Science - 7 (earthquakes)

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3. How can you be better prepared for earthquakes?

On October 15, 2013, a 7.2-magnitude earthquake hit Bohol and destroyed a number of infrastructure, including historic churches and landmarks in the province. Based on official reports from the National Disaster Risk Reduction and Management Council (NDRRMC), the effects of the earthquake resulted in about 200 human casualties, with eight persons missing and 976 injured.

In this chapter, you will learn about why earthquakes occur, how much damage they can cause, and how to protect yourself during earthquakes.

198 Science in the Mode

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Lesson 1: Earthquakes along Faults

➜ Why do earthquakes occur?

Our country is located in an earthquake-prone region. Did you know that earthquakes are recorded in the Philippines every day? Fortunately, most of them are too weak to be felt or noticed.

What exactly are earthquakes? Why do these happen? How do these occur?

MINI LAB 10-1

1. Lay the ruler along the table such that 3/4 of its length is over the edge.

2. Hold the ruler with one of your hands. With your other hand, bend the free end of the ruler and then release it.

3. Describe the movement of the ruler.

As the ruler is bent, energy is stored. This stored energy is released when you let go of the ruler, and the vibrations caused by the release of energy are felt by the hand. The same process occurs in rocks. The rocks beneath the Earth's surface are constantly subjected to deformation caused by pushing and pulling forces. Remember how forces allow objects to move. When the rigid rocks break, the energy stored in them is released, and the energy is manifested on the Earth's surface as earthquakes. The energy that causes earthquakes travels from particle to particle in the solid and liquid materials of the Earth. This energy spreads through the Earth as vibrations called seismic waves.

The point of origin of seismic waves is called the focus, or hypocenter. On the other hand, the epicenter is located on the Earth's surface directly above the focus. The waves spread radially from the focus as energy moves from one particle to another. The fault plane is the surface that slips during an earthquake.

(Image with label: Figure 10-1. The epicenter of an earthquake is located directly above its focus, or the point of origin of an earthquake.)

LEARNING COMPETENCIES

Use models or illustrations to explain how movements along faults generate earthquakes and identify and explain which types of faults are most likely to occur in the Philippines and explain why.

Classify geological faults according to the angle of the fault plane and direction of slip.

LEARNING OBJECTIVES

Explain the occurrence of earthquakes.

Differentiate between the epicenter and focus of an earthquake.

Describe the causes of earthquakes.

Differentiate active and inactive faults.

Compare and contrast the three geological faults.

Explain how the movements of faults can cause earthquakes.

SCIENCE CHECK

Earthquakes occur when the Ear

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Earthquakes may be caused by the sudden movement along faults and plate boundaries, or by volcanic eruptions. Earthquakes caused by faults and the movement of plate boundaries are called tectonic earthquakes. Earthquakes that are induced by the rising lava or magma within a volcano are called volcanic earthquakes.

MINI TEST 10-1

What is the difference between the epicenter and the focus of an earthquake? How do faults generate earthquakes? Present your answers by doing one of the following:

Create an animation.

Act out the concept.

Write a news article.

Earth’s solid and rigid outer layer is not whole, but consists of huge pieces called tectonic plates. Boundaries between tectonic plates are made up of a system of faults. Faults are breaks in rocks which result when they rub against each other or push each other out when tectonic plates move and separate. When those rocks break due to stress, earthquakes occur.

Active and Inactive Faults

A fault is considered active if there is evidence that it has moved one or more times in the last 10,000 years and may move again in the future. On the other hand, an inactive fault is a fracture that has not experienced any geologic activity in the last 10,000 years. However, some inactive faults may reactivate. Classifying a fault as inactive does not mean it will never move again. In the Philippines, segments of the Philippine Fault Zone, the Valley Fault System, and the Casiguran Fault are categorized as active, while the Masbate Fault and the Eastern Mindanao Fault are considered inactive. (The locations of these fault systems are shown in figure 10-3.)

Dip-Slip, Strike-Slip, and Oblique-Slip Faults

In a dip-slip fault, the ground moves either up or down along the slope of the fault. The dip is the angle the fault makes with respect to the surface. The block above the fault is called the hanging wall, while the block below is the footwall. Depending on how these blocks move in relation to the hanging wall and footwall, dip-slip faults are categorized as normal or reverse. In a normal dip-slip fault (figure 10-2A), the hanging wall moves down in relation to the footwall. In a reverse dip-slip fault, the opposite happens; the hanging wall moves up in relation to the footwall (figure 10-2B).

In a strike-slip fault, the blocks move side to side, sliding past one another (figure 10-2C). A strike is where the fault cuts across a horizontal surface. In a sinistral strike-slip fault, a fault block moves to the left with respect to the opposite block. In a dextral strike-slip fault, the movement is to the right.

SCIENCE TIPS

Hanging wall and footwall are mining terms that have been adopted in geology. In mining, a hanging wall is the mass of overlying mineral deposit, while a footwall is the rock below the mass. These terms are used in geology to refer to the wall above the fault plane (hanging wall) and the wall below (footwall).

(Page Number: 200

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An oblique-slip fault exhibits both dip-slip and strike-slip displacements: either an upward or downward sliding of the fault block, and a horizontal movement to the left or to the right of one fault block with respect to the opposite block (figure 10-2D).

(Image with labels: Figure 10-2. Different faults based on orientation of displacement: (a) normal dip-slip fault, (b) reverse dip-slip fault, (c) strike-slip fault, and (d) oblique-slip fault.)

MINI LAB 10-2

Apply what you have learned by performing the activity on faults.

Scan the QR code to download the activity.

How Movement along Faults Generate Earthquakes

Normal fault, reverse fault, and strike-slip fault are associated with one of three types of plate boundaries (or meeting places of leading edges of tectonic plates). Normal faults occur at divergent boundaries, and strike-slip faults occur at transform plate boundaries.

At or near tectonic plate boundaries, there is a large concentration of faults. The reason for this is that the movement of tectonic plates provides stress. Rocks on the Earth's surface break in response to this stress.

Normal faults are caused by rocks being stretched. They are formed as a block of rock falls after being fractured. Sudden movement along these faults causes earthquakes as the movement releases stored energy. The earthquake focus is the place where the rocks break.

In a reverse fault, the hanging wall has moved upward relative to the footwall due to compressional force. Earthquakes happen due to this sudden movement.

In strike-slip faults, two plates move horizontally sideways in opposite directions. As the plates move against one another, strain is built up. As energy is released, earthquakes occur. Strike-slip faults can produce major earthquakes with a magnitude as great as 8.

(Page Number: Chapter 10,

Earthquakes along Faults)

, Science in the Modern World 7)

th's crust suddenly slips along faults due to stress.

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