Engineering Geology and Geohazards Lecture 11: Recording and Locating Earthquakes Flashcards
Describe the Seismometers
Work on the principle of inertia as described by Newton’s First Law, which states that “an object at rest tends to remain at rest unless acted on by an outside force”.
- During an earthquake, vibrations cause the frame of the seismograph to move. A mass suspended on a pendulum remains stationary due to it’s inertia, tracing a record on a moving paper cylinder.
- Modern seismometers work on the same principle but recording is made by the electric current generated by movement within a fixed magnetic field, producing a digital readout.
Recorded as a series of numbers indicating ground
movement amount and time, same principle as music CDs.
Easily analysed by computers.
Who invented the first seismometer
John ‘Earthquake’ Milne
Describe modern seimometers
Three-component broadband instruments measure
shaking across a wide range of frequencies, along three axes at right angles to each other.
* Long period and short period instruments provide
detail for narrower frequencies
Strong motion instruments, or accelerometers, are
designed to record peak ground acceleration (PGA)
Describe the seismogram
The seismogram is the graphic expression/graph of an earthquake as recorded by a seismometer at a seismograph station.
- Seismograms allow estimates of the distance, direction, magnitude and type of faulting causing the earthquake.
What variables can affect Seismograms?
Distance of seismogram from focus
Earthquake size/magnitude
Earthquake focal depth
Ambient noise
How can seismic wave speeds be use to estimate hypocentre distance
Ratio between the average speeds of P waves and
S waves is constant.
* Consequently the time delay between the first arrival of the P wave and the first arrival of the S wave provides a quick and reasonably accurate estimate of the distance
obtain the approximate distance of the earthquake in km by multiplying the S-minus-P (S-P) time, in seconds, by the factor 8 km s-1.
