Ocean bottom seismometers (OBS) are instruments that measure tectonic movements and earthquakes from the seafloor. They were developed in the 1970s but modern versions have lower power usage, higher precision clocks, and can detect smaller motions. OBSs are used to study the structure of the Earth's mantle and crust, calculate earthquake energy levels, and predict seismic events. They are deployed for periods ranging from 2 months to over a year and have provided insights into phenomena like sea floor spreading and whale vocalizations.

1. Keygan Sands

2.  Measures tectonic movement*
 90% earthquakes occur
underwater*
 Motions may be <1mm or >1m*
 Uses:
 Calculating quake energy*
 Predicting quakes*
 Studying structure of mantle and
crust*
*Ocean Instruments
Collins 2004

3.  Rests on and moves with
seafloor*
 Heavy mass swings between
two magnets*
 Oscillations produce electrical
current*
 Cylindrical body houses
instruments, batteries and data
logger housed separately*
 60-600kg instrument package*
*Ocean Instruments
Ocean Instruments

4.  Developed in 1970s by institutions (WHOI, SIO,
University of Texas)*
 Inefficient: high power, limited storage capacity,
inexact clocks*
 Formation of National OBS Instrumentation Pool in
1999 by NSF with first major long-term deployment in
2004*
 Modern OBSs are lower power, higher precision clocks,
sensitive to finer motions*
 Divided into short and long period OBSs*
*Collins 2004

5.  Short-period OBS (WHOI D2 and L-CHEAPO):
 High-frequency motions*
 Small quakes*
 Studies outer layers of crust*
 60 day to 6 month deployment period**
 Long-period OBS (WHOI and Scripps long-
deployment):
 Broader range of motions*
 10/sec to 1/min data collection*
 Mid-sized quakes, distant activities*
 >12 month deployment period**
*Ocean Instruments
**OBSIP, National Science Foundation
OBSIP, NSF

6.  Good:
 Stable clocks
 Easily recovered data
 Connectable to surface
observatories
 Easily deployed (requires
winch)
 Bad:
 Imprecise installation
 Short-period have short
battery lives
 High data Ocean Instruments
Ocean Instruments

7.  “Structure of the East Pacific Rise from an Ocean
Bottom Seismometer Survey” –Orcutt and
Kennett, 1976
 Physical structure of sea floor studied
 Determined that oceanic crust structure can change
rapidly depending on age (velocity of earthquake
waves through medium)
 “Earthquake distribution in the subduction zone
off eastern Hokkaido, Japan, deduced from ocean-
bottom seismographic and land observations” –
Iwasaki et. al, 1991
 Observations of microearthquakes on continental
slope of Japan
 Determined that several tectonic blocks exist in the
subduction zone along Kuril Trench (differences in
seismicities between locations)

8.  “A Sea-Floor Spreading Event
Captured by Seismometers” –Tolstoy
et. al, 2006
 Monitoring of East Pacific Rise during
ridge eruption and precursor activity
 Developed a profile of major spreading
event
 Eruptions can be used to forecast
spreading
 “Tracking fin whales in the northeast
Pacific Ocean with a seafloor seismic
network” –Wilcock, 2012
 Use of OBSs to passively monitor whale
locations
 Microearthquakes: 10Hz, whale calls:
20Hz
Life of Sea

9. Collins, John (2004). “Listening Closely to „See‟ Into the Earth.” Oceanus.
<http://www.whoi.edu/oceanus/viewArticle.do?id=2509&archives=true>
Iwasaki, T., Hirata, N., Kanazawa, T. Urabe, T., Motoya, Y., and Shimamura, H. (1991).
Earthquake distribution in the subduction zone off eastern Hokkaido, Japan, deduced
from ocean-bottom seismographic and land observations. Geophys. J. Int. 105, 693-711.
“Life of Fin Whale.” Life of Sea. <http://life-sea.blogspot.com/2011/07/life-of-fin-whale.html>
“Ocean Bottom Seismograph Instrument Pool.” NSF. <http://www.obsip.org/index.php/>
“Ocean-Bottom Seismometer.” Ocean Instruments. <http://www.whoi.edu/instruments/vi
ewInstrument.do?id=10347>
Orcutt, J. A. and Kennett, L. N. (1976). Structure of the East Pacific Rise from an Ocean Bottom
Seismometer Survey. Geophys. J. R. asir. Society 45, 305-320.
Tolstoy, M., Cowen, J. P., Baker, E. T., Fornari, D. J., Rubin, K. H., Shank, T. M., Waldhauser, F.,
Bohnenstiehl, D. R., Forsyth, D. W., Holmes, R. C., Love, B., Perfit, M. R., Weekly, R.
T., Soule, S. A., and Glazer, B. (2006). A Sea-Floor Spreading Event Captured by
Seismometers. Science 314, 1920-1922.
Wilcock, W. S. D. (2012). Tracking fin whales in the northeast Pacific Ocean with a seafloor
seismic network. Acoustical Society of America 132, 2408-2419.