Question:

After an earthquake, which type of seismic wave arrives first at a seismometer?

The primary wave or the P-wave is the first to arrive at a seismometer. Thanks for using AnswerParty!

earthquake Mechanics Physics

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Seismology

Seismic waves are waves of energy that travel through the Earth's layers, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves are studied by geophysicists called seismologists. Seismic wave fields are recorded by a seismometer, hydrophone (in water), or accelerometer.

The propagation velocity of the waves depends on density and elasticity of the medium. Velocity tends to increase with depth, and ranges from approximately 2 to 8 km/s in the Earth's crust up to 13 km/s in the deep mantle.

P-wave Seismometer

Body wave magnitude ($m_b$) is a way of determining the size of an earthquake, using the amplitude of the initial P-wave to calculate the magnitude. The P-wave is a type of body wave that is capable of traveling through the earth at a velocity of around 5 to 8 km/s, and is the first wave from an earthquake to reach a seismometer. Because of this, calculating the body wave magnitude can be the quickest method of determining the size of an earthquake that is of a large distance from the seismometer.

Limitations in the calculation method mean that body wave magnitude saturates at around 6-6.5 $m_b$, with the figure staying the same even when the moment magnitude may be higher.

The primary purpose of a seismometer is to locate the initiating points of earthquake epicenters. The secondary purpose, of determining the 'size' or Moment magnitude scale must be calculated after the precise location is known.

The earliest seismographs were designed to give a sense of the direction of the first motions from an earthquake. The Chinese frog seismograph would have dropped its ball in the general compass direction of the earthquake, assuming a strong positive pulse. We now know that first motions can be in almost any direction depending on the type of initiating rupture (focal mechanism).

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