Earthquakes

Rocks react to stress much like a rubber band does; they stretch and bend. Once their elastic limit is passed the rocks remain bent and may break.

The vibrations that are produced when rocks break are called earthquakes.

Earth’s crust is stretched and thinned by tension.

Earth’s crust is folded and thickened by compression.

Rocks in Earth’s crust slide past each other by shearing.

The force that pulls rock apart to form a normal fault is tension.

Sierra Nevada Mountains

The force that pushes rock from opposite sides to create a reverse fault is compression.

Rocky Mountains

The force that causes rocks to slide past each other forming a strike-slip fault is shearing.

San Andreas Fault

Earthquake Information

The point in the Earth’s interior where energy is released is the focus.

Seismic waves travel outward from this point.

The point on Earth’s surface directly above the focus is the epicenter.

The P-Wave moves back and forth in the direction of the wave The P-Wave moves back and forth in the direction of the wave. Compression wave

The S-Wave moves at right angles to the direction of the wave The S-Wave moves at right angles to the direction of the wave. (Shear Waves) Transverse Waves

The L-Wave moves in an elliptical motion (back & forth and side-to-side) Longitudinal

The P-Wave is the fastest wave.

The L-Wave (surface wave) is the slowest wave.

The L-Wave (surface wave) causes the most destruction.

A minimum of 3 seismographs are needed to determine the epicenter of an earthquake.

Richter Scale Richter Magnitude TNT Equivalent 1.0 6 ounces (small blast) 2.0 (some may be felt) 13 lbs. 3.0 397 lbs. 4.0 (felt by most) 6 tons (small a-bomb) 4.5 32 tons (average tornado) 5.0 (damaging shocks) 199 tons 6.0 (destructive) 6270 tons 7.0 (major – serious damage) 199,000 tons 8.0 (great EQ’s) 6,270,000 tons 9.0 199,999,000 tons

How much energy is released for every 1.0 increase in magnitude? 32 times more energy 6.5 to 8.5 = 32 x 32 = 1024 times 5.5 to 8.5 = 32 x 32 x 32 = 32768 times more

Intensity – the amount of damage done by an earthquake. Uses the Modified Mercalli Scale.

Mercalli Scale I Not felt. II Felt by few. III Some people indoors may feel movement. Hanging objects may swing. IV Noticeable indoors, esp. on upper floors. Dishes and windows rattle. Parked cars may rock. V Felt by nearly everyone, many awakened. Doors swing. Pictures move. Dishes break. Trees shake. VI Felt by all. Furniture moves. People have trouble walking. Objects fall off walls & shelves. Little structural damage.

VII Little damage in well-built buildings. Moderate damage in ordinary structures. VIII People have trouble standing. Drivers feel car shaking. Slight damage in well-guilt buildings. Damage greater in poorly constructed buildings. IX Well-built buildings suffer damage. Underground pipes break. Ground cracks. X Most building and foundations are destroyed. Bridges and dams damaged. Landslides occur. Railroad tracks bent. Water thrown out of rivers and lakes. XI Most buildings fall down. Bridges destroyed. RR tracks badly twisted. XII Almost everything destroyed.

tsunami – ocean waves or seismic sea waves. cause: earthquake under the ocean. reach heights: 100 feet (30 meters)

Surface Wave Fault Epicenter Focus Secondary Wave Fault Primary Wave

The focus is always located on the fault.

Epicenter The epicenter is always directly above the focus.

P & S waves slow down when they reach the upper mantle. P & S waves speed up again when they reach the lower mantle.

Shadow zone is the area between 105° and 105° from the focus where no s-waves are detected.

The Moho Discontinuity is the boundary between the crust and the mantle.