HR: 11:50h
AN: S12A-06    [Abstracts]
TI: Using 3D Wavefield Modeling in Modeling in Interpreting Historical Macroseismic Observations - The Luroy Earthquake of 31 Aug. 1819.
AU: * Husebye, E S
EM: eystein.husebye@geo.uib.no
AF: Eystein S. Husebye, Department of Earth Science University of Bergen Allegaten 41, Bergen, Norway, Bergen, N-5007
AU: Kebeasy, T R
EM: tarek\_kebeasy@yahoo.com
AF: Tarek R.M. Kebeasy, National Institute of Astronomy and Geophysics, Helwan, Cairo, Egypt, Cairo, EG-11421
AB: The Luroy earthquake of August 31, 1819 with MS around 6.0 is, by many colleagues, rated as the largest in NW Europe in historical times (pre-1900) and even up to present. Local shaking manifestations were most spectacular with rock and mud avalanches, mast-high waves in nearby Rana fjord and even liquefaction was reported. Most surprisingly, at epicentral distances exceeding 100 km except for Stockholm 800 km away, very few macroseismic observations are available. Another peculiarity was the lack of any significant housing damage even in the Luroy parish itself. In a recent paper, we postulated that the earthquake was of moderate size, reestimated at MS = 5.1, but of shallow depth between 5 - 10 km causing the intense local shaking. In this article, we add a new dimension to the many of Luroy earthquake studies namely simulating the seismic wavefield response of Luroy itself and adjacent areas characterized by steep topographic relief. We use a 3D finite difference scheme and compute ground motion for a point source. We used a shear waves source with a focal depth of 5 km. Water covered areas are replaced by crystalline crust due to the dearth of dense bathymetric data. Main results are that the topography of the Luroy, close to the mountain peak at 685 meter, cause wavefield amplification by a factor of 20 and even stronger. Further away in the Rana fjord and surrounding areas, we also got strong amplification in particular where the relief is sharpest thus explaining triggering of avalanches in a quantitative manner. In other words, macroseismic observations would be biased upward due to the topographic focusing effects and unless properly corrected for also will increase the final earthquake magnitude estimate. We take these results to strongly support our claim that the historic Luroy earthquake was of moderate size of MS = 5.1 and not at MS = 6.0 class as claimed by many colleagues. Finally, downscaling of maximum earthquake magnitude would also lower the seismic risk levels significantly.
DE: 7260 Theory and modeling
DE: 5460 Physical properties of materials
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 1734 Seismology
SC: Seismology [S]
MN: 2004 AGU Fall Meeting