Lesson 2: Earthquake Epicenters

• Every day, there are record of several seismic activites in the Philippines.
• Earthquakes occur when rocks in the lithospere rupture or move in order to release accumulated energy and stress. They commonly occur along plate boundaries which are made of many faults.
• Faults are breaks in Earth's crust where rocks move or slide past one another. There should be an apparent displacement for a break or a fracture for it to be considered as a fault.
• Rocks along plate boundaries or zones of high stress are either consolidated or held together by friction. Given enough stress, a fault will overcome the frictional force, and stressed rocks will rupture to produce a new fault.

  • These events release incredible amounts of energy creating earthquakes.
• The surface where the slip occurs is called the fault plane and the location below Earth's surface where the earthquake originates is called a hypocenter. It is also commonly known as focus.
• The surface directly above the hypocenter is the epicenter. Most of the time, it is the area where the greatest damage takes place during earthquakes.
• There are two different measures in describing the strength of an earthquake, namely: intensity and magnitude
• Intensity is a measure of the degree of shaking experienced in an area. This description is a combination of what the people felt and the damage observed in the structures.
• Magnitude, on the other hand, is a quantitative measurement based on instruments. It relies on the data from seismic records along with other techniques to estimate the amount of energy released.
• The Philippines is comprised of several subduction zones both on its eastern and western part.

  • These regions are where tectonic plates meet, making it responsible for most of the significant earthquakes, terrible tsunamis, and some of the worst volcanic eruptions in the archipelago.
• Located in the Eastern part of the country are the East Luzon Trough and Philippnes Trench, whlie the western region has the Manila Trench, Negros Trench, Sulu Trench, and Cotabato Trench.
• The Philippine Fault or Philippine Fault Zone (PFZ), is a 1,200-km fault zone cutting almost the whole archipelago.
• It transects the Philippines from Luzon to Mindanao, passing across Bicol and Leyte. PFZ accommodates the oblique convergence between the Philippine Sea Plate and the Philippine archipelago.
• Aside from the Philippine Fault Zone, there are also other active faults in the country, namely: Valley Fault system or Marikina Valley Fault, Macolod Corridor, Lubang-Verde Passage Fault System, Mindoro or Aglubang Fault, Sibuyan Sea Fault, Legaspi Lineament, Tablas Lineament, Mindanao Fault, and Offshore Cebu-Bohol Faults.
• Even though the Philippines is seismically active, historical accounts of earthquakes only started during the Spanish colonization in the late 15th century.

  • These earthquakes are considered as the historical record before the availability of instruments. However, the instrumentally-derived parameters for earthquakes started in 1892 onwards.
• However, from time to time, large earthquakes occur in the country which results in massive damages.
• Landslides and sand boils

  Combination of sand and water that come out from the ground during an earthquake due to liquefaction caused by ground shaking