Massive Earthquake Swarm Located Under Dormant Tenorio Volcano In Guanacaste, Costa Rica |

Since January 9, 2018 the National Seismological Network (RSN: UCR-ICE) has located 42 earthquakes on the eastern slope of the Tenorio volcano. These earthquakes have had magnitudes between 2.2 and 5.3. Seismicity began on January 9, with an earthquake of magnitude 4.7 at 8:56 pm. On day 9, 11 replicas were presented with magnitudes between 2.7 and 4.3. On the 10th the seismicity decreased and only seven earthquakes occurred, with magnitudes between 2.2 and 3.5. On the 11th there was a rebound in the seismicity. Up to 5:00 p.m., 23 earthquakes have been located, including three of magnitude greater than 5.0.

The main earthquakes located by the RSN are the following:

January 9, 8:56 p.m., magnitude 4.7

January 9, 8:58 pm, magnitude 4.3

January 9, 8:59 pm, magnitude 3.6

January 9, 9:05 pm, magnitude 3.4

January 9, 9:14 pm, magnitude 3.8

January 9, 9:19 pm, magnitude 3.7

January 10, 2:36 am, magnitude 3.5

January 11, 10:00 am, magnitude 5.1

January 11, 10:04 am, magnitude 4.5

January 11, 10:08 am, magnitude 3.7

January 11, 11:03 a.m., magnitude 3.7

January 11, 11:03 a.m., magnitude 5.1

January 11, 11:09 am, magnitude 4.0

January 11, 11:13 am, magnitude 3.6

January 11, 11:35 am, magnitude 3.7

January 11, 11:58 am, magnitude 4.0

January 11, 12:38 a.m., magnitude 3.9

January 11, 01:03 p.m., magnitude 3.6

January 11, 01:57 pm, magnitude 5.3

Effects

The instrumental intensities in Bijagua were from V-VI on the Modified Mercalli Scale. The largest magnitude earthquakes have been reported as felt in Bagaces, Tilarán, Alajuela, Fortuna, Ciudad Quesada, Puntarenas, Esparza, Abangares and Santa Cruz de Guanacaste. For the earthquake of 1:57 pm, of magnitude 5.3, the fall of light objects was reported in Aguacaliente, located in the epicentral zone, southwest of Tenorio volcano.

Another notable effect after the earthquakes of January 11, was the change in the coloration of water in some celestial springs, such as the El Roble river, located on the slopes of Tenorio volcano. These changes are normal and may be due to disturbances in the aquifers during important earthquakes or to material contributed to the rivers by landslides. This phenomenon was also reported after the seismic activity of 2002 and 2016 in the same region.

Registration of the BSN station in the Arenal volcano showing the most important earthquakes on January 11, 2018

Historical earthquakes

Historically important earthquakes have been recorded in the Guanacaste mountain range related to faults, such as those of 1853 in Tilarán, 1911 in Guatuso, 1935 and 1941 in Bagaces, 1973 in Tilarán. In 1984, 1985 and 1987 there were swarms in Armenia of Upala. Subsequently, in 1997 and 1998 there were swarms in Tierras Morenas and in the Miravalles volcano. The relevant earthquakes in the Bijagua area and more recent surroundings occurred in January 2002 (Mw 5.4), July 2011 (Mw 5.5) and July 2016 (Mw 5.4).

Instrumental intensities map of #Sismocr Revisado, 11-1-2018, 11:03 am, Mag: 5,1 Mw, Prof: 4 km, 12 km southeast of Bijagua, Upala.

Origin and its relationship with the Honduras earthquake

The hypocenters of the earthquakes have depths between 2 and 15 km and their epicentral location is in the vicinity of the Caño Negro and Chiquero faults, located in the surroundings of the Tenorio volcano. Additionally, five earthquakes have occurred in the area of the Miravalles volcano, with magnitudes between 2.8 and 4.0, which could be related to other faults located in that area. The focal mechanisms of the larger earthquakes suggest that they were caused by fault displacement.

The onset of seismicity on January 9 coincided with the earthquake on the Caribbean coast of Honduras, magnitude 7.6, which was not felt in Costa Rica. The first earthquake in the Tenorio area occurred five minutes after the earthquake. The distance between the epicenters in Costa Rica and the earthquake in Honduras is approximately 800 km. It is possible that earthquakes of high magnitude trigger seismicity at great distances, induced by the passage of seismic waves.