Seismological Perspectives on Broadband Tilt, Strain, and

Rotation Measurement

Charles Langston

Center for Earthquake Research and Information

University of Memphis

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.

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.

Typical horizontal/vertical noise levels CMG-40T Installation

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.

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).

Broadband signals at the summit of Merapi Volcano show anomalous long-period pulses on the radial components

Geometry

Hypothesis is that these long-period pulses are due to near-field tilts from the growing lava dome

Theoretical tilt response of broadband seismometers

Near-Field tilts explain the signal

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.

Example:Strong Motion Array Recording During the October 28-29, 2002, Embayment Seismic

Excitation Experiment (ESEE)

Array Geometry

K2 Acceleration data integrated to displacement

Compute partial strain(or rotation)from

array channels

Plane Wave Model

Horizontal phase velocity can be found be taking the ratio of ground velocity to strain.

Velocity Estimate

Ratio method agrees with moveout analysis velocities for the major phases

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.

Broadband Micro-Arrayor

Finite Difference Star

Construct a small array of broadband seismometers

To compute rotation (or strains):

• Thus, instrument responses must be known to better than 1 part in a million.

• It is probably better to design a broadband rotation meter.

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.