Seismic Waves and Measurement Questions and Answers

Answer: C

Correct answer: the largest absolute value on the record

Peak ground acceleration is simply the largest absolute value attained anywhere on the trace, in either direction, and it is read directly without regard to when it occurs or what phase caused it. It is the most widely quoted strong motion parameter, though a poor predictor of damage on its own, since a single brief spike may do far less harm than a longer episode of moderate shaking at a period matching the structure.

Answer: D

Correct answer: low-pass filtering

A low-pass filter passes the low frequencies and attenuates those above the chosen corner, so it removes high-frequency noise while leaving the engineering-significant motion intact. High-pass filtering does the opposite and is used at the other end of the band, to remove long-period drift before integration. Differentiating would amplify the noise, and raising the sampling rate merely records it more faithfully.

Answer: A

Correct answer: false, since it records the motion of the ground relative to the mass

A seismometer works by inertia: the suspended mass tends to stay still while the frame, anchored to the ground, moves with it. What is measured is the relative displacement between frame and mass, and for periods well below the instrument’s natural period that relative motion follows the ground motion faithfully. The mass is the nearest thing available to a fixed reference, which is precisely why it is used.

Answer: B

Correct answer: in two horizontal directions and the vertical

Ground motion is a vector, and three orthogonal components are needed to describe it completely - conventionally north-south, east-west and vertical. The three together also allow the particle motion to be reconstructed, which helps identify phases and estimate the direction of approach. Separating P, S and surface waves is done by their arrival times and character, not by using a different instrument for each.

Answer: C

Triggering conserved recording medium in the days of film and paper and still limits the data to be handled. The cost is that the first fraction of a second may be lost, which is why a pre-event memory is now standard.

Answer: D

Correct answer: The intensity at the recording site

Intensity is a measure of observed effects - what people felt, what objects moved, what damage occurred - assigned from questionnaires and damage surveys on a scale such as Modified Mercalli. It is not an instrumental quantity and cannot be read from a trace, though empirical relations exist between intensity and instrumental parameters. Arrival times, trace amplitudes and the S minus P interval are all measured directly.

Answer: A

The gap between the harmless fast arrival and the damaging later ones gives seconds to tens of seconds of warning at distance, enough to stop trains and shut valves. It is detection of an event already under way, not prediction.

Answer: B

Correct answer: continuous low-level ground vibration from natural sources

Microseisms are the continuous background hum of the earth, generated chiefly by ocean waves interacting with the sea floor and coastline, with a strong peak near periods of six to eight seconds. They set the noise floor of a seismic station and rise in stormy weather. They have nothing to do with small earthquakes, which are called microearthquakes - a different word for a different thing.

Answer: C

Velocity depends on the ratio of an elastic modulus to density, so stiffer rock transmits waves faster while greater density alone slows them. This is why waves speed up with depth, stiffness rising faster than density.

Answer: D

Correct answer: the wave velocity changes at the interface

Refraction is governed by Snell’s law, and it occurs wherever the velocity changes across a boundary: the ray bends towards the normal on entering a slower medium and away on entering a faster one. Frequency is unchanged across an interface - it is the wavelength that changes with the velocity - and only part of the energy is transmitted, the rest being reflected.