GEOL105 Chapter Notes - Chapter 3: Seismometer, Elastic-Rebound Theory, Strong Ground Motion
31 May 2018
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CH 3: Earthquakes
3.1 Introduction to Earthquakes
● Worldwide, people feel 1 million earthquakes a year
● Earthquakes occur at faults
○ Fault: a plane of weakness in earth’s crust, a semi planar fracture or fracture
system where rocks have been displaced
● Footwall: the block below the fault plane where the miner would stand
● Hanging wall: the block above the fault plane where a lantern could be hung
● Faulting: the process of creating a rupture in earth’s crust by the movement of one
crustal block in relation to another
○ Stress: a force that results from tectonic movements
○ Strain: change in shape or location of the rocks due to applied stress
● Fault Types
○ Normal dip-slip fault-> tensional stress
■ Downward movement of the hanging wall relative to the footwall
○ Reverse dip-slip fault-> compressional stress
■ Upward relative movement of the hanging wall due to compression,
resulting in shortening and thickening of earth’s surface typical of
convergent plate boundaries
○ Left-lateral strike slip fault-> shearing stress
■ Close to vertical, no hanging wall or footwall
■ Offset earth’s crust laterally or in a horizontal motion
○ Blind faults
■ A fracture or zone of rupture along which displacement does not extend
to the earth’s surface
■ Located by identifying folds at the surface known as anticlines and
synclines
3.2 The Earthquake Processes
● A fault is considered an active fault if it has moved during the past 10,000 years of the
Holocene Epoch
● Paleoseismicity: the occurrence of earthquakes in space and time in the geologic past
● Earthquake cycle: proposes that there is a drop in elastic strain after an earthquake
and an accumulation of strain before the next event
● Typically three to four stages
○ A long period of inactivity along a segment of geologic fault
○ Accumulated elastic strain produces small earthquakes
○ Consists of foreshocks, may occur only hours or days prior to the next large
earthquake
■ In some cases, this stage may not occur
○ Mainshock, the major earthquake and its aftershocks occurring near the location
of the mainshock anywhere from a few mins to a year or so after the main event
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● Epicenter: the place on the surface of the earth above where the ruptured rocks broke
to produce the earthquake
● Focus (hypocenter): directly below epicenter, the point of initial breaking or rupturing
within in the earth
● Seismic waves: a periodic disturbance of particles of earth by an earthquake or other
vibration that is propagated without the net movement of the particles
○ Two types of waves
■ P waves, primary, faster, can travel more quickly through solids than
through liquids
■ S waves, secondary or shear, can only travel through solid materials,
slower
○ When p and s waves reach the land surface, complex surface waves form and
move along earth’s surface
■ These waves move slower than p and s waves
■ Love wave
● Tectonic creep: gradual movement along a fault that is not accompanied by perceptible
earthquakes
○ Slow earthquakes
○ Fault creep
○ Rupture can last from days to months
3.3 Earthquake Shaking
● Factors that determine the shaking you will experience
○ Magnitude
○ Location in relation to the epicenter and direction of rupture
○ Local soil and rock conditions
● Richter scale: magnitude determined by measuring the max amt of ground shaking due
to the s wave
○ Logarithmic
● Seismograph: records ground motion in either a vertical or horizontal direction
● Moment magnitude scale: a measurement of the actual Energy released during an
earthquake
● Modified Mercalli Intensity scale: a scale with 12 divisions on which the amt and
severity of shaking and damage from an EQ can be ranked
○ Roman numerals
● Depth of an EQ influences the amount of shaking
○ The deeper the focus of the EQ, the less shaking will occur at surface
● Direction of rupture and distance to epicenter both can affect the amount of shaking
● Supershear: occurs when the propagation of rupture is faster than the velocity of shear
waves or surface waves produce by the rupture
○ Increased shear and shock may significantly increase the damage from a large
EQ
● Local geologic conditions
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○ Material amplification: inc in the amplitude of seismic shaking caused by some
earth materials. Such increase is generally associated w soft sediments, such as
silt and clay deposits
○ Figure 3.17 on page 67
3.4 Geographic Regions at Risk from Earthquakes
● In US, the areas with highest EQ risk include
○ Pacific coastal areas of California, Oregon, Washington, Alaska, and Hawaii
○ Area along the California-Nevada border
○ Territories of Puerto Rico and the Virgin Islands
● Megathrust earthquakes: a term used to describe large EQs (M8 and greater) that
occur along low-angle faults at subduction zones
○ Chile earthquake of 1960, M 9.5
● Intraplate earthquakes: sudden movement along a fault caused by the abrupt release
of accumulated strain in the interior of a lithospheric plate, far away from any plate
boundary
○ Lack of preparedness
○ Occur less often
○ Buildings may not be able to withstand strong shaking
○ Generally more damaging
○ Felt over a much larger area
● Recurrence interval: time between events
3.5 Effects of Earthquakes and Linkages with Other Natural Hazards
● Primary effects
○ Ground shaking
○ Effects of people and structures
○ Surface rupture
○ Caused directly by fault movement
● Secondary effects
○ Result from faulting and shaking
○ Liquefaction of the ground
○ Regional changes in land elevation
○ Landslides
○ Fire
○ Tsunamis
○ Disease
● Fault scarp: steep slope or small cliff that was formed during an eq by rupture and
displacement of earth’s crust
● Shaking is commonly measured as ground acceleration and is compared to the overall
acceleration of gravity
● Resonance: the matching of the freq of shaking w the natural vibrational freq of an
object
○ Can affect buildings a significant distance from epicenter
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Worldwide, people feel 1 million earthquakes a year. Fault: a plane of weakness in earth"s crust, a semi planar fracture or fracture system where rocks have been displaced. Footwall: the block below the fault plane where the miner would stand. Hanging wall: the block above the fault plane where a lantern could be hung. Faulting: the process of creating a rupture in earth"s crust by the movement of one crustal block in relation to another. Stress: a force that results from tectonic movements. Strain: change in shape or location of the rocks due to applied stress. Downward movement of the hanging wall relative to the footwall. Upward relative movement of the hanging wall due to compression, resulting in shortening and thickening of earth"s surface typical of convergent plate boundaries. Close to vertical, no hanging wall or footwall. Offset earth"s crust laterally or in a horizontal motion.
