How to Read Drum Seismograms


When an earthquake occurs the seismogram will show ground motions that typically last from several tens of seconds to many minutes, depending on the size of the earthquake and the sensitivity of the seismograph. The height of the recorded waves on the seismogram (wave amplitude) is a greatly magnified representation of the actual ground motion. A recording of an earthquake has recognizable characteristics. Typically, one can recognize the arrival of different wave types: P (the fastest traveling waves), S (shear waves), and surface waves. On these seismograms you may see local earthquakes in Northern California and earthquakes elsewhere in the world. Not all the wiggles seen on the seismograms are due to earthquakes. Anything that produces ground vibrations could be recorded, for example a car that passes by the seismometer (this is why they try to locate most of the seismometers well away from roads). Since the electrical signals from the seismometers are typically transmitted to the USGS by microwave or over telephone wires, electrical and radio noise on the transmission path may also show up on the seismogram. Such interference is usually easy to distinguish from earthquake-generated signals because it is often erratic or “spikey” in appearance.


The plots are “magnified” according to the level of background or ambient seismic noise, which is generated by wind, cultural disturbances and oceanic microseisms. Thus, some sites appear noisier than others. The seismogram is “read” like a book, from left to right and top to bottom (this is the direction that time increases). As with a book, the right end of any horizontal line “connects” with the left end of the line below it. Each line represents 15 minutes of data; four lines per hour. The colors of the horizontal lines have no particular significance; they are used to make it easier to distinguish lines from one another and to make it easier to recognize at which quarter-hour of data we are looking. The vertical lines are not part of the seismogram. They were added to indicate equal intervals of time. Time is indicated on the left in local Pacific time, and on the right end in Universal (or Greenwich) time. The graph below shows the first wave or small foreshock arriving at this station at 8:45am. The earthquake occured at 9:27:23am, the P wave arrived at 9:38:55am and the first S wave arrived at 9:48:25am. By using information from many different stations enables Geologist to triangulate the earthquake center location, depth and magnitude.


Below: Clibration Pulse Example



Below: Foreshock, Mainshock, Aftershock Example




Many of the seismometers in our network are of the magnet-coil-spring type. This type of instrument consists of a permanent magnet and a coil of wire. The coil, which is wound around a rather massive core, is suspended by a spring. When the ground moves, the coil tends to remain in place due to its mass, while the magnet, which is rigidly attached to the seismometer housing, moves relative to the coil. The relative motion produces a current in the coil, and it is this electrical signal that ends up being recorded as a seismogram.

The mechanical response of the magnet, springs and coil, as well as the electronics that amplifies the current, all affect the final signal. If the springs weaken or the electronics drifts, for example, the seismogram will not be accurate. Since these seismometers are located all over northern California, it is not practical to visit each one to check its performance.

Instead, the seismometer is programmed to check itself. Once a day, the electronics in the seismometer sends a controlled current through the coil. The response of the magnet-spring-coil system to this test signal is sent back as a calibration pulse. These pulses can be measured at the central recording site in Menlo Park, California, to assure that each seismometer is functioning properly.

BELOW: Quarry Blast Example


BELOW: High Winds Example


ABOVE: High Winds Example

The seismogram above documents a windy night at Geyser Peak station (GGP). From about 22:30 PDT on April 24 to 02:25 on April 25, the wind blew hard in coastal central California as a weather front passed through. Wind can produce low-amplitude seismic waves or “microseisms” in the earth through the action of trees, which transfer wind-generated forces into the ground through their roots. (Ocean waves also generate microseisms by the pounding of the surf.) Here, the wind-generated noise appears as an increase in the amplitude of the smallest background motions detected by this seismometer. Also, two small earthquakes are visible at 22:04 and 22:06, PDT.

BELOW: M3.1, 2.1, 2.7 Earthquake Example (two stations)


ABOVE: 3 Earthquake Example (two stations)

Three earthquakes, the largest having a magnitude of 3.1, occurred within two hours of each other on May 7, 2000, as detailed in the table above.

Seismograms from two stations, HFP (Fremont Peak; 9 miles SW of Hollister, 18 miles SSE of Gilroy) and BBG (Big Mountain; 7 miles ENE of Pinnacles, 26 miles N of King City) captured all three events. The New Idria events are closer to BBG than HFP, while the San Juan Bautista event is closer to HFP than to BBG.

These distance relationships can be seen in two ways. First, the New Idria events registered higher amplitudes at BBG than at HFP, while the San Juan Bautista event had higher amplitudes at HFP than at BBG. Secondly, if you look closely you will see that the first waves from the San Juan Bautista event arrive slightly earlier at HFP than at BBG. Conversely, the first waves from the New Idria events arrived at BBG first.


Three earthquakes, the largest having a magnitude of 3.1, occurred within two hours of each other on May 7, 2000, as detailed in the table below.

Time PDT Magnitude Latitude Longitude Depth Approximate Locaton Distance to HFP Distance to BBG
08:05 M=2.1 36.866 -121.597 6.3 km Near San Juan Bautista 16 Km 60 Km
08:50 M=2.7 36.246 -120.821 6.4 km Near New Idria 82 Km 42 Km
09:50 M=3.1 36.244 -120.829 6.0 km Near New Idria 82 Km 42 Km


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