Earthquakes - METHODS OF ASSESSING EARTHQUAKES Flashcards
three methods of assessing earthquakes
long-term forecasting
short-term prediction
other methods (gephysical)
based mainly on the knowledge of when and where earthquakes have occurred in the past
long-term forecasting
long term forecasting are based on ______ and _____
historical records and geologic evidences
t or f: in seismically active areas, large earthquakes are more likely to occur as the amount of time increases since the last major event
T
two important aspects of long-term forecasting
paleoseismology
seismic gaps
study of prehistoric earthquakes
paleoseismology
what is paleoseismology
study of prehistoric earthquakes by studying offsets in sedimentary layers near fault zones and looking into materials that can be dated
materials that can be dated in paleoseismology
coal seams, peat
paleoseismology determines
recurrence intervals of major earthquakes prior to historical records
paleoseismology is done thru:
trenching
depth of trenching:
width of trenching:
2m;
depends on the fault
formation of sand blows is linked to
liquefaction
sand and water that come out onto the ground surface during an earthquake as a result of liquefaction
sand blows
what type of dating are used in sand blows to determine earthquake event?
radiocarbon dating
a zone along a tectonically active area where no earthquakes have occurred recently but is said to be still accumulating strain
seismic gaps
it was known as a gap before the september 15, 1985 earthquake
Former Michoacan Gap
two gaps along the cocos plate and acapuclo trench
Former Michoacan Gap and Guerrero gap
involves monitoring of processes that occur in the vicinity of earthquake prone faults that may signify a coming earthquake
short-term prediction
anomalous events or processes that may precede an earthquake
precursor events
successful short term prediction have been difficult to obtain because:
earthquakes are generated deep beneath the surface and are difficult to monitor
t or f: no consistent patterns for short-term prediction have been recognized because earthquakes in different region or faults all behave differently
T
earthquakes precursors
- increased foreshocks
- slight swelling/uplift or tilting of ground surface
- decreases electrical resistance
- fluctuating water level in wells
- increased radon concentration in groundwater
- generation of radio signals
these form prior to complete rupture
microcracks
t or f: microcracks decrease the volume of rocks
F (increase)
water entering void spaces results in a _____ in electrical resistance
decrease
water entering new cracks causes water in well to ______; and will ______ when voids are closed again
lower ; rise again
t or f: radon is an inert gas, meaning it is non-reactive
T
radon is a radioactive decay product of ____
uranium
radon escape from ____ and enter ____-
rocks ; wells
radio signals are caused by
changes in rock strain;
movement of saline groundwater
advantages of non-conventional / alternative geophysical surveys
non-intrusive;
sensors are planted on the ground;
utilizes passive sources;
cost-effective
cover wide area and up to 3km deep
fast and efficient, 4 hrs max
examples of passive sources
ambient noise, microtremors
three types of non-conventional methods
microtremor survey method
refraction microtremor survey
horizontal-to-vertical spectral ratio method
microtremor survey method uses
7 portable seismometers
each instrument in microtremor survey method is equipped with ______ for _____ and _______
Global Positioning System ;
time synchronization and location coordinates
microtremor surveys records
microtremors for a few minutes
a hammer striking a steel plate is used as a seismic source
refraction microtremor survey
in refraction microtremor survey, a series of _____ are planted on the ground and connected to _______
geophones ; seismograph
in refraction microtremor surveys, ________ were measured and analyzed
propagating waves
uses the same instrument as microtremor survey method but only a single station
horizontal-to-vertical spectral ratio method
what does horizontal-to-vertical spectral ratio method record?
ground period of an area
recording time required for horizontal-to-vertical spectral ratio method
20 mins at most
reducing earthquakes risk (4)
- structural engineering
- early warning systems
- planning and education
- earthquake control?
seismic engineering provide greater structural strength with respect to the ____ generated by ______ and structure’s own ________
shear forces ; lateral ground motion ; inertia
seismic engineering methods
shear walls
cross bracing
base isolation
steel jacket
steel jacket wrapped spirally
a somewhat expensive but viable option to make existing buildings resistant to lateral movement is to _____
retrofit existing buildings with seismic controls
the basic idea behind an early warning system is to take advantage of the ______ and the fact that
time lag ; P waves do very little damage
the first ______ is simply used as an alert that a highly destructive _____ and ______ will follow soon
P-waves ; S waves ; surface waves
examples of early warning systems
close valves on gas lines
shut down electrical powers
trains stopped
planning and education for earthquake includes:
hazard assessment
development of building codes
informing people on what to do before, during and after
regular earthquake drills
examples of human induced eartquakes
- hoover dam blocked colorado river generated 600 earthquakes , one with mag 5 and 2 with mag 4
- injection of toxic waste in disposal wells, generated earthquake 4-8 km below 3.8 km well
- underground nuclear testing generated 6.3 magnitude earthquake, aftershock was mag 5
