CEE 1030 1st Edition Lecture 11Earthquakes Northridge, 4:30am 17th Jan 1994- magnitude 6.7~ 6km, 1 min later, mag 5.9- $44 billion damages, 25,000 building uninhabitable, 900 injured, 57 deaths (very small for earthquake of this size in an urban area)  earthquake- sudden release of energy accumulated in deformed rocks: radiates as seismic waves- focus= site of initial rupture  seismic- greek for ‘shaking’  fault -locus of earthquake movement - all earthquakes cause surface rupture Elastic rebound- sudden brittle failure after slow build up of elastic strain, rocks bend and store elastic energy, when stress exceeds fault strength, strained rocks suddenly snap back to original shape  seismographs- instruments that record seismic waves by the movement of earth relative to a stationary mass. Rotating paper drum or digitally  types of seismic waves- primary and secondary waves  body waves- travel through earth’s interior  surface waves- travel over earth’s surface responsible for most earthquake damageBody waves primary (P) waves- compressional or push-pull waves. Fastest seismic waves (6 km/s)  secondary (S) waves- shear or shaking waves. Travel at half speed of P waves (3.5 km/s in crust) Surface Waves: love waves - side- to- side motion: like a wriggling snake Surface Waves: Rayleigh waves - up- and- down motion: like ocean waves These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. focus: place within earth where earthquake waves originate  epicenter- location on surface directly above focus  located by difference in arrival times of P & S waves Global network of seismic stations - established to monitor international nuclear bomb test ban treatiesDistribution of earthquakes - not random but focused around plate margins (but also seen in plate interiors) - most earthquakes are shallow - heat weakens rocks- less able to store strain energy Measuring size of earthquakes  intensity- measure of degree of earthquake shaking at a given locale based on damage  intensity- measure of the severity of ground shaking - influencing by local geology, building design and construction- good for assessing historical quakes before seismology  magnitude- estimates the amount of energy released at the source of the earthquake  richter scale- based on amplitude of large seismic wave recorded, adjusted for distance to epicenter  each unit of richter magnitude increase corresponds to a tenfold increase in wave amplitude and a 32-fold energy increase  moment magnitude- related to the physical properties of fault that caused the earthquake  better estimate of total energy released - amount of displacement on fault, area of rupture on fault, shear strength of rocks - seismograms and also field studies better for describing large earthquakes Earthquake damage - extent of structural damage attributable to earthquake vibrations depends on:  depth of earthquake intensity and duration of the vibrations nature of the material the structure rests on  design of the structure  quality of construction  liquefaction- unconsolidated materials saturated with water turn into a mobile fluid.  Tsunamis- destructive ocean waves triggered by movement of surface fault or landslide. Appear like a rapidly- rising tideCan earthquakes be predicted?- short-range predictions (days to weeks) monitoring precursors that might precede an earthquake: uplift, subsidence, strain, weird animal behavior… no reliable method exists for making short- range earthquake predictions- long-range forecasts (decades to centuries) based on premise that earthquakes are repetitive or cyclical. Continued motion of earth’s plate causes strain to build up again  historical records or