Forces inside Earth Information Scale and Forecasting 19.2 Earthquakes Forces inside Earth Information Scale and Forecasting

What are earthquakes? Earthquakes occur at plate boundaries. As plate move their rock edge experience immense pressure and this causes waves. Pressure is so great it breaks the rock. Rock breaks = energy is released (seismic waves)

Requirements for Earthquakes Fault – boundary of two tectonic plates Active Stress along the fault Failure Rocks “snapping” back elastically Reach Elastic Limit Elastic Rebound Theory Phenomenon whereby stressed rocks snapback elastically after an earthquake to their pre-stress condition Stress exceeds rupture strength of rock

General Definitions Focus- The point beneath the Earth’s surface where the rocks break and move. Epicenter- The point on the Earth’s surface directly above the focus. Seismic Waves- Waves produced by earthquakes

Focus vs. Epicenter Focus Epicenter Point in Earth energy is released from P-waves S-waves Move outward in all directions, not just toward surface Epicenter Point directly above focus on the surface Energy that reaches surface generate surface waves Move along surface Elliptical motion & back and forth Most destruction

Seismic Waves (ENERGY) Primary (P-waves) Travel fastest Longitudinal wave Through solids, liquids, and gasses Faster through denser material Originate at Focus Secondary (S-waves) Slower that P-waves Transverse waves Perpendicular to flow Can not pass through liquid or gasses Faster through denser material Originate at Focus

Surface Waves Moves only along Earth Surface Combination of up and down motion and back and forth motion Cause more destruction than P or S waves.

Locating an Epicenter Based on different speed of seismic waves Need at least 3 seismograph stations Determine arrival times of p and s waves Calculate difference between p and s waves Use the graph from Earthquake information work sheet Compare radius with 2 other stations Draw radius from all three stations Intersection point is EPICENTER

Using Seismic Waves to Map Earth’s Interior At certain depths, speed and path of waves change Through lithosphere – speed up Asthenosphere – slow down Lower Mantle – speed up Outer core – s waves stop Inner core – speed up (p waves)

Normal Faults Tension Force – causes stretching and elongation Pulls rocks apart (divergent boundary) Fracture Rock above fault moves downward in relation to rock below fault line Formed Sierra Nevada Mountains

Reverse Fault Compression force – Convergent boundaries Pushes on rocks causing to bend and sometimes break Rocks above fault are forced up and over rocks below fault line Rocky Mountains

Strike-slip Fault (Transform) Shear Force No upward or down movement Surfaces snag and rocks are twisted and strained due to friction Short and sudden but can be slow and steady San Andreas Fault

Magnitude & Intensity Damage caused by Earthquakes depends on… Distance to Epicenter Depth of Focus Bedrock/Soil Types # and types of structures Magnitude vs. Intensity Magnitude – measured force based on seismic waves Intensity – What was felt? What occurred? Observer specific

Modified Mercalli Scale Arbitrary set of definitions based upon… What people in area feel Observations of damage to buildings Advantages Great if in inhabited area of developed country Easy to use NO Specific equipment Disadvantages Difficult in places without buildings or trees (desert) Observer specific – what appears to be “heavy” to one may not be for someone else

Richter Magnitude Scale Based on Seismic waves Deals with strength of break (fault line) Each step is 30 fold higher increase in magnitude Advantages Allows for comparison regardless of location Good for small, nearby quakes (seismograph) Used from 1930’s – 1980’s

Moment Magnitude Scale Estimates Total energy released by quake Good for ALL earthquakes Measures size and type of waves and movement along faults When you hear “Richter” on the news, they are actually referring to Moment Magnitude Scale as of late 1980’s

Forecasting vs. Predicting Forecasting – General statement about what will happen in future (not specific) Predicting – Specific statement with specific date Animal behavior – ground animals come out of ground Radon Levels increase in ground water Why it is difficult to predict Too many factors Movement, stresses, rock mechanical property, past movement, seismic gaps