DOWNWARD PENETRATING CLASTIC DIKES AS INDICATORS OF TSUNAMIS?

Researchers have studied the process of hydraulic fracturing in cracking unlithified materials, such as soils, largely because of its role in the failure of earthen dams and in vapor/fluid extraction. Recently, Obermeier (1996) identified hydraulic fracturing as a chief mechanism that forms upward penetrating clastic dikes during seismic liquefaction. Dikes commonly form where a fine-grained surficial cap soil overlies liquefiable sand. Here, excess porewater pressure from the liquefied sand exploits pre-existing minor flaws along the base of the cap, creating tensile stresses at the flaw tip and enlarging it into a tabular fracture. The crack propagates if the excess fluid pressure does not readily dissipate in the host sediments and the fluid-induced tensile stress exceeds the horizontal stress plus the compressive strength of the host. The rapidly-induced porewater pressure also drives fluidized sand and water into the newly widened fracture, creating an upward penetrating dike.

In southeastern Washington, similar but downward-injected tabular clastic dikes are ubiquitous in the sediments of glacial Lake Lewis, a temporary impoundment of the latest Pleistocene (15.5-12.7 Ka) Missoula floods. Based on our recent field studies there, we found that these dikes commonly developed by flood-induced, downward-oriented hydraulic fracturing from the surface, providing evidence of a sudden rise in water depth and pressure. Genesis and morphological details of the dikes critically depend on host sediment grain size and permeability, among other factors. Importantly, our hypothesis does not require seismic shaking to form the dikes.

The sudden deluges of the Missoula floods probably loaded the ground surface in a manner analogous to tsunamis, and the clastic dikes that they left behind may have counterparts in tsunami deposits. As such, downward penetrating dikes can be a useful signature of where paleotsunamis have struck, worldwide. Further, downward penetrating dikes may form in field settings where surficial signatures such as marine sand beds are unlikely to develop or be preserved or recognized. In addition, such dikes may provide estimates of tsunami flow depth – a key element in assessing tsunami hazard.