Costa Rica’s digital earthquake catalog quashes a common misconception

Analysis of 14 years of earthquake records reveals that these events aren’t seasonal
 

By Esteban J. Chaves, Ph.D., Volcanological and Seismological Observatory of Costa Rica OVSICORI, Universidad Nacional
 

Citation: Chavez, E. J., 2024, Analysis of 14 years of earthquake records reveals that these events aren’t seasonal, Temblor, http://doi.org/10.32858/temblor.354
 

In Latin American countries, one can hear stories that pass from generation to generation. One of them is that, when the weather is too hot or it’s raining a lot, an earthquake may occur. For instance, on Easter, the general public may be expecting a large event simply because the temperature is high. Another story says that if the pattern of clouds shows some rock-like shape, an earthquake may occur. These stories have been demystified by local scientists, but still prevail in the culture. In my own experience, I tried to tell my grandparents, “Ok, look, if you feel an earthquake, it doesn’t mean that it is related to the weather or climate conditions. The behavior of all earthquakes, including FELT earthquakes is more random.”

To study earthquake seasonality, as well as a host of other questions, a full catalog of earthquakes in the region is needed. And in Costa Rica, OVSICORI has compiled more than 14 years of digital seismic records. This is a uniquely complete earthquake catalog compared to those in other Central and Latin American countries.

Between 2010 and 2023, residents of Costa Rica have reported feeling a total of 2,114 quakes which represents 1.5% of the 140,000 earthquakes in the full seismic catalog recorded by OVSICORI’s seismic network. Thanks to the national expansion of internet services, the development of social networks and the increase in the number of followers on OVSICORI’s Facebook and X (formerly known as Twitter) pages, reporting events has become simple and accessible for people in Costa Rica. Even people living in Nicaragua and Panama near the Costa Rican border contribute to OVSICORI, as these countries do not have quick automatic response from a seismological observatory.

These datasets — both the complete catalog and the list of felt earthquakes — provide invaluable information about when and where earthquakes occur, along with how people experience them. We invite researchers to dive into this data, much of which is open access through OVSICORI’s website. More scientists applying different methods can help us not only better understand earthquake and fault physics, but also Earth’s structure beneath Costa Rica.
 

Which quakes do people feel?

Felt events (Figure 1) between 2010 and 2015 show a variable quarterly average magnitude, with values between 3.0 and 4.5 (red line). However, as of 2016, the behavior stabilized, with values between 3.4 and 4.0. In other words, we now know that earthquakes with magnitude of about 3.5 and above are likely to be felt.
 

 

There are two possible reasons for the stabilization that occurred in 2016. First, in, 2016 the seismic network in the country was expanded. The deployment of more seismic stations means both more robust measurements of earthquake magnitude, and the capability of detecting smaller magnitude events.

Of the felt earthquakes in the catalog, 75% have magnitudes less than or equal to 4.3, while 25% have magnitudes less than or equal to 2.8. This highlights that people live very near fault traces and can feel a good proportion of small magnitude events. This information can be incredibly useful in several key areas, including:

1. Targeted Education and Preparedness: If people are sensitive to smaller tremors and live near faults, these communities might be more receptive to educational efforts focused on earthquake preparedness.
2. Influence on Building Codes and Land Use: This information contributes to building codes and land-use planning. To reduce damage from frequent, low-magnitude tremors, codes are often stricter in areas near active faults. Knowing where faults are located can help local officials prepare for larger events.
3. Public Communication and Risk Perception: Knowing that people feel small events and their potential impacts helps both people and governmental agencies to take proactive steps to manage risks. This preparedness can reduce panic, foster a sense of control, and encourage preventive actions, ultimately saving lives and reducing injuries.
 

 

The second reason for the 2016 change may be the result of a notable increase in the number of felt earthquake reports received through OVSICORI’s social networks (Figure 2). The increase in felt reports may be due to a combination of factors, including increase in internet availability at the national level and the use of social media networks as tools for immediate communication. For example, a time series extracted from the World Bank database demonstrates how, as of 2016, around 3.7 million people gained access to internet service in Costa Rica (Figure 3). During the same period, there was a significant increase in the number of mobile telephone accounts (smartphones), which surpassed the number of fixed telephone lines nationwide.

Moreover, a sustained increase in the number of mobile subscriptions on different platforms (e.g., 3G/5G/LTE phone connectivity) began in 2013, reaching a number greater than 8 million in 2016. This increase in connectivity means that people could more easily access social media apps like Twitter and Facebook. Greater connection between OVSICORI and the public means more felt reports.
 

 

Recurrence time of large events in Costa Rica between 2010 and 2023

Between 2010 and 2023, a total of 37 earthquakes with magnitudes greater than or equal to 6.0 occurred in Costa Rica (Figure 4 and Figure 5).
 

 

The recurrence time observed for these earthquakes varies between less than 20 days and up to 700 days. There are three main episodes of lower recurrence: 600 days between 2010 and 2012 (E1), 700 days between 2015 and 2017 (E2) and 400 days between 2019 and 2021 (E3). There are also three episodes in which such events occurred more frequently, within 20 days or less (yellow bars in Figure 4), labeled as S1, S2 and S3. These groups of earthquakes were generated either along the Pacific Subduction Zone or in the Panama Fracture Zone. Both regions present different seismotectonics or geological contexts.

The Pacific Subduction Zone is a convergent margin where the Cocos plate dives beneath the Caribbean plate and the Panama microplate; the Panama Fracture Zone is a transform fault, where the Cocos plate and the Nazca plate slide past each other horizontally. Each of the events shown in Figures 4 and 5 generated a sequence of cascading aftershocks. Locals reported to OVSICORI that they felt the main event and a great number of these aftershocks.
 

 

This recurrence time analysis provides insights into the timing and frequency of major earthquakes, helping scientists identify cycles or other patterns in seismic activity. By studying these intervals, researchers can better estimate the likelihood of future earthquakes, which is essential for preparing infrastructure, implementing effective building codes and improving emergency response strategies. Additionally, understanding recurrence times aids in deciphering the tectonic processes driving earthquake generation, including stress accumulation and release along fault zones. Ultimately, such knowledge is invaluable for reducing potential human and economic losses, as it informs public policies and risk management practices, enhancing resilience in earthquake-prone areas.
 

Annual and monthly distribution of felt earthquakes

To explore the question of seasonal variation of earthquakes in Costa Rica, I analyzed the monthly count of events for the 14 years of data collected. If earthquakes vary by season, a consistent and repeatable pattern as a function of time should appear. Indeed, except for the month of January, across all years of data collection, the monthly count of felt earthquakes shows a random (or “stochastic”) distribution, suggesting no direct relationship between the number of felt earthquakes and whether Costa Rica was in the dry or rainy season (Figure 6).
 

 

The correlation coefficient between the annual time series also allows us to look for repeated, consistent seasonal patterns over time (Figure 7). The correlation coefficient varies between -1 and 1. A value of 0 indicates no correlation, whereas a value of 1 indicates the maximum correlation between two values or time series. A correlation of -1 indicates the inverse correlation between the time series. By calculating the cross-correlation between the 14 annual time series and graphing the correlation matrix (Figure 7), we see that all the values outside the diagonal (which indicates the autocorrelation of each annual series) show relatively low correlation values or negative correlation.
 

 

So, there is no seasonal variation in the number of felt earthquakes, or, there is no increase in the number of felt earthquakes during the rainy season as compared to the dry season. Although it will take some generations to completely change the misconceptions on the topic of earthquake seasonality in any given country or region, a modern and robust seismic network can help. Costa Rica’s seismic network has facilitated the creation of a unique earthquake catalog in this region, which has dramatically helped local scientists to educate and sensitize the population — and especially younger generations — about earthquakes.

The catalog of felt earthquakes can be reviewed from the OVISCORI website using the following link: http://www.ovsicori.una.ac.cr/index.php/sismos-sentidos.
 

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