Seismic Waves and Measurement Questions and Answers

Answer: C

Correct answer: 64 km

The two waves cover the same distance at different speeds, so the difference in travel time is

This is the basis of single-station distance estimation: the S minus P interval in seconds, multiplied by a constant that depends on the velocity model, gives the distance. It fixes the distance but not the direction, which is why three stations are needed for a location.

Answer: D

Correct answer: 6 km/s

Velocity is distance over time:

which is a typical P wave velocity in the continental crust. The S wave in the same rock would travel at roughly 3.5 km/s, the ratio of the two being fixed by the elastic constants of the material and close to \( \sqrt{3} \) for a Poisson solid.

Answer: A

Correct answer: 3.24 km/s

The ratio is applied directly:

A ratio near 0.58, that is \( 1/\sqrt{3} \), corresponds to a Poisson solid with Poisson’s ratio of 0.25, which most crustal rocks approximate. The ratio is a useful diagnostic in itself: an unusually low value can indicate fracturing or the presence of gas in the pore space.

Answer: B

Correct answer: 2 seconds

Directly beneath the station, the path length is the focal depth:

For a shallow local earthquake the P wave therefore arrives almost at once, and the S minus P interval is correspondingly short - here about two seconds, since the S wave takes roughly four. Short intervals mean a nearby event, which is the basis of the rough rule that counting the seconds between the tremor and the shaking gives the distance.

Answer: C

Correct answer: 0.01 s

The sampling interval is the reciprocal of the sampling rate:

By the Nyquist criterion this record can faithfully represent frequencies up to 50 Hz, half the sampling rate, and anything above that must be removed by an anti-alias filter before digitising or it will be folded back into the band and corrupt it.

Answer: D

Correct answer: the instrument that records ground motion

The seismograph is the complete recording instrument - the seismometer that senses the motion together with the timing and recording system. The record it produces is the seismogram, the point at which rupture begins is the focus or hypocentre, and magnitude scales are quite separate constructs. The three words seismograph, seismometer and seismogram are frequently confused and are worth keeping distinct.

Answer: A

Correct answer: seismogram

The seismogram is the record - formerly a trace on smoked paper or photographic film, now a digital time series. The seismometer is the sensor and the seismograph the complete instrument. An isoseismal is a contour of equal intensity on a map, which is a quite different product derived from observed effects rather than from an instrument.

Answer: B

Inertia keeps the mass nearly stationary while the case, fixed to the ground, moves around it, and the relative displacement is what is recorded. The instrument therefore measures motion of the ground with respect to an approximately fixed reference.

Answer: C

It records acceleration directly, stays within range close to a large earthquake, and is usually triggered by the shaking itself so that it does not run continuously. Sensitive seismographs designed for distant small events saturate at short range.

Answer: D

Correct answer: goes off scale

An instrument designed to detect distant earthquakes must resolve motions of a fraction of a micron, and its amplification is correspondingly enormous. Close to a large event the ground moves by centimetres, the trace is driven far beyond the recordable range, and the record is clipped and useless. Strong motion accelerographs exist for exactly this reason: they are insensitive, they trigger on strong shaking, and they stay on scale.