Earth Science 11
Name __________________
Earthquake & Destruction
Modified Mercalli Scale
 Measures the __________ and amount of _____________ done to structures.
 Using Roman numerals I to XII, Mercalli scale rates the types of damage and other effects of an
earthquake noted by observers.
o Ex. III – Quite noticeable indoors. Vibrations are like passing of a truck.
o Ex. X – Most ordinary structures are destroyed. Rails are bent. Landslides are common.
Earthquake Intensity
 Intensity depends primarily on the ______________.
 Surface and body waves gradually ____________ in size with _______________ from the focus.
 Maximum intensity will be observed in the region ______ the epicenter.
 Mercalli values decrease as we go farther ________ from the epicenter.
Depth of Focus
 The _______ of the quake’s focus influences the quakes intensity.
 An earthquake can be classified as ___________, ______________, or _________.
 Deep - produce _________ vibrations at the epicenter
 Shallow - produce _________ vibrations at the epicenter
o Ex. A shallow magnitude 6 earthquake will demonstrate greater intensity than a deep
magnitude 8.
Damage
 Ground Shaking:
o The result of the __________ set in motion by the earthquake.
o Some vibrations move ___ and _________, while others move _____ to ______.
o Most building can withstand large up and down vibrations, but not side to side (cause collapse).
 Foundation failure:
o The soils under a building may ________ from serve shaking, causing ____________.
(Liquefaction)
o Buildings built on ______ rock experience __________ damage
o Buildings located on bog muds or soil fill suffer __________ damage.
Risk and Prediction
 No location is without some earthquake risk.
 Most prediction efforts are based on the assumption that earthquakes are periodic events.
 Seismic gap method:
o Plotting the location of earthquakes foci along a branch of the San Andreas fault.
o Scientists can sometimes find a place where the fault has not ________ over a period of _____.
 Found to proceed earthquakes:
o Micro quakes
o Slight increase in the elevation of the land
o Decreases in the electrical resistance of the ground
o Increase in the amount of radon
o P-waves slow by 10-15% for a period of time preceding an earthquake.
Earthquake waves inside Earth
 P and S-Wave velocities:
o As waves travel through earth, they change velocity, _______ or get ___________.
 At 2900 km:
o Boundary between mantle (_____) and outer core (________)
o P-waves _____ down
o S-waves ______ (don’ travel through liquids)
 At 5200 km:
o P-wave velocity increases (approaches inner core – solid)
The Moho
 Another abrupt change in P and S-wave velocities occurs at the boundary between the crust and the
mantle.
 Discovered in 1909 by Andrila Mohorovicic:
o Found that seismograms showed two distinct groups of P and S-waves.
o One of the groups traveled at an average velocity of 7km/s.
o The other group of waves travelled at 8 km/s.
o Mohorovicic reasoned that the ________ wave group had gone through denser material below
the crust (50km).
 Mohorovicic discontinuity (Moho):
o Boundary between the ______ and the ________.
o The Moho is about 32km deep under continents but only 8km deep below the oceans.
o This is why continents stand _______ on the crust.
Shadow Zone
 Even though an earthquake sends waves throughout Earth’s interior, not all seismograph stations
receive information.
 Seismic stations that receive neither P nor S-waves are said to be in the shadow zone.
 ______________:
o A wide belt around Earth on the side opposite the focus of the earthquake.
 Cause:
o Earth’s outer core.
o P-waves are refracted (______) when they pass through the mantle.
o S-waves cannot travel through the _____________ (liquid)