“The chair moved a bit and I thought it was a rat that had sneaked in.” This is how the seismic event narrates Jose Ignacio Garcia, who was in his earthly house from Santa Cruz de La Palma. Shortly after, his wife and daughter got him out of his mistake by pointing out that other furniture had also been displaced. Although they were closer to the epicenter (20 kilometers away), his house barely moved and, in fact, unlike Toño Hernández, for García this is his first earthquake.
Why do these differences exist on the same island? The depth and magnitude of earthquakes are key data to predict how these tremors will influence the population and infrastructure, but even more relevant is the type of terrain through which the seismic waves travel or the elements that the wave encounters in his path. “La Palma is divided into two islands”, explains the IGN seismologist, Itahiza Domínguez. The northern palm tree area is much older than the rest, its soils were created at least two million years ago, so they are more consolidated and have acquired greater hardness. Known as the domain of Taburiente, the northern area is located, since the great collapse known as Aridane landslide, practically inactive. Volcanism then migrated south through of the eruptive fissure of Cumbre Vieja, which is the place where the last eight volcanoes in the history of La Palma have appeared, including the current one. “This terrain is newer and softer,” so seismic waves encounter fewer obstacles and wreak less havoc. In other words, deep earthquakes cause more damage in rigid soils because they resist moving to the sound of the seismic wave, while the softer ones deform and couple to the movement of the same.
This information is relevant not only to know how a movement of the earth can influence the population, but it also helps in planning the territory. To calculate what is known as “ground acceleration” – those lands with the greatest possibility of multiplying seismic signals – physical studies must be carried out, which the Canary Islands do not yet have. The IGN tries to establish this parameter with its macroseismic questionnaires, with which they can find out which places are more likely to accelerate seismic waves and cause more damage. For its part, the group of Physical Oceanography and Applied Geophysics (OFYGA) of the University of Las Palmas de Gran Canaria (ULPGC) will install accelerometers in Fuencaliente to measure this index on site. Another phenomenon that occurs with deeper earthquakes is the “attenuation” of their strength during their journey. “In the central zone we see that attenuation”, remarks Domínguez. On the contrary, there may be “local accelerators” that can cause earthquakes to be noticed in a place with greater intensity than a few meters away. One of the most remarkable examples of this evidence is the one that occurred in Mexico in 1985. The epicenter of a magnitude 8 earthquake occurred 200 kilometers from the city. On the way it did not cause any damage and it was barely felt, however, in “Mexico City it knocked down buildings of 20 floors”, because in the area there was an aquifer that generated “resonance” in the area. The earthquakes on La Palma will continue as long or longer as the eruption continues, something that the researchers indicate is “normal” – it also happened in 2011 in El Hierro – and that the next ones may be even stronger.
Intensities of earthquakes
- It has no effect on people, objects and nature or buildings.
- A few people (less than 1%) notice the tremor while at rest.
- Felt by some people inside buildings who feel a sway at rest and hanging objects sway.
- Some people wake up, the vibration is moderate, slight shaking in the bed or room. Knocking of dishes, windows and doors.
- It is felt inside buildings and some people outside. It can be scary. Objects move, liquids oscillate, animals become restless.
- Many get scared and run outside the buildings. Small objects can fall and furniture is dislodged. Moderate damage to buildings
- It is difficult to stand inside buildings. Objects fall from the shelves and there is serious damage to infrastructure.
- It is difficult to stand outside buildings. Furniture overturns and large objects fall. Very serious damage to buildings
- General panic People thrown to the ground. The monuments and columns fall. Very serious damage to buildings.
- Very serious structural damage to most buildings.
- Very serious structural and non-structural damage to all buildings.
- All the buildings are destroyed, of all kinds.