Earthquakes

• Topics:

– How to locate earthquakes
– How to measure earthquakes
– Prediction?

• Earthquake

– Shaking of ground from motion along a fault
– Faults common along plate boundaries
– Most earthquakes along plate boundaries

• Not all

Elastic rebound theory

• Crustal blocks move through time
• “locked” along the fault: doesn’t slip
• Rocks are strained: deformation of shape
• The strain can be released catastrophically
• Energy released causes earthquake
• Analogy: breaking rubber band

Terms

• Slip: the distance of displacement along the fault
• Focus: the location where the earthquake rupture began
• Epicenter: the location on the surface of the earth directly above the focus
• Seismic waves: vibration from released energy

Measuring earthquakes

• Seismograph: meter to measure ground displacement

– Pen loosely attached to earth

• Suspended from spring – vertical motion
• Attached to hinge – horizontal motion
• Seismograph measure ground displacement
• Ground displacement from seismic waves

Seismic waves

• Types of waves:

– P waves: fastest traveling, through earth
– S waves: intermediate speed, through earth
– Surface waves: slowest waves, across surface of earth

P waves

• Compressional waves

– Similar to sound waves
– “Push-pull waves”
– Series of contractions and relaxations
– Fastest, ~5 km/sec (depends on rock type)
– Travel through solid, liquid and gas

S waves

• Shear waves

– Material motion is right angles to direction of wave
– Half speed of P waves
– Travel only through solid

Surface waves

• Similar to water waves

– Require free surface
– Speed slightly less than S waves
– Two types

• Circular motion
• Shearing motion

Locating Epicenter

• Example: Distance to lightning bolt

– Difference between flash and sound
– Light travels faster than sound, arrives first

• Earthquake

– Difference between arrival time of P and S waves
– Can calculate distance to epicenter
– Still don’t know location

• Location

– Derived by triangulation using several seismographs
– Also can determine the time the earthquake occurred

Magnitude

• Seismograph measure ground displacement
• Ground displacement from seismic waves
• Very sensitive

– UF seismograph station records touchdowns

• Richter magnitude

– Basis is amplitude of ground motion
– Scale is logarithmic, magnitude 3 is 10 times > magnitude 4
– Energy increases by factor of 33 for each magnitude of amplitude

 

• Moment magnitude

– Measure of what occurs at focus
– Depends on several factors

• Slip
• Area of fault break
• Strength of rock

– Produce similar numbers to Richter magnitude, but based on actual fault break

Fault mechanisms

• Important to know type of fault
• Occasionally possible to observe fault in field
• Faults for deep focus earthquakes don’t come to surface
 
 

• Can deduce motion on basis of “first motion studies”
• Determine direction of motion at seismograph after earthquake

– Some motion is push (upward)
– Some motion is pull (downward)
 

• Plot the first motion on maps

– Arrangement always into four quadrants
– The arrangement of pushes and pulls determines fault motion
– One of the boundaries is the fault plane
– Need additional information to determine which one

• First motion studies & world-wide seismograph network

– Created for nuclear test ban treaty
– First motion studies of nuclear explosion

Earthquakes and plate tectonics

• Most earthquakes occur in narrow belts around earth
• Correspond to plate boundaries
• Type of earthquakes correspond to plate boundaries
 

• Divergent boundaries

– Shallow focus
– Normal faults at ridge crests
– Strike parallel to ridge orientation
– Strike slip earthquakes along transform boundaries

• Convergent boundaries

– Where most deep focus earthquakes occur
– Also are some shallow focus
– Deep focus epicenters farther inland from trench than shallow focus
– Most faults are thrust faults

• Within plates

– Small number occur in areas away from plate boundaries
– Commonly found along “fossil” plate boundaries

• E.g. east coast U.S.
• Mississippi rift

Destruction

• Level of destruction depend on:

– Location relative to populated areas
– Building construction

• Well engineered buildings more durable
• Requires detail information of geology
• Example: Northridge earthquake (6.9), $20 billion dollars

– Thrust fault

• Other types of destruction:

– Fires (e.g. 1906 great fire of San Fransisco)
– Mass wasting

• Liquifaction (Marina district in San Fransisco)

– Tsunami (“tidal waves”)

Mitigating destruction

• Preparation

– Determine seismic potential
– Alter construction practices (added initial expense, justified??)

• Bolt house to foundation
• Gas lines flexible
• Attach shelves to walls
• Rails to hold in books
• Move beds from windows
 

• Early warning indicators

– Tilting of ground
– Aseismic slip
– Additional strain
– Changes in physical properties (conductivity, P-wave velocity)
– Changes in water levels
– Small earthquake swarms

• Prediction

– Average recurrence interval
– “Date” earthquakes by observing fault offsets
– Calculate rate of deformation
– Calculate amount of energy released (strain release) from earthquake

• Real time warnings

– Radio waves travel faster than seismic waves
– Possible to build system to give 50 to 80 second warning of approaching seismic waves
– Only work for earthquakes far from cities (e.g. hundreds of kilometers