Class Notes (809,047)
Geography (1,279)
Lecture 7

# Lecture 7 Earthquakes.doc

8 Pages
136 Views

School
Western University
Department
Geography
Course
Geography 2152F/G
Professor
Mark Moscicki
Semester
Fall

Description
Lecture 7 – Earthquakes • They result from the rupture of rocks along a fault. • Energy from an earthquake is released in the form of seismic waves. • They are mapped according to the epicentre; the focus is located directly below the epicentre. • They are measured by seismographs and compared by magnitude • Focus is below group epicenter on the surface • Energy emitted in all directions from the focus Earthquake Magnitude • The magnitude of an earthquake is expressed as a number to one decimal place. • This type of measurement was first developed by Richter in 1935 • The Richter Scale was a measure of the strength of a wave at a distance of 100 km from the epicentre. • Since then, more accurate methods have been developed and the Richter Scale is no longer in use The Moment Magnitude Scale • Today, earthquakes are measured using the Moment Magnitude scale (M). • The scale is determined by: o The area ruptured along a fault o The amount of movement along the fault o The elasticity (rigidness, how hard rock is in the crust) of the crust at the focus • Similar to the Richter Scale, it is a logarithmic scale. o Example: An M7 earthquake represents 10 times the amount of ground motion as an M6 earthquake. M3 to m5 is 100 times the intensity. (EXAM QUESTION) Magnitude and Frequency of Earthquakes • Except for very large earthquakes, the magnitude on the Moment Magnitude Scale is similar to the Richter Scale. • The strongest earthquake to ever occur is M9.5 in Chile in 1960. In canada, it is M8.1 in B.C. in 1949 • There are only a few M9+ earthquakes each century. Earthquake Intensity • The Modified Mercalli Intensity Scale is a qualitative (descriptive) scale based on damage to structures and the affect on people. • It is based on 12 categories • Maps are produced showing the differences in Modified Mercalli intensities over broad areas The Modified Mercalli Intensity Scale Earthquake Processes • Earthquakes are most common at or near plate boundaries. • Quakes can still happen at places in the middle of continents, away from boundaries. • Motion at plate boundaries is not usually smooth or constant. • Friction along plate boundaries exerts a force (stress) on the rocks, exerting strain or deformation • When the stress exceeds the strength of the rocks, there is a sudden movement along a fault. Fault Types • There are two basic types of geologic faults distinguished by the direction of the displacement of rock or sediment. • Strike-slip faults o Displacements are horizontal • Dip-slip faults o Displacements are vertical Strike-Slip Faults • The San Andreas Fault is the best example of this type. • Neither rock sinks or rises relative to the other one but move in opposite directions Dip-Slip Faults • There are three types: o Reverse faults, thrust faults, and normal faults • They are comprised of two walls on an incline defined by miners: o Footwall (where miners place their feet) o Hanging- wall (where miners placed their lanterns) o These are just names for each rock, so we can say which one rises or which falls • Reverse fault: hanging wall has moved up relative to the footwall inclined at an angle steeper than 45 degrees • Thrust Fault: similar to reverse fault except the angle is 45 degrees or less • Normal fault: the hanging wall has moved down relative to the footwall Fault Activity • In terms of activity, faults can fall into one of three categories: • Active - Movement during the past 11,600 years • Potentially Active - Movement during the past 2.6 million years • Inactive - No movement during the past 2.6 million years Tectonic Creep • Definition: The slow movement of rock or sediment along a fracture caused by stress • It is also referred to as fault creep. • This can damage roads and building foundations (i.e. movement of a few cm per decade). • Along these faults, periodic sudden displacements producing earthquakes can also occur Seismic Waves • Some seismic waves generated by fault rupture travel within the body of the Earth and others travel along the surface. • Body waves: • These include P waves and S waves o 1. P waves  They are also called primary or compressional waves  They move fast with a push-pull motion and can travel through solids or liquids  P waves are horiztontal o 2. S waves  They are also called secondary or shear waves.  They move more slowly, in an up- and-down motion and can only travel through solids • Surface Waves: o Definition: Seismic waves that form when P and S waves reach Earth’s surface and then move along it o These waves move more slowly than body waves. o They are responsible for damage near the epicentre Earthquake Shaking • Factors that determine the shaking people experience during an earthquake: - Magnitude - Distance to epicentre - Focal depth – how deep beneath surface did rupture occur - Direction of rupture – is fault running E/W or N/S - Local soil and rock types – hardness of rock - Local engineering and construction practices • Seismographs record the arrival of waves to a recording station. • Because P waves travel faster than S waves, they appear first on a seismogram. • Earthquake shaking decreases with distance from the epicentre Distance to the Epicentre • The difference between the arrival times of the first P and S waves at different locations determine the distance to the epicentre. • The distance to the epicentre is calculated at 3 different seismic stations • A circle with radius equal to that distance is drawn around the station Locating an Earthquake • The epicentre is located where the circles intersect; this process is called triangulation. Focal Depth • Seismic waves lose some of their energy before they reach the surface. • The greater the focal depth, the less intense the shaking at the surface • This loss of energy is referred to as attenuation Direction of Rupture • Earthquake energy is focused in the direction of rupture. • This is known as directivity and contributes to increased shaking • Radiated waves are sometimes stronger in one direction along the fault Local Soil and Rock Types • The local geology influences the amount of ground motion. • Dense homogeneous crust (bedrock) can transmit earthquake energy quickly • Seismic energy slows down in areas with heterogeneous, folded, faulted crust. • Implication: • Earthquakes in eastern North America are felt over larger areas than those in western North American Amplification • Definition: An increase in ground motion during an earthquake • P and S waves slow as they travel through alluvial (sediment that has been deposited by a river) sand, gravel, clay, soil, etc. • As the waves slow, some of their energy is transferred to surface waves • Amplification has historically enhanced damage in San Francisco area earthquakes Shake Maps • The combination of these effects results in widespread variation of the shaking felt in the vicinity of an earthquake. • Therefore, earthquakes that have the same magnitude may have much different impacts The Earthquake Cycle • Definition: A hypothesis that explains successive earthquakes on a fault • It is based on the idea that str
More Less

Related notes for Geography 2152F/G

OR

Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Join to view

OR

By registering, I agree to the Terms and Privacy Policies
Just a few more details

So we can recommend you notes for your school.