An earthquake generates both transverse (S) and longitudinal...

An earthquake generates both transverse (S) and longitudinal $(P)$ sound waves in the earth. The speed of $S$ waves is about $4 \mathrm{~km} \mathrm{~s}^{-1}$ and that of $P$ waves is about $8 \mathrm{~km} \mathrm{~s}^{-1}$. A seismograph records $P$ and $S$ waves from an earthquake. The first $P$ wave arrives 4 min before the first $S$ wave. The epicentre of the earthquake is located at a distance of about (a) $192 \mathrm{~km}$ (b) $384 \mathrm{~km}$ (c) $1920 \mathrm{~km}$ (d) $3840 \mathrm{~km}$

An earthquake generates both transverse (S) and longitudinal $(P)$ sound waves in the earth. The speed of $S$ waves is about $4 \mathrm{~km} \mathrm{~s}^{-1}$ and that of $P$ waves is about $8 \mathrm{~km} \mathrm{~s}^{-1}$. A seismograph records $P$ and $S$ waves from an earthquake. The first $P$ wave arrives 4 min before the first $S$ wave. The epicentre of the earthquake is located at a distance of about
(a) $192 \mathrm{~km}$
(b) $384 \mathrm{~km}$
(c) $1920 \mathrm{~km}$
(d) $3840 \mathrm{~km}$

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Key Concepts

Time Difference of Arrival

The difference in the arrival times of P and S waves at a seismic station is a key measurement in locating an earthquake's epicenter. Since P waves travel faster than S waves, the time delay between their arrivals directly relates to the distance from the earthquake's focus. Using the time difference along with the known speeds of the waves allows seismologists to estimate how far away the earthquake occurred.

Wave Propagation and Speed

The speed at which seismic waves travel depends on the properties of the materials they pass through. In seismology, the known speeds of P and S waves are used to analyze how energy is distributed and to infer details about the Earth's interior. This knowledge is essential for calculating distances from the source of an earthquake based on the arrival times of these waves.

Seismic Waves

Seismic waves are energy waves that travel through the Earth following an earthquake. They are generally classified into P (primary or compressional) waves and S (secondary or shear) waves. P waves travel faster and can move through both solid and liquid materials, while S waves move slower and only propagate in solids. Understanding these distinct types is crucial in seismology for determining the earthquake's characteristics and location.

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