Lecture 9 earthquakes 3 Flashcards
Can we predict earthquakes?
no - instead probabilistic seismic hazard analysis and mitigation, protection and education strategies based on this probability analysis.
How is earthquake monitoring conducted?
- Seismometers - can be short period, long period, both on and off shore
- Strong motion accelerometers
- Tilt and strain meters
- Geodetic survey, including GPS
- Satellite radar interferometry (InSAR)
- Sea floor hydrophone arrays
- Ocean floor tsunami (pressure) gauges.
They are also used to test evidence for earthquake precursors.
Give some examples of earthquake precursors
- Decrease in seismic velocity
- Deformation: uplift/subsidence, tilt,
horizontal strain - Geomagnetic field variations
- Geo-electromagnetic variations
- Level of seismic activity - foreshocks
- Groundwater flow or level changes
- Radon gas emission
- Animal behaviour
What is the role of fluids in earthquake cycle
- Pore fluid in the brittle crust tends to reduce the fracture strength
of rocks and stress needed to make them break - Effective pressure/stress = lithostatic pressure - pore fluid pressure.
- Increasing fluid pressure can trigger earthquakes.
- Man-made examples: filling of reservoirs behind dams (also a function of the loading); water injection into boreholes.
- Conversely, in the build-up to an earthquake, as stress increases many small fractures form in the rock body
- These create additional space for the pore water, so the pore water pressure may DROP before an earthquake happens.
- Fluids also produce cement sealing a fault - may reduce probability of rupture
- development of a fault zone strongly influenced by fluids.
Give an example of how fluids may cause earthquakes?
Aswan High Dam:
111 m high; 3.6 km wide.
Impounds Lake Nasser (300 km
long; max. water level
= 178 m).
Filling started in 1964; maximum
in 1978.
ML = 5.6 EQ in 1981 under large
bay; preceded by foreshocks.
No damage to dam.
Area had been considered
aseismic; no evidence of shaking
for 3000 years.
Nubian sandstones are highly
porous, so water seepage into
surrounding rock plus load of
water exerts pressure on
underlying faults.
Are earthquake predictions reliable
one example of successful earthquake prediction, i.e., forecasting on a time scale suitable for evacuation: * Yingkow, China, 1975
- many times where they have not predicted correctly.
How do you determine earthquake potential
Instrumented records (e.g., seismology, GPS)
* Historic records
* “Paleoseismology” - geological earthquake records
* Sedimentary records, e.g., of landslides, tsunamis, earthquake secondary effects
* Predictive probability based on past patterns of frequency, magnitude, location
* Other important factors being assessed: source characteristics, parameters at destination/region.
What is probabilistic Seismic Hazard Assessment
PSHA aims to determine the likelihood/probability that a given
locality will experience earthquake-induced shaking leading to
some value of seismic intensity or ground acceleration, over a
defined time interval into the future.
what are the inputs and outputs of probabilistic seismic hazard assessment.
Inputs:
* Regional scale tectonic setting
* Current/recent/historical/geological seismicity: magnitude, location, frequency
* Historical seismic intensity levels (isoseismic maps of previous earthquakes) and ground shaking/acceleration
* Any known periodicity or pattern
* Locations of known faults, fault length, fault slip rates and paleoearthquake frequency and slip per event (paleoseismology - geological earthquake history)
* ‘Seismic gaps’ - recorded and geological records
* Conditions affecting seismic wave amplitudes: e.g., soil amplification, liquefaction
Output: earthquake hazard zoning
What are the forecasts of earthquakes?
Forecast might take the form of: “Accelerations exceeding 0.25 g
have a 50% probability of occurring here within 30 years”.
* A global map was issued by the Global Seismic Hazard Assessment Program (GSHAP) in December 1999.
* This was a major international achievement of the UN’s International Decade for Disaster Reduction, 1990-2000.
* It shows the peak ground acceleration expected with a 10% probability of being exceeded in 50 years (equivalent to return
period of 475 years).
* ‘High’ seismic hazard corresponds to 10% or greater probability of violent ground shaking, defined as 25% of acceleration due to gravity (0.25g), within the next 50 years.
What is a seismic hazard map?
- Data compiled from earthquake catalogues, potential earthquake sources, ground motion records and prediction and assessment of uncertainty
- 8% of the Earth’s land mass falls within the high or very high seismic hazard range on the map (> 2.4 m/s2 acceleration, >0.2-0.3g)
- About 70% lies in a low hazard range
- Many of the world’s megacities lie within bands of high or very high hazard (e.g., Caracas, Jakarta, Karachi, Kathmandu, Istanbul, Manila, Tehran, Tokyo, Mexico
City).
What is seismic hazard referring to?
Remember that seismic hazard refers only to the probability of ground shaking of a particular intensity or acceleration.
What is seismic risk referring to?
Risk deals with the probability of damage to people and buildings – hazard is only part of the assessment. This introduces engineering, presence of people and their behaviour.
* Risk = Hazard x Vulnerability
what is seismic vulnerability referring to?
Full assessment by authorities also needs to include ‘vulnerability’ (looks at criticality of the risk) plus includes the effects and benefits of “mitigation” relative to cost.
This corresponds to an assessment of the
loss of life, injury and damage likely to be
sustained by a specific structure, area or
community if exposed to a given level of
ground shaking.
* This needs to be combined with the
hazard assessment.
* Risk = Hazard x Vulnerability
give examples of secondary hazards
Secondary hazards affect net hazard and risk (vulnerability)
* Ground conditions
* Bedrock
* Soil/soft sediment amplification
* Basin wave focussing and ground
shaking amplification
* Liquefaction
* Mudslides, landslides, avalanches
* Tsunamis, seiches
