Are Rock Avalanches and Landslides Due to Large Earthquakes or Local Topographic Effects? A Case Study of the Lurøy Earthquake of August 31, 1819, A 3D Finite Difference Approach

Abstract

The Luroy earthquake of August 31, 1819, with MS similar to 5.8 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, stone and mud avalanches, mast-high waves in nearby Rana fjord and even liquefaction was reported. Most surprisingly, at epicentral distances exceeding 100km except for Stockholm 800km 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 similar to 5.1, but of shallow depth between 5 and 10km causing the intense local shaking. In this article, we add a new dimension to the many Luroy earthquake studies namely simulating the seismic wavefield response of Luroy itself and adjacent areas characterized by steep topographic refiefs. We use a 3D finite difference scheme and compute ground motion in the 2-8Hz band for a shear wave source with a focal depth of 5 kin. Water covered areas are replaced by crystalline crust due to the sparsity of dense bathymetric data. Main results are that the topography of the Luroy, close to the mountain peak at 685 m, causes 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 may 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 similar to 5.1 and not at MS similar to 6.0 class as claimed by many colleagues. The largest magnitude estimates stem from including outlier observations in Kola and Stockholm. Finally, downscaling of maximum earthquake magnitude would also lower the seismic risk levels significantly.