Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston...
-
Upload
bertha-shaw -
Category
Documents
-
view
214 -
download
0
Transcript of Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston...
![Page 1: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/1.jpg)
Seismological Perspectives on Broadband Tilt, Strain, and
Rotation Measurement
Charles Langston
Center for Earthquake Research and Information
University of Memphis
![Page 2: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/2.jpg)
Ground Motions have 6 Degrees of Freedom
A thorough understanding of the seismic wave field at a point requires the measurement of 3 components of vector displacement and 3 components of rotation.
![Page 3: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/3.jpg)
Rotation (tilt) as Noise
• Seismological practice using 3 component seismometers usually assumes that recorded ground motions represent ground displacement. It is very well known, however, that horizontal seismometers are particularly susceptible to tilt, which is a rotation about a horizontal axis. Tilt is a common noise signal for shallow-depth, temporary installations, as in many PASSCAL experiments, where diurnal thermal or water-saturated ground expansion and contraction produces high levels of noise on the horizontal channels.
![Page 4: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/4.jpg)
Typical horizontal/vertical noise levels CMG-40T Installation
![Page 5: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/5.jpg)
Rotation as "Pseudo" Signal
• Tilt may also be a direct signal of tectonic interest. Broadband installations on active volcanoes often record large "long-period" pulses that have been interpreted as long-period pressure waves but may also be simple tilts induced in the near-field of intrusive or extrusive loads within the volcano. Thus, a major improvement in broadband seismometry is needed to separate the effects of simple ground displacements from the effects of tilt.
![Page 6: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/6.jpg)
Hidayat, D., B. Voight, C. Langston, A. Ratdomopurbo, C. Ebeling, Broadband seismic experiment at Merapi volcano, Java, Indonesia: very-long-period pulses embedded in multiphase earthquakes, J. Volcan.Geotherm. Res., 100, 215-231 (2000).
![Page 7: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/7.jpg)
Broadband signals at the summit of Merapi Volcano show anomalous long-period pulses on the radial components
![Page 8: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/8.jpg)
Geometry
![Page 9: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/9.jpg)
Hypothesis is that these long-period pulses are due to near-field tilts from the growing lava dome
Theoretical tilt response of broadband seismometers
![Page 10: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/10.jpg)
Near-Field tilts explain the signal
![Page 11: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/11.jpg)
Rotation and Strain as Future Real Signal
• Tilt or rotation, in itself, is also a very interesting observable in wave propagation and should be part of seismic interpretation. Rotation is closely related to strain and both quantities can be used to gain a deeper understand seismic wave propagation. For example, combined use of displacement polarization, rotation, and/or strain at a single observation point yield estimates of wave horizontal phase velocities.
![Page 12: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/12.jpg)
Example:Strong Motion Array Recording During the October 28-29, 2002, Embayment Seismic
Excitation Experiment (ESEE)
![Page 13: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/13.jpg)
Array Geometry
![Page 14: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/14.jpg)
K2 Acceleration data integrated to displacement
![Page 15: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/15.jpg)
Compute partial strain(or rotation)from
array channels
![Page 16: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/16.jpg)
Plane Wave Model
Horizontal phase velocity can be found be taking the ratio of ground velocity to strain.
![Page 17: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/17.jpg)
Velocity Estimate
Ratio method agrees with moveout analysis velocities for the major phases
![Page 18: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/18.jpg)
Rotation and Strain as Future Real Signal (2)
• Measurement of local strains and tilts could be made by differencing the signals of closely spaced seismometers. Ironically, closely matched instrument responses and tilt insensitivity of individual sensors would be a requirement for accurately estimating rotations and strains with such micro arrays.
![Page 19: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/19.jpg)
Broadband Micro-Arrayor
Finite Difference Star
Construct a small array of broadband seismometers
![Page 20: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/20.jpg)
To compute rotation (or strains):
![Page 21: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/21.jpg)
• Thus, instrument responses must be known to better than 1 part in a million.
• It is probably better to design a broadband rotation meter.
![Page 22: Seismological Perspectives on Broadband Tilt, Strain, and Rotation Measurement Charles Langston Center for Earthquake Research and Information University.](https://reader036.fdocuments.in/reader036/viewer/2022062517/56649f125503460f94c257ae/html5/thumbnails/22.jpg)
Conclusions
• Future broadband velocity sensors should be insensitive to rotation.
• Future broadband rotation sensors should be insensitive to linear motion.
• Combined Linear and Rotational measurements will yield interesting new information on seismic wave propagation.