CIRES Council of Fellows>

Roger Bilham

Ph.D., Cambridge University, U.K., 1970
Professor, Geological Sciences
Associate Director, Solid Earth Sciences

E-mail: bilham@stripe.colorado.edu
Office: ESCI (Benson) 462B
Phone: 303-492-6189
Web: Prof. Bilham

Geodetic measurements throughout the Indo-Asian collision, combined with historical archives provide new insights into future earthquake hazards in India. Geodetic and historical archive data for damaging earthquakes in India have been systematically reprocessed to revise magnitudes and rupture parameters. Special studies have been undertaken for M>7.5 earthquakes in 1505, 1803, 1819, 1833, 1842, 1881, 1892, 1897, 1905, 1914, 1931, 1935, and 2002 which have claimed more than 100,000 lives. In the past several centuries earthquakes have ruptured less than 35% of the Himalayan plate boundary, and a similar pattern is emerging for the western and eastern transfoms in Baluchistan and Burma. Geodetic measurements reveal an absence of significant strain within the subcontinent (<3 mm/yr in the past decade) but suggest significant convergence across the ancient rift zone of Kachchh (1 cm/yr in the past century). We are entertaining the possibility that flexure of the subcontinent may underly local deformation, and most of India's mid-plate seismicity. Continuous GPS tracking sites in Pakistan, Bhutan, Nepal and India are planned to form part of a monsoon-monitoring network in the subcontinent.

Plate boundary deformation processes are monitored in several countries with a view to determining the physics of the earthquake process. GPS monitoring programs in Venezuela, Mexico and Mexico provide details of the plate boundary velocity fields in these areas. In Venezuela we report an estimate for the rotation pole of the Caribbean plate relative to south America: in Mexico we report episodic changes in aseismic slip on the subduction zone. In California we operate an array of five creepmeters on the Hayward fault in the Bay Area, and a pair of long baseline tiltmeters monitoring volcanic activity in Long Valley. A slow earthquake was detected on a normal fault in the Long Valley caldera with a coherent strain field detected on the 11 independent sensors of the 850-m-long biaxial tiltmeter. An analysis of tidal strain amplitude (20 minute kinematic GPS solutions over 5 months) across the African rift produced the null result, that strains within and contiguous to the Ethiopian rift show no evidence for rheologic weakness of the rift zone.

The new 500-m-long tiltmeter that was installed near Acapulco, Mexico used float sensors. In 1999 and 2002 we installed similar sensors in the Long Valley tiltmeters. The new sensors have the advantage that they require no maintenance and operate at low power (<0.1 watt). They monitor tilt signals between deep reference points and the equipotential water surface directly, halving the number of signals required to measure each axis of tilt. The new biaxial tiltmeter costs roughly $40k to construct and install, has a tilt resolution of 1 nanoradian, and a long term stability better than 0.1 µrad/yr.

Selected Publications

• Ambraseys, N., and R. Bilham, Earthquakes and crustal deformation in N. Baluchistan, Bull. Seism Soc. In press. 2002.
• Ambraseys, N., and R. Bilham, MSK Isoseismal intensities evaluated for the 1897 Great Assam Earthquake., Bull. Seism Soc. Am. In press 2002.
• Ambraseys, N., and R. Bilham, Earthquakes in Afghanistan, Bull. Seism Soc. Am. submitted Jan. 2002.
• Bendick, R., and ten authors, The January 26, 2001 "Republic Day" Earthquake, India. Seism. Res. Lett., 72(3), 328-335, 2001.
• Bendick, R., and R. Bilham, How perfect is the Himalayan Arc? Geology, 29,791-794, 2001.
• Bilham, R. and P. England, Plateau pop-up during the great 1897 Assam earthquake. Nature, 410, 806 - 809 (2001).
• Bilham, R., .R. Bendick and K. Wallace, Flexure of the Indian Plate and Intraplate Earthquakes, J. Ind Inst of Sci. in press.
• Bilham, R., and V. K Gaur, Geodetic contributions to the study of seismotectonics in India, Current Science 79(9), 1259-1269, 2000.
• Bilham, R., Slow tilt reversal of the Lesser Himalaya between 1862 and 1992 at 78°E, and bounds to the southeast rupture of the 1905 Kangra earthquake, Geophys. J. Int (2001) 144, 1-23.
• Bilham, R., V. K. Gaur and P. Molnar, Himalayan Seismic Hazard, Science, 293, 1442-4, 2001.
• Hough, S. E., S. Martin, R. Bilham and G. Atkinson, The January 26 2001, Bhuj India earthquake: Observed and predicted Ground Motions, Seismological Soc. Am. 2002.
• Paul, J.,and ten authors, 2001 The motion and active deformation of India. Geophys. Res. Lett. 28 (4) , 647-651, 2001.
• Wallace, K, R. Bilham, .R. Bendick, V. Gaur and F. Blume, Geodetic constraint of the Bhuj 2001 rupture, J. Ind Inst of Sci. in press.
• Wang, Qi, Pei-Zhen Zhang, J. T. Freymueller, R. Bilham, K. M. Larson, Xi'an Lai, X. You, Z. Niu, J. Wu, Y. Li, J. Liu, Z. Yang, Q. Chen, Present Day Crustal Deformation in China constrained by Global Positioning Measurements, Science, 294, 574-577, 2001.
• Bilham, R. , R. Bendick, K. M. Larson, P. Mohr, J. Braun, J. Tesfaye and L. Asfaw, Secular and tidal strain across the Main Ethiopian Rift, Geophysical Research Letters , 26(18), 2789-2792, 1999.
• DeMets, C., Jansma, P. E., Mattioli, G. S., Dixon, T. H., Farina, F., Bilham, R., Calais, E., Mann, P., GPS geodetic constraints on Caribbean-North America plate motion , Geophys. Res. Lett. 27 , No. 3, p. 437 (2000).
• Kostoglodov, V., R. Bilham, J. A. Santiago, V. Manea, M. Manea, V. R. Hernandez, Long baseline tiltmeter for seismotectonic studies of the Mexican Subduction zone. Geofisica International, 41,1, 11-25, 2002.
• Lowry, A. R., K. Larson, V. Kostoglodov, and R. Bilham, Transient fault slip in Guerrero, south Mexico, Geophys. Res Lett. 28(19), 3753-3757, 2001.
• Pérez,O. J., R. Bilham, R. Bendick, J. R. Velandia, N. Hernández, C. Moncayo, M. Hoyer, M. Kozuch, Velocity Field across the southern Caribbean Plate Boundary and estimates of Caribbean/S. American Plate Motion using GPS Geodesy 1994-2000. Geophys. Res. Lett., 28(15),2987-2991, 2001.
• Weber, J., Dixon T DeMets, C., Ambeh, W., Jansma. P., Matioli G., Bilham R., and Saleh, J and Perez O. A GPS estimate of the relative motion between the Caribbean and the South American plates, and geologic implications, Geology, 29, 75-78, 2001.

Bilham is also one of the CIRES Faculty who teach at the University of Colorado at Boulder with teaching loads equivalent to those of CU faculty rostered solely in departments and programs.