DE (W5)

• stress is the local force, per unit area along a fault that causes rock deformation
• strain is the relative amount of deformation (% of distortion) a rock undergoes due to stress
• strength is a rock's ability to resist deformation or movement under stress
• strength fail point occurs when rock is stressed beyond a critical value resulting in the breaking into two or more parts
• fault slips are rock movement that releases stress
• elastic rebound theory is the gradual accumulation and release of stress and strain
• the distance of displacement caused by an earthquake is called fault slip
• energy in an earthquake is released as both friction heat and seismic waves
• the time between earthquakes is referred to as a recurrence interval
• the reoccurrence interval of earthquakes is complicated due to the fact that not all strain is released during every quake (rebound incomplete), the strength of fault rocks may change and stress may be effected by nearby quakes
• foreshocks are earthquakes that occur before the main shock. They are smaller than the main shock but are hard to distinguish from small quakes until after the main shock occurs
• aftershocks occurs after the main shock, with the largest generally being 1 magnitude smaller than the main, they can occur months to years after the main shock but decrease in quantity over time
• the focus point of an earthquake is the point at which slipping begins
• the focus point on continental crust ranges from a depth of 2-20km, with deep (20km) points being rare due to higher temperatures and pressures causing rock to act more ductile than brittle
• focus points at subduction zones can occur up to depths greater than 700km due to cold brittle oceanic crust being pushed deep into the lithosphere
• an epicentre is the point on the earths surface directly above the focus of an earthquake
• the focus point of an earthquake is located using p and s seismic waves. Travel-time curves are graphs used to determine time for each wave time to travel a set distance
• seismograph: an instrument that measures and records details of earthquakes, such as force and duration.
• through the use of 3 or more seismographs scientists are able to determine the location of the focus point
• primary (P) seismic waves travel at about 6km/s and are compressioal (travelling through solid, liquid and gas)
• secondary (S) seismic waves travel at about half the velocity of p waves (3km/s) and are shear (only passing through solids)
• surface waves cause rolling and shaking of the surface and have a velocity less than 3km/s
• the size of an earthquake is measured in magnitude
• GPS can be used to track the slow movement of the earth, this movement is known as silent earthquakes or creep, they occur over days to weeks and produce on seismic waves
• bigger earthquakes occur less frequency, decreasing by a facto of 10 for every increase in magnitude
• zone of deformation is the area of the rock that is being pulled apart by stress
• fault systems are interacting faults along a boundary, often there is one master fault and other subsidiary faults
• divergent plate boundaries produce shallow earthquakes and normal style faulting
• transform boundaries cause horizontal shearing, resulting in strike-slip faulting
• convergent boundaries have the largest and deepest earthquakes
• horizontal compression at convergent boundaries result in megathrusts
• intraplate earthquakes occur mostly on continents, they are shallow and small in size, occurring on old fault lines or are the result of human activity