Seismic waves and measurement
Mann Whitney U Test
1) Introduction to Earthquakes
Seismicity = earth shaking
Connects with plate tectonics theory, as only 5% of earthquakes are not found along plate margins.
Most earthquakes are small, so not of concern to people. They become a hazard when there is a large earthquake close to a population centre.
Using GIS to Investigate the Earthquake Hazard
Open up this link – to a map in ArcGIS: http:// rhsb.maps.arcgis.com/home/webmap/viewer.html?webmap=deed48a894b94f
Geographical Information Systems (GIS) = a computer system that allows different types of geographical data to be linked to a location and displayed in an easily understandable form.
Databases (e.g. earthquake data, cities) are linked to base maps. The user can then decide which layer of information to view, to help determine spatial patterns and relationships.
See GSkills pg. 61 & view this 2½ min YouTube clip about GIS: https:// www.youtube.com/watch?v=kEaMzPo1Q7Q A hazard is natural or human-made event that adversely affects human life, property or activity. A hazard involves people.
Using the GIS map, investigate the following:
a) Which parts of the world have experienced most earthquakes during the last month. b)Which parts of the world have experienced the earthquakes with greatest magnitude during the last month.
c) Areas of the world experiencing greatest hazards from earthquakes during the last month.
See pg. 36 A2Geog
http://web.gccaz.edu/~lnewman/gph111/topic_units/Fold_fault_eq/F old_fault_eq2.html Intermediate focus earthquakes
The foci of earthquakes are found at different depths below the ground surface. There are three levels: - Shallow at 0-70 km below - Intermediate 70-300 km - Deep foci at 300-700 km Shallow earthquakes create the most damage and 3/4 of total energy released of earthquakes in the world. They, unlike intermediate and deep focus earthquakes, occur at all type of plate boundaries. Seismic Waves
P (Primary/Pressure) waves
S (Secondary/ Shear) waves
L (Love / Long) waves
See A2 Geog pg. 36
https://www. youtube.com/ watch?v=gl4F vHKzAlU 10 sec animation http://misswise.weebly.com/the-structure-of-the-earth-and-seismic-waves.ht ml We can find out a lot about the structure of the earth from earthquakes. An earthquake produces three types of wave: S waves, transverse waves that can only travel through solids, P waves, longitudinal waves which can travel through liquids, and surface waves that travel around the crust.
From the arrival times of the P and S waves at detectors in various parts of the world, we can find out:
1) How far away the earthquake started
2) That the mantle is very dense because S waves can travel through it.
3) That the outer core is liquid because S waves cannot travel through it.
4) That the density of Earth increases gradually as you head towards the centre because the seismic waves are refracted (bent) through it.
2) Measuring Earthquakes – Magnitude & Frequency
Earthquake magnitude can be measured via …
The Richter scale rates the magnitude of an earthquake -- the amount of energy it released.
This is calculated using information gathered by a seismograph.
The Richter scale is logarithmic, meaning that whole-number jumps indicate a tenfold increase. In this case, the increase is in wave amplitude.
That is, the wave amplitude in a level 6 earthquake is 10 times greater than in a level 5 earthquake, and the amplitude increases 100 times between a level 7 earthquake and a level 9 earthquake. The amount of energy released increases 31.7 times between whole numbers.
See AQAG2 pg. 23
The intensity of an earthquake is measured using the …
…Modified Mercalli Scale.
The effect of an earthquake on the Earth's surface is called the intensity. The intensity scale consists of a series of certain key responses such as