Earthquakes & Volcanoes

Main Idea

Earthquakes release built-up stress along faults in Earth's crust, and the resulting shaking, not just direct ground rupture, is often what causes buildings to collapse.

Key Concepts

• Earthquakes occur when stress builds up along a fault and is suddenly released.
• Volcanoes form at subduction zones, divergent boundaries, and hotspots.
• Seismic waves radiate from the focus (underground) to the epicenter (surface point above it).
• Building collapse in earthquakes is usually caused by resonance and structural weaknesses, not the shaking alone.

Definitions

• Fault: a fracture in the crust where blocks of rock move relative to each other.
• Focus/hypocenter: the point underground where an earthquake originates.
• Epicenter: the point on the surface directly above the focus.
• Magnitude: a measure of the energy released, often measured on scales like the moment magnitude scale.

Processes

• As plates grind past or into each other, friction locks them until stress exceeds friction, causing a sudden slip and quake.
• Seismic waves (P-waves and S-waves) travel outward from the focus and are recorded by seismographs.
• Volcanoes erupt when magma rises through weaknesses in the crust, often at subduction zones where melting rock forms magma chambers.

Real-World Examples

• The 2011 Tōhoku earthquake in Japan occurred at a subduction zone and triggered a tsunami.
• Mount St. Helens erupted in 1980 as part of the Cascade volcanic arc formed by subduction.
• Reinforced buildings in earthquake-prone Japan and California use flexible designs to reduce resonance damage.

Common Mistakes

• Believing scientists can precisely predict when an earthquake will strike; only probabilities can be estimated.
• Thinking all volcanoes are equally explosive; eruption style depends on magma composition.
• Assuming taller buildings are always more dangerous in earthquakes; it depends on resonance frequency matching the shaking.