Rotational motions induced by the M8.1 Tokachi‐oki earthquake, September 25, 2003

Abstract

[1] We report the first consistent observations of rotational motions around a vertical axis induced by distant large earthquakes. It is standard in seismology to observe three components (up-down, N-S, E-W) of earthquake-induced translational ground motions using inertial seismometers. However, only recently ring laser technology has provided the required sensitivity for observations of the theoretically predicted rotational part of ground motion generated by seismic waves in a wide distance range and frequency band. Here we show that the rotations observed are consistent in waveform and amplitude with collocated recordings of transverse accelerations recorded by a standard seismometer. This suggests that rotations may become a new observable for seismology and related fields with the potential of providing complementary information on earthquake source processes, structural properties, and ground shaking.

Rotational Motions in Seismology: Theory, Observation, Simulation

“…This explains that Aki and Richards (2002, p. 608) note that "as of this writing seismology still awaits a suitable instrument for making such measurements." However, the subsequent development of ring laser technology indicates that a significant part of the gap has recently been filled, as demonstrated by Schreiber et al (2003b) and Igel et al (2005b) and exemplified later in this paper. Rotations can also be determined with array measurements, but with important limitations; see section 3.1 and Suryanto et al (2005).…”

“…The "spectral element method" (Komatitsch 1997;Komatitsch and Tromp 2002a,b) for example allows very accurate modeling of elastic waves in three-dimensional model structures of the Earth's interior, additionally incorporating various effects, such as topography/bathymetry, anelasticity, anisotropy, ocean loading and rotation as well as gravitation of the Earth. The method proved to give very good results in terms of fit to observed data when using current 3D tomographic earth models (Komatitsch et al 2003;Igel et al 2005b;Schuberth et al 2005). For this study, we used a modified version of the spectral element program SPECFEM3D, in which we implemented the calculation and output of rotational ground motions for comparison with the ring laser data from Wettzell, Germany.…”

“…Thus, for a given earthquake for which the distance to the station is large enough compared with the source dimension and the wavelength of interest so that the plane wave assumption is valid, from the measurement of rotation and displacement of a surface wave train one can in principle infer both the phase velocity and the direction of the incoming wave train. In practice, we have so far dealt only with earthquakes of known location, and have assumed that the surface trains were traveling along the great circle, thus providing the direction (Igel et al 2005b, and section 4 below). There are now observations of ground rotations of several dozens of earthquakes with a wide magnitude and epicentral distance range.…”

The influence of crustal scattering on translational and rotational motions in regional and teleseismic coda waves

“…For rotational motion sensor examples and more details on rotational motions see, for example McLeod & Stedman (1998), Pancha et al (2000) and Brokešová et al (2012). Earthquake induced rotational motions over a wide range of magnitudes and epicentral distances are reported by Igel et al (2005Igel et al ( , 2007. Records of rotational motions can find use in several applications.…”

“…As an optical instrument the sensor has a flat response over the entire frequency range which allows simultaneous observations of the Earth's rotation and seismic waves. A change of the beat frequency can be caused by several effects (Igel et al 2005). First, variations of the scaling factor A/λP will change the beat frequency, an effect that is minimized by temperature and pressure shielding and extremely low expansion coefficients of the device.…”