Rotational Seismology: AGU Session, 

Working Group, and Website 

By William H. K. Lee1, Heiner Igel2, Maria I. 

Todorovska3

, and John R. Evans1

Open-File Report 2007-1263 

2007 

U.S. Deprtment of the Interior 

U.S. Geological Survey 

1

U.S. Geological Survey, Menlo Park, Calif. 

2

University of Munich, Munich, Germany 

3

University of Southern California, Los Angels, Calif. 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007

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Suggested citation: 

Lee, W.H.K., H. Igel, M.I. Todorovska, and J. R. Evans, 2007. Rotational Seismology: AGU Session, 

Working Group, and Website: U.S. Geological Survey Open-File Report 2007-1263, 7 p. 

[http://pubs.usgs.gov/of/2007/1263/]. 

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Introduction 

Although effects of rotational motions due to earthquakes have long been observed (e. g., 

Mallet, 1862), nevertheless Richter (1958, p. 213) stated that: “Perfectly general motion 

would also involve rotations about three perpendicular axes, and three more instruments 

for these. Theory indicates, and observation confirms, that such rotations are negligible.” 

However, Richter provided no references for this claim. 

Seismology is based primarily on the observation and modeling of three-component 

translational ground motions. Nevertheless, theoretical seismologists (e.g., Aki and 

Richards, 1980, 2002) have argued for decades that the rotational part of ground motions 

should also be recorded. It is well known that standard seismometers are quite sensitive to 

rotations and therefore subject to rotation-induced errors. The paucity of observations of 

rotational motions is mainly the result of a lack, until recently, of affordable rotational 

sensors of sufficient resolution. Nevertheless, in the past decade, a number of authors have 

reported direct observations of rotational motions and rotations inferred from rigid-body 

rotations in short baseline accelerometer arrays, creating a burgeoning library of rotational 

data. For example, ring laser gyros in Germany and New Zealand have led to the first 

significant and consistent observations of rotational motions from distant earthquakes 

(Igel et al., 2005, 2007). A monograph on Earthquake Source Asymmetry, Structural 

Media and Rotation Effects was published recently as well by Teisseyre et al. (2006). 

Measurement of rotational motions has implications for: (1) recovering the complete 

ground-displacement history from seismometer recordings; (2) further constraining 

earthquake rupture properties; (3) extracting information about subsurface properties; and 

(4) providing additional ground motion information to earthquake engineers for seismic 

design. A special session on Rotational Motions in Seismology was convened by H. Igel, 

W. H. K. Lee, and M. Todorovska during the 2006 AGU Fall Meeting. The goal of this 

session was to discuss rotational sensors, observations, modeling, theoretical aspects, and 

potential applications of rotational ground motions. The session was accompanied by the 

inauguration of an International Working Group on Rotational Seismology (IWGoRS) 

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which aims to promote investigations of all aspects of rotational motions in seismology 

and their implications for related fields such as earthquake engineering, geodesy, strongmotion seismology, and tectonics, as well as to share experience, data, software, and 

results in an open Web-based environment. 

The primary goal of this article is to make the Earth Science Community aware of the 

emergence of the field of rotational seismology. 

AGU Special Session on Rotational Seismology 

A special session with seven oral presentations and 12 posters was held at the AGU Fall 

Meeting on 12 December 2006: U. Schreiber and H. Igel presented a paper on Sagnac 

ring-laser interferometry for the determination of rotations in seismology; V. Graizer a 

paper on tilts in strong-ground motion; laboratory and field testing of two promising 

rotational seismometer models were discussed by R. L. Nigbor and others; observations 

and modeling of broadband rotational ground motions were presented by H. Igel and his 

colleagues; J. B. Fletcher and P. Spudich reported the seismic rotations and strains at the 

UPSAR accelerometer array from the 28 September 2004, M6 Parkfield earthquake; 

rotational ground motions radiated by vibrating structures were discussed by 

M. I. Todorovska; amplification of ground motion rotations by local geology was reported 

by L. Godinho and colleagues; modeling of the seismic waves in non-classical media 

(E. Grekova and others), rotation response of a rigid body under seismic excitation 

(T. Tobita and S. Sawada), stochastic modeling of rocking seismic ground motion and 

respective structural load modeling (Z. Zembaty), a synthetic study of source- and basinrelated effects on ground rotational motions (H. Wang and others), finite-difference 

numerical simulation of seismic gradiometry (D. F. Aldridge and others); measuring midand near-field rotational ground motions in Taiwan (B. S. Huang and others); observations 

of rotational motions around vertical and horizontal axes and comparison with translations 

(S. Lehndorfer and others); towards a global network of rotation sensors (J. Hautmann and 

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others); seismic rotations observed with inertial seismic sensors (R. Pillet and 

J. Virieux); engineering implications of rotational sensitivity of translational 

accelerometers (R. L. Boroschek); and a new class of instruments instrument for direct 

observation of torsional normal mode oscillations of the Earth (R. Cowsik and others). 

Some interesting observational results were presented in both the oral and poster sessions. 

For example, Fletcher and Spudich compared peak velocity to rotation rate from 

earthquakes recorded at their UPSAR array of accelerometers in Parkfield, California, and 

found the average for four events to be 2.88 km/s; they observed rotation rates up to about 

200 μrad/s and peak rotations of μrad. As shown by Igel et al. (2005), the ratio of 

acceleration to rotation rate should be –2c, where c is the apparent S-wave velocity. This 

ratio implies a value of 1.44 km/s for c is similar to the minimum apparent velocity of 

2 km/s observed for the mainshock (Fletcher et al., 2006) during the S wave and 4 s 

afterwards (body-wave arrivals). Huang and others observed rotational velocities of 

several hundred earthquakes in Taiwan using a rotational sensor on the surface, directly 

above a downhole broadband seismometer (100 m depth). Their observed peak rotational 

velocities are far larger than predicted by Bouchon and Aki (1982) for earthquakes in the 

near field. If these observations are confirmed, then rotation motions of M5 or larger 

earthquake at 15 km away could reach the threshold of structural damage (M. D. Trifunac, 

pers. comm., 2007). 

International Working Group on Rotational Seismology (IWGoRS) 

A luncheon also held at the special session inaugurated the International Working Group 

on Rotational Seismology (IWGoRS). Creation of the IWGoRS is the result of an earlier 

workshop on rotational seismology organized by W. H. K. Lee, K. Hudnut, and J. R. Evans 

of the USGS in February of 2006. That first workshop was organized in response to grass 

roots interest and was held at the Menlo Park and Pasadena, California, USGS offices with 

about 30 participants from numerous institutions participating via teleconferencing and 

telephone (Evans et al., 2007). 

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IWGoRS consists of volunteers and has no official status. Anyone may join either as an 

“associate member” to receive newsletters and emails, or as an “active member” to lead a 

new “task force” or join an existing one. Its charter is accessible on the project web site 

(http://www.rotational-seismology.org). 

IWGoRS Website 

IWGoRS uses this Website to promote rotational seismology by sharing ideas, data, 

software, etc. Website access is free and does not require membership. To start IWGoRS, 

H. Igel and W. H. K. Lee are serving as “co-organizers”. The Working Group has a 

number of active members leading task forces focusing on the organization of workshops 

and scientific projects including: testing and verifying rotational sensors, broadband 

observations with ring laser systems, and developing a field laboratory for rotational 

motions. The IWGoRS web site also contains presentations and posters from the 2006 

AGU special session as well as the earlier workshop; eventually this site will also provide 

access to rotational data from many sources. 

Conclusion 

Rotational seismology is an emergent field previously receiving little attention. The 

substantial interest in the first workshop and the AGU special session demonstrate that 

rotational motions are of current interest to a wide range of 

geophysical disciplines, including strong-motion seismology, exploration 

geophysics, broadband seismology, earthquake engineering, earthquake physics, seismic 

instrumentation, seismic hazards, geodesy, and astrophysics, confirming the timeliness of 

IWGoRS. 

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At the IWGoRS inauguration luncheon it became clear that to establish an effective 

international collaboration within the IWGoRS, a larger workshop was needed to allow 

sufficient time to discuss the many issues of interest and to draft a research plan for 

rotational seismology. Thus, the First IWGoRS Workshop will be hosted by the U.S. 

Geological Survey in Menlo Park, California, on 18–19 September 2007, convened by 

W. H. K. Lee, M. Çelebi, and M. Todorovska. The Second IWGoRS Workshop will be 

organized by the European members of the Working Group and is scheduled for 2009. 

References 

Aki, K., and P. G. Richards, Quantitative Seismology, 1st edition, W. H. Freeman and Co., San 

Francisco, California, 1980; 2nd edition, University Science Books, Sausalito, California, 2002. 

Bouchon, M., and K. Aki, Strain, tilt, and rotation associated with strong ground motion in the 

vicinity of earthquake faults, Bull. Seismol. Soc. Am., 72, 1717–1738, 1982. 

Evans, J. R., and others, Report of a workshop on rotational ground motion, U.S. Geol. Surv. Open 

File Rep., 2007-1145, 14 pp., 2007. 

Fletcher, J. B., P. Spudich, and L. M. Baker, Rupture propagation of the 2005 Parkfield, California 

earthquake from observations at the UPSAR, Bull. Seismol. Soc. Am., 96, S129-S142, 2006. 

Igel, H., U. Schreiber, A. Flaws, B. Schuberth, A. Velikoseltsev, and A. Cochard, Rotational 

motions induced by the M8.1 Tokachi-oki earthquake, September 25, 2003, Geophys. Res. 

Lett., 32, L08309, doi:10.1029/2004GL022336, 2005. 

Igel, H., A. Cochard, J. Wassermann, U. Schreiber, A. Velikoseltsev, and N. P. Dinh, Broadband 

observations of rotational ground motions, Geophys. J. Int., 168(1), 182–197, 2007, 

doi:10.1111/j.1365-246X.2006.03146.x. 

Mallet, R., Great Neapolitan Earthquake of 1857, vol. I and II, Chapman and Hall, London, 1862. 

Richter, C. F., Elementary Seismology, W. H. Freeman & Co., San Francisco, California, 1958. 

Teisseyre, R., M. Takeo, and E. Majewski, eds., Earthquake Source Asymmetry, Structural Media 

and Rotation Effects, Springer Verlag, Berlin, 2006. 

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