Earthquake Hazards

Earthquake defined by Cramer (1991)

generates weak to strong shaking on the Earth’s surface due to

the movement of rock materials underneath.

Focus/Hypocenter

They occur when energy stored in elastically strained rocks

(like rubber bands being stretched) is suddenly released, causing ground shaking near the source of the earthquake, called

Seismic Waves

Energy travels from focus through

Strain

is the change in shape, size, or volume of the rock,

Stress

is the force that causes these deformations

Pacific Ring of Fire

The Philippines, as described in the Basic Concepts of Disaster and Disaster Risk, is part of the

Ground shaking

Movement of the ground in various directions, potentially causing structural collapse.

Ground Rupture

Visible breaking and displacement of the Earth's surface along the fault line.

Liquefaction

Water-logged sediments lose strength and behave like a fluid during shaking.

Earthquake-Induced Ground Subsidence

The settling or sinking of the ground surface due to soil compaction induced by earthquake vibrations. This is most likely to occur in areas with loose or compressible soils, such as un-compacted fills or reclaimed lands.

Tsunami

Ocean waves caused by undersea earthquakes leading to coastal flooding.

Earthquake-Induced Tsunami

Destabilization of slopes leading to landslides.

Fault

A fracture in the rocks that make up the Earth’s crust

Epicenter

the point at the surface of the earth directly above the focus.

Focus (hypocenter)

The point within the earth where an earthquake rupture starts.

Plates

massive rocks that make up the outer layer of the earth’s surface, and whose movement along faults triggers earthquakes.

Seismic Waves

waves that transmit the energy released by an earthquake.

Tensional (or extensional) stress

stretches rock

Compressional Stress

Squeezes rock

Shear Stress

causes slippage and translation

3 types of stresses

are the ones that eventually fracture rocks, producing faults.

Epicenter

The point at the surface of the Earth directly above the focus is called the

Dip-Slip Faults

have inclined fault planes and are caused either by tensional or compressional stress.

Footwall Block

is the one you can step on,

Hanging wall block

is the one you can hang stuff onto.

Strike-Slip faults

are faults where the displacement on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust.

left-lateral strike-slip fault.

If the block on the other side has moved to the left

right-lateral strike-slip fault

If the block on the other side has moved to the right

Normal Fault

• Horizontal tensional/extensional stress

• The hanging wall block moves down relative to the footwall block.

Reverse Fault

• Horizontal compressional stress

• The hanging wall block moves up relative to the footwall block.

Thrust Fault

• a special case of reverse fault wherein the dip of the fault is less than 45°.

• Horizontal Compressional stress

• The hanging wall block moves up relative to the footwall block.

Strike-Slip Fault

Shear Stress

Left Lateral and Right Lateral

2 types of strike-slip fault

Left lateral

the block on the other side of the fault moves to the left

Right lateral

the block on the other side of the fault moves to the right

Blind Fault

Depends on the relative movement of the rocks

Blind Fault

is a fault that does not break the surface of the earth. Instead, rocks above the fault have behaved in ductile fashion and folded over the tip of the fault.

Seismic Waves

Energy is transmitted from the focus of the earthquake through

Seismology

The study of how seismic waves behave is called

Body Waves

waves that move through the interior of the Earth; cause high-frequency vibrations, thus mainly affecting low structures.

Primary Waves (P-waves)

• can travel through solids and liquids (thus can travel through the outer core); the speed decreases and the direction changes when p-waves travel from solid to liquid; faster than s-waves

• the wave moves by compressing and expanding the material as it travels.

Secondary Waves (S-waves)

• cannot travel through the outer core; slower than p-waves

• The wave moves material side by side (perpendicular to the direction it is travelling).

Surface Waves

waves that travel on the Surface of the Earth; cause low-frequency vibrations, thus mainly affecting taller structures. These waves cause the most damage.

Love Waves (l-waves)

slow-travelling waves that result in side to side motion; cannot pass through bodies of water

Rayleigh Waves

waves that result in an up and down rolling motion; because of this, it can affect bodies of water such as lakes

Seismographs

are instruments that are being used to record ground motion during earthquakes.

Seismometer

Within a seismograph is a pendulum or a mass mounted on a spring which is called

Seismograms

output from instruments can be read in seismograms

Building Codes

Ensuring buildings and infrastructure are constructed according to seismic design standards that can withstand ground shaking.

Retrofitting

Upgrading existing structures to improve their resistance to seismic forces.

Site Selection

Avoiding construction on soils prone to severe shaking or other earthquake-related hazards.

Emergency Preparedness

Having an earthquake response plan, conducting regular drills, and ensuring everyone knows the “Drop, Cover, and Hold On” procedure during shaking.

Furniture Anchoring

Securing heavy furniture and objects to prevent them from falling and causing injuries during an earthquake.

Monitoring Systems

Using seismic monitoring systems to provide early warnings and allow for preemptive measures.

ground rupture

When an earthquake is strong enough, faulting initiated at depths may breach the Earth’s surface and forms a

Avoidance

The most effective way to minimize damage is to avoid building near known fault lines where ground rupture is likely to occur. Because of the nature of the damage that a ground rupture does, making a structure sturdier will not be enough to save it from damage or destruction. Any type of construction that is built right on top of an active fault stands no chance of surviving without damage when it finally moves.

Land-Use Planning

Implementing strict zoning regulations that prevent construction of critical structures in areas prone to surface rupture.

Engineering Solutions

Designing structures with flexible utility connections and reinforced foundations that can withstand differential movements caused by ground rupture.

Insurance

Obtaining earthquake insurance that covers damages from ground rupture can provide financial protection against this type of hazard.

Public Awareness

Educating the public about the risks of ground rupture and the importance of proper land use and construction practices. Local government units, other government agencies, business entities, NGOs, and homeowners may access available detailed maps of local active faults from PHIVOLCS. Such maps can be used in formulating local and national legislation measures to minimize the effects of ground ruptures.

Liquefaction

takes place when loosely packed, water- logged sediments at or near the ground surface lose their strength in response to strong ground shaking. ______ occurring beneath buildings and other structures can cause major damage during earthquakes.

earthquake-induced ground subsidence

Earthquakes, which sometimes cause liquefaction, would also cause loose deposits of soil to be compacted from the shaking of the ground, causes an

Tsunami

as a wave or series of waves generated by the sudden, vertical displacement of a column of water that may be caused by seismic activity, explosive volcanism, a landslide above or underwater, an asteroid impact, or certain meteorological phenomena.

Philippine Institute of Volcanology and Seismology

PHIVOLCS

Earthquake-Induced Landslides

landslides are a mass movement of rock, soil, and debris down a slope due to gravity. There are several causes of landslides, including ground vibrations during tectonic and volcanic earthquakes, natural triggers like plant and animal movement, heavy rainfall, rock weathering, and man-made activity such as excessive and unregulated mining practices.