Earthquakes and Landslides

Epicentre

The point of an earthquake on the earths surface directly above focus

Focus

The point in the earths lithosphere where stress is released in the form of seismic waves.

Shallow Focus

0-70km deep

Most dangerous as the seismic waves have less distant to travel before reaching and impacting the surface

Intermediate Focus

70-300km deep

Danger varies on where the focus is but waves have further to travel then shallow focus seismic waves.

Deep focus

>300km deep

Least dangerous as waves have much further to travel till they reach the surface and the epicentre.

L (Love) Waves

Slowest waves, which move from side to side.

However most damaging and move along earths surface.

R (Rayleigh) Waves

Propagate from the epicentre in low frequency. Move in an elliptical motion.

Move along the surface of the earth.

Triangulation

Arrival times of the P and S waves at different seismometers are used to determine the location of an earthquake.

Assuming the relative wave speed and time difference, they can calculate the earthquakes location.

Intensity

Qualitative measure of strength of shaking caused by an earthquake determined from effects on people, objects, and buildings.

Fet Reports

Focus on individual perception of an event. Can be subjective.

Magnitude

Measure the amount of energy released during an earthquake.

Measuring Earthquakes - Richter Scale

Pros:

• Simple

• Logarithmic precision

• Widely recognised

• Effective for small to medium Quakes

Cons:

• Limited accuracy for large quakes

• Focus on local measurements

• Outdated for modern needs

• Surface Wave dependency

Measuring Earthquakes - Mercalli Scale

Pros:

• Focuses on human impacts

• Useful for historical events

• No equipment required

• Provides detailed local information

Cons:

• Subjective measurements

• Inconsistent across regions

• Limited for remote/uninhabited areas

• Cannot measure energy release

Measuring Earthquakes - Moment Magnitude Scale

Pros:

• Accurate for all earthquake sizes

• Global consistency

• Based on physical parameters

• Effective for deep and distant quakes

• Standard in modern seismology

Cons:

• Complex calculations

• Less intuitive for the public

• Delayed results

Ground Ruptures

Occur when the earth’s surface visibly fractures or is uplifted usually along the line of a fault.

Is the Primary Hazard of an earthquake.

Secondary Seismic Hazards - Liquefaction

Occurs in sediments where the water table is close to the surface.

As the ground shakes and water pressure builds up between individual sediment particles, the ground loses strength and behaves like a fluid.

Secondary Seismic Hazards - Landslides

Occur when slopes fail as a result of ground shaking.

The likelihood of landslides will depend upon the magnitude of the earthquake, the steepness of the slope, whether or not ground has been saturated and any stabilising vegetation.

Secondary Seismic Hazards - Fires

Often occur in the aftermath of earthquakes due to collapsed electricity wires or broken gas pipes

Secondary Seismic Hazards - Tsunamis

If one of significant size occurs, they are the most devasting secondary hazard. Much more damaging than the initial earthquake.

Out at sea can travel at speeds of 640-960kph.

Disruption to society

Disrupt peoples health, well-being, and livelihoods as results of societal stress

Societies continue to suffer losses for years from secondary impacts

E.g. A decade after the 2010 Haiti Earthquake, 55,000 people still lived in makeshift camps and 2.5 million needed aid.

Seismic Hazard Map

Identify areas at high risk based on past earthquakes and fault activity. Created by:

Historical Data and Patterns - Looking at past quake cycles to asses potential reoccurrence.
Contemporary Studies - Our understanding of plate tectonics means were able to predict where quakes may occur.

Seismic Sensors

Detect the first, less destructive P-Waves and send alerts before the damaging S-Waves arrive. The closer to the epicentre you are, the less effective this will be.

Problem - The Collapse of infrastructure causes economic damage and death

Solutions:

1. Build new infrastructure away from the earthquake prone areas, implementing guidelines on the location and limits on building heights.

2. Earthquake resistant infrastructure designed to withstand strong tremors (E.g. Transamerica Pyramid, SF = $75 million)

Limitations:

• Difficult in developing cities and countries because migrants build illegal homes

• LDCs can’t afford these expensive buildings (elitist)

Problem - More than 1.8 million older buildings are unable to withstand earthquakes

Solutions:

1. Strengthen existing infrastructure to withstand strong tremor

e.g. Reinforced by wrapping steel frames around the buildings or by placing steel rods in existing structures

2. Use fireproof materials and implement automatic shut-off valves

e.g. 1994 North Ridge earthquake, USA - reinforced buildings damaged whilst earthquake resistant buildings remained undamaged.

3. Education on emergency procedure - posters, and signs e.g. Costal Tsunami Zones, Japan

e.g. Some residents are less prepared for quakes than others in Japan.

4. Earthquake monitoring and warning system - seismometers collect data, governments inform residents.

e.g. Haicheng, China - changes in the ground level and increase in small tremors alerted authorities moving 90,000 people to safety from a 7.3 magnitude quake destroying 90% of infrastructure

Limitations:

• Seismic retrofitting is limited as the strengthened infrastructure may not be as strong

• People tend to be complacent and may not see the importance of drills

• Authorities may choose to ignore warnings, Predictors of scientists may not always be accurate

Seismic retrofitting freeway structures

After a huge earthquake in California, collapsing the freeways, they decided to retrofit them with:

• Cable joints to keep the roadbeds from separating at the joints

• New Columns with continuous 3/4” steel spirals on 3” centres support vertical rods

• Wrap the columns in steel casing to support the freeway pillars

Also having to spend hundreds of millions of dollars on the roads leading up to the Golden Gate Bridge

HIC Earthquake Case Study

2011 Tohoku Earthquake, Japan

LIC Earthquake Case Study

2010 Port-au-Prince Earthquake, Haiti

Landslides

Mass movement of material such as rock, earth, or debris down the slope of a hill or cliff.

Five Main Types of mass movement

Falls, Topples, Rotational Slides, Translational Slides, Flows

Can be one or a mixture which makes them difficult to identify

When do Landslides occur?

May occur because the strength of the material is weakened, reducing the ‘glue’ power cementing soil and rock on a cliff. The rock is no longer strong enough to resist gravity.

Common causes: Earthquakes, Volcanoes, Quarrying, and Rainfall.

Between 1998-2017, 4.8 million have been affected, with 18,000 deaths (WHO estimates)

Earthquakes creating landslides example

Jan. 1994, 6.7 magnitude earthquake in Northridge, CA triggered over 11,000 landslides contributing to $20 billion in property damaged centred around the San Fernando Valley Region.

Caused:
- Collapsed roads limiting support to the area
- Deep Block Slides impacted dozens of homes
- Shallow, disrupted slides also seriously damaged many structures.

Response to Earthquakes - Taiwan 1999

Improved building regulations

Strong emergency response unit

Tests on earthquakes taken by the public - average 90% from 200,000 tests