Oceanography Chapter 7

What is an Earthquake?

The sudden release of energy stored in rocks due to stress, causing ground shaking.

What’s the difference between elastic and plastic deformation?

Elastic = rock bends and returns to original shape; Plastic = rock bends permanently into a new shape.

What happens when stress exceeds rock strength?

Fracture occurs, often along faults (zones of weakness).

What are the focus and epicenter?

Focus = point inside Earth where rupture starts; Epicenter = point on surface directly above focus.

What are the two main categories of seismic waves?

Body waves (P and S) and Surface waves (rolling and side-to-side).

What are P waves?

Primary, compressional, fastest (4–7 km/s), travel through solids, liquids, gases

What are S waves?

Secondary, shear, slower (3–4 km/s), travel only through solids.

What are surface waves?

Slower waves at Earth’s surface, cause most damage (rolling and side-to-side).

How are seismic waves measured?

With a seismograph (instrument) and seismogram (recording).

What are the three main earthquake scales?

• Mercalli = measures damage/intensity felt.

• Richter = magnitude from wave amplitude.

• Moment magnitude = modern, based on energy, fault slip, and rupture area.

How is an earthquake’s location found?

By comparing arrival times of P and S waves (time-travel curves) from at least 3 stations (triangulation).

Where do most earthquakes occur?

At Tectonic Plate Boundaries

What happens at transform boundaries?

Strike-slip motion, shallow quakes (ex: San Andreas Fault).

What happens at convergent boundaries?

Deep earthquakes along subduction zones (Benioff zone).

What happens at divergent boundaries?

Shallow earthquakes from friction at sliding blocks.

Can earthquakes occur in plate interiors?

Yes, but rarely (ex: 1811–12 New Madrid, Missouri).

Can earthquakes be predicted exactly?

No. Long-term patterns are known, short-term predictions are unreliable (foreshocks, ground/animal changes may help but not consistent).

What is a tsunami and how does it form?

A seismic sea wave caused by undersea fault motion; low in open ocean but grows taller near coasts.

What is the Moho?

The crust–mantle boundary (Mohorovičić discontinuity), discovered by seismic wave refraction.

What do seismic wave shadow zones tell us?

P-wave refraction and absence of S-waves beyond 105° show Earth’s outer core is liquid.

How is Earth’s magnetic field generated?

In the liquid outer core—convection of molten metal, deflected by Earth’s rotation.

How are magnetic poles positioned relative to geographic poles?

Magnetic north and south poles are ~11.5° offset from geographic poles.

The Hawaiian Islands are an example of a volcanic island arc located at a convergent plate boundary

False

S or shear waves

are characterized by particle movement perpendicular to direction of wave propogation

The moment-magnitude scale

is an accurate indicator of the energy released during an earthquake