�ݺ�ߣ

How Strong is the Earthquake?
An earthquake may be described in two ways: intensity and magnitude. The intensity of an
earthquake gives us an idea of how strong or weak the shaking is or simply by describing the effects
of earthquake on people or surroundings. The Philippine Institute of Volcanology and seismology
(PHIVOLCS) uses the PEIS or PHILVOLCS Earthquake Intensity Scale to describe the intensity of
earthquakes in the Philippines (see Figure 2).
Another way of describing the strength of an earthquake is by magnitude. The Richter scale – also
called the Richter magnitude scale is a measure of the energy released. The greater the magnitude,
the stronger the earthquake. To distinguish the two, intensity is expressed using Roman Numeral (I,
II, III) while magnitude uses Hindu-Arabic numerals (2, 3, 4)
A magnitude 2 earthquakes may or may not be felt. But those with magnitude 4 are felt by most
people . Magnitude 6 can lead to a lot of damage in highly populated areas. Earthquakes with
magnitude 7 can cause severe damage while magnitude 8 or 9 that can occur once or twice a year
results in widespread destruction, especially the places near the epicenter.

Where do earthquakes occur?
A fault line is defined as a geological fracture wherein the movement of masses of rock has displaced
parts of the Earth’s crust. A rapid movement of a fault line may produce a powerful energy that can
trigger a strong earthquake. There are five active fault lines in the country namely the Western
Philippine Fault, the Eastern Philippine Fault, the South of Mindanao Fault, Central Philippine Fault
and the Marikina/Valley Fault System.
Fault Classifications
1. Active Faults are structure along which we expect displacement to occur. The process that
produces displacement across a fault continuously. All shallow earthquakes occur on active faults.
2. Inactive Faults are structures that we can identify, but which do no have earthquakes. If a fault
has been inactive for millions of years, it’s certainly safeto call it inactive.
Tsunami
You learned how faults are found not only on land but also at the bottom of the bodies of water. When
a fault at the bottom of the sea suddenly moves, the water above can be affected. A sudden movement
of an underwater fault can produce a wave called a tsunami.
The tsunami grows in height and slows down as it is approaching the shore. So, how are earthquakes
related to tsunamis? When a fault suddenly moves on land, you experience an earthquake. But if a
fault suddenly moves in or near a body of water, you may experience a tsunami in addition to the
earthquake.
Not every fault movement beneath the sea can produce tsunami. Faults that move in a horizontal
direction or sideways will not result in a tsunami. The fault has to move in the vertical direction.
The Earth
In this lesson, you will learn how earthquake waves provide information about the interior of the
earth. Seismic waves are vibrations of the earth caused by the sudden release of energy stored in
rocks that have been deformed by forces that originate from moving tectonic plates.
The two types of seismic waves are body waves and surface waves. See the diagram below.
I

Body waves refer to the vibrations that travel through the interior of the earth. The two types of body
waves are primary waves or P waves and secondary waves or S waves. P waves push rocks in the
direction they are travelling and they travel in all states of matter while S waves displace rocks at
right angles to the direction they are travelling. S waves cannot pass through liquids. Surface waves
refer to vibrations that travel at the surface of the earth. They can also travel at the surface of the
mantle and core. The two types of surface waves are the Rayleigh waves and Love waves. Surface
waves cause the damage incurred during an earthquake.
Seismic Waves provide information about the interior of the Earth
P waves travel through solids and liquids, but they travel faster through solids. Changes in the speed
of earthquake vibrations give scientists an idea of the physical properties of various depths of the
earth’s interior.
Refer to the diagram on the side. The lithosphere, composed
of the crust and the uppermost part of the mantle is solid.
Seismic waves travel fast through this rocky sphere.
Below the lithosphere, the seismic waves slow down. This
observation indicates a very high temperature that melts
rocks, making the molten behave like a fluid. Scientists call
this region of the mantle asthenosphere. Below the
asthenosphere, seismic waves travel fast again indicating
that the lower part of the mantle is solid. It is probably the
very high pressure that keeps it solid in spite of the high
temperature. A similar phenomenon is observed in the core. Seismic waves travel slowly through
the outer core indicating that it is molten due to the extremely high temperature. Then again
the inner core is solid in spite of the very high temperature. Most probably, the very high
pressure in the deepest part of the earth keeps it solid.
Learning Task 1: Using the given organizer, write the necessary information to complete the concept
about seismic waves. Do this in your answer sheet.

�ݺ�ߣ

visual aid_Magnitude_Seismic Waves.docx.

Recommended

More Related Content

Similar to visual aid_Magnitude_Seismic Waves.docx. (20)

Recently uploaded (20)

visual aid_Magnitude_Seismic Waves.docx.