MOTHER NATURE’S RUMBLINGS EARTHQUAKES MOTHER NATURE’S RUMBLINGS

What is that force? Earthquake – a vibration of the solid earth produced by the very rapid release of energy The energy that drives these quakes is derived from the earth’s interior The motions of the earth’s plates are frequently the cause of this rapid energy release

Why bother in the first place? Why do we study these earthquakes? Pure scientific curiosity Try and learn how to predict them Gives us a view of the earth’s interior So we can try and avoid tragedies like the one shown here In 1985, an earthquake measuring over 8.0 struck outside of Mexico City An estimated 25,000 people were killed in this tragedy

Where in the world are they? This map show world earthquake distribution What familiar pattern do these earthquake locations line up with? That’s right, the tectonic plates!

Where do they happen? Focus – The point of origin of an Earthquake The earth’s lithosphere ruptures, and energy radiates out in all directions Most earthquakes do not occur directly at the surface Epicenter – a location on the surface directly above the focus A diagram showing a fault with an earthquake focus occurring at depth with the epicenter directly above it

How deep? Although it’s possible, most earthquakes do not happen right at the earth’s surface The depth that they occur, depends upon the plate boundary Convergent boundaries produce the largest quakes and also the deepest ones—600 km depth is maximum Transform boundary quakes can also be large but are generally shallow—less than 80 km below the surface Divergent boundaries produce weaker, shallower quakes as well This map and cross section of the Japanese Island Arc shows the range of earthquake depths found at convergent subduction zones

Waves on the Water Just as waves radiate out from a stone being dropped in water, so will earthquake waves radiate out in all directions EQ waves will radiate out from the focus in 3-D Energy dissipates very rapidly through solid rock, but sensitive instruments can pick up, that is “listen for” seismic events all around the globe

Whose fault is it? Most earthquakes occur along faults or fault zones Fault – a fracture in the earth’s crust along which there is movement Some are a few km in length, others 1000s of km long Faults come in all varieties of angles, depths, and shapes

Earthquake Processes No one knew how earthquakes worked until early in the 20th century H. F. Reid studied the 1906 San Fran quake and came up with a theory called elastic rebound Just like a stick under pressure, rocks will bend to a certain point and will then rupture and release stored energy with violent shaking

Understand the Physics Elastic rebound can be explained once the idea of stress and strain is understood Stress = pressure Strain = deformation Rocks in the crust are under incredible amounts of pressure and will slowly deform Once the rocks’ strength is exceeded they will rupture causing vibrations There are different kinds of stress as demonstrated by the above diagram

The Theory Behind the Shake Elastic Rebound – sudden released of stored strain in rocks The result of this rebound is sudden movement along a fault The rupture of this fault trace in California was formed due to elastic rebound The strength of the rocks could only hold so much stress and then they gave way

What comes before and after? Foreshocks – small earthquakes preceding a major quake by days or even possibly years Aftershocks – the settling or adjustment of crustal rocks after a large earthquake

Seismology The entire branch of geology dedicated to studying earthquakes and earthquake waves is called seismology Seismographs – the instruments used to measure and record seismic waves Almost 2000 years ago the Chinese made the very first seismograph The original seismograph was invented by the Chinese 2000 years ago Metal spheres were delicately balanced in mouths of dragons, and the slightest vibrations would cause them to fall out

Modern Seismograph Modern seismographs incorporate the principle of inertia Inertia – an object at rest will stay at rest, & an object in motion will stay in motion Seismographs are placed at depth away from background noise Always in a group of three to measure all three dimensions of movement (3-D) A modern seismograph has a pen attached to a weight on a spring; the ground shakes the rotating drum while the pen remains stationary due to inertia and traces out the quake

Look at the Data When seismographs record earthquake activity onto paper it is called a seismogram Top: simple seismogram showing the arrival of different P & S waves Bottom: a long track seismogram showing a major earthquake accompanied by both a foreshock and an aftershock

To Review: Earthquakes are the result of a rapid release of stored energy from within the crust The focus is where the EQ occurs and the epicenter is on the surface directly above the focus EQ happen along faults which are long cracks in the earth’s crust Earthquake energy radiates out in all directions like ripples on a pond H. F. Reid came up with theory of elastic rebound when studying the great 1906 quake of San Fran This theory is used to explain EQ even today Stress (pressure) causes strain (deformation) Seismograph is the instrument used to measure EQ and it operates on the law of inertia Seismograms are recorded earthquakes on paper