Definition of terms

Terms about demography

Age dependency ratio
The ratio of persons in "dependent" ages (generally under age 15 and over age 64) to those in "economically productive" ages (15 to 64 years) in the population. It is sometimes divided into the old-age dependency (the ratio of people aged 65 and older to those aged 15 to 64 years), and the child dependency (ratio of people under 15 to those aged 15 to 64 years). (Source: Philippine Statistics Authority)
Median age
The age that divides a population into two numerically equal groups, that is, half of the population are younger than this age, and the other half are older. (Source: Philippine Statistics Authority)
Population growth rate
The rate at which the population increases within a given time or period. Applies/refers to the calculated growth rates between each census.

Terms about distance

Great-circle distance
The shortest distance between two points along the surface of a sphere or ellipsoid. Applies/refers to the shortest distance between two cities or towns (using the city/municipal center coordinates as points).

Terms about rivers

The place where a river begins is called its source. River sources are also called headwaters. (Source: National Geographic Society. "Source." National Geographic, 9 Oct. 2012,
River basin
A river drainage basin is an area drained by a river and all of its tributaries. A river basin is made up of many different watersheds. (Source: National Geographic Society. "Basin." National Geographic, 9 Oct. 2012,
River mouth
The place where a river enters a lake, larger river, or the ocean. (Source: National Geographic Society. "Mouth." National Geographic, 9 Oct. 2012,
A freshwater stream that feeds into a larger stream, river or other body of water. (Source: National Geographic Society. "Tributary." National Geographic, 9 Oct. 2012,
A watershed is an entire river system — an area drained by a river and its tributaries. It is sometimes called a drainage basin. (Source: National Geographic Society. "Watershed." National Geographic, 9 Oct. 2012,

Terms about roads

Primary Road
National Road classification, a road that connects major cities with population of at least 100,000. (Source: Department of Public Works and Highways)
Secondary Road
National Road classification, a road which complements the national primary roads to provide access to main population and production centers of the country. (Source: Department of Public Works and Highways)
Tertiary Road
National Road classification, an existing road under the DPWH which performs a local function. (Source: Department of Public Works and Highways)

Terms about volcanoes

Erupted within historical times (within the last 600 years). (Source: Philippine Institute of Volcanology and Seismology)
Potentially active
Morphologically young-looking but with no historical or analytical records of eruptions. (Source: Philippine Institute of Volcanology and Seismology)
No recorded eruptions; physical form has been intensively weathered and eroded, bearing deep and long gullies. (Source: Philippine Institute of Volcanology and Seismology)

Terms related to Disasters in the Philippines

Dangerous storms that include lightning and can bring powerful winds over 80 kph (50 mph), create hail, and cause flashfloods and tornadoes. (Source: "Thunderstorms and Lightning." Official website of US Homeland Security (
A seasonal shift in the prevailing wind direction, that usually brings with it a different kind of weather. It almost always refers to the Asian monsoon, a large region extending from India to Southeast Asia where monsoon conditions prevail. (Source: "What is a monsoon?" Weather Questions,, 2 Nov. 2012,
In the Philippines, the monsoon consists of the Northeast Monsoon, called Amihan, and the Southwest Monsoon, or Habagat. The Northeast Monsoon brings cool and dry air originating from Siberia, Mongolia, and northern China at a time when these countries are going through winter. Such wind system causes cloud formation and rainfall over the eastern sections of the country from November to February each year.
The Southwest Monsoon or Habagat consists of seasonal winds blowing from the southwest direction which cause extensive cloud formation and rainfall over the western sections of the country. Habagat occurs during the months of June to October each year. (Source: Ganal, Jr., Romeo R. "Effects of Monsoons in the Philippines." Official website of DOST-PAGASA (
Typhoon/hurricane/tropical cyclone
The only difference between a hurricane and a typhoon is the location where the storm occurs. Hurricanes and typhoon are the same weather phenomenon: tropical cyclones. A tropical cyclone is a generic term used by meteorologists to describe a rotating, organized system of clouds and thunderstorms that originate over tropical or subtropical waters and has closed, low-level circulation.
In the North Atlantic Ocean, central North Pacific, and eastern North Pacific, the term hurricane is used. The same type of disturbance in the Northwest Pacific is called a typhoon. Meanwhile, in the south Pacific and Indian Ocean, the generic term tropical cyclone is used, regardless of the strength of the wind associated with the weather system.
The ingredients for tropical cyclones include pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds. If the right conditions persist long enough, they can combine to produce the violent winds, large waves, torrential rains, and floods we associate with this phenomenon. (Source: "What is the difference between a hurricane and a typhoon?" Official website of the US Department of Commerce - National Oceanic and Atmospheric Administration (NOAA). National Ocean Service -

A tropical cyclone is classified according to its strength and grouped according to the maximum sustained winds near the center:

  1. Tropical Depression (TD) – 61 kph or less
  2. Tropical Storm (TS) – 62 to 88 kph
  3. Severe Tropical Storm (STS) – 89 to 117 kph
  4. Typhoon (Ty) – 118 to 220 kph
  5. Super Typhoon (ST) – more than 220 kph
(Source: DOST-PAGASA website)
Storm surge
The rising of the sea level associated with the passing of a tropical storm or typhoon. This is due to the push of strong winds in the water surface, the piling up of big waves, pressure setup and astronomical tide moving towards the shore. In other words, the stronger the winds or the larger the storm, the higher the surge. (Source: "What Is Storm Surge?" Department of Health Website, Accessed 24 July 2019.)
A storm surge brings widespread floods which can extend to kilometers from the seashore depending on the shape and height of the wave. Along with strong waves and forceful winds, a storm surge can destroy and wash away anything in its path.
The rise of water in rivers, creeks, lakes, and other bodies of water which overflows to low-lying areas. This could result to a destruction of property and loss of life. (Source: Official Gazette of the Republic of the Philippines, Accessed 24 July 2019.)
The mass movement of rock, soil, and debris down a slope due to gravity. It occurs when the driving force is greater than the resisting force. The following can trigger a landslide: intense rainfall, weathering of rocks, ground vibrations created during earthquakes, and volcanic activity. (Source: "Introduction to Landslide." PHIVOLCS, Accessed 24 July 2019.)
Pacific Ring of Fire
A band of volcanoes and fault lines circling the edges of the Pacific Ocean. It is also called the Circum-Pacific seismic belt; it is shaped like a horseshoe and is 40,000 kilometers (25,000 miles) long. It runs from Chile northwards along the South American coast through Central America, Mexico, the west coast of the United States, and the southern part of Alaska, through the Aleutian Islands to Japan, the Philippines, and Indonesia, before curving back towards New Guinea, the southwest Pacific islands, and New Zealand. The area's seismic activity results from collisions between tectonic plates of the world's some 1,500 active volcanoes, almost 90% of which are in the Ring of Fire. Seventy-five percent of the Earth's active and dormant volcanoes are located in the area. Ninety percent of the world's earthquakes and 81 percent of the world's largest earthquakes occur within it. (Source: "What is the Ring of Fire?", North East Arizona Energy Services Company, Accessed 24 July 2019.)
An intense shaking of the Earth's surface. The shaking is caused by movements in Earth's outermost layer. An earthquake is the sudden movement of Earth's crust at a fault line. The location where an earthquake begins is called the epicenter. An earthquake's most intense shaking is often felt near the epicenter. The energy from an earthquake travels through Earth in vibrations called seismic waves. Scientists can measure these seismic waves on instruments called seismometer. A seismometer detects seismic waves below the instrument and records them as series of zig-zags. (Source: "What Is an Earthquake?" NASA Space Place – NASA Science for Kids, 17 July 2019,
An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel. (Source: "Earthquake Glossary." USGS Earthquake Hazards Program, Accessed 24 July 2019.)
Magnitude and Intensity
Magnitude and intensity measure different characteristics of earthquakes. Magnitude measures the energy released at the source of the earthquake. Magnitude is determined from measurements on seismographs. Intensity measures the strength of shaking produced by the earthquake at a certain location. (Source:
Seismograph and Seismometer
Seismographs are instruments used to record the motion of the ground during an earthquake. They are installed on the ground throughout the world and operated as part of a seismograph network.
A seismometer is the internal part of the seismograph which may be a pendulum or a mass mounted a spring; however, it is often used synonymously with "seismograph."
A seismogram is the recording of a ground shaking at the specific location of the instrument.
(Source: "Seismometers, Seismographs, Seismograms - What's the Difference? How Do They Work?" United States Geological Survey, 2012,
A series of sea waves commonly generated by under-the-sea earthquakes and whose heights could be greater than 5 meters. It is erroneously called tidal waves and sometimes mistakenly associated with storm surges. tsunamis can occur when the earthquake is shallow-seated and strong enough to displace parts of the seabed and disturb the mass of water over it.
Tsunamis can also be produced by other natural phenomena aside from an undersea earthquake, and these include underwater landslides, volcanic eruptions, and very rarely by large meteorite impacts in the ocean.

There are 2 types of tsunami generation.

  1. Local tsunamis – are confined to coasts within a hundred kilometers of the source, usually an earthquake and a landslide or a pyroclastic flow. They can reach the shoreline within 2 to 5 minutes. The coastal areas in the Philippines especially those facing the Pacific Ocean, South China Sea, Sulu Sea, and Celebes Sea can be affected by tsunamis that may be generated by local earthquakes.
  2. Far field or distant tsunamis – can travel from 1 to 24 hours before reaching the coast of the nearby countries. These tsunamis mainly come from the countries bordering the Pacific Ocean like Chile, Alaska in USA, and Japan. The Pacific Tsunami Warning Center (PTWC) and Northwest Pacific Tsunami Warning Advisory Center (NWPTAC) are the responsible agencies that closely monitor Pacific-wide tsunami events and send tsunami warning to the countries around the Pacific Ocean.

Natural Signs of an Approaching Tsunami

  1. A felt earthquake
  2. Unusual sea level change
  3. Rumbling sound of approaching waves
(Source: "Introduction to Tsunami." PHIVOLCS, Accessed 24 July 2019.)
Refers to those earthquakes commonly along major subduction zone plate boundaries such as those bordering the Pacific Ocean that can generate tsunamis. (Source: "Earthquake Glossary." USGS Earthquake Hazards Program, Accessed 24 July 2019.)
GO BAG (Emergency Preparedness Bag)
When there is an impending danger, we need to leave our homes and evacuate to safer areas. To prepare for these emergencies, we should have a ready GO BAG that we can bring along immediately.

Contents of a GO BAG

  • Important documents in waterproof container
  • Flashlight, candles, matches, and whistle
  • Radio with fresh and extra batteries
  • First aid kit with remedies for fever, LBM, minor wounds and pain, and maintenance medicines
  • Spare cash including coins
  • Items for special needs of young and older members of the family, including persons with disabilities
  • Easy-to-serve, ready-to-eat food enough for three days
  • Drinking water in sealed containers enough for three days
  • Mobile phones, powerbanks, and chargers
  • Clothing, raincoat, boots, and sanitary supplies
  • Sleeping bags or mats, and blankets
  • Ropes, old newspaper and ecobags made of strong materials


  1. The GO BAG items should be checked and replenished every three months.
  2. The GO BAG should be stored properly where it can be easily accessed in case of an emergency.
  3. Coordinate with leaders on community evacuation plan. Decide with the family where to meet in case there is a need to evacuate.
(Source: Civil Defense PH (,
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