Measuring Ground Vibrations Using the S102 Seismometer

A Presentation by

Dr. Alan ScottDepartment of Physics

University of Wisconsin-Stout

Measuring Ground Vibrations Using the S102 Seismometer

OutlineS102 SeismometerGround Vibrations

Earthquakes

S102 Seismometer

S102 Seismometer

Schematic Diagram

Schematic Diagram

Up and down movement of magnet induces a voltage signal in the pick-up coil.

Schematic Diagram

…But the magnet is actually remaining somewhat stationary while the ground (which is rigidly connected to the coil) moves up and down.

Damped Harmonic Oscillation

If one displaces the inertial mass then releases it while keeping the housing stationary, the mass follows a damped harmonic oscillator behavior.

Damped Harmonic Oscillation

0 kxbvma

)cos()( tAetx t

S102 Seismometer damping

022

2

xdt

dx

dt

xdo

Electromagnetic Induction

dt

d Bemf

After the induced voltage is amplified, the rated output of the S102 Seismometer is 15 V per mm/sec, single ended output.

Instrument Calibration

A circuit in the meter can displace the inertial mass in a very uniform way. The signal produced can be correlated to a specific amount of ground motion.

Instrument Sensitivity

Recording of footsteps taken every 2 seconds at a distance of about 3 feet away from the seismometer using a concrete floor.

Instrument Sensitivity

If 15V corresponds to a ground motion of 1 mm/sec, the above footsteps have a ground motion of about 6x10-3 mm/sec. One can estimate the amplitude of ground motion by assuming a simple harmonic motion with a frequency of 5 Hz.

Instrument Sensitivity

)cos()( tAtx )sin()( tAtx

6x10-3 mm/sec = -A(2)(5 Hz)(1)A~200 nanometers

Ground Motion Produced by an Earthquake

P-waves•Longitudinal waves that expand and compress rocks.•Travel at about 7.3 km/s away from the origin.

S-waves•Transverse waves that move rocks perpendicular to the waves motion.•Travel at about 4.6 km/s away from the origin.

Ground Motion Produced by an Earthquake

L-waves (or Surface-waves)•Transverse waves that move rocks perpendicular to the waves motion.•Travel along the earth’s surface.•Travel at about 2.6 km/s away from the origin.

Ground Motion Produced by an Earthquake

Graph from National Earthquake Information Service, USGS/neic.usgs.gov/neis/seismology/keeping_track.html

Ground Motion Produced by an Earthquake

2-dimensional animation of wave propagation.

Scales to Measure the Strength of Ground Motion

•Richter Scale

•Mercalli Scale

Richter ScaleIt is a measure of the amplitude of ground vibration using a seismometer.

With the seismometer a standard distance of 100 km away from the epicenter, one gets:

Amplitude (mm)

1 10 100 100010000

Magnitude

3 4 5 6 7

Modified Mercalli Scale

I. Not felt except by a very few under especially favorable conditions.

II. Felt only by a few persons at rest, especially on upper floors of buildings.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

Modified Mercalli ScaleVI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Plate Tectonics – The Cause of Most Earthquakes

San Andreas fault

Picture from Earthquake On-line Book from USGS, by Kaye M. Shedlock & Louis C. Pakiser, http://pubs.usgs.gov/gip/earthq1/

Plate Tectonics – The Cause of Most Earthquakes Picture from Earthquake On-line Book from USGS,

by Kaye M. Shedlock & Louis C. Pakiser, http://pubs.usgs.gov/gip/earthq1/

Plate Tectonics – The Cause of Most Earthquakes Picture from Earthquake On-line Book from USGS,

by Kaye M. Shedlock & Louis C. Pakiser, http://pubs.usgs.gov/gip/earthq1/

Seismicity MapsGraph from National Earthquake Information Service, USGShttp://neic.usgs.gov/neis/general/seismicity/us.html

Seismicity Maps

Alaska

Graph from National Earthquake Information Service, USGShttp://neic.usgs.gov/neis/general/seismicity/us.html

Seismicity Maps

Central U.S.

Graph from National Earthquake Information Service, USGShttp://neic.usgs.gov/neis/general/seismicity/us.html

Earthquakes Are Usually Pre-cursors To Volcanic Eruptions

Mt. Redoubt

Picture from Earthquake On-line Book from USGS, by Kaye M. Shedlock & Louis C. Pakiser, http://pubs.usgs.gov/gip/earthq1/

Earthquake – San Francisco, 1906

Picture from Earthquake On-line Book from USGS, by Kaye M. Shedlock & Louis C. Pakiser, http://pubs.usgs.gov/gip/earthq1/

Earthquake – San Francisco, 1906

Picture from Earthquake On-line Book from USGS, by Kaye M. Shedlock & Louis C. Pakiser, http://pubs.usgs.gov/gip/earthq1/

Earthquake – Alaska, 1964

Picture from Earthquake On-line Book from USGS, by Kaye M. Shedlock & Louis C. Pakiser, http://pubs.usgs.gov/gip/earthq1/

Earthquake – Loma Prieta (Bay Area), 1989

Earthquake – Loma Prieta (Bay Area), 1989

Earthquake – Northridge, (Los Angeles), 1994

Earthquake – Northridge, (Los Angeles), 1994

Earthquake – Taipei, Taiwan September 20, 1999

Origin Time: 17:47:19 GMTLATITUDE: 23.78 N LONGITUDE: 121.09 E DEPTH: 33 kmMAGNITUDE: Ms=7.6 (NEIC)

Earthquake – Taipei, Taiwan

Seismograph readouts in Germany:

Earthquake – Taipei, Taiwan

Epicenter location indicated by the star:

Monitoring Weapons Proliferation

Source: Physics Today, March 1998

Monitoring Weapons Proliferation

Source: Physics Today, March 1998

The End