Part 2: What Do Seismographs Measure?

By Charles G. Spencer, Ph.D.

The instruments used to measure vibrations are called seismometers. They are also referred to as seismographs (term dating back to when vibrations were recorded by a pen on paper, rather than electronically).

The vibrational energy imparted to surrounding rock and soil by a blast produces a complex variety of vibrations, but seismometers measure vibrations along three mutually-perpendicular axes (X-, Y-, and Z-axes). Those represent measurements of three directions of vibration: forth-and-back (along the direction the energy travels), side-to-side, and up-and-down. Those motions are identified as, respectively, radial, transverse, and vertical.

The maximum speed that a seismic wave causes a “particle” to move is measured as peak particle velocity, or PPV. The particle concept is a little hypothetical, but you can think of it as a tiny piece of rock, or soil; or, the detector in the seismograph. The PPV is measured for each of the three directions of vibration.

The number of vibrations per second is called the frequency. It doesn’t matter which type of motion is involved. For example, one up-and-down motion is one vibration cycle. Frequency is measured in units called hertz. One hertz equals one cycle (complete vibration) per second. Blasting produces vibrations over a wide spectrum of frequencies, but nearly all fall below about 100 hertz. Frequencies below about 20 hertz can be problematic, depending on the amount of energy they contain.

The amplitude of the seismic wave is basically the amount of particle displacement it causes, and represents the energy of the wave. It is the height of the waveform, recorded on a graph of displacement versus time, above (or below) the “0” line on the graph. Taller waves mean greater energy and displacement.

Because the movement is oscillatory, the direction of motion changes over a short time. The change produces acceleration (change in velocity over time). Although not typically reported, it is these rapid changes that apply the greatest stress to structures.

Finally, because the air blast is a sound wave its strength is measured in decibels, just like other sound waves.

At the moment, there are two seismometers (“Stations”) in use. Right now the two are both placed about 800 feet from the current location of blasting; Station #1 is to the west near Lowenstein Drive; Station #2 is south of I-470, almost due south of the current blasting zone. And, because the instruments are on the surface, they are measuring surface waves (and the air blast).

The recent approval by the Missouri Mining Commission for the use of explosives at the Star Excavations site on Quarry Park Road in Lee’s Summit has led to concern on the part of many nearby residents. The Tribune asked Dr. Charles Spencer, a local geological consultant, to provide some basic information on the nature of blasting and its effects. His article will appear over the next four weeks.