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
This method uses seven (7) portable seismometers that will record microtremors for a few minutes
microtremor survey method
what are each instrument for microtremor survey method equipped with?
GPS for time synchronization and
location coordinates
full GPS
Global Positioning System
describe how refraction microtremor method works
- A series of geophones planted on the ground connected to a seismograph
- A hammer striking a steel plate is used as the seismic source
- Propagating waves are measured and analyzed
Uses the same instrument as the ones used in microtremor array method
horizontal-to-vertical spectral ratio method
do you need to set up horizontal-to-vertical spectral ratio method as an array?
no, only single station
describe the recording of horizontal-to-vertical spectral ratio method
Records fundamental ground period of an area
Only requires a recording time of 20 minutes at most
4 ways to reduce earthquake risks
- seismic engineering
- early warning systems
- planning and education
- earthquake control?
5+1 specific example of seismic engineering
Addition of cross-bracing and shear walls, base isolation, wrapping of columns with a steel jacket, and spiral wrapping technique on vertical reinforcing rods
dampers
benefits of seismic engineering
➮provide greater structural strength with respect to the shear forces generated by lateral ground motion and a structure’s own inertia; and
➮reduce the actual amount of shear force that can develop on the structure.
instead of demolishing old buildings with outdated designs or without any seismic controls, what can be done? examples?
A somewhat expensive, but viable option is to retrofit existing buildings with seismic controls
ie. exoskeleton outside building, weights on roof as pendulum to accommodate lateral movement
The basic idea behind this is to take advantage of this time lag and the fact that P-waves do very little damage. The first P-wave then is simply used as an alert that the highly destructive S-waves and surface waves will soon follow.
Early Warning Systems
3 specific examples of early warning systems
- Only seconds are needed for preprogrammed systems to close valves on gas lines, thereby reducing the risk of uncontrolled fires.
- Trains can be programmed to automatically stop.
- Electric utilities can also shut down critical control systems on electrical grids and at power plants.
the first step in planning and education as a way to reduce earthquake risks
➮The first step is to determine the level of severity of risk in a given area
➮by conducting hazard assessment
➮and subsequent construction of hazard maps.
based on hazard assessments, what do government agencies do to mitigate earthquake hazards?
➮government agencies will develop building codes
➮that require appropriate levels of seismic engineering in buildings and other structures
in terms of education what can be done to reduce earthquake risk?
➮Raising of awareness on what to do before, during, and after an earthquake on all levels of society, from school-aged children up to emergency management
officials.
➮Regular earthquake drills
examples of how humans can induce earthquakes
- fluids can change affect the energy distribution of rocks (dam construction, injecting toxic wastes into wells, injection of water in fault zones)
- nuclear testing (explosions causing earthquakes) and blasting in mine sites
