EQ Hazard and Mitigation Flashcards
EQ precursors
Decrease in seismic velocity Deformation Geomagnetic field varitations Foreshocks Groundwater flow Radon Gas emissions Animal behaviour
pore fluid in the brittle crust
reduces fracture strength of rocks and stress needed to break them
Effective pressure/stress
= lithostatic pressure - pre fluid pressure
Man made fluid changes
filling reservoirs, water into boreholes
Stress increase in EQ build up
small fractures, more space for pore water, pore water pressure may drop before an EQ
Fluid examples
Aswan high dam, egypt. sandstones
Probabilistic Seismic Hazard Assessment
regional tectonics, current seismic info, intensity levels, patterns, location of know faults and slips,
OUTPUT: EQ hazard zoning
Accelarartions exceeding 0.25g have a 50% prob of occurring here in 30 years
Eg of PSHA
Global seismic hazard map
from earthquake catalogues, potential EQ sources, 8% of earth in high seismic range
70% low hazard
Seismic hazard in california
probability of ground shaking exceeding 0.2g in next 50 years
Hazard
Prob of ground shaking of a particular intensity or acceleration
Risk
probability of damage to people and buildings
Vulnerability
criticality of risk, effects and benefits of mitigation
Risk =
Hazard x Vuln
Secondary hazards
Ground conditions
Liquefaction
Mudlsides, landslides,
Tsunamis
Anthropogenic factors
Building construction, pop density, dams/nuclear etc, communcation roads e.g. bridges, education and preparation, gas/fire potential
Soft sediments vs. bedrock
amplicates seismic waves in soft low density sediments
Focussing of seismic waves by refraction; amplitude increases
Liquefaction
soft sediments with high water content:
lose bong, sediments beome saturated and lose strength
reduced ground stability
Engineering buildings
minimal cost protection, withstand shaking
cross braced
flexibility
resonant freq in materials
masonry
add steel
future EQ risk
1/3 of 27 big cites near plate boundaries
high likelihood of catastrophic fatalities
