Chapter 3: Earthquakes Damaged farm structures in the Edgecumbe, New Zealand earthquake, 1987.
Videos • Earthquake Destruction • Earthquakes 101 • Why do Earthquakes Happen?
What is an earthquake? An earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath the surface of the Earth. In other words, the motion of the plates causes enormous stress on the rock. Like bending a stick, rock can only bend so far before it breaks. When the rock breaks, waves of energy are released through the Earth as an earthquake.
What are seismic waves? Seismic waves are waves of energyproduced by an earthquake. There are several different types of seismic waves. What is a fault? A fault is a break between two rock masses. Faults can be new/active (such as the San Andreas Fault) or old/inactive (such as the Aspy Fault in Northern Cape Breton)
What causes an earthquake? The classic explanation of what causes an earthquake is called the elastic rebound theory.Over many years, tectonic forces act on the rock, causing it to bend. Gradually, the bending becomes more and more severe. Eventually, the strain exceeds the breaking strength of the rock, and the rock breaks suddenly causing an earthquake. The two masses of rock move past each other along a fault. The rock may move horizontally, vertically or both. Recently, however, some scientists believe that faults are weak, and that only small forces are needed to cause an earthquake. Also, extremely deep earthquakes (100-670 km below the surface) may be caused by minerals changing from one form to another
Videos • Elastic Rebound Theory (USGS) • Breaking Stick
In general, where do earthquakes occur? Most earthquakes are associated with movement along obvious fault lines (ex. San Andreas Fault). But many earthquakes occur along very old fault lines (ex. Ontario & Quebec; offshore of CB) or buried fault lines (ex. Northridge quake outside of L.A. in 1994). Earthquakes are also associated with volcanic eruptions and plate boundaries (ex. Mid-Atlantic Ridge).
What is the difference between a focus and an epicentre? The focus is the point IN the Earth where seismic waves originate. This is the centre of the quake and this is where the first breakage occurs. The point ON the Earth’s surface directly above the focus is the epicentre.
What are the two types of seismic waves? • Body Waves: These travel through the BODY of the Earth (or, through the Earth’s interior), spreading out in all directions. This happens just like the exploding Death Star in Star Wars Episode I.
Surface Waves: Surface waves travel on the Earth’s surface away from the epicentre, like water waves when a pebble is thrown into a pond.
There are two types of body waves: • Primary wave (or P wave) • Compression wave or “pushing” wave in which the rock vibrates in the same direction. • Just like a sudden push on the end of a stretched slinky • Travels very fast (4-7 km/sec) INSIDE THE EARTH • Primary waves are the first (or primary) wave after a quake
Secondary Wave (or S-wave) • Back & forth (or shearing) kind of wave in which the rock vibrates perpendicular to the direction of the wave • Essentially, the rock vibrates back and forth just like shaking a rope • Secondary waves are slower (only 2-5 km/sec) and is the SECOND type of body wave that is formed
Surface Waves • Surface waves are the slowest waves that are set off by an earthquake • Surface waves cause the most damage because they take longer to pass • There are two types of surface waves: • Love Waves • Rayleigh Wave
Love Waves: • The ground moves side-to-side horizontally • Tend to knock buildings off their foundations and destroy highway overpasses
Rayleigh Waves: • Act just like ocean waves, moving up and down • Very destructive because they produce more ground movement and take longer to pass
How do we determine the location of an earthquake? P-waves are faster and always arrive at a seismograph before the slower S-waves. The farther the distance the seismograph is located from the epicentre, the bigger the delay between the first and second wave. This time interval can be used to calculate the distance to a quake. If three different seismograph stations are used, the exact location of the quake can be determined. Also, most quakes are shallow focus quakes, in which the focus of the quake is 0-70 km deep (85% of quakes are shallow).
Measuring the Size of an Earthquake The size of quakes is measured in two ways. One way is to measure the intensity of the damage (what kind of damage and how much damage). The scale that is used to measure intensity is called the Mercalli Instensity Scale (pg. 82). The second method measures the amount of energy released by the quake. The larger the quake, the more the ground vibrates and the larger the wiggle on a seismogram. Scientists can then assign a number called a magnitude. The scale that is used to measure an earthquake magnitude is called the Richter Scale.
Facts About the Richter Scale • A higher number on the Richter Scale indicates a more severe earthquake • Largest recorded quake was 8.6 on the Richter Scale • The Richter Scale is a logarithmic scale. This means that for each increase in the scale, there is actually a 10X increase in the amount of movement. Ex. If an Richter Scale 4 (or R4) quake moves the ground 1cm, an R5 quake would move the ground 10X more (or 10 cm). And, an R6 quake would move the ground 10X10 cm or 100cm; R7 would move the ground 10X10X10 or 1000cm; R8=10000cm; etc. • So, and R8 quake does NOT move the ground twice as much as an R4 quake. Because the Richter Scale is logarithmic, an R8 quake actually moves the ground 10,000 more than an R4 quake.
Energy from Earthquakes • In terms of energy, a 10X increase in ground movement causes a 32X increase in the amount of energy that is released from a quake. Look at this example:
Where do Earthquakes Occur on a Global Scale? • Pacific Rim (80% of the world’s shallow quakes) • Mediterranean-Himalayan Belt • Mid-Ocean Ridges • Some volcanoes (ex. Hawaii; Mount St. Helens)
What is a Benoiff Zone? When one ocean plate slides under the continental plate forming a trench, many earthquakes often occur. This zone of increased seismic activity is called the Benoiff Zone.
Earthquakes & Plate Boundaries Earthquakes at Divergent Boundaries: • Shallow quakes (not very deep) • Lower magnitude (not as severe) • Small area affected • Form rift valleys (like on Mid-Atlantic Ridge) • Example: Quakes along the Mid-Atl. Ridge
Earthquakes at Transform Boundaries: • Often shallow earthquakes • Can be single or many fault lines • Example: San Andreas Fault in California Earthquakes at Convergent Boundaries: • Wide zones of shallow earthquakes • Plates may buckle (during mountain building) or subduct (when trenches form), both of which create earthquakes • Example: Marianas Trench near the Philippines; Himalayas Mountains
Earthquakes Away from Plate Boundaries: • Very rare; represents only about 5% of quake energy released in a year • Also called intraplate earthquakes • Triggered by a build-up of stress between the crust and the upper mantle • Occur in Ottawa region, St. Lawrence River Valley and around the Grand Banks (off the coast of Louisbourg) • Earthquake probability in Southern Ontario in the next 50 years: M557% (occurred in 2010 & 2011) M66% M71% Note: Miriamichi, NB experienced two large quakes (M5.7 & M5.1) in 1982
Can we predict where earthquakes occur? There are many ways to detect slight changes in the rock and in the Earth’s surface: • Small cracks open up causing small tremors to increase • Changes in rock magnetism & other properties • Changing water levels in wells • Increase in radon gas emissions in wells • Changes in Old Faithful and other geysers • Surface of Earth tilts & changes in elevation • Animal behaviour (dogs howling) • Analyzing patterns of earthquakes (where they occur & how frequently) • Studying places along fault lines where quakes have not occurred for a long time
Tsunamis • Tsunami is Japanese for “big wave in harbour” • A tsunami is a large wave created by an earthquake, landslide, or meteorite impact. • Usually, tsunamis are caused by earthquakes greater than 6.5 on the Richter Scale
Videos • Tsunami Formation • Tsunami Animation • Alaskan Mega Tsunami • Japanese Tsunami 2011 • East Coast Tsunami