Abstract
SUMMARYThe 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.
Highlights
The strainmeter record observed at Isabella (ISA), California, for the 1960 Chilean earthquake (Mw = 9.5) is one of the most important historical seismograms because it allowed the first definitive observations of the Earth’s free oscillations (Benioff Press & Smith 1961, hereafter referred to as BPS-1961), together with other observations at UCLA, California, (Ness et al 1961) and Ogdensburg, New Jersey, (Alsop et al 1961)
Since the ISA strain record is one of only a few records of the 1960 Chilean earthquake from which quantitative amplitude information at very long period can be extracted, we investigate it in detail in an attempt to understand the source characteristics of the 1960 Chilean earthquake which is believed to be the largest instrumentally recorded earthquake (Mw = 9.5; e.g. Ruiz & Madariaga 2018)
A combination of a pure thrust fault (δ = 17◦) and a simultaneous vertical strikeslip fault can explain our data as well and is a viable solution. In this case by matching the amplitude of the observed and synthetic seismograms of G2 and R2 at ISA, we obtain Mw = 9.37 for the thrust event and Mw = 9.07 for the strike-slip event. This model is motivated by the presence of the Liquine-Ofqui fault (LOF) which is a major right-lateral strike-slip fault extending over 1000 km north– south along the Chilean coast
Summary
In the early 1960s, the main interest of seismologists seemed to be in the period of normal modes rather than the amplitude. Since the ISA strain record is one of only a few records of the 1960 Chilean earthquake from which quantitative amplitude information at very long period can be extracted, we investigate it in detail in an attempt to understand the source characteristics of the 1960 Chilean earthquake which is believed to be the largest instrumentally recorded earthquake We will show that it is difficult to explain the large amplitude G waves (we use ‘G wave’ to refer to long-period Love wave) and toroidal modes recorded on the Isabella strain record with the conventional low-angle thrust mechanism (strike, φs = 10◦, dip, δ = 17◦, rake, λ = 90◦) typical of subduction-zone great earthquakes.
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