Lesson 12-What is seismicity and how do we monitor seismic activity? Flashcards
Epicentre
- Point on the surface of the Earth directly above crust, where seismic waves are first felt
Focus
- Point within the earth crust where the earthquake originates
Seismic waves
- Waves of energy which travel through the crust caused by released by the release of pressure from an earthquake
Primary (P) waves
- Compressional (longitudinal) waves are where particles vibrate paralell to the direction of movement
- 6-7km/s
- Travel through solids, liquids and gases
Secondary (S) waves
- Transverse (similar to light and sea waves), particles), particles vibrate at right angles to the directiomn of the movement
- slower than 2.5 - 4km km/s, travel through solids
Surface (Love and Rayleigh)
- Travel at speeds similar to S waves
- Love waves have a horizontal motion that moves the surface from side to side perpendicular to the direction the wave is traveling. Of the two surface waves, Love waves move faster.
- Rayleigh waves cause the ground to shake in an elliptical pattern. This motion is similar to that observed in ocean waves.
Can we warn people about earthquakes?
- Waves that move the fastest (P waves) dont shake the ground but they are recorded on seismometres
- This can tell us an earthquake is about to happen- named the P-wave warning system
Can we predict earthquakes?
- We can predict earthquakes based on history of the plate boundary
- Can measure vibrations in earths crust (seismometer)
- Emissions of radon gas
How can seismic waves be recorded?
- Can be measured by seismography
- These graphs can record the amplitude of seismic waves by recording P and S waves
- Can then be translated to numerical scale of Richter scale
How are seismic waves recorded in Japan?
- When there is a P-wave detection from any of the 4,235 seismometers in Japan
- JMA predict location of epicentre
- Can then warn people in that area through TV and radio
What are shadow zones?
- Parts of the earth which do not tetect earthquakes because certain waves cant travel through the outer core
P-wave shadow zone
- The area of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any seismic waves, therefore cannot be recorded there
- P waves get refracted so create smaller shadow zones
S-wave Shadow zone
- Bigger and exists as S waves (earthquakes waves) cannot travel through the liqued outer core so they create a big shadow zone
Exam question-Outline the features of seismic waves and account for the shadow zone (4 Marks)
- P waves are the first waves are the P-waves. These travel around 6-7km/s through solids, liquids and gases. Longitudinal that can be detected up to a minute before the ground begins to move.
- Next, the other type of waves are the S-waves. These can only travel through solids at a speed of 2.5km - 4km/s and are also reffered to as transverse waves
How do we measure the magnitude of earthquakes ?
- Richter scale-logrythmic and measures the energy released
- Macarli scale-depends on many factors other than energy like rock type
- Moment Magnitude Scale-the total amount of energy released by an earthquake, better at measuring larger earthquakes
Evaluations of Modified Mercalli scale
+There is a set of criteria that diffrentiates earthquake
+Qualitative scale
-No use within isolated, rural areas where damage is limited
-Based on observations/opinions
Evaluations of Richter Scale
+Logarithmic scale
+Can compare with other earthquakes globally
+Objective definition of power of an earthquake as no opinion is involved
-Quantative measurement
-Open ended as theres no upper limit
