thrust earthquake and its afterslip significantly enhanced the strike-slip stress loading within the subducting slab, helping to trigger that event. We find that although regions such as the Shumagin Gap have a low seismogenic potential for plate interface thrusting, the existence of this decoupled region increases the potential for intra-plate strike-slip faulting in association with more typical subduction earthquakes on adjacent coupled segments of the plate boundary. Therefore, the seismic and tsunami potential near these uncoupled regions might be greater than previously thought.

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Finite-element simulations of long-period ground motions: Japanese subduction-zone earthquakes and the 1906 San Francisco earthquake

Journal of Seismology ◽

10.1007/s10950-008-9091-5 ◽

2008 ◽

Vol 12 (2) ◽

pp. 161-172 ◽

Cited By ~ 5

Author(s):

Yasushi Ikegami ◽

Kazuki Koketsu ◽

Takeshi Kimura ◽

Hiroe Miyake

Keyword(s):

Finite Element ◽

Subduction Zone ◽

San Francisco ◽

Ground Motions ◽

Finite Element Simulations ◽

Long Period ◽

Subduction Zone Earthquakes

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Great earthquakes of variable magnitude at the Cascadia subduction zone

Quaternary Research ◽

10.1016/j.yqres.2006.02.009 ◽

2006 ◽

Vol 65 (3) ◽

pp. 354-365 ◽

Cited By ~ 87

Author(s):

Alan R. Nelson ◽

Harvey M. Kelsey ◽

Robert C. Witter

Keyword(s):

Subduction Zone ◽

Plate Boundary ◽

Conclusive Evidence ◽

Cascadia Subduction Zone ◽

Radiocarbon Age ◽

Rupture Length ◽

Great Earthquakes ◽

Varying Length ◽

Recurrence Intervals ◽

Coseismic Subsidence

AbstractComparison of histories of great earthquakes and accompanying tsunamis at eight coastal sites suggests plate-boundary ruptures of varying length, implying great earthquakes of variable magnitude at the Cascadia subduction zone. Inference of rupture length relies on degree of overlap on radiocarbon age ranges for earthquakes and tsunamis, and relative amounts of coseismic subsidence and heights of tsunamis. Written records of a tsunami in Japan provide the most conclusive evidence for rupture of much of the plate boundary during the earthquake of 26 January 1700. Cascadia stratigraphic evidence dating from about 1600 cal yr B.P., similar to that for the 1700 earthquake, implies a similarly long rupture with substantial subsidence and a high tsunami. Correlations are consistent with other long ruptures about 1350 cal yr B.P., 2500 cal yr B.P., 3400 cal yr B.P., 3800 cal yr B.P., 4400 cal yr B.P., and 4900 cal yr B.P. A rupture about 700–1100 cal yr B.P. was limited to the northern and central parts of the subduction zone, and a northern rupture about 2900 cal yr B.P. may have been similarly limited. Times of probable short ruptures in southern Cascadia include about 1100 cal yr B.P., 1700 cal yr B.P., 3200 cal yr B.P., 4200 cal yr B.P., 4600 cal yr B.P., and 4700 cal yr B.P. Rupture patterns suggest that the plate boundary in northern Cascadia usually breaks in long ruptures during the greatest earthquakes. Ruptures in southernmost Cascadia vary in length and recurrence intervals more than ruptures in northern Cascadia.

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Strike-Slip Enables Subduction Initiation Beneath a Failed Rift: New Seismic Constraints from Puysegur Margin, New Zealand

10.5194/egusphere-egu2020-10317 ◽

2020 ◽

Author(s):

Brandon Shuck ◽

Harm Van Avendonk ◽

Sean Gulick ◽

Michael Gurnis ◽

Rupert Sutherland ◽

...

Keyword(s):

New Zealand ◽

Subduction Zone ◽

Plate Boundary ◽

Strike Slip ◽

Rift Basin ◽

The South ◽

Subduction Initiation ◽

The North ◽

Collision Zone ◽

Seismic Images

Critical ingredients and conditions necessary to initiate a new subduction zone are debated. General agreement is that subduction initiation likely takes advantage of previously weakened lithosphere and may prefer to nucleate along pre-existing plate boundaries. To evaluate how past tectonic regimes and lithospheric structures might facilitate underthrusting and lead to self-sustaining subduction, we present an analysis of the Puysegur Margin, a young subduction zone with a rapidly evolving tectonic history.&#160;The Puysegur Margin, south of New Zealand, currently accommodates convergence between the Australian and Pacific plates, exhibits an active seismic Benioff zone, a deep ocean trench, and young adakitic volcanism on the overriding plate. Tectonic plate reconstructions show that the margin experienced a complicated transformation from rifting to seafloor spreading, to strike-slip motion, and most recently to incipient subduction, all in the last ~45 million years. Details of this tectonic record remained incomplete due to the lack of high-quality seismic data throughout much of the margin.&#160;Here we present seismic images from the South Island Subduction Initiation Experiment (SISIE) which surveyed the Puysegur region February-March, 2018. SISIE acquired 1252 km of deep-penetrating multichannel seismic (MCS) data on 7 transects, including 2 regional dip lines coincident with Ocean Bottom Seismometers (OBS) deployments which extend (west to east) from the incoming Australian plate, across the Puysegur Trench and Puysegur Ridge, over the Solander Basin and onto the continental Campbell Plateau margin.&#160;We integrate pre-stack depth migrated MCS profiles with OBS tomography models to constrain the tectonic development of the Puysegur Margin. Based on our results we propose the following Cenozoic evolution: (1) The entire Solander Basin contains thinned continental crust which formed from orthogonal stretching between the Campbell and Challenger plateaus during the Eocene-Oligocene. This phase of rifting was more pronounced to the south, producing thinner crust with abundant syn-rift volcanism across a wider rift-basin, in contrast to the relatively thicker crust, moderate syn-rift volcanism and narrower rift basin in the north. (2) Strike-slip deformation subsequently developed along Puysegur Ridge, west of the locus of rifting and within relatively unstretched continental lithosphere. This young strike-slip plate boundary translated unstretched crust northward causing an oblique continent-collision zone, which led to a transpressional pattern of distributed left-stepping, right-lateral faults. (3) Subduction initiation was aided by large density contrasts as oceanic lithosphere translated from the south was forcibly underthrust beneath the continent-collision zone. Early development of oblique subduction generated modest and widespread reactivation of faults in the upper plate. (4) Present-day, the Puysegur Trench shows a spatiotemporal transition from nearly mature subduction in the north to a recently initiated stage along the southernmost margin, requiring a southward propagation of subduction through time.&#160;Our new seismic images suggest subduction initiation at the Puysegur Margin was assisted by inherited buoyancy contrasts and structural weaknesses that were imprinted into the lithosphere during earlier phases of continental rifting and strike-slip along the developing plate boundary. The Puysegur Margin demonstrates that forced nucleation along a strike-slip boundary is a viable subduction initiation model and should be considered throughout Earth&#8217;s history.

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