By Aron Mirwald, M.Sc., Temblor, Inc.
A magnitude 6.1 earthquake occurred on 23 March 2019 at 2:14 pm in Colombia. A recent scientific paper reports that the tide might be responsible for 16% of the earthquakes in Colombia. But did the Moon trigger this earthquake? Possibly, but there are important limitations.
Colombia’s hyperactive Cauca Cluster and Bucaramanga Nest
The M=6.1 quake, which was widely felt in Bogota, Cali, and Medellin, was located in the well-known ‘Cauca cluster’ in Colombia, where M≥3 earthquakes occur frequently (~24 per year). Together with the ‘Bucaramanga nest’ (~550 per year), the two clusters account for over half of all Colombian earthquakes (Geological Service Colombia). Most of the earthquakes in the two clusters strike at depths between 70-180 km (43 -111 mi). How earthquakes can be produced at these great depths is itself an enigma, and a matter of ongoing research (read this and this for an introduction).
But, as for many geoscience problems, there is more to it: Researchers from Medellin (University of Antioquia and National University of Colombia) have found that earthquakes in Colombia correlate with the tide. They show in their recent publication that the relation between earthquakes and tide is especially strong for earthquakes within the two earthquake clusters (Moncayo et. al., 2019).
Each dot represents an earthquake. The colored dots are corresponding to earthquakes in seismic clusters. The upper two are the Cauca cluster and Bucaramanga nest, where over half of the earthquakes in Colombia occur. Image from Moncayo et. al. (2018)
The Moon and the Sun cause the Earth to deform
Maybe you have heard that we are slightly lighter when the moon is above us (only one millionth of our weight). But, to be exact, this is also true if the moon is directly below us, at the opposite side. The reason for this is that the gravitational force is not the only force at play. The earth is moved by the moon circling around it, and we experience a centrifugal force because of this (here is a webpage with a great animation of this). The net force is upwards both at the side that faces the moon and at the opposite one.
Both Moon and Earth move in ellipses due to the force they exert on each other. The white arrows represent the net force, i.e. the sum of the centrifugal force and the gravitational force. Image from http://beltoforion.de (interactive animation)
The moon is not the only one who influences the earth. The sun does it in a similar way, although the force it generates is about half as large. The combined effect of the Sun and the Moon is called ‘tide’. The tide has two effects on the earth. First, it moves large quantities of water, also known as ocean tide. Second, it deforms the solid earth: The tidal forces, that pull on both sides, elongate the planet, making it around 40 cm longer. This generates shear and unclamping stresses in the earth that can promote earthquakes (Heaton, 1975).
The magnitudes of the stresses generated by the tide are much smaller than stresses due to the movement of the tectonic plates. This means that tides themselves are not responsible for earthquakes. Perhaps, however, if an earthquake is about to trigger, the tide can nudge it to fail. Therefore, we would expect seismicity to be higher when the tidal stresses and the tectonic stresses point in the same direction, and lower when the opposite is true.
Searching for periodicity: can we prove tidal triggering?
There are two key tidal cycles: The first one is 27.5 days long, which is the time the moon needs to circle around the earth. The second one is 24 hours long, which is the time the earth takes to turn around its own axis. If an increase in the rate of earthquakes correlates with these periods, then that increase could be tidally triggered. The next step would then be to actually compute the stresses involved.
Could the tides permit earthquake forecasts?
Since 1980 seismologists have searched for such a link, with mixed results. Recent studies, which have found a relation, are limited to certain regions or circumstances (Ide et. al., 2016). For example, it was found that the number of earthquakes in the region of the 2011 Tohoku earthquake in Japan was correlated with the tide before the earthquake occurred. After the magnitude 9 earthquake, on the other hand, no correlation was found (Tanaka et. al., 2012). Studies like this speculate that it might be possible to evaluate if a large rupture is about to come in certain areas, but this has yet to be proven.
The recent event was probably facilitated by the tide
In their research, Dr. Gloria Moncayo and her colleagues evaluated earthquakes in Colombia between 1993 and 2017. They found that the rate of earthquakes might have a periodic component, with a period of 27.5 days. About one-sixth (or 16%) more earthquakes occur when the moon is closest, i.e. at perigee. This correlation between earthquakes and tides was strongest for the events within the Cauca cluster and the Bucaramanga nest.
The recent earthquake occurred just two days after the last perigee, which also happened to be very close to full moon (20 March). In the figure below, this corresponds to a phase of 34°, and thus in an area where more earthquakes are expected due to the tide. We contacted the authors of the research in order to learn more.
Dr. Moncayo told us that the position and the timing of the event indicated tidal triggering. Her colleague, Dr. Jorge I. Zuluaga, added that they calculated the tidal stress for this event and found that its direction was such that the earthquake would be facilitated. ‘If I could bet a dollar, I would bet that it was tidally triggered. Regretfully, we cannot falsify this assertion’, he wrote.
Here, you see the number of earthquakes in relation to the 27.5-day period of the moon. A phase of 0 and 360 degrees corresponds to the perigee (moon is closest to earth), and 180 degrees to the apogee (moon is farthest away). You can see that only a small fraction of the total number of earthquakes varies with time. Image from Moncayo et. al. (2019)
Putting it into perspective: A tidal nudge, but not an earthquake prediction
For last Saturday’s event, we know that the tidal stress favored the triggering. Before we jump into hasty conclusions, we should be aware that there are limitations to the result of the study of Dr. Moncayo and her colleagues. An important one is that the seismological network has expanded in the time they evaluated. This could introduce error in the detection of periodicity (Ader and Avouac, 2013). Even if the periodicity that the authors found was true, still most of the earthquakes are independent of the tide. Only a fraction (less than 16%) of the seismicity could be attributed to it. Finally, we need to know the actual tidal stresses and not only the periodicity to make statements of the causality.
Ader, T. J., & Avouac, J. P. (2013). Detecting periodicities and declustering in earthquake catalogs using the Schuster spectrum, application to Himalayan seismicity. Earth and Planetary Science Letters, 377, 97-105.
Heaton, T. H. (1975). Tidal triggering of earthquakes. Geophysical Journal International, 43(2), 307-326.
Ide, S., Yabe, S., & Tanaka, Y. (2016). Earthquake potential revealed by tidal influence on earthquake size–frequency statistics. Nature Geoscience, 9(11), 834.
Moncayo, G. A., Monsalve, G., & Zuluaga, J. I. (2018). Tidal Coulomb Failure Stresses in the Northern Andean intermediate depth seismic clusters and its implications for a possible correlation between tides and seismicity. arXiv preprint arXiv:1812.01104.
Moncayo, G. A., Zuluaga, J. I., & Monsalve, G. (2019). Correlation between tides and seismicity in Northwestern South America: the case of Colombia. Journal of South American Earth Sciences, 89, 227-245.
Tanaka, S. (2012). Tidal triggering of earthquakes prior to the 2011 Tohoku‐Oki earthquake (Mw 9.1). Geophysical research letters, 39(7).