Understanding Earthquake Hazards

• Earthquake hazards overview

What is an Earthquake?

• An earthquake is the shaking of the ground caused by the sudden movement of rock materials below the surface
• Earthquakes originate at tectonic plate boundaries
• The focus, also called the hypocenter, is the point inside the earth where the earthquake starts
• The epicenter is the point on the earth's surface directly above the focus

Measuring Earthquake Strength

• Earthquake strength is measured by magnitude and intensity
• Magnitude measures the energy released and is calculated from data recorded by a seismograph
• Intensity measures the earthquake's strength as perceived and felt by people
• The Philippines uses the PHIVOLCS Earthquake Intensity Scale (PEIS)

Types of Earthquakes

• Earthquakes can be either tectonic or volcanic
• Tectonic earthquakes are caused by movement along faults and plate boundaries
• Volcanic earthquakes are induced by rising lava or magma

Earthquake Risk Factors

• "Earthquakes don't kill people, buildings do"
• Deaths result from building collapse or other construction failures
• Isolated earthquakes are less dangerous
• Population density, construction standards, and emergency preparedness all affect earthquake risk

Earthquake Examples

• Mindanao, Philippines, 1976: A magnitude 7.9 earthquake and tsunami killed 3,564
• Central Luzon, Philippines, 1990: A magnitude 7.0 earthquake killed 1,660
• Shaanxi, China, 1556: The worst earthquake in recorded history killed 830,000, many living in caves
• T'ang Shan Province, China, 1976: A magnitude 7.8 earthquake and magnitude 7.1 aftershock killed 240,000
• Haiti, January 12, 2010: A magnitude 7.0 earthquake caused an estimated 230,000 deaths

Hazards Associated with Earthquakes

• Possible earthquake hazards may be classified

Ground Motion

• Shaking of the ground from seismic waves near the epicenter
• Is the primary effect of an earthquake and depends on:
  • Local geologic conditions
  • Earthquake size
  • Distance from the epicenter
  • Type of construction

Seismic Shaking

• The epicenter has the strongest seismic shaking
• Strong seismic shaking can occur in loose soil or filled land far from an epicenter, which magnifies the effects of seismic waves
• This can cause severe damage to structures

Liquefaction

• Sediment layers and rock saturated with water can cause liquefaction
• Liquefaction turns stable soil into liquid
• The liquid soil cannot support buildings or other structures, causing collapse
• Underground tanks and sewer lines may float to the surface

Landslides and Mudflows

• Earthquakes can trigger different types of mass movements, which often cause more destruction and loss of life
• Earthquakes often cause landslides, where loose rock and soil move on slopes
• Mudflows, a mixture of soil and water can start in areas of high soil water content, and move rapidly downhill

Tsunamis

• A tsunami is a wave formed when the ocean floor shifts suddenly during an earthquake
• A magnitude 9.4 earthquake west of Sumatra, Indonesia in 2004 caused devastating tsunamis in the Indian Ocean
• The tsunami struck coastal areas of Indonesia, Sri Lanka, Thailand, and other countries, killing nearly 300,000
• Underwater landslides or volcanic eruptions can trigger tsunamis
• A tsunami wave may only be a meter high in the open ocean and move at hundreds of miles per hour
• Waves slow down and pile up as they enter shallower water near the shore, increasing in size

Earthquake Damage Factors

• Earthquake damage depends on several factors
• Strength and duration of seismic shaking and the materials and design of structures are important factors
• Reduce earthquake damage and loss of life by determining the earthquake risk, building earthquake-resistant structures, and following earthquake safety procedures

Reducing Earthquake Damage

• Assess the danger or probability of an earthquake in an area, using knowlodge that earthquakes are most common along the boundaries of Earth's tectonic plates
• Use several methods to determine earthquake risk, study historical records of earthquakes to observe where they have occurred
• Instruments are used to measure the uplift, subsidence, and strain on rocks in fault area
• Seismic gaps, areas along a fault with no activity for a long time are studied
• Scientists hypothesize that strain buildup along a seismic gap will cause an earthquake
• Small warning signs of an earthquake may include changes in water levels and pressure in wells, radon gas emissions, or electromagnetic properties of rocks

Seismic Design Safety

• Many modern cities have modern building codes that take earthquakes into account
• Steel building frames are often reinforced with cross-braces
• Base-isolators, large rubber and steel pads, are used to mount buildings to absorb shock
• Wooden frame houses are reinforced and bolted to their foundations
• Flexible pipes and automatic shutoff valves prevent rupture of gas and water lines
• Efforts at short term earthquake prediction are not adequate to allow timely warning