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

Abstract

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.

Comment on “Crustal faults in the Chilean Andes: geological constraints and seismic potential” by Santibáñez et al. (2019), Andean Geology 46 (1): 32-65

“…Additionally, new research by Kanamori and Rivera (2017) along the master fault of the Liquine-Ofqui fault zone (LOFZ), suggest that an Mw 7.7 aftershock of the 1960 Mw 9.5 giant Valdivia earthquake occurred on June 6, 1960 as a dextral strike-slip event along the LOFZ. The 2007 Aysen earthquake sequence led Kanamori and Rivera (2017) and Kanamori et al (2019) to conclude that a north-south trending right-lateral strike-slip structure, likely the LOFZ is responsible for this June 6 Mw 7.7 event, and that the best match for the observed waveforms from this event is 11 km (i.e., in the crust), although they mentioned that the June 6 th event could be considerably deeper than 11 km. Although this could be a slow earthquake, and perhaps from near the base of the brittle-ductile transition zone, it should still be considered within the realm of observations and potentialities along crustal faults in Chile.…”

Substantial Upper Plate Faulting Above a Shallow Subduction Megathrust Earthquake: Mechanics and Implications of the Surface Faulting During the 2016 Kaikoura, New Zealand, Earthquake

“…Along the southern Chilean subduction zone, an Mw 9.5 earthquake in 1960 ruptured over 800 km of the subduction megathrust along the coast offshore of Valdivia (Barrientos & Ward, 1990;Kanamori & Cipar, 1974). A recent analysis indicates that this earthquake contained a surprisingly substantial strike-slip component (Kanamori et al, 2019). If this strike-slip faulting occurred on a structure separate from the megathrust, it would independently have a magnitude of 8.6-9.1 (depending on the obliquity of slip on the subduction plate interface), making it one of the largest strike-slip earthquakes ever to occur.…”

“…The discrepancy seen in the Kaikoura earthquake between the standard earthquake cycle model (with its limited upper plate faulting) and observations from this event require a rethinking of the distribution and amount of upper plate deformation throughout the earthquake cycle. It is tempting to ascribe the extreme upper plate faulting during the Kaikoura earthquake to unique conditions in that plate boundary setting, but there are other events that also show evidence of unusual upper plate faulting potentially associated with a megathrust earthquake: the 1960 Mw 9.5 Chile earthquake (Kanamori et al., 2019), the 1855 Wairarapa, New Zealand, event (Beavan & Darby, 2005; Rodgers & Little, 2006), and along sections of the Cascadia margin (McKenzie, Furlong, & Kirby, 2022). Collectively, observations of these other earthquakes suggest that extreme upper plate faulting above a rupturing megathrust may be a more common global process than previously inferred or assumed.…”

Excitation of Back‐Arc Tsunamis From Megathrust Ruptures: Theory and Application to the Sea of Japan

“…The choice of CC over amplitude matching is due to the fact that CC is a better representation of variations in signal's trend (i.e., period). Also, the large‐scale apparent amplitude mismatch (Figure 11) is likely caused by our simple uniform‐slip source model which does not incorporate the variations in down‐dip moment release (Kanamori et al., 2019).…”