Earthquake and Volcano Hazards PDF

Earthquake Hazards Ground Shaking Ground Rupture Liquefaction Tsunamis Earthquake-Induced Ground Subsidence Earthquake-Induced Landslide GROUND RUPTURE  What is Ground Rupture? Tension cracks are also referred to as Ground ruptures or fissures. It is a phenomenon where ground mov...

Earthquake Hazards Ground Shaking Ground Rupture Liquefaction Tsunamis Earthquake-Induced Ground Subsidence Earthquake-Induced Landslide GROUND RUPTURE  What is Ground Rupture? Tension cracks are also referred to as Ground ruptures or fissures. It is a phenomenon where ground movement occurs and the surface of the ground breaks. It is commonly present in areas located near or along fault lines.  WHAT IS GROUND RUPTURE Most structures are not built on fault lines as these are prone to breakage. However, roads that rupture or affect transportation as well as power and communication lines that are rooted here. Sizable ruptures can have days to repair, not to mention the possible damage to water lines underneath. Fissure or ground rupture. These are the most prevalent in areas LIQUEFACTION WHAT IS LIQUEFACTION? In liquefaction, the sediment composition becomes “liquefied” in the sense that it assumes the dynamics of water by flowing.  WHAT IS LIQUEFACTION On a normal day, areas that are water-saturated underneath would not experience any destruction. However, once ground shaking affects the area, this creates a disturbance in the underlying materials, causing grains of matter to loosen and mix with the water (Nelson, 2015).  WHAT IS LIQUEFACTION Once all of this flowing, this cause the release of water to the ground or worse, collapsing of structures on the liquefaction-prone sites. COMMON SIGNS OF LIQUEFACTION Ground undulation causing Ground water leaking uneven land formations Structures built in liquefaction-prone areas are subject to definite collapse. Likewise, roads and other related infrastructure also become unstable over time once ground materials fuse extensively with water underneath. With this, several roads may be unpassable if affected by liquefaction. Some other common manifestations of liquefactions are the sinking of structures that harbor mostly loose materials underneath. EARTHQUAKE-INDUCED GROUND SUBSIDENCE  EARTHQUAKE-INDUCED GROUND SUBSIDENCE Land subsidence or the sinking of the ground is attributed to various man-made activities such as the over-pumping of groundwater and many others. On the other hand, earthquakes and their complexities may also cause land subsidence, specifically earthquake-induced ground subsidence. HOW EXACTLY DO EARTHQUAKES CAUSE THE LAND TO SINK? The process of liquefaction Land may also sink may cause loose deposit of soil to be compacted from primarily because the ground shaking, thereby fault is underneath causing the ground to the ground. Blind subside. In the case of thrust fault shows a earthquake-induced ground notable sinking of subsidence, the water land due to the flowing through displacement of the liquefaction suddenly goes land underneath through surface, leaving ground unstable and causing it to sink  EARTHQUAKE-INDUCED GROUND SUBSIDENCE Earthquake-induced ground subsidence can be a major factor in an area’s susceptibility to flooding. As it lowers, it becomes a catch-basin for rainwater. In addition, gradual sinking becomes a precursor for the development of sinkholes in adjacent areas.  Land subsidence (gradual setting or sudden sinking); circular plate represents the maker of where the land used to be) TSUNAMI  WHAT IS TSUNAMI? Tsunamis that are of seismic origin start from a movement underneath the ocean. This movement is also described as a displacement, marks the initiation of the tsunami.  WHAT IS TSUNAMI? According to Bryan (2014), a tsunami is a wave or series of waves in a wave train generated by sudden, vertical displacement of a column of water caused by seismic activity, explosive volcanism, a landslide above or underwater, etc.  WHAT IS TSUNAMI? The term tsunami came from the Japanese terms “tsu”, which means harbor and “nami” which means wave. It is also called “harbor wave”.  STAGES OF SEISMIC-RELATED TSUNAMI STAGE 1:INITIATION STAGE 2:SPLITTING STAGE 3:AMPLIFICATION STAGE 4:RUNUP INITIATION The displacement of the seafloor underneath is followed by the displacement of ocean water, thereby lifting it and triggering the formation of waves. SPLITTIN G The “splitting” of the tsunamis into two waves, with one heading toward the deep ocean, called distant tsunami and the other one heading toward the near shoreline is called local tsunami. AMPLIFICATION As the tsunamis travel in their respective directions, they follow an amplification process, where the waves increase their height while their wavelength decrease. This causes the shore water to recede then exposing parts of the beach.  RUNUP It refers to the approach of the tsunami’s leading wave to the shore. Runups happen multiple times with accumulating force. As the parts of the wave recede, it is simultaneously adding to the height of a succeeding leading wave. Imagine that you live in a coastal area that has never experienced tsunami but is surrounded by major plates and faults. How will you know if a tsunami is headed to a coastal area?  NATURAL SIGNS OF IMPENDING TSUNAMI Ground shaking Unusual sea level Sound of rumbling near a body of wave nearby water The speed of tsunami, coupled with force and debris it brings, can create a disaster that affects people, infrastructures, homes, as well as livelihood and normal environmental processes in an area. Once it causes flooding, it will take time to recede it completely. EARTHQUAKE-IN CED LANDSLIDE  EARTHQUAKE-INDUCED LANDSLIDE Earthquakes create a massive effect on land when combined with the force of gravity. According to Rimando (2016), gravity always serves as the primary force that enables landslides to occur and is usually coupled with the triggering effect of earthquakes.  EARTHQUAKE-INDUCED LANDSLIDE PHIVOLCS (2021) describes landslides as a mass movement of rock, soil and debris down a slope due to gravity. It manifests due to ground vibrations during tectonic and volcanic earthquakes.  EARTHQUAKE-INDUCED LANDSLIDE In addition, it may also combine with various triggers over time such as: natural triggers (e.g., plant and animal movement); rainfall; weathering of rocks; and man-made activity (e.g., excessive mining and massive urbanization in unstable areas). The following triggers may create conditions in an area that make it prone to landslides: steep slopes – increases the potential energy of the impending landslide; weakening of slope material – unstable landmass; weathering of rocks – alteration of rock condition and composition over time; and overloading on the slope – may be brought about by man-made development. Landslide caused by unstable soil content and steepness.  EARTHQUAKE-INDUCED LANDSLIDE Going deeper into understanding landslides, it must be noted that it comprises three parts: (1) source, (2) path, and (3) deposit. Whatever mode of failure a landslide assumes – whether it is falling, toppling, sliding, spreading, or flowing – communities may experience various forms of disaster whether they are at the source, the path, or the deposit area. Parts of a Landslide  Volcano Hazards Lahar Ashfall Pyroclastic Flow Ballistic Projectile Volcanic Gas Lava Flow As the earth’s tectonic plates move and collide with one another, a tectonic plate subducts in an area called the subduction zone. The boundaries of plates are where majorities of volcanoes are formed, as these are the areas where subduction continually occurs. LAHAR  LAHAR The term lahar is of Indonesian origin for “lava” or “lava flow”, which volcanologists adapted to describe a wet cement-like mixture of volcanic material and water.  LAHAR When lahar flows, it carries with it fresh volcanic materials – pyroclastic flows and tephra falls (ashfalls). With the thickness of volcanic materials it brings as well as toxic chemicals, it is impossible to survive being submerged in this kind of mixture. ASHFALLS  ASHFALLS The gray appearance of lahar can be attributed to the presence of debris from ashfall. Tephra is another name that refers to ashfall; however, there are notable differences.  ASHFALLS Ashfalls consist of fragmented volcanic particles less than 2 mm in diameter in size while tephra simply means fragmented volcanic particles in general. After the eruption occurs, these minute particles get thrown into the air and form a column of ash. Then they settle down due gravity, cloaking affected areas with a blanket of tephra ash.  ASHFALLS Taal Volcano, for example, showed intense volcanic activities in 2020 that affected various areas near the volcano. As a result of its eruption, the ashfall posed a health hazard when inhaled as it can lead to a variety of respiratory diseases. In some instances, it can cause severe amounts of skin damage due to its toxic content, contaminate water sources and it can damage infrastructures, etc. PYROCLASTIC FLOWS  PYROCLASTIC FLOWS Apart from the ashes that trail down affected areas, what comes from explosions are pyroclastic flows. These materials released from the mouth of the volcano are generally classified as pyroclastic density currents made up of the volcanic particles such as pyroclastic, hot gases, and ashes. All of this rush down from the mouth of the volcano after its explosion.  PYROCLASTIC FLOWS Depending on density, composition, and viscosity, PDCs may be classified into two. 1. Pyroclastic Flow – it is a dense type of current that moves a bit slower than a surge. This tends to be more attached to the ground. 2. Pyroclastic Surge – it is diluted type of current that has more mobility, therefore posing more risk to affected communities.  PYROCLASTIC FLOWS An explosion of a lava dome from side vent or the main vent may cause pyroclastic flow or surge.  BALLISTIC PROJECTILES  BALLISTIC PRJECTILES Apart from all the ashfall and pyroclastic flow that comes out of the mouth of the volcano, some large blocks or bombs are also formed called ballistic projectiles. These do not go straight into volcanic column but are rather thrown into an affected area. Their mechanism can be likened to a cannonball usually landing within 2-5 km away from the vent. It can also travel much further if the volcano is more explosive. VOLCANIC GASES  VOLCANIC GASES There are variety of gases released before and even after an eruption. Before an eruption, gases have already built up pressure inside the magma waiting to be released. Magma consists of various dissolved gases that are harmful to health, vegetation, and infrastructure.  VOLCANIC GASES Most of the volcanic gases are released into the atmosphere through the main vent; some of them, being heavier than air, go down into low-lying communities near the vicinity of the eruption.  VOLCANIC GASES Below are some of the gases released from an eruption: Water Vapor (colorless, odorless, and harmless; the most abundant volcanic gas) Carbon Dioxide Sulfur Dioxide Hydrogen Sulfide Hydrogen Halides such as Halogens Flourine, Chlorine, and Bromine  LAVA FLOWS  LAVA FLOWS Lava flows emit bright red-orange appearance, signaling that the once enclosed magma has been released in the form of lava. Depending on the level of silica, lava flows have varying viscosities: Low silica magma = low viscosity lava flow; movement at high speeds (km/hour) High silica magma = high viscosity flow; movement at low speeds (km/day)  LAVA FLOWS Note that the steepness of the slope adds to the movement speed of the lava flow at it succumbs the gravity. Furthermore, lava flows carry high temperatures capable of forest fires. It can also burn through the soil, vegetation, residential houses, and infrastructure, creating a river of lava. SIGNS OF IMPENDING VOLCANIC ERUPTION According to the USGS (2021), there are several ways to tell if a volcano is about to erupt. Some are related to seismic activity while others are related to the release of gases. The signs include the following: The frequency and intensity of earthquakes felt within the area increase. As pressure keeps building as it releases more seismic energy that causes ground shaking in the area.  The release of steam and gases from the vents near the volcano increases. From afar, it may be seen ones as a cloud of air. The fumarolic activity increases. Fumaroles near the mouth of the volcano not only emit gases but also show a noticeable increase in carbon dioxide concentration in the air. Heat emission increases. The temperature within the area increases dramatically. It is an indicator that magma is close to the surface. Manila Observatory (2005) provided a list of provinces with the greatest amount of risk for volcano-related damage: 1. Camiguin (or Camiguin Islands) 2. Sulu Camiguin has the 3. Biliran highest risk because 4. Albay the land area is so 5. Bataan small that a volcanic 6. Sorsogon eruption can affect the whole province. 7. South Cotabato Sulu ranked second 8. Laguna because it has the 9. Camarines Sur greatest number of 10. Batanes active and potentially active volcanoes.