Documentation of earthquake-induced disturbance of lake sediments using subbottom acoustic profiling

Abstract

Subbottom acoustic profiling in lakes on the Canadian Shield and in the Canadian Cordillera has revealed disruptions in deltaic and lacustrine sediments that can be attributed to large historical earthquakes. The Timiskaming earthquake of 1935 (Magnitude 6.25) and the Vancouver Island earthquake of 1946 (Magnitude 7.2) triggered extensive slumping and flow of postglacial sediments in lakes located within a few tens of kilometres of the epicentres of these events. There is some evidence in one of the surveyed lakes for in situ mobilization of sediment on relatively gentle slopes, with minimal horizontal displacement; this may be a product of liquefaction induced by coseismic shaking or displacements along faults beneath the basin. In another lake, widespread coseismic slumping resuspended large amounts of fine sediment, which then settled out onto the lake floor to produce a texturally anomalous layer. Features similar to those produced by the 1935 and 1946 earthquakes can also result from a variety of nontectonic processes, for example, the collapse of sediment over buried ice blocks, depositional oversteepening of deltas, groundwater sapping, and variations in pore-water pressures due to lake-level fluctuations. Records of sediment disruption in lakes therefore must be interpreted with caution. In general, many lakes in a region should be surveyed and a variety of different features identified before sediment disruptions are attributed to an earthquake.Un sondage acoustique sur des lacs dans le Bouclier canadien et dans la Cordillkre canadienne a devoilt des ruptures dans des dCp6ts deltaiques et lacustres, lesquelles auraient pu Ctre provoqukes par de puissants tremblements de terre historiques. Le tremblement de terre de Timiskaming, en 1935, de magnitude 6,25, et le tremblement de terre de l'fle Vancouver, en 1946, de magnitude 7,2, ont produit d'importants glissements et des coulCes de sediments postglaciaires dans des lacs localises B 1'intCrieur d'une region de quelques dizaines de kilomktres des epicentres de ces stismes. Dans un des lacs CtudiCs, les indicateurs suggkrent une remobilisation in situ des sediments sur des pentes relativement faibles, avec un deplacement horizontal minimum; il s'agit peut-Ctre d'un effet de liquefaction induite par une agitation cosismique ou de mouvements le long des failles existantes sous le bassin. Dzns un autre lac, un grand glissement cosismique a remis en suspension d'importantes quantites de matkriel fin, qui ont sediment6 sur le fond du lac pour former une couche presentant une texture anormale. Des phCnomknes analogues i ceux crCCs par les tremblements de terre de 1935 et de 1946 peuvent possiblement rCsulter de divers processus non tectoniques, par exemple, la chute de sediments accumulCs sur des blocs de glace enfouis, des dCp6ts instables sur les pentes de deltas trop raides, le sapement par les eaux souterraines, et les variations des pressions de l'eau interstitielle crkCes par les fluctuations du niveau du lac. Les Cvknements de rupture des sediments dans le...

Earthquake-induced deformation structures in lake deposits: A Late Pleistocene to Holocene paleoseismic record for Central Switzerland

“…Paleoseismic studies have been successfully carried out in outcrops of lacustrine sediments (for example Sims 1973 and1975;Marco et al 1996;Ringrose 1989;Rodriguez-Pascua et al 2000;Hibsch et al 1997). In modern lakes, investigations are based on high-resolution seismic data (Shilts & Clague 1992;Chapron et al 1999) and more rarely on sediment cores (Doig 1986;Becker et al 2002, Migowski et al 2004.…”

A Reconnaissance Geophysical Survey of the Kawartha Lakes and Lake Simcoe, Ontario

“…Our Unit A corresponds to this sequence. Shilts and Clague (1992) defined an upper, acoustically transparent sequence composed of organic and mineral sediments (gyttja and mud). Our Unit B corresponds to this upper sequence.…”

“…Based on the termination of reflectors within Unit A, on the conformai, draped nature of the sediments across the section, and on the apparent continuity of reflectors below the base of the depressions, we tentatively suggest that these depressions originated as areas beneath which blocks of buried glacial ice had melted, pending the collection of further evidence (Klassen and Shilts, 1982;Shilts, 1984;Shilts and Clague, 1992). Further processing is being applied to all Lake Simcoe single-and multichannel seismic data to improve the lateral continuity of the reflectors.…”

“…Similarly, water released from melting blocks of glacier ice buried by late-glacial sediments may cause localized liquefaction and diapirism. Shilts and Clague (1992) pointed out that faults in glaciolacustrine sediments, produced by tectonically-induced movements in bedrock beneath lakes, will be difficult to differentiate from faults of glacial or glaciotectonic origin. A neotectonic origin for a fault is possible if the fault plane extends from bedrock into, and through, overlying postglacial sediments.…”

“…Because of their clayey or organic composition and high water content, lacustrine sediments are highly deformable and are susceptible to disturbance during earthquakes (Shilts and Clague, 1992). Structural or sedimentological features may be created by movement resulting from earthquakes, and these features may be preserved in the sediments (Doig, 1991).…”