Republic of the Philippines

Department of Education
Regional Office No. VIII (EASTERN VISAYAS)
Schools Division of Catbalogan City
District of Catbalogan III
EASTERN VISAYAS REGIONAL SCIENCE HIGH SCHOOL
Brgy. 07, Catbalogan City

LESSON HANDOUT IN DISASTER READINESS & RISK REDUCTION (DRRR)

LESSON 4.1: Earthquake Hazards

What is an Earthquake?
An earthquake (also known as a quake, tremor, or tremblor) is the perceptible
shaking of the surface of the Earth, resulting from the sudden release of energy in
the Earth’s crust that creates seismic waves. (US Geological Science – USGS)

Potential Earthquake Hazards

The following are different hazards that normally result from the occurrence
of an earthquake:

1. Ground Shaking or Ground Motion

The Earth shakes with the passage of earthquake waves, which radiate
energy that had been “stored” in stressed rocks and were released when a fault
broke, and the rocks slipped to relieve the pent-up stress. The strength of
ground shaking is measured in the velocity of ground motion, the acceleration
of ground motion, the frequency content of the shaking and how long the
shaking continues.
2. Ground or Surface Rupture
Surface rupture is an offset of the ground surface when fault rupture
extends to the Earth’s surface. Any structure built across the fault is at risk
of being torn apart as the two sides of the fault slip past each other. Normal-
and reverse- (collectively called dip-slip) faulting surface rupture features
vertical offsets while strike-slip faulting produces lateral offsets. Many
earthquake surface ruptures are combinations of both.
3. Liquefaction
Soil liquefaction is a phenomenon in which the strength and stiffness of
a soil is reduced by earthquake shaking or other rapid loading. It normally
occurs in saturated soils, that is, soils in which the space between individual
particles is filled with water. Prior to an earthquake, the water pressure is
relatively low- the weight of the buried soil rests in the framework of grain
contacts that comprise it. However, earthquake shaking can disrupt the
structure, the soil particles no longer support all the weight, and the
groundwater pressure begins to rise. The soil particles can move farther and
become entrained on the water- the soil flows. Liquefied soil sill force open
ground cracks in order to escape to the surface. The ejected material often
results in flooding and may leave cavities in the soil.
4. Earthquake-induced ground subsidence and lateral speeding
Subsidence, or lowering of the ground surface, often occurs during
earthquakes. This may be due to downward vertical displacement on one side
of a fault and can sometimes affect a huge are of land. Coastal areas can
become permanently flooded as a result. Subsidence can also occur as ground
 shaking causes loose sediments to “settle” and to lose their load bearing
strength or to slump down sloping ground.
5. Tsunami
A tsunami, also known as a seismic sea wave, is a series of waves in a
water body caused by the displacement of a large volume of water, generally
in an ocean or a large lake. Earthquakes, volcanic eruptions, and other
underwater explosions (including detonations of underwater nuclear devices),
landslides, glacier cavings, meteorite impacts and other disturbances above
or below water all have a potential to generate tsunami.
6. Earthquake-induced Landslides
Landslides are frequently triggered by strong ground motions. They are
important secondary earthquake hazard. The term landslide includes a wide
range of ground movement, such as rock falls, deep failure of slopes, and
shallow debris flows. However, gravity acting on a steep slope is the primary
reason for all landslides. Strong earthquake-induced ground shaking greatly
increases the likelihood of landslides where landscape is susceptible to these
types of ground failure.

Natural Signs of an Impending Tsunami

1. Animal Behavior
Some zoologists hypothesize that some animal species like elephants
have the ability to sense subsonic Rayleigh waves from an earthquake or
a tsunami. If correct and substantiated with more evidence, monitoring
their behavior could provide advance warning of earthquakes and
tsunamis.
2. Drawback
This is an observable natural sign of an impending tsunami that ix
noteworthy. In fact, drawback can serve as a brief warning. There are
already proofs that people who observed drawback survived when they
immediately run for high ground or climbed to upper floors or nearby
buildings.

How Drawbacks are Formed

All waves have a positive and negative peak, i.e. a ridge and a
trough. In the case of a propagating wave like tsunami, either may be the
first to arrive. If the first part to arrive at shore is the ridge, a massive
breaking wave or sudden flooding will be the first effect noticed on land.
However, if the first part to arrive is a trough, a drawback will occur as the
shoreline recedes dramatically, exposing normal submerged areas.
Drawback can exceed hundreds of meters, and people unaware of the
danger sometimes remain near the shore to satisfy their curiosity or to
collect fish from the exposed seabed.
A typical wave period for a damaging tsunami is about 12 minutes.
This means that if the drawback phase is the first part of the wave to arrive,
the sea will recede, with areas well below sea level exposed after 3 minutes.
During the next 6 minutes the tsunami wave trough builds into a ridge,
and during this time the sea is filled in and destruction occurs on land.
During the next 6 minutes, the tsunami wave changes from a ridge to a
trough, causing flood waters to drain and drawback to occur again. This
may sweep victims and debris some distance from land. This process
repeats as the next wave arrives.

The Seismometer — the main tool in detecting and monitoring an

earthquake. (See photo on the next page)
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TASK: READ AND STUDY!