New model could help predict major earthquakes
A Nagoya University-led team reveals the mechanisms behind different earthquakes at a plate boundary on the west coast of South America, shedding light on historical seismic events and potentially aiding prediction of the future risk from these natural disasters.
When tectonic plates that have been sliding past each other get stuck, a huge amount of energy builds up, and is eventually released in the form of an earthquake. Although much is known about the mechanisms behind this process, more needs to be understood about what happens at particular plate boundaries to determine the risk of earthquakes and tsunamis at specific sites and potentially to predict when these events might occur.
In a breakthrough in this field, researchers at Nagoya University and their colleagues in South America have studied several earthquakes that occurred at the Ecuador-Colombia subduction zone over the last hundred years, revealing the relationships between different earthquakes and the size and location of the ruptures at plate boundaries that caused them. The findings were published in Geophysical Research Letters.
A comparison between the previous source model and our model of the large earthquakes along the Ecuador-Colombia subduction zone. (Left) The previous source model of the large earthquakes in this subduction zone. The 1906 earthquake has been interpreted as a megathrust event (Mw 8.8) ruptured all segments of 1942, 1958, and 1979 earthquakes. (Right) Our source model of the large earthquakes in this subduction zone. Our analysis of the 1906 earthquake indicated Mw 8.4 and the occurrence of the large slip near the trench (black triangles with line) off the source areas of the three earthquakes. © Masahiro Yoshimoto
The team used a combination of data sources and models to study large earthquakes that struck the west coast of South America in 1906, 1942, 1958, 1979, and 2016. These included information on tsunami waveforms recorded at sites across the Pacific, data on seismic waves obtained by monitoring stations in Ecuador and Colombia, and previous work on the intensity of coupling, or locking together, of adjacent plates and the distance that they slipped past each other to cause each earthquake.
"The Ecuador-Colombia subduction zone, where the Nazca plate passes underneath the South American plate, is particularly interesting because of the frequency of large earthquakes there," says study author Hiroyuki Kumagai of the Graduate School of Environmental Studies, Nagoya University. "It's also a good site to investigate whether the ruptures at plate boundaries causing huge earthquakes are linked to subsequent large earthquakes years or decades later."
By carefully modeling the fault area where these earthquakes arose in combination with the other data, the team showed that the strongest of the earthquakes, that of 1906, involved a rupture at a different site than the other earthquakes. They also used data on the known speed at which the plates are moving past each other and the simulated "slip" of a plate associated with the 2016 earthquake to show that the 1942 and 2016 earthquakes were triggered by ruptures at the same site.
"Now that we can precisely link previous earthquakes to ruptures at specific sites along plate boundaries, we can gauge the risks associated with the build-up of pressure at these sites and the likely frequency of earthquakes there," lead author Masahiro Yoshimoto says. "Our data also reveal for the first time differences in rupture mechanisms between oceanic trenches and deeper coastal regions in this subduction zone."
The findings provide a foundation for risk prediction tools to assess the likelihood of earthquakes and tsunamis striking this region and their potential periodicity and intensity.
A large earthquake (Mw 7.7) occurred on 16 April 2016 within the source region of the 1906 earthquake in the Ecuador-Colombia subduction zone. The 1906 event has been interpreted as a megathrust earthquake (Mw 8.8) that ruptured the source regions of smaller earthquakes in 1942, 1958, and 1979 in this subduction. Our seismic analysis indicated that the spatial distribution of the 2016 earthquake and its aftershocks correlated with patches of high interplate coupling strength and was similar to those of the 1942 earthquake and its aftershocks, suggesting that the 2016 and 1942 earthquakes ruptured the same asperity. Our analysis of tsunami waveforms of the 1906 event indicated Mw around 8.4 and showed that large slip occurred near the trench off the source regions of the above three historical and the 2016 earthquakes, suggesting that a depth-dependent complex rupture mode exists along this subduction zone.
Source: Nagoya University
"Depth-dependent rupture mode along the Ecuador-Colombia subduction zone" – Masahiro Yoshimoto et al. – Geophysical Research Letters – DOI: 10.1002/2016GL071929
Featured image: The burning of San Francisco, April 18, 1906, view from St. Francis Hotel. Credit: Pillsbury Picture Co. – Panoramic photographs, Library of Congress
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The answer may be simpler than what is discussed above. I wrote this shortly after arriving in Ecuador when the latest of these earthquakes occurred…
THE 2016 MEGA-THRUST ECUADORIAN EARTHQUAKE IS CYCLE OCCURRING ON AVERAGE EVERY 18 TO 18.5 YEARS
Unlike other extreme weather phenomena occurring around the world these days, this earthquake, as is widely understood by mainstream scientists, is a direct response to stress building up between the Nazca and South American plates. It is almost identical to the 1942 earthquake. Both were 7.8 magnitude, in the middle of the rupture zone (as defined in this analysis), and followed a cycle in which the previous mega-thrust earthquake was relatively weak.
For the more progressive thinkers among you, please note that this stress buildup would occur in both the tectonic plate and expanding Earth models.
The important point to grasp is that it was highly predictable because of the constancy of the Pacific ocean floor spreading (as documented in paleomagnetism), and occurs with great regularity on average every 18 to 18.5 years. It has been 18 years since the last one. This very obvious pattern has not been recognized because of the utterly stupid academic debate about whether the 1942, 1958, and 1979 earthquakes either equal or do not equal the 1906 earthquake in total energy release (see details below). This argument focuses the attention on a single event, the huge 1906 8.8 earthquake. If you insist on making the “what equals the 1906” argument, according to the USGS, an 8.3 releases as much energy as more than five 7.8’s. So with the 2016 7.8, the answer is most certainly now yes. Here is the energy release since 1906:
1924 7.0 (year and magnitude estimated)
But this argument has distracted everyone from noticing the obvious cyclic nature of this seismic event. With the 2016 mega-thrust, this cycle is now obvious. The 1906 to 1942 gap is 36 years, or exactly two 18 year cycles. My guess is that because the 1906 earthquake was an 8.8, the missing 1924 earthquake went unnoticed, possibly being as small as a 7.0 (like the 1956 7.0, which followed a larger 7.8 in 1942). But it certainly did occur whether the USGS is aware of it or not. We know this again because the rate of spreading of the ocean floor crust is so utterly predictable.
Ignoring the two southernmost mega-thrust earthquakes that were distinctly part of this cycle did not help either. These were the 1956 7.0 and the 1998 7.2 earthquakes. Had the 1956 mega-thrust been properly considered than the rupture zone would be understood to extend further south than previously thought. The 1998 would then have been understood to be part of the same mechanism and the 18-year cyclic pattern would have been recognized sooner. Both of these smaller mega-thrust earthquakes are critical to understanding the true extent of the rupture zone and the 18-year cycle of stress release between the two plates. (For some reason the southernmost earthquakes in the rupture zone are smaller and the northernmost are larger.)
Note that there were two earthquakes in the 1950’s cycle because 1958 was the peak of the Modern Maxima when solar activity was extraordinarily high. (The author of this document adheres to the electric universe view that earthquakes are a form of underground lightening. Their relationship to heightened solar activity is or should be therefore obvious).
Something must be done to stop this once-a-generation devastation from being visited on the Ecuadorian people. Ecuador’s next major earthquake in this cycle is likely not to occur for another 36 years. The next mega-thrust earthquake in this rupture zone in the year 2034-35 earthquake will likely be weak, so Ecuador has 36 years to change their culture of building to one that is antiseismic, particularly along the coast line. But this is not Japan. A low-cost solution that lends itself to the building habits of the Ecuadorian people is needed. The author of this document is advocating such a low cost method of accomplishing that goal, one from which he is NOT seeking to profit.