GEOL 108Lg 1st Edition Lecture 3Outline of Last LectureI. Mid Ocean RidgesII. Convection CurrentsIII. Rayleigh Number and its MeaningIV. Mantle PlumesV. Plate Motiona. Plate Boundaries/TypesOutline Current Lecture I. FaultsII. Plate BoundariesIII. Rupture ProcessIV. Earthquake TerminologyV. Foreshocks and AftershocksVI. Seismic Cycle: Reid’s Elastic Rebound TheoryVII. Stick-Slip CycleCurrent LectureEarthquakes The likelihood of a building feeling an earthquake is left up to the distance between building and a fault. SoCal has a very complex system of faults, with the plate boundary of the San Andreas slicing the state as the Pacific Plate and the North American plate move against oneanother. The mountains caused by the activity along the San Andreas—the ground shaking will be much larger if a structure is sitting on sand rather than on mountainous bedrock. The Newport-Inglewood fault bumped into the Santa Monica fault.Large Earthquakes San Fernando (1971): Magnitude 6.7Landers (1992): Magnitude 7.3Northridge (1994): Magnitude 6.9; occurred on a transform fault.The length of the faults will tell you how large an earthquake is possible—the faults, however, link up so if an earthquake jumps from fault to fault it can cause a much larger earthquake to occur. Another issue is that we are not aware of the existence of all possible faults, so earthquake jumping can occur much more frequently without us knowing.Earthquakes are also California’s costliest disasters. FaultsFault: a fracture in the earth’s crust that is the locus of relative ground movement or rupture (rupture: two sides of the earth moving against one another)Faults come at ALL scales: from mm to the separation of tectonic plates.Earthquakes: movement of rock bodies past each other along a fault, and the shaking that happens as a result of this movement. Earthquakes are the violent expression of faults. Earth’s crust is put under stress (stress=force/area)—the plates move as a result of mantle convection.Plate BoundariesEach type of plate boundary is associated with one of the three following types of faulting. Normal Fault are found at divergent boundaries and is associated with crustal extension.Thrust Fault, or Reverse Fault, is associated with crustal shortening. It can be found at convergent boundaries.Strike-slip faults are associated with the lateral motion of the Earth’s crust and can thus be found at transform boundaries. Rupture ProcessRupture Propagation:At around 20 seconds - rupture arrests; rupture has progressed along the entire length of the fault; the earthquake stops.Important Earthquake TerminologyFocus or hypocenter : point on the faults where rupture initiates, then causing an earthquakeEpicenter : point on surface directly above the focus.Fault rupture : the sudden displacement of rocks on either side of a fault that begins at the focusand spreads out across the fault.Fault trace : the line of of intersection of the fault plane with the earth’s surface.Foreshocks and AftershocksForeshocks are earthquakes that precede other larger earthquakes in the same location. Aftershocks are smaller earthquakes that occur in the same general area during the days or even years following a larger event, usually defined as within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. Aftershocks occur on and around the edge of the rupture area.Seismic Cycle: Reid’s elastic rebound theory Reid hypothesized that the forces that cause an earthquake are not near the earthquake but very far away. Rocks deform elastically, then rebounds during an earthquake rupture. Stick Slip CycleStick-slip displacement on a fault radiates energy in the form of seismic waves, creating an earthquake. What this refers to the fast movement that happens between two sides of a fault when the two sides of the fault become “unstuck”. The rock becomes distortedbut holds its original position until the earthquake occurs. When the rock snaps back into an unstrained position it is called elastic rebound.This diagram illustrates a simple formula: When the force trying to make the block (m) slip becomes greater than the frictional force causing it to stick, the block will