The sometimes higher amplitude noise observable especially when viewing an 24 hour helicroder record is caused by microseisms! Microseisms are surface waves generated by ocean swells and surf, and can become very large when there is a storm at sea approaching the coastline.

Microseisms travel very efficiently along the surface layers of the Earth, which is why they show up all across the US during the same time interval. The period of these waves is typically between 3 and 8 seconds. They are efficient, but their amplitude does decrease with distance from the source. A smaller storm would generate microseisms of lower amplitude than a large storm, so it would not be seen across as large an area.

Example from May 2005 microseisms

PHAR Helicorder of Microseisms

NLNC Helicorder of Microseisms

WLIN Helicorder of Microseisms

The strong microseisms that were very visible on many AS-1 records from the midwest and eastern US were caused by a low pressure system and consequent high winds that tracked northward up the east coast during the past 3 days (May 5-8, 2005). At the WLIN (West Lafayette, IN) station, the microseisms were strongest from about 0800 on May 6 to about 1000 on May 7 (UTC) while the storm was off the SE US coast.

How can I adjust my arrival time if my computer clock is behind?

Question about picking arrival time. I noticed that my computer clock has raced ahead 3 min. 10 sec. since I set it from WWV two weeks ago. Do I need to adjust my arrival time on my sac file name or should I reset my clock, then re-extract the earthquake?

If you reset your clock, that will help with future extractions but not with past ones. Just go ahead and extract your event and in the Event screen, you can adjust the time with the Event dialog box.

Introductory level Q & A

What is a seismometer? What does it record?

A seismometer is a very sensitive instrument that can detect movements of the Earth’s surface. The surface is in constant motion due to natural and man-made phenomena. For example, the solid Earth tide is the stretching of the Earth due to the gravitational pull of the moon. Tucson slowly rises and falls twice a day about 40 cm due to the moon orbiting overhead. Mining explosions in Morenci, Arizona create seismic waves which shake Tucson daily. The amplitude of these vibrations are less than a micrometer and the oscillations have a frequency of several Hz.

The wide variety of ground motion, both in terms of period of oscillation and amplitude of vibration make it impossible for a single seismometer to record all types of motion. The AS-1 seismometer is a simple inertial system that can detect local mining explosions as well as moderate or large earthquakes that occur globally.

The seismometer is called inertial because it is based on Newton’s 1st Law: A body at rest will remain at rest. In the AS-1 seismometer a magnet is suspended by a spring from a boom. When the ground vibrates the boom moves, but the mass wants to remain at rests, so the spring is stretched. The relative motion between the boom and mass is proportional to the ground shaking and causes a tiny electric current to be generated that is sent to the seismograph to be converted into a seismogram. Unfortunately, the spring will eventually pull the mass, so the seismometer requires damping. Mineral oil is used to dampen the AS-1 Seismometer.

What are seismograms? How do you read them?

Seismometers are the recorded motions of the seismometer. They can be used to calculate the location and magnitude of earthquakes. They show how the ground moves with the passage of time.

On a seismogram, the horizontal axis represents time (measured in seconds) and the vertical axis represents the ground displacement. When there is no earthquake reading there is just a straight line, except for small wiggles caused by local disturbance or “noise”.

What can I expect to record with my AS-1?

What you can expect to record depends on the location of your seismometer and the amount of background noise that you are recording. More background noise will make it difficult to identify smaller events on your helicorder screen.

While each instrument response will be different, below is a map that plots earthquakes and mining blasts recorded in Tucson with an AS-1 during the 2001-2002 school year.

131 Earthquakes (represented by the red dots)

123 Mining blasts recorded from three mines (blue stars)

What type of dampening oil should I use?

How to determine what type of AS-1 you are operating
Thin Spring AS-1 - Spring diameter ~4mm
Thick Spring AS-1 - Spring diameter ~8mm

If you are operating a thin spring AS-1 combine the following mixture in your dampening oil container
1/3 STP Oil Treatment
2/3 10w40 Motor Oil

If you are operating a thick spring AS-1 combine the following mixture in your dampening oil container
1/2 STP Oil Treatment
1/2 10w40 Motor Oil

Frequently Asked Questions

Basic

How to

Display Problems

Noise Problems

Staying Updated

Recording / Data

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