Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
History of geodesy wikipedia , lookup
Seismic inversion wikipedia , lookup
Magnetotellurics wikipedia , lookup
Post-glacial rebound wikipedia , lookup
Shear wave splitting wikipedia , lookup
Seismic communication wikipedia , lookup
Earthquake engineering wikipedia , lookup
Earthquakes ()الزالزل Roughly 1,000 damaging earthquakes occur each year What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy radiates in all directions from its source, the focus Energy moving outward from the focus of an earthquake travels in the form of seismic waves Seismographs record the event Earthquakes and faults Earthquakes are associated with faults Movement along a fault produces earthquake and waves Motion along faults can be explained by plate tectonics Most earthquakes result from movements along large fractures called faults The San Andreas fault zone separates two great sections of the crust of earth, the North American plate and the Pacific plate What causes the rock to beak? Stress- a force of directed pressure acting on a rock Types of stress: – Compressional – Tensional – Shear Strain-the result of stress of deformation – Elastic deformation- when stresses are removed, rock returns to original shape – Plastic deformation-permanent deformation, when stresses are removed, rock stays bent – Rupture-breakage and fracturing of the rock, causing an earthquake – Brittle materials break during elastic deformation Elastic rebound Mechanism for Earthquake's explained by H. Reid – Rocks on sides of fault are deformed by tectonic forces – Rocks bend and store elastic energy – Frictional resistance holding the rocks together is overcome by tectonic forces Elastic rebound Earthquake mechanism – Slips starts at the weakest point (the focus) occurs – Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound) Elastic rebound. As rock is deformed it bends, storing elastic energy. Once the rock is strained beyond its breaking point it rupture, releasing the store-up energy in the form of earth waves. Aftershocks and foreshocks The change in stress that follows a mainshock creates smaller earthquakes called aftershocks Foreshocks- small earthquakes may occur before a large one Red dots show location of aftershocks formed by 3 earthquakes in Missouri and Tennessee in 1811/1812 Strike Slip Fault Quake - Japan Strike Slip Fault Quake - California Normal Fault Quake - Nevada San Andreas: An active earthquake zone San Andreas is the most studied fault system in the world Displacement occurs along discrete segments 100 to 200 kilometers long Most segments slip every 100-200 years producing large earthquakes Some portions exhibit slow, gradual displacement known as fault creep Fence offset by the 1906 San Francisco earthquake Seismology Seismometers - instruments that record seismic waves Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape The first seismograph was developed by the Chinese almost 2000 years ago The study of earthquake waves, seismology, dates back to the Chinese almost 2000 years ago Modern seismographs, instruments that record seismic waves, are not unlike those used by the early Chinese A seismograph designed to record vertical ground motion Types of seismic waves Surface waves – Complex motion (up-and-down and side-toside) – Causes damage to structures during an earthquake – High amplitude and low velocity – Longest periods (interval between crests) – Termed long, or L waves – Slowest Surface waves Surface waves travel along the outer part of Earth Types of seismic waves Body waves – Travel through Earth’s interior – Two types based on mode of travel – Primary (P) waves Primary, Push-pull motion Travel through solids, liquids & gases Travel fastest of the seismic waves – Secondary (S) waves Secondary, shaking, shear, side-to-side Moves at right angles to their direction of travel Travels only through solids Body waves Body waves travel through the interior of Earth