5. Seismic Hazards

Describe the nature and distribution of earthquakes? Giving an example how do intra-plate earthquakes occur?

• Most earthquakes (95%) occur along plate margins mostly conservative eg: San Andreas fault line (fault line slip rate 2-3cm per year) and destructive (most powerful at destructive plate margins)

• Many earthquakes occur along the Pacific Ring of Fire

• Intra-plate earthquakes can occur due to the reactivation of old fault lines or hotspots eg: South Wales earthquake 2018 which occurred due to activity along a pre-existing fault line

How may humans be responsible for earthquakes?

Human induced earthquakes may be caused by:

1. Building large reservoirs which puts pressure on underlying rocks

2. Fracking (hydraulic fracturing of rock to release gas). Eg: Permian Basin of West Texas 2025

Describe what an earthquake is and how they happen using key words?

• Earthquakes occur due to movements in the Earth’s crust causing stress to build up and rocks deform.

• Stored energy builds up until it eventually exceeds the strength of the rock and releases known as elastic rebound  → rock fractures along the fault

• Intense vibrations/seismic waves spread out from the initial point of fracture (the focus) causing the ground to shake

• The point directly above the focus is the epicentre (largest seismic waves)

Describe how the depth of focus impacts seismic events?

• Shallow focus (0-70km deep), intermediate focus (70-300km), deep focus (300-700km)

• The shallower the earthquake = more destructive

• Shallow = all plate boundaries, intermediate = mainly at subduction zones under the trench but can be continental collision zones, deep = always at subduction zones

Describe how the different plate boundaries impact seismic events?

• Conservative margins (e.g. San Andreas Fault): plates lock due to friction, stress builds up and is released suddenly → powerful shallow earthquakes and high surface damage.

• Destructive margins: subduction of dense oceanic crust creates large megathrust earthquakes, sometimes triggering tsunamis.

• Constructive margins (e.g. Mid-Atlantic Ridge): plates pull apart, tension is released more gradually → frequent but generally low-magnitude earthquakes.

• Continental collision zones (e.g. Himalayas): strong compression creates shallow crustal earthquakes, often causing landslides and rockfalls in mountainous terrain.

How does the rate of movement impact seismic hazards?

• Faster plate movement → stress accumulates more quickly, increasing the likelihood of frequent earthquakes.

• Slower plate movement → stress builds over longer periods, potentially producing very large but less frequent earthquakes when released.

What are the 3 scales to measure earthquakes and describe them?

1. Richter Scale - measured magnitude but was replaced with the moment magnitude scale as it underestimates the magnitude of large-scale events

2. Moment Magnitude Scale - measures magnitude (energy released at the focus) using using seismographs. Logarithmic scale from 1-10+

3. Modified Mercalli Intensity Scale - measures intensity (impact on people) from I to XII

Describe the frequency of seismic hazards and the seismic gap theory?

• Low magnitude earthquakes occur much more frequently than high magnitude earthquakes → there are around 20,000 earthquakes around the world every year many are low magnitude so not felt by humans

• According to records from 1900, we expect around 16 major earthquakes (magnitude 7.0 or greater) every year however, in 1989 there were only 6

• Seismic gap theory suggests that if there has been little or no earthquake activity at a plate margin for a long period of time, it is more likely to experience a large earthquake in future

  • This is because stress will have built up for a longer period of time, so more energy will be released - helps identify area at higher risk of large earthquakes however, it is not precise and not all seismic gaps result in large earthquakes

Describe earthquakes as a primary hazard form of seismic hazard?

This is the ground shaking. Mainly occurs on plate margins (check flashards further up). Magnitude is impacted by: type of plate margin, rate of movement, depth. Eg: 2011 Tohoku earthquake in Japan which occurred on a destructive margin had a magnitude of 9.0-9.1

Describe shockwaves as a primary form of seismic hazard?

Wave type	Characteristics
Primary - P waves	Alternating compressions/dilations in the same direction as the wave Fastest → first on seismogram Smaller amplitude + higher frequency
Secondary - S waves	Transverse motion perpendicular to direction of the wave Only travel through solids Slower than P wave but faster than L wave so are second on the seismograph
Love - L waves	Transverse horizontal motion perpendicular to the direction of the wave Surface wave → most damaging Slowest → last on seismograph

Seismic shockwaves are released energy from the earth causing movement