Broad-band observations of earthquake-induced rotational ground motions

Abstract

S U M M A R YIt has been noted by theoretical seismologists for decades that-in addition to translations and strains-the rotational part of ground motions should also be recorded. It is expected that collocated measurements of translations and rotations may (1) allow transformation of translational seismograms to the complete ground motion of an observation point; (2) help to further constrain rupture processes and (3) provide additional hazard-relevant information to earthquake engineers. The lack of instrumental sensitivity used to be the main obstacle to observing rotational motions. Recently, ring laser technology has provided the means to develop instruments that allow in principle the observation of rotational motions in a wide frequency band and epicentral distance range. Here we investigate whether this technologyoriginally designed for geodesy-is capable of providing accurate and useful observations for seismology. We report observations of rotations around a vertical axis of several earthquakes obtained by a 4 × 4 m ring laser installed in SE-Germany and compare them to collocated broad-band translations. Assuming plane transverse wave propagation, acceleration and rotation rate should be in phase and their amplitude ratio proportional to horizontal phase velocity. Here we show that most of the observations can be explained under these assumptions and that the collocated observations allow the estimation of wavefield properties (e.g. phase velocities, propagation directions), otherwise only accessible through seismic array measurements, polarization analysis, or additional strain measurements.

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.…”

“…The third quantity, the three components of rotational motion, on the other hand is extremely hard to measure, mainly due to low sensitivity of the instruments. The development of high sensitivity rotational motion sensors such as ring laser gyroscopes makes it nowadays possible to record the rotational motions in the seismic wavefield induced by earthquakes (Igel et al 2007) and even by ambient seismic noise (Hadziioannou et al 2012). In this study, we use the ringlaser rotation sensor located at the geodetic Fundamentalstation in Wettzell, Germany.…”

“…This observation is of particular interest for our study as rotational motions around the vertical axis are not sensitive to compressional waves and are only excited by horizontally polarized shear waves. Igel et al (2007) pointed out, that P and Rayleigh waves at the Earth's surface can lead to tilting effects and therefore pollute the rotational motion. The tilt signal can be estimated roughly from the vertical translational motions, but its contribution to the rotations is small in comparison to the noise of the rotational sensor (Pham et al 2009a) and far below the detected signal.…”

“…Bernauer et al (2009) introduced a technique to incorporate rotational motions into the seismic inverse problem to improve tomographic images in the vicinity of seismic stations. Another application using rotational motions is the determination of seismic phase velocities and source directions from point measurements using collocated observations of rotational motions and translational accelerations induced by earthquakes (Igel et al 2007;Kurrle et al 2010) or ambient seismic noise (Hadziioannou et al 2012). Igel et al (2007) and Pham et al (2009b) report direct observations of rotational motions around the vertical axis in the coda of the direct teleseismic P waves.…”

“…Another application using rotational motions is the determination of seismic phase velocities and source directions from point measurements using collocated observations of rotational motions and translational accelerations induced by earthquakes (Igel et al 2007;Kurrle et al 2010) or ambient seismic noise (Hadziioannou et al 2012). Igel et al (2007) and Pham et al (2009b) report direct observations of rotational motions around the vertical axis in the coda of the direct teleseismic P waves. A local and a teleseismic example for rotational motions around the vertical axis recorded at the Fundamentalstation Wettzell in southeast Germany is shown in Fig.…”