PART 2 EARTHQUAKES Flashcards
3 methods of assessing earthquakes
- long-term forecasting
- short-term prediction
- other methods
Forecasting based mainly on the knowledge of when and where earthquakes occurred in the past.
Long-term forecasting
T or F: In seismically active areas, small earthquakes are more likely to occur as the amount of time increases since the last major event.
False, LARGE earthquakes are more likely to occur as the amount of time increases since the last major event
two important aspects in long-term forecasting
- paleoseismology
- seismic gaps
study of prehistoric earthquakes
paleoseismology
what does paleoseismology involve
seismologists will study sections of fault zones, particularly across/perpendicular to fault zone (offsets)
the study of offsets in sedimentary layers near fault zones to determine recurrence intervals of major earthquakes prior to historical records
they look into materials that can be dated: carbon, coal seams, peat lenses/layers
a zone along a tectonically active area where no earthquakes have occurred recently, but it is known that elastic strain is building in the rocks.
seismic gap
what happens if a seismic gap can be identified?
then it might be an area expected to have a large earthquake in the near future
examples of areas with seismic gaps
- Alaskan subduction zone
- Acapulco Trench in Mexico (guerrero gap)
- San Francisco and Parkfield Seismic Gaps along San Andreas Fault
involves monitoring of processes that occur in the vicinity of earthquake prone faults for activity that signify a coming earthquake.
short-term prediction
are predictions usually successful?
much of them fail
Anomalous events or processes that may precede an earthquake are called ?
precursor events
what do precursor earthquakes signal?
a coming earthquake
why has short-term earthquake prediction been difficult to successfully obtain
- the processes that cause earthquakes occur deep beneath the surface and are difficult to monitor.
- earthquakes in different regions or along different faults all behave differently, thus no consistent patterns have so far been recognized
what are the 6 earthquake precursors
- increase in foreshocks
- slight swelling/uplift or tilting of the ground surface
- decreased electrical resistance
- fluctuating water levels in wells
- increased concentration of radon gas in groundwater
- generation of radio signals
*also the behavior of animals
example of how erratic behavior of animals preceded and earthquake
1976 Tangshan 7.4 Mag Tangshan Earthquake in China.
- snakes didn’t wanna go in their holes
- pandas were screaming
- fawns refused to go near water
*first systematic study, but could not recreate, succeeding studies were unsuccessful
Causes microcracks to form prior to complete rupture, or main shock.
increase in foreshocks
microcracks increasing the rock volume
Slight swelling/uplift or tilting of the ground surface
water entering new void spaces that is more conductive than surrounding minerals.
Decreased electrical resistance
water entering new cracks causes water
levels to lower; levels rise when voids close again.
Fluctuating water levels in wells
new cracks allowing the gas, a radioactive decay product of uranium, to escape from rocks and enter wells.
Increased concentration of radon gas in groundwater
caused by changes in rock strain or movement of saline groundwater.
Generation of radio signals
6 differences between conventional and non conventional geophysical survey
intrusive, sensors, source, cost, scope, time
ISSCST
3 examples of non-conventional/non-intrusive methods
- microtremor survey method
- refraction microtremor survey
- horizontal-to-vertical spectral ratio method
