Seismic Hazards Flashcards Preview

Hazards > Seismic Hazards > Flashcards

Flashcards in Seismic Hazards Deck (36):
1

Primary and Secondary Seismic Hazards

PRIMARY

Earthquakes
Seismic Hazards

SECONDARY

Tsunamis
Landslides
Liquefaction

2

Liquefaction

SECONDARY

Soils and Sediments with high water content starting to behave like a FLUID
^^^ when they are vibrated by earthquake shock waves

Frictional contact lost as individual particles are shaken apart
= ground loses mechanical strength

Building and other built structures collapse as their foundations subside and in some instances into the liquefied ground cracks

3

Landslides

SECONDARY

Earthquake shock waves triggering slope failure = landslide/avalanche happening

Shock waves vibrate the ground briefly
^^^^ground loses stability of the slope material (snow??)

= gravity pulls it down instead

4

Tsunamis

SECONDARY

Intranet

5

Earthquakes

PRIMARY

They’re SHOCKS or a SERIES OF SHOCKS
=Caused by sudden movement of rocks BENEATH OR AT THE EARTH’S SURFACE

They make the ground shake (usually less than a minute)

Focus is deep in the plates and the epicentre is directly above the focus sending out seismic waves

Seismic intensity DECREASES with distance from the earthquake focus



6

Isoseismal lines

Imaginary lines which locations with EQUAL DEGREE OF SEISMIC INTENSITY

CONCENTRIC CIRCLES form around the epicentre

7

How earthquakes happen

Plates move
= their rough edges rub against each other and get stuck

Stress builds up until the rocks slip up, break and suddenly move

CAUSED BY movement of rocks along the EXISTING FAULT LINES a long way FROM PLATE BOUNDARIES

8

Big and damaging earthquakes

SHALLOW FOCUS earthquakes cause the most damage

The biggest earthquakes are caused by MOVEMENT OF TECTONIC PLATES

9

What can trigger earthquakes

Volcano eruptions

Landslides

Avalanches

10

Seismic waves coming in 2 types

Foreshocks

Aftershocks

11

What are foreshocks

The shock waves recorded a short time BEFORE THE MAIN SHOCK

Caused by the initial slip up or fracture of the stressed rock

12

What are aftershocks

MINOR shock waves that occur hours/days/months AFTER THE MAIN SERIES OF EARTHQUAKE SHOCK WAVES

Due to the overstressed rocks
~ because they readjust + settle down after the main slip up or fracture

13

What are the 3 ways to measure the magnitude/frequency of the earthquakes

Richter Scale

Mercalli Scale

Moment Magnitude Scale (MMS)

14

What’s the Richter Scale

Measures SEISMIC WAVES on a SEISMOGRAM
= measured on a LOGARTHIC Scale for its magnitude

Each 1 incrimen on the Scale you 10x up each time but 30x in terms of energy

1➡️2 (10 x + 30 x) ➡️ 3 (60 x + 900 x)

15

What’s the Mercalli Scale

It measures the intensity by the effects the earthquakes has brought on

Up to 12 = means total destruction

I.e. 8 = partial roof collapse

SUBJECTIVE down to opinion on what devastation looks like

16

What’s the MM Scale

For every increment it goes up, there is 10x more energy

Best for the bigger and deeper earthquakes

17

Where do the majority of earthquakes occur

At destructive plate boundaries

80% of the world’s BIGGEST earthquakes have occurred at the ring of fire

18

There is a way of predicting when earthquakes will occur

very difficult but can be identified using plate tectonics

can measure:
-groundwater levels
-releases of radon gas
-detecting unusual animal behaviour

=can also be mapped on the basis of geological information
=study into ground stability in order to predict earthquake impact + PRODUCE a hazard zone map

close studies of fault lines can sometimes indicate the point along a fault line where the next earthquake is due - 'seismic gap'

NOT POSSIBLE TO PREDICT IT PRECISELY

19

What’s released from an earthquake

Seismic waves

Body waves (P + S waves)

Surface waves (L + R waves)

20

Seismic waves (shock waves)

Body waves - travel through the body of the earth

Surface waves - travel along the earth’s surface

21

P waves

PRIMARY WAVES

Travel the fastest (1.5-14km/S)

Travel parallel to the direction of travel /. /

Go straight through the earth

They reach the surface first

Travel through all physical matter: solid, liquid, gas

22

S Waves

Are slower than P waves (1-8 km/S)

Have to go round the earth

Travel through the mantle not the core

Nearly half as fast and reach the surface next

Transverse or waves direction of travel 🌊🌊🌊

23

How can the epicentre be identified using P + S waves

The distance between P + S being increased shows where the epicentre is

CLOSE TOGETHER WAVES = nearer to the EPICENTRE

24

How geologists and scientists have been able to discover more about the structure of the earth thorough SEISMIC WAVES

Because the waves can be refracted and reflected within the earth

Reflection - waves are rebounded when they strike a boundary BETWEEN different layers of the earth

Refraction - waves pass from one layer to another

The wave bends towards the “normal” if the new medium is denser and away from the “normal” if the medium is less dense

THE WAVES TRAVEL FASTER THROUGH DENSE MATERIAL

25

Waves when an earthquake occurs

P + S waves radiate from the focus

As the DENSITY of the earth increases with DEPTH = paths of the waves BEND

26

L waves

Known an LOVE waves or LONG waves

Vibrate the ground in a horizontal direction

Travel at 2-6 km/S

27

R waves

Known as Raleigh Waves

Move in an elliptical or vertical motion (up and down)

Travel at 1-5 km/S

28

Type of focus earthquakes + their depth

Shallow focus = 0-70km

Intermediate focus = 70-300km

Deep focus = 300-700km

29

Seismic activity distribution

90% are clustered along plate margins

30

Deep focus vs Shallow focus

Shallow focus -

tend to cause the most damage account for 75% of all earthquake energy released

Deep focus -

are more likely to have a high magnitude

31

Factors that affect volcanic and seismic hazards

PHYSICAL

Topography

- lahars, tsunamis, pyroclastic flows, landslides
- lack of accessibility
- steep slopes are a greater hazard

Type of plate margin

-type of lava
- nature of the event = slip or thrust earthquake?

Timing

-what people are doing and where they are when it strikes

How tsunamis can travel when earthquakes can’t in terms of special extent

Magnitude

VEI/ Richter Scale

Frequency

32

Factors that affect volcanic and seismic hazards

HUMAN

Level of development

-quality of the buildings
-recovery
-infrastructure
-level of preparation

Levels of prediction / preparation / mitigation

-communication
-frequency
-subjective thoughts on it
-education
-politics and help for after when responding - alliances?
-how it was before the event - in terms of the cycle was it mended to be better this time or go back to normal/worse

Reclaimed Land

-liquefaction
-more draining of the land from any water

33

Preparedness

Public Education
-securing homes, appliances, 'earthquake kits'
-earthquake drills in schools,offices,factories

Tsunami Warning Systems

-automated systems used to give warnings (bottom pressure sensors measuring water pressure at buoys)
-tsunami prevention walls up to 12m high - not effective

Earthquake/tsunami Drills

Emergency Response Training

Aid
-usually available a few days after the event ie. medical services, water purification, search + rescue equipment

34

Mitigation

Seismic Engineering
(water tanks on roofs) (metal clasps on the bottom reduce swaying and protect it from swaying)

Retrofitting
(quake wraps - sheets of composite material)
-make them more earthquake proof

Tsunami Walls

Insurance policies for damage
-to cover losses
-expensive for HIC individuals to do this

Fire prevention with smart meters
-smart meters developed to cut off the gas if an earthquake of a high magnitude is happening
-reduces lots of fires

35

Adaptation

Hazard Mapping

Land Use Planning

-schools and hospitals meant to be put in areas of low-risk
-sufficient open space is important - safe area after the event

Hazard-resistant buildings and structures

-large concrete weight put on top of the building (it moves) in the opposite direction of the earthquake force to counteract it
-large rubber shock absorbers in the building foundations to allow the building to move a little
-cross-bracing to the structure to hold it together when it shakes

36

Prevention

IMPOSSIBLE

however, this has not stopped thoughts into schemes from taking place to stop plates sticking to each other + releasing
^^^^^ instead sliding past each other
^^^^^^ water and oil to lubricate these plates