FoG: T12 - EARTHQUAKE-Associated Hazards-Mitigation

According to PHIVOLCS, what is the definition of an earthquake?

A weak to violent shaking of the ground produced by the sudden movement of rocks or rock materials below the earth’s surface.

What is the primary cause of an earthquake related to faults?

The sudden movement along faults or large sections of the earth’s crust.

In geology, what is a fault?

A planar fracture or discontinuity between two blocks of rocks due to rock-mass movements.

What is the first step in the mechanism behind earthquakes at tectonic plate boundaries?

The plates do not always move smoothly past each other at faults.

Why do plate edges get stuck at faults?

Plate edges get stuck because of friction, while the rest of the plate continues to move.

What builds up at the stuck edges of tectonic plates?

The energy and pressure that move the plates get stored up at the edges.

What happens when the stored energy at plate edges overcomes friction?

The plates move past each other along the fault, releasing the energy.

The energy and pressure from plate movement are released in the form of _____ that shake the surface.

seismic waves

What theory explains how energy is gradually accumulated and released during an earthquake?

The Elastic Rebound Theory.

Who developed the Elastic Rebound Theory based on observations of the 1906 San Francisco earthquake?

Henry Fielding Reid.

What is the term for the deformation of rocks resulting from applied stress?

Strain.

The earthquake cycle hypothesis proposes a drop in elastic strain after an earthquake and a re-accumulation of strain before the _____.

next event/occurrence

Where do most earthquakes originate?

At the boundary between two tectonic plates.

Tectonic plates are pieces of the Earth's crust and uppermost mantle, collectively called the _____.

lithosphere

What is the primary driving force behind the motion of tectonic plates?

Convection within the mantle.

What is the name for the point inside the Earth where rocks rupture to produce an earthquake?

The focus or hypocenter.

What term describes the location on the Earth's surface directly above the focus of an earthquake?

The epicenter.

What is the scientific study of seismic wave behavior called?

Seismology.

What are the two broad classifications of seismic waves?

Body waves and surface waves.

Which type of seismic waves travel through the Earth's interior?

Body waves (P-waves and S-waves).

Which type of body wave is the fastest and can travel through solids, liquids, and gases?

P-waves (Primary or Pressure waves).

P-waves are described as _____ waves because they push and pull on the material they travel through.

compressional

Which type of body wave is slower than P-waves and can only travel through solid materials?

S-waves (Secondary or Shear waves).

How do S-waves move material relative to the direction of wave travel?

They move the material perpendicular to the direction the wave is traveling (shear motion).

Which type of seismic waves are the first to be detected after an earthquake?

Body waves (specifically P-waves).

Upon reaching the Earth's surface, P and S waves convert into what type of waves?

Surface waves.

Which of the seismic wave types (P, S, Surface) travels the slowest?

Surface waves.

What are the three important factors that determine the level of ground shaking experienced at a site?

Earthquake magnitude, distance from the source (epicenter), and site effect (local soil & rock conditions).

What term measures the size of an earthquake, proportional to the energy released at the focus?

Magnitude.

What instrument is used to record the data needed to calculate an earthquake's magnitude?

A seismograph.

Magnitude is represented by what type of numerals?

Arabic numbers (e.g., 4.8, 6.0, 9.0).

The _____ scale estimates earthquake magnitude by measuring the amplitude of the largest seismic wave recorded.

Richter

What term describes the strength of an earthquake as perceived and felt by people in a certain locality?

Intensity.

Intensity is generally higher near the _____.

epicenter

What scale, represented by Roman numerals, is used to describe the effects of an earthquake on people, objects, and structures?

The Modified Mercalli Intensity (MMI) Scale.

What type of earthquake is produced by sudden movement along faults and plate boundaries?

Tectonic earthquake.

What type of earthquake is induced by rising lava or magma beneath active volcanoes?

Volcanic earthquake.

What three earthquake parameters are determined by the PHIVOLCS Data Receiving Center (DRC) from seismic station data?

Magnitude, depth of focus, and epicenter.

What are the four factors that structural damage from earthquake vibrations depends on?

Wave amplitudes, duration of vibrations, nature of foundational material, and design of the structure.

Structures built on _____ experience less ground shaking because the material's rigidity helps dissipate seismic energy.

hard rock

What phenomenon can occur in soft, water-saturated materials during an earthquake, causing the ground to behave like a liquid?

Liquefaction.

What are the four primary effects of earthquakes listed in the source material?

Ground shaking, landslides, liquefaction, and surface/ground rupture.

What are the two secondary effects of earthquakes mentioned in the source material?

Fires and tsunamis.

What is the term for the disruptive up-down and sideways movement experienced during an earthquake?

Ground shaking.

Ground rupture is the displacement on the ground due to the movement of _____.

faults

What is the term for faults that have moved in historical times and are expected to move again?

Active faults.

In a _____ fault, the hanging wall moves down relative to the footwall.

normal

In a _____ fault, the hanging wall moves up relative to the footwall.

reverse

In a _____ fault, the blocks move horizontally past one another.

strike-slip

What is a key measure to minimize the effects of ground rupture when planning construction?

Avoidance of active fault traces and deformation zones.

What is liquefaction?

A process that transforms the behavior of a body of sediments from a solid to a liquid when subjected to intense shaking.

What type of soil is particularly susceptible to liquefaction?

Wet fine sand.

What is the term for the failure in steep or hilly slopes triggered by an earthquake?

Earthquake-induced landslide.

One engineering measure to mitigate landslide effects is the construction of landslide barriers and _____ structures.

drainage

What is the term for a series of giant waves, often greater than 5 meters, produced by strong under-the-sea earthquakes?

Tsunami.

The Pacific Tsunami Warning Centre (PTWC) relays information and warnings about oncoming tsunamis to which Philippine agency?

PHIVOLCS.

What is the major tectonic feature in the Philippines, stretching approximately 1,200 km from NW Luzon to SE Mindanao?

The Philippine Fault Zone (PFZ).

What type of fault is the Philippine Fault Zone (PFZ)?

A left-lateral strike-slip fault.

The Philippine Fault Zone is a result of the subduction of the Philippine Sea plate under the _____ plate.

Eurasian

The Central Mindanao Fault is described as the most active and _____ fault line in its region.

longest

The Masbate Fault segment in the Visayas has been the source of moderate earthquakes with unusually large _____.

ground ruptures

What is the term for the earthquake's source, located inside the earth?

Focus (or hypocenter).

Which seismic waves have a higher frequency and travel at faster speeds than surface waves?

Body waves (P-waves and S-waves).

What information do body waves provide that is valuable for understanding the Earth's interior?

They provide insights into the structure and composition of the Earth's interior.

A numerical rating based on the relative effects of an earthquake on people, objects, and structures is called its _____.

intensity

What is a primary danger to people during strong ground shaking?

Failure of buildings due to inferior design, poor construction, or weak foundation.

Earthquakes with a magnitude of _____ or greater are generally strong enough to produce large-scale ground ruptures.

6.5

Besides avoiding construction on active faults, what practice can help prevent total destruction from ground rupture?

Sound engineering and construction practice.

What is the term for the ejection of water and sediment at the surface during liquefaction?

Sandblows or sandboils.

Besides building liquefaction-resistant structures, what is another option to mitigate liquefaction hazards?

Improving the existing soil.

What type of biological measure can be used to mitigate the effects of landslides?

Planting of deeply rooting trees.

A major secondary effect of a tsunami disabling a nuclear power plant, as seen in Fukushima, is a _____.

radiation leak

What type of structure can be built along coastlines to mitigate the effects of a tsunami?

Seawalls.

The fault in central Luzon is known for its four left-stepping segments: San Manuel, San Jose, Digdig, and _____ faults.

Gabaldon

The Leyte Fault segment shows relatively quieter seismic activity but has been associated with medium-sized events and _____ activity.

creep

A 2017 earthquake of magnitude 6.7 in Surigao del Norte was associated with which segment of the Philippine Fault Zone?

The Surigao Segment.