Earthquake rupture in shallow, unconsolidated sediment

Faults in shallow, unconsolidated sediment are often associated with aseismic creep, due to the velocity-strengthening behaviour of unconsolidated materials observed during lab experiments. They are expected to appear as broad zones of distributed deformation. However, large seismic ruptures can still propagate to the surface through shallow sediment, causing vast damage and destructive tsunamis. Our understanding of how seismic rupturing of shallow faults in unconsolidated sediment occurs in nature is limited due to the lack of direct observations constraining their structure, deformation patterns and mechanisms, and frictional behaviour.

We studied syn-depositional normal faults, which deform unconsolidated, saturated lake sediments of the palaeo-Dead Sea. The sediments belong to the Lisan Formation (~70-18 Ka) and comprise alternating 1-3 mm thick laminae of aragonite and ultrafine-grained detritus. The faults formed at the surface, the only overburden being the overlying water column, and are known to have hosted seismic slip during large events (M ≥ 6), due to their association with seismites. The faults are discrete, localized structures, which sharply truncate laminae in the host sediment. Slip surfaces are sharp and straight and accompanied by a narrow slip zone, up to 1 cm wide, but often ≤ 1 mm wide, containing ultrafine-grained gouge. The majority of slip, up to 3 m, is concentrated in these slip zones. Faults can be categorised as having either simple geometry (one continuous fault strand accommodates all the displacement) or complex geometry (two or more fault segments share the overall displacement). Slip profiles constructed for simple geometry faults all have similar shapes, regardless of maximum displacement, whereas those for complex geometry faults are highly variable, due to segment interaction. It is apparent from the slip profiles that these faults grow and interact in the same way as ‘brittle’ faults in cohesive rocks.

We will present results of microstructural analyses of the fault rocks, to constrain the deformation mechanisms occurring during seismic slip in unconsolidated sediment. We will also present results of low- and high-velocity friction experiments, which will constrain the conditions under which brittle deformation and seismic slip occur in these materials.