Lecture 7 Earthquakes and Volcanism

A Hawaiian eruption is primarily associated with fluid lava flow.

A Strombolian eruption is characterized by explosive bursts of lava and gas, typically producing lava fountains.

The main volcanic hazards include:

• Ground Shaking
• Fractures/Fissures
• Volcanic Flow
• Outgassing
• Acid Lakes
• Pyroclastic Flow
• Ash Fall
• Lahars & Landslides
• Effect of Volcanic Ash on Aircraft
• Climate Change

Collapse earthquakes can lead to the collapse of the roof of the mine, instigating more tremors, and are prevalent in small towns where underground mines are located.

Collapse earthquakes are small earthquakes that occur in underground caverns and mines, caused by seismic waves from the explosion of rock on the surface. They are sometimes referred to as mine bursts.

The focus (or hypocenter) is the point within the earth where the fracture begins, while the epicenter is the point directly above the focus on the surface of the earth.

Seismic waves can be classified into:

1. Body waves

   • P wave (Primary wave)
   • S wave (Secondary wave)

2. Surface waves

   • Rayleigh wave
   • Love wave

• Type: Longitudinal wave
• Movement: Push (squeeze) and pull (stretch) rocks in the direction the wave is traveling.
• Speed: Fastest waves, traveling at approximately 6 – 13 km per second.
• Medium: Can travel through solids, liquids, or gases.
• Nature: Compressional wave, where material movement is in the same direction as wave movement.

S-waves are approximately 30% slower than P-waves and can only travel through solids. They are shear waves that move material perpendicular to the direction of wave movement.

S-waves cause particles to oscillate at right angles to the direction of wave travel, effectively 'shaking' the particles perpendicular to their direction of movement.

• Travel just below or along the ground's surface
• Slower than body waves (approximately 10% slower than S-wave)
• Include rolling (Rayleigh wave) and side-to-side movement (Love wave)
• Particularly damaging to buildings

• P wave arrives earlier than S wave because P wave travels faster.
• Body waves arrive earlier than surface waves due to their higher speed.

• Rayleigh wave: Rolling motion, causing particles to move in elliptical paths.
• Love wave: Side-to-side motion, resulting in shear displacement parallel to the Earth's surface.

The main earthquake belts in the world include the Circumpacific belt, which surrounds the Pacific Ocean, the Alpine-Himalayan belt that stretches across Eurasia, and various oceanic ridges.

The three main types of tectonic plate movements are:

1. Divergent - Plates move apart, allowing magma to rise.
2. Convergent - Plates collide, with one sliding under the other.
3. Transform - Plates slide past each other horizontally.

The different types of earthquakes include:

• Tectonic: Caused by the movement of tectonic plates.
• Volcanic: Associated with volcanic activity.
• Explosion: Resulting from nuclear or other explosions.
• Collapse: Caused by the collapse of structures.

A tectonic earthquake occurs when the earth's crust breaks due to geological forces on rocks and plate movements.

Tectonic plates can move in several ways:

1. Towards each other
2. Away from each other
3. Sliding past each other
4. Colliding with each other

Tectonic earthquakes are the most prevalent kinds of earthquakes in the world and have caused most of the planet's mass destruction. Their intensity can vary from small to large.

A volcanic earthquake results from tectonic forces occurring in conjunction with volcanic activity. They usually take place before or after a volcanic eruption.

During a volcanic earthquake, magma erupts from inside the earth's crust, leaving a space behind. Rocks move towards this space to fill it, resulting in severe earthquakes.

Explosion earthquakes result from the detonation of nuclear and/or chemical devices. They are triggered by human impact, specifically nuclear explosions.

Explosion earthquakes can be measured by the motion of large tectonic earthquakes using a GPS device, as the rocks on either side of the fault are offset during this type of earthquake.

High magnitude earthquakes can lead to building collapse, resulting in debris that can be deadly to humans. For example, the earthquake on February 6, 2016, in the southwest coast of Taiwan, with a magnitude of 6.4, resulted in 15 deaths and 150 hospitalizations.

A tsunami is caused by the displacement of a large body of water, typically due to sudden movements such as earthquakes, landslides, or land slumping. The process involves:

1. An earthquake jolts the ocean floor.
2. This displacement pushes a large volume of water outward.
3. It sets off an oscillation, creating turbulence underwater at great speed.
4. The sea is then sucked back from the shore, leading to tsunami waves.

An earthquake with a preliminary magnitude of 8.9 occurred at 9:46 PM Pacific Standard Time.

The steepness of the slope and vibrations, whether small or large, can contribute to the speed and danger of an avalanche.

The runout zone is the area where debris from the avalanche accumulates at the bottom of the slope.

Landslides can occur, causing large rocks and earth to roll rapidly down into valleys, leading to destruction and potential death for people living downstream.

1. More earthquake studies to gather data and understand patterns.
2. Better monitoring systems to detect seismic activity in real-time.
3. Enhanced earthquake prediction models such as:
   • Time predictable models (Bufe et al., 1977)
   • Magnitude predictable models (Shimazaki and Nakata, 1980)
   • Time-magnitude both predictable models.
4. Case study of Haicheng earthquake: On February 4, 1975, a magnitude 7.5 earthquake occurred, and much of the city was evacuated before the event, showcasing the effectiveness of prediction efforts.

Volcanoes are primarily distributed along plate boundaries. They can be found at:

• Divergent plate boundaries: characterized by fissure eruptions and shield volcanoes (e.g., Iceland).
• Convergent plate boundaries: where cinder cones and composite volcanoes occur (e.g., Mt. Fuji in Japan, Pinatubo in the Philippines).
• The Ring of Fire: a major area in the basin of the Pacific Ocean where many earthquakes and volcanic eruptions occur.
• Intra-plate volcanoes: such as those found in Hawaii.

The Volcanic Explosivity Index (VEI) is based on two main criteria:

1. Total volume of erupted materials
2. Height of the eruption column

The eruption frequency for different levels of the Volcanic Explosivity Index (VEI) is as follows:

A diffusive eruption involves the cracking of rocks above the magma chamber, leading to the outpouring of lava. This type of eruption is characterized by a more gentle flow of lava rather than explosive activity.

An explosive eruption occurs when magma ascends along the conduit, causing a separation of volatiles from the magma. As pressure decreases, the expansion of gases leads to a violent eruption, characterized by ash clouds and explosive lava ejection.

A Shield volcano is characterized by its broad, gently sloping profile and is formed primarily from fluid lava flows.

A Dome volcano is defined by its steep sides and is formed from viscous lava that piles up near the vent.

A Caldera volcano is a large depression that forms when a volcano collapses after the magma chamber has emptied.

A Composite volcano has a steep-sided structure built up by alternating layers of lava and ash.

The major types of volcanic eruptions include:

1. Plinian eruption
2. Vulcanian eruption
3. Pelean eruption
4. Icelandic eruption
5. Hawaiian eruption
6. Strombolian eruption

A Plinian eruption involves a mixture of fluid lava flow, viscous lava flow, cooled lava, volcanic gases, gas clouds, and volcanic ash.

The rate and distance of lava flows depend on several factors:

1. Temperature
2. Silica Content
3. Extrusion Rate
4. Slope of the Land

Cold lava flows or lava with high silica content will not travel far due to high viscosity, making them less hazardous in terms of distance traveled but still a significant hazard to property.

• Slow lava flow
• Ropy structure
• Forms large lava tubes
• Slow heat loss with a well-developed solidified tube crust
• Tears on the crust can be healed by molten lava underneath

• Rapid lava flow
• Blocky appearance
• Rough, rubbly surface
• Rapid heat loss leading to fragmentation of the solidified surface crust

Lava flows are streams of molten rock that emerge from a volcano during an eruption. They can vary in speed and temperature, and are commonly found in volcanic regions such as Hawaii and Mt. Etna in Sicily.

A pyroclastic flow is a fast-moving current of hot gas and volcanic matter that flows down the slopes of a volcano. It occurs when the eruption column loses support from rising hot air, causing the material to surge downslope.

The main types of volcanic deposits associated with pyroclastic flows include:

1. Ignimbrite: deposits formed from pyroclastic flows.
2. Tephra: ash-fall deposits that accumulate from volcanic ash falling from the eruption cloud.

Pyroclastic flows typically move faster than 80 km/hr and have temperatures ranging from 200 to 700°C. They can destroy almost everything in their path and can block streams, creating temporary dams that may lead to sudden flooding downstream.

Volcanic ash can cover wide areas and has several impacts:

• Reduces visibility and blocks sunlight
• Damages engines
• Can collapse roofs due to its density
• Disrupts power generation
• Clogs water supplies
• Causes breathing difficulties
• Damages crops

A lahar is a volcanic mudslide consisting of a mixture of water, rock fragments, and soil that flows down a volcanic slope or river valley. Lahars can form as a result of volcanic eruptions or heavy rainfall, and they grow in size as they flow and erode material from the volcanic slopes.

The city of Puyallup, Washington, is built on an old lahar deposit, making it vulnerable to future lahars. Many communities in the vicinity of volcanoes are at risk due to the potential for lahars to flow through populated areas, causing destruction and posing significant hazards to life and property.

Acid lakes in volcanic craters can have extremely low pH values, sometimes as low as 0.1, due to the dissolution of gases from magma, such as CO2, SO2, H2S, HCl, and HF in water. These gases contribute to the high acidity of the lakes.

High CO2 concentrations in the soil can kill plants by:

1. Denying their roots oxygen (O2) needed for survival.
2. Interfering with nutrient uptake, leading to poor plant health.

In areas like Mammoth Mountain, CO2 can make up about 20 to 95% of the gas content of the soil, severely affecting vegetation.

Volcanic ash can severely impact mechanical systems by:

• Contaminating fuel, hydraulic, and electrical systems.
• Jamming gears and causing engines to flame out.
• Melting in combustion chambers due to low melting points, leading to ceramic mass sticking to turbine blades, fuel nozzles, and combustors, which can result in total engine failure.
• Causing abrasion to cockpit windows and penetrating air conditioning systems.

The impacts include:

1. Business and leisure travel: Delays or cancellations.
2. Perishable goods: Loss of products due to inability to transport.
3. Industrial production: Suspension of operations in industrial plants.
4. Economic impact: Total estimated impact of US$4.7 billion over one week.

Volcanic eruptions can lead to a decrease in global temperatures due to the release of ash and gases that block sunlight. For example:

• Pinatubo eruption (1991): Global temperatures dropped by -0.5°C from 1991 to 1993.
• Mt. Tambora eruption (1815): Global temperatures dropped by 0.4-0.7°C for three years, resulting in the 'Year Without a Summer', which caused famines and epidemics of typhus and cholera.

Volcanic tremor refers to seismic signals that indicate magma movement beneath a volcano.

The main techniques for monitoring volcanoes include:

1. Volcanic tremor - seismic signals indicating magma movement.
2. Ground deformation - measured using GPS, InSAR, and tiltmeters.
3. Gas monitoring - tracking gas emissions from the volcano.

Challenges in predicting volcanic eruptions include:

• The need for careful monitoring of volcanic activity.
• The necessity to accept false alarms when predictions are made.
• The inability to prevent eruptions, leading to potential financial losses and human impact.
• Difficulty in predicting the style and severity of eruptions even when they are anticipated.

The elastic rebound theory explains that rocks bend under stress until their strength is exceeded, leading to a rupture. After the rupture, the rocks quickly rebound to their undeformed shape, releasing energy in waves that radiate outward from the fault, causing violent shaking.