EARTHQUAKE HAZARDS

Earthquake - sudden and violent shaking of the ground that is caused by movement within the
earth's crust

HAZARD TYPES

1. Ground Shaking
➔ vibration of the ground during an earthquake
◆ Causes of Ground Shaking
● sudden slippage along a fault zone
○ slippage is hindered due to irregularities on the fault plane
● Elastic Rebound Theory - elastic energy builds up in the deforming
rocks on either side of the fault
○ released as seismic waves; felt through form of vibrations
○ Seismic Waves
◆ Body Waves - travel through the rocks below the surface
of the earth; from the rupturing fault
● Compressional Primary Waves - same direction of
the wave propagation
○ can go through solids, liquids, and gases;
first
● Shear Waves - perpendicular to the direction of
wave propagation
○ only go through solids; second
◆ Surface Waves - outward from the epicenter of an
earthquake
● slower that the body waves; more destructive
● love waves, raleigh waves
◆ Strength of Ground Shaking is measured in terms of :
● Velocity - product of of frequency and amplitude of a seismic wave
● Acceleration - rate of change of velocity expressed as a ratio of the
acceleration of gravity [1g = 9.81 m/s2 = 981 gal]
● Frequency - how often vibration occurs; (Hz) or cycles per minute
● Duration - how long the shakings lasted; time as unit of measurement
➔ dependent on the magnitude, depth of focus, epicenter, and duration of the shaking
➔ geology of the ground material determines shaking reaction of the ground
➔ horizontal component of seismic wave - most destructive to buildings
◆ s and love waves are destructive as both have horizontal components
➔ Control of Vibration Intensity
◆ dependent on the nature of the eathquake source
◆ varies on the ff: size of the fault rupture, magnitude of earthquake, distance from
the epicenter
◆ characteristic of materials can increase or decrease the intensity of shaking
 ◆ s waves - measure for ground shaking potential in various materials
◆ strong materials - high frequency; weak materials - low frequency
◆ hardness and fracture as a way to test a material’s relative strength and
toughness
◆ grain size - used for grouping sediment units of different shaking strength
● s wave increase = grain size increase
● will also lower seismic wave amplitude

2. Ground Rupture
➔ visible offset of the ground surface when an earthquake rupture along a fault affects the
Earth's surface
➔ mostly in active faults
➔ more active - shorter interval
◆ Factors
● normal fault, reverse fault, strike-slip fault
● Deformation - horizontal and/or vertical displacement along the fault
trace and folding/bending of the adjacent area
○ width of deformation - largely dependent on the type of faulting
○ strike- slips = steeper fault
● Inclination of fault planes - thrust fault
● length depends on the magnitude of the earthquake
◆ Minimizing Effects
● avoidance of active fault traces and deformation zones
○ avoiding construction within 5 meters on each side of a fault trace,
or a total width of 10 meters
● adopting appropriate engineering and construction practices

3. Liquefaction
➔ rearrangement of grains caused by shaking; shaking increase water pressure
◆ Factors
● loose granular sediment areas
● saturation of sand and silt by ground water
● earthquake
◆ Types
● Flow Failure
○ most dangerous; occurs on liquefiable slope materials with
steepness greater than 3 degrees
● Lateral Spread
○ blocks of broken pieces of the flat/very gentle sloping ground (less
than 3 degrees) above a liquified zone move laterally
● Oscillation
○ unable to spread and instead oscillate like a wave; ejected
through the fissures
● Loss of Bearing Strength
 ○ tilting of houses and floating of buoyant structures
◆ Effects
● most striking ground failures and damages to man-made structures
○ sand, silt, and water erupts upward under pressure through cracks
and flows out onto the surface
◆ Mitigation
● hazard zone maps to identify areas that are prone to liquefaction
● liquefaction-resistant structures if building in said area is unavoidable

4. Earthquake Induced Ground Subsidence

➔ lowering of the ground surface
◆ Causes
● failure of the ground under a foundation
● densification of sand and gravel layers due to the ground shaking, and
liquefaction
◆ Types
● Liquefaction-related settlement, Tectonic Subsidence

5. Earthquake Induced Landslides

➔ mass movement of material, such as rock, earth or debris, down a slope
◆ Factors
● removal of support, groundwater pressure, volcanic eruption, intense
rainfall, snowmelt, human intervention, and earthquakes
◆ Effects
● death, flooding, changing river morphology, alteration of agriculture, and
relocation of human population and infrastructure
◆ Mitigation
● reporting any slope instability to the local authorities
● stabilization of the landscape
● planting deep root vegetation along the slope
● city ordinances preventing construction in landslide-prone areas

6. Tsunami
➔ waves in a water body caused by the displacement of a large volume of water, generally
in an ocean or a large lake
◆ Tsunami Generation - movement of an offshore fault during an earthquake
◆ Propogation - away from the source with amplitude far too small compared to
the wavelength that passes almost unnoticed by an observer
◆ Run Up and Inundation - break and reach heights far exceeding the normal sea
level
◆ Causes - underwater landslide, volcanic eruption, meteorite impact, and
earthqauke
◆ Effects - drowning, building collapse, impact of various debris
 ◆ Mitigation - PHILVOLCS, evacuation plans, info dissemination, building
restrictions, vegatation

VOLCANO HAZARDS

Volcano - opening in the earth's crust through which lava, volcanic ash, and gases escape

CATEGORIES OF VOLCANO
1. Fissure Volcano
➔ linear volcanic vent where lava erupts; short in width but several kilometers long
➔ lack of explosive activity during eruption
➔ connected to other volcano types via their magma reservoir

2. Shield Volcano
➔ broad volcano with shallow inclining sides
➔ from multiple highly fluid magma flows with low viscosity
➔ lack of explosive activity during eruption
➔ basaltic magma = high temperature + very low silica and gas content
➔ runny/low viscosity and non-acidic
➔ shorter intervals - less violent
➔ divergent plate boundaries

3. Dome Volcano
➔ circular mound-shaped protrusion
➔ from slow extrusion of viscous lava

4. Ash/Cinder Volcano
➔ from pyroclastic fragments near volcanic vents
➔ simplest type of volcano

5. Composite Volcano/ Strato Volcano

➔ steep sided symmetrical cone of large dimensions
➔ alternating layers of lava, ash, pumice, blocks, and tephra
➔ along the earth’s subduction zones
➔ intermediate silica content and is of medium to high viscosity
➔ dormant - more violent

6. Caldera
➔ large depression formed when a volcano erupts and collapses

Volcanic Eruption - release of gas and/or lava from a volcano

➔ Signs
◆ increase in the frequency & intensity of felt earthquakes
 ◆ noticeable fumarolic activity & new/enlarged areas of hot ground
◆ subtle swelling of the ground surface
◆ small changes in heat flow

LAVA FLOW - outpourings of molten rock from a vent onto Earth's surface during an effusive
volcanic eruption
LAVA TYPES
SILICATE LAVA NON SILICATE LAVA
dominated by oxygen and silicon
● silicon = more viscous

Felsic - silicic lava; greater than 63% Carbonatite - 75% carbonate minerals
➔ rhyolite, dacite ➔ Ol Dionyo Lengai Volcano in
◆ erupt explosively to Tanzania
produce pyroclastic
deposits
◆ rhoylite lava occasionally
erupts effusively to form
lava spines, lava domes,
or coulees
◆ continental crust
◆ light colored

Intermediate - andesitic lava; 52 - 63% Iron Oxide - Kiruna, Sweden

➔ low aluminum - high magnesium
◆ steep composite volcanoes
in convergent plate
boundaries with a
subduction zone
◆ less viscous - low silica

Mafic - basaltic lava; 45 - 52% Sulfur - molten sulfur deposits

➔ high magnesium and iron oxide ➔ Lastarria Volcano, Chile
content
◆ produces shield volcanoes
◆ pillow lavas underwater
◆ forms oceanic crust

Ultramafic - silica content under 45%

➔ 18% magnesium oxide
◆ komatite, boninite

LAVA FLOW HAZARD - casualties due to slow velocity of flow

➔ Mitigation
◆ avoid traversing recently cooled lava beds
● Lava bench - lava deltas breaking off and falling into the sea
◆ creating a barrier or dike to impede the lava flow
VOLCANIC GASES HAZARD - responsible for approximately 3% of all volcano related deaths;
acidic corrosion or asphyxiation
➔ Mitigation
◆ avoid going to areas where volcanic gases will be abundant

PYROCLASTIC FLOW - dense, fast moving flow of solidified lava pieces, volcanic ash, and hot
gasses
1. Basal Flow - contains large rocks and boulders; flatten trees and buildings
2. Ash Plume - above the basal flow that is extremely hot; incinerate living organisms
trapped inside
➔ Mitigation
◆ evacuation is top priority
◆ protective gear
◆ underground shelter

TEPHRA FALLS - pieces of fragments of rock ejected into the air by an erupting volcano
● Ash- <2nm
● Lapili/ Volcanic Cinder - 2 - 64 nm
● Volcani Block - >64 nm

BALLISTIC PROJECTILES - fragments of solid (blocks) or fluid (bombs) material ejected

during the range of magmatic or phreatic (steam) explosive eruptions
● cm to meters in diameter; follows near-parabolic trajectories

PHYSICAL CHEMICAL

➔ damage to natural environment and ➔ alteration of water cycle

man-made structures ➔ elevated mineral levels in water
➔ disruption of air and water movement supply causing eutrophication
➔ respiratory difficulty

➔ Mitigation
◆ evacuation is top priority
◆ protective gear
◆ stay indoors

LAHAR - floods due to glaciers, lake breakout, or heavy rainfall

➔ Effects
◆ economic and environmental damage to rivers and floodplains
◆ destruction of infrastructures
◆ autocorrect
➔ Mitigation
◆ land use planning
 ◆ modification of lahar hazards through engineered protection structures
◆ warning systems to enable evacuations
◆ effective response to and recovery from lahars when they do occur

VOLCANO DEBRIS AVALANCHE - landslides that occur in volcanic slopes

➔ Factors
◆ a bulge on one side of the volcano
◆ fractures
◆ alteration of of rock material into clay
➔ Effects
◆ alter pre-existing topography; deep horseshoe shaped crater
◆ block streams to form lakes
➔ Mitigation
◆ anticipation of debris avalanche
◆ identification of avalanche prone areas
◆ EVACUATION

OTHER GEOLOGICAL HAZARDS

COASTAL EROSION - natural process which shapes shorelines by wearing away of coastal
land or beaches, mainly by the impact of waves along the shoreline
➔ Factors
◆ removal of vegetation
◆ concentration of drainage water flow into the sea
◆ structures that interfere with the natural flow of coastal materials
◆ activities that damage protective features
◆ production of extra waves
➔ Signs
◆ Caves - enlarged from natural weaknesses
◆ Arches - eroded cave
◆ Stack - collapsed arch roof
➔ Effects
◆ continuous modification of the coastal environment
➔ Mitigation
◆ preservation & restriction of activities/development in natural protected features
◆ construction/maintenance of coastal erosion protection structures
◆ prohibiting construction in areas of active coastal erosion and areas in reach
◆ restricting development of public utilities in areas prone to coastal erosion
GROUND SUBSIDENCE - relative lowering of the earth’s surface with the respect the mean
sea level
➔ Factors
◆ Carbonate Dissolution Collapse - limestone layers underground to be
dissolved due to slightly acidic water
● common in the Philippines due to an abundance of limestone terrain
◆ Excessive Groundwater withdrawal - absence of fluid pressure (deep well
pumps) leads to compaction of overlying sedimentary layer and subsequently
sinking of ground surface
● main cause of subsidence in Metro Manila (Caloocan, Malabon, Navotas,
and valenzuela)
◆ Extraction of Oil and Natural Gas - removal of fluid/gas pressure leads to
compaction of the overlying ground surface
● mainly anthropogenic but can occur naturally
◆ Earthquakes
● subsidence due to normal or reverse faults
● liquefaction of fine-grained sediments or in reclaimed land
◆ Underground Mining Methods - underground tunnels are located, subsides
◆ Isostatic Rebound - rise of land masses that were depressed by the huge
weight of ice sheets during the last ice age
◆ Change of Season - effect on clay
○ rainy season - clay expands
○ dry season - clay shrinks
● expansion and contraction of the clay ground surface can lead to cracking
and foundation failure
➔ Signs of Subsidence : cracks
➔ Effects
◆ sudden ground collapse can be catastrophic but mostly isolated and limited in
magnitude
◆ slow subsidence can proceed unnoticed but will only be discovered too late
when causes widespread property damage
➔ Mitigation
◆ Map out areas that are subsidence prone
● Avoid development of said areas
● Modifying/strengthening of structures