A PHYSICAL MODEL OF NEAR-HOMOGENEOUS 1-D HORIZONTAL SHORTENING OR STRETCHING OF A ~2-D LAYER

This physical model was developed in support of a curricular module that uses velocity data from three non-colinear GPS sites within the Plate Boundary Observatory network to determine mean crustal strain within that triangle (bearspace.baylor.edu/Vince_Cronin/www/PBO_ed/GPSstrainShortcourse.html). Prior attempts to create physical models that generate homogeneous stretching or shortening of a layer have been frustrated by a variety of issues. Plastic wrap used in food preservation can be stretched but is usually too weak, has limited extensional range, and does not contract because it is not elastic. Rubber sheeting is elastic, but is usually too strong and is sometimes difficult to find for purchase. The substrate we use is an elastic fabric used as a swimsuit liner, which is widely available through fabric stores. The same material is also widely used in athletic compression shirts; however, the fabric used in some compression shirts has ribbing that results in anisotropy during stretching or shortening. We use elastic cloth without anisotropy that is significant at the scale of the model.

One edge of the elastic cloth is fastened firmly to a base made of melamine-covered fiberboard, and the opposite edge has a casing or rod pocket through which is fitted a steel rod ~3-5 mm in diameter. Smaller, student models can use foam board in place of the melamine board. We use dry powder (joint compound used for taping and skim-coating gypsum-board walls, widely available at home-improvement stores) to simulate the crust. Dry powder produces cracks or true faults, as contrasted with sand that deforms by inter-granular sliding and flow. Dry flour is an alternative to joint compound, but its disadvantages include [1] potential contamination of a classroom that might be harmful to gluten-intolerant students, and [2] the attraction of hungry vermin. Sometimes we use a thin layer of dry sand overlain by a thin layer of dry powder.

Placing a uniform thin layer of powder on the elastic fabric and then stretching the fabric by pulling on the steel rod produces a set of mode I cracks in the layer. Placing the powder on stretched fabric and letting the fabric relax and shorten produces multiple small reverse faults. Various simple modifications of the model produce interesting results that can lead to student-initiated discovery learning.