Measuring Earthquakes

Flashcards about measuring earthquakes

Richter Magnitude Scale

Measure of ground motion; highest wave amplitude recorded by seismometer; logarithmic scale.

Local magnitude

Objective measurement from seismometer; available immediately after earthquake.

Limitations of Richter Scale

Depends on distance from focus and local geology; NOT intrinsic earthquake size.

Moment Magnitude Scale

Measure of total energy released from the vicinity of fault during rupture; depends on area of rupture and amount of displacement along fault line.

Calculation Time of Moment Magnitude Scale

Takes days to months to be calculated; absolute magnitude; intrinsic earthquake size.

Advantages of Moment Magnitude Scale

Independent of distance or local geology; logarithmic scale (~33x more energy per magnitude change).

Modified Mercalli Intensity Scale

Measure of relative effects of ground shaking on people, buildings, and natural landscapes.

Data of Modified Mercalli Scale

Based on qualitative impressions of people at various distances from epicenter obtained via surveys and questionnaires.

Qualitative Measure of Intensity

Subjective evaluation of injury to people and damage to property or infrastructure; categorical scale from I to XII.

Energy

Measure of the size of an earthquake based on energy released during an earthquake; objective.

Ground shaking

Measure of the size of an earthquake based on maximum ground motion measured by a seismograph.

Intensity

Measure of the size of an earthquake based on relative effects of ground shaking on people, buildings, and natural landscapes; subjective.

Frequency per Earthquake Size

Smallest earthquakes are most common; largest earthquakes are rare; exponential relationship.

Seismicity Maps

Show occurrence of earthquakes in space and time.

Relationship between Mercalli Intensity and Distance

Mercalli Intensity diminishes with distance from epicenter.

Attenuation

Seismic waves attenuate (lose energy) as they go through the Earth.

Material amplification

Seismic waves travel faster through consolidated (solid) rock, slower through loose rocks, soil, and mud.

1989 Loma Prieta Earthquake

Earthquake along San Andreas fault zone (M6.9); most severe shaking on muddy bay shore.

1985 Michoacán Earthquake

Earthquake near Mexico City (M8.0); ancient lake (Lake Texcoco) beneath Mexico City.

Direction of Fault Rupture

Highest intensity is NW of epicenter rather than centered around it.

Supershear

A series of earthquakes is triggered along the fault in the same direction.

Amplification of Ground Motion

Surface waves catch up to one another and hit at the same time resulting in amplification of ground motion.

Mercalli Intensity IV

Vibration feels like the passing of a truck.

Mercalli Intensity VI

Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight.

Mercalli Intensity IX

Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb.

Mercalli Intensity XI

Few, if any, masonry structures remain standing. Bridges destroyed.

Richter Scale Logarithmic Scale

Logarithmic scale so amplitude on seismograph of M4 earthquake is 10x higher than M3.

Epicenter

The point on the Earth's surface directly above the focus of an earthquake.

Focus

The point within the Earth where an earthquake rupture starts.

Strike-Slip Fault

A fault in which the movement is predominantly horizontal.