Seismic Waves and Measurement Questions and Answers

Answer: A

The scale was tied to the maximum trace amplitude on that specific instrument at a specified distance, which is why the definition is bound up with the equipment of its day. Modern practice computes equivalent values from other instruments.

Answer: B

Correct answer: stop the mass oscillating at its own natural period

An undamped suspended mass would ring at its own natural period after any disturbance, so the record would show the instrument’s oscillation superimposed on the ground motion. Damping - usually electromagnetic and set close to 0.7 of critical - suppresses that free oscillation while leaving the response flat over the widest possible band. Damping reduces the sensitivity slightly; that is the price of a faithful record.

Answer: C

Correct answer: three

One station gives the distance to the epicentre from the S minus P interval, which places the event somewhere on a circle. Two stations give two circles, which intersect at two points. A third resolves the ambiguity, and the three circles meet at the epicentre. In practice many more stations are used, and the location is computed by least squares from all the arrival times at once, which also yields the focal depth and the origin time.

Answer: D

Because the two waves travel at different speeds, the gap between them grows steadily with distance, so the interval is a direct measure of it. Using the absolute arrival time would require knowing the origin time, which is not yet known.

Answer: A

Correct answer: arrival time plotted against epicentral distance

Travel time curves plot the arrival time of each phase against distance from the epicentre, built up from many recorded earthquakes. They are the working tool for identifying phases on a seismogram and for estimating distance, and their shape is what reveals the earth’s internal velocity structure - the breaks and shadow zones in them are the evidence for the Moho, the mantle transition zones and the core.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The S minus P interval fixes the distance, but a single station has no reliable way of determining the azimuth, so the epicentre may lie anywhere on a circle around it. Three stations give three circles that intersect at one point. Array processing and three-component particle-motion analysis can estimate direction from one site, but only crudely.

Answer: C

Correct answer: a liquid outer core that cannot transmit shear

Beyond about 103 degrees no direct S wave is received anywhere, and the only explanation consistent with the observations is that the waves encounter a layer with no rigidity. Since \( V_s = \sqrt{\mu/\rho} \) vanishes when the shear modulus is zero, the outer core must be liquid. The corresponding P wave shadow zone between 103 and 143 degrees is caused by refraction at the core boundary, where the velocity drops sharply, and is a different phenomenon.

Answer: D

The P wave is compressional and passes through fluids, the S wave is a shear wave that does not, and the Rayleigh wave rolls along the surface in a vertical ellipse. Those three properties are the ones most often tested.

Answer: A

Correct answer: remove a spurious drift before integration

An accelerogram usually carries a small offset or slow drift from instrument and digitisation effects. Acceleration itself is barely affected, but integrating to velocity turns a constant offset into a ramp, and integrating again turns it into a parabola, so the computed displacement runs away. Baseline correction fits and removes that trend first, and without it the derived velocities and displacements are meaningless.

Answer: B

Correct answer: velocity

Acceleration integrated once with respect to time gives velocity, and integrated again gives displacement - each integration requiring a constant of integration and, in practice, careful baseline correction and filtering. Differentiating in the other direction gives jerk. Displacement matters because it drives the deformation of structures, while acceleration drives the inertial forces on them.