Volcanic Hazards in Costa Rica

Volcanic activity is related to plate tectonics. Most volcanoes are located near active plate boundaries (Keller 128). About two thirds of Earth's active volcanoes are above sea level located within the "Ring of Fire". Features of volcanoes are vents, craters, and calderas. Eruptive behavior of specific volcano types can be predicted (Keller 133). The Volcanic Explosion Index, VEI, is a relative scale where the different eruptions can be compared "based on quantitive and qualitative observations of explosivity" (Keller 134). There are different kinds of volcanoes such as:

The stratovolcanoes are cone shaped and occur above subduction zones. These volcanoes are characterized by explosive eruptions (Keller 134).

Lava Domes are smaller than stratovolcanoes. These domes can be explosive and nonexplosive.

Shield volcanoes are the largest. They are common in mid ocean ridges and over mid plate hot spots (Keller 137). They are usually nonexplosive lava flows.

The continental caldera volcanoes are created by violent eruptions. They often present a volcanic hazard for the next million years.

Effects of volcanic activity include lava flows, pyroclastic hazards, emissions of poisoned gases. Loss of land and property can result from lava flow. Pyroclastic hazards involve ash falls and in turn ruin agriculture land. Secondary effects include debris flows and mudflows. Volcanoes are also linked to fires, earthquakes, landslides and climate change (Keller). However, volcanoes provide fertile soils, mineral resources and new land.
Today we minimize the danger of volcanic activity by forecasting and Volcanic Alert Codes. Costa Rica is home to several volcanoes and is apart of the "Ring of Fire". Poas remains one of Costa Rica's largest and most active volcanoes. Although the last major eruption was in 1910, the volcano remains steadily active to this day, and visitors can still see geysers exploding into the air, some reaching as high up to 820 ft (Arenal.net). Costa Rica's tallest volcano is Irazu and is located in the highlands. Costa Rica's volcanic activity has taken them to sending out alerts in case of any threats. They use cell phone alerts as well as broadcasting them through television as well as non profit groups to help the elderly in case of evacuations. They have scientists dedicated to follow the activity of volcanoes in Costa Rica such as the Volcanological and Seismological Observatory of Costa Rica (OVSICORI) and the National Seismological Network (RSN).

Sources:

• https://www.arenal.net/volcanoes-costa-rica
• as-turrialba-volcano-erupts-again-scientists-warn-of-increased-health-risks-economic-damage
• Keller, Edward A., et al. Natural Hazards, Earth's Processes as Hazards, Disasters, and Catastrophes. Fourth Edition. 20

Earthquakes

The constant movement of Earth's tectonic plates have reshaped continents and oceans over the past billion years. If the movement beTtween adjacent plates was smooth and continuous we would be unaware of the movement between the plates. Unfortunately, that is not the case. Plates move "episodically"(Keller 53) every few hundred years. "These large episodic movements of the plates cause the violent ground shaking we know as an earthquake" (Keller 53).  There are three key factors that determine the shaking you feel during an earthquake: 

1. Earthquake magnitude
2. Location in relation to epicenter and direction of rupture
3. local soil and rock conditions

Large earthquakes release enormous amount of energy. Seismologists measure this energy on a magnitude (M) scale. "Earthquakes are given description adjectives based on their magnitude scale" (Keller 61). Most earthquakes are described as major(M 7-7.9) or strong (M 6-6.9) (Keller 61). Earthquakes classified as major can cause "widespread and serious damage" (Keller 61). Whereas, strong earthquakes can cause "considerable damage" (Keller 61). The most powerful earthquake is called a grand (M 8+). The amount of ground motion produced by an earthquake is related to its magnitude, depth and environment where it happens. Buildings more prone to damage are the ones that are constructed on unconsolidated sediment, artificially filled land, water saturated sediment. All are said to amplify the shaking (Keller 63). Also, the buildings that are not designed to withstand significant horizontal acceleration of the ground or have natural vibration frequencies that match the frequencies of the seismic waves are highly subject to damage. 

Earthquake intensity varies with the severity shaking and is affected by proximity to the epicenter, the local geological environment and the engineering of structures (Keller 64). Earthquake intensity is shown in maps and recently the U.S. Geological Survey, USGS opened an online survey, "Did You Feel It?" to solicit reports from people in the US who recently felt an earthquake.

According to the USGS website, along the western coast of Central America, the Cocos plate subduct towards the east beneath the Caribbean plate at the Middle America Trench. Convergence rates vary between 72-81 mm/yr, decreasing towards the north. This subduction results in relatively high rates of seismicity and a chain of numerous active volcanoes; intermediate-focus earthquakes occur within the subducted Cocos plate to depths of nearly 300 km. 

Since 1900, there have been many moderately sized intermediate-depth earthquakes in this region, including the September 7, 1915 M7.4 El Salvador and the October 5, 1950 M7.8 Costa Rica events. The boundary between the Cocos and Nazca plates is characterized by a series of north-south trending transform faults and east-west trending spreading centers. Under Costa Rica, the tectonic plates Nazca, Cocos and Caribbean all converge just off the Pacific Coast.This makes for a lot of seismic activity. In the area where the September 5 quake hit on the Nicoya Peninsula, the Cocos plate was diving beneath the Caribbean plate. Currently, Costa Rica has changed to Masonry buildings rather than adobe ones because adobe buildings were the riskiest  types to build (gfdrr.org). There have been many groups arranging plans snd helping individuals tear their adobe buildings to modern masonry buildings. 

Sources

• https://gemrisk.wordpress.com/2016/03/04/profiling-earthquake-risk-in-costa-rica/
• https://earthquake.usgs.gov/earthquakes/eventpage/us2000bmhe#region-info
• gfdrr.org
• Benz, Harley. “USGS Open-File Report 2010-1083-A: Seismicity of the Earth 1900–2010 Caribbean Plate and Vicinity.” USGS Publications Warehouse, pubs.usgs.gov/of/2010/1083/a/.
• Keller, Edward A., et al. Natural Hazards, Earth's Processes as Hazards, Disasters, and Catastrophes. Fourth Edition. 20

Tectonic Plate?

Earth's lithosphere is broken into tectonic plates that move relative to one another. Tectonic processes are driven by forces within Earth (Keller 33).  Plate tectonics are often the cause of natural hazards such as volcanoes and earthquakes. All major plates include an ocean basin and a part of the continent. Plates can be small or large There are three types of plate boundaries: divergent, convergent and transform.

1. Divergent: Divergent boundaries occur when neighboring parts of plates are moving away from each other and new lithosphere is being produced (Keller 36). 
   
2. Convergent: Convergent boundaries occur when plates move towards each other (Keller 36). 
   
3. Transform: Transform boundaries occur when the edges of two plates slide past each other like the San Andreas fault in California (Keller 38). 
   

Everything living on Earth is affected by plate tectonics. The types of and properties of the rocks which we depend for land, our mineral and rock resources, and soils in which our food is grown  is determined by the tectonic processes occurring in the plate boundaries (Keller 48).

Costa Rica is in the Caribbean Tectonic Plate with the Cocos Plate to the West. The Cocos plate moves Northeast relative to the Caribbean plate. Frequent earthquakes and volcanic activity happen in Costa Rica due to the competitive movement between the two plates.

Source: 

Keller, Edward A., et al. Natural Hazards, Earth's Processes as Hazards, Disasters, and Catastrophes. Fourth Edition. 20