Mechanical properties of shallow fault rock in erosive margins: Influence on earthquake rupture propagation and slow slip events

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Plate boundary thrust faults at convergent margins generate the largest and most destructive earthquakes, and tsunamis. Over half of worldwide subduction zones are erosional margins, inlcuding the Japan Trench and Costa Rica margins. The Mw = 9.0 Tohoku event in the Japan Trench demonstrates that erosive subduction zones are capable of generating great earthquakes and tsunamis. The shallow portions of subduction zones are of great importance for two reasons: (1) they may be prone to earthquake rupture and coseismic slip propagation and (2) they may generate slow earthquakes. Direct observation, monitoring and sampling of fault zones at seismogenic depths in subduction zones has yet to be achieved, but core samples from within and adjacent to major thrust faults have been recovered from shallow depths (~1 km or less). There is also evidence of shallow slow slip events offshore Costa Rica from borehole strainmeters and GPS data. Slow earthquakes also occur in the shallow near-trench region of the Japan Trench. Therefore, available natural samples obtained from well-described subduction zone settings, such as the Japan Trench and Costa Rica, are ideally suited for studying shallow earthquake rupture propagation and slow slip events. Describing the slip behavior of major fault zones relies heavily on the results of laboratory friction experiments. The proposed research intends to use core samples recovered during IODP Expedition 343 (Jfast) to the Japan Trench and ODP Leg 170 and 205 to the Nicoya Peninsula region offshore Costa Rica, in order to measure frictional strength and sliding stability. Measurements will cover lithologic variations in a vertical transect from the overlying sediment prism through the decollement and underthrust sediments in the Japan Trench. Further experiments will examine the effect of spatially varying lithology on the incoming plate in Costa Rica. Low slip velocities (<1 cm/s) will be used, which are critical for predicting the response of a fault zone to earthquake propagation. An important part of the testing program will be measurements of cohesive strength in both fault gouge and wall rock, which remains unexplored by the geophysical community but is a potentially key parameter. Additional experiments will be included which accurately mimic slow earthquakes in nature. Supplemented by detailed analyses of mineralogic composition by XRD and physical characterization of the sheared specimens by WLI, the proposed research will result in a comprehensive set of frictional property measurements revealing the mechanisms behind earthquake propagation and the occurrence of slow slip events in the shallow portions of erosive subduction zones.