THE STOPPING OF A RISING, LIQUID-FILLED CRACK IN THE EARTHS CRUST BY A FREELY SLIPPING HORIZONTAL JOINT

The stress intensity factor K at the upper tip of a vertical, liquid-filled crack in the earth's crust is large for relatively small cracks (K greater than 3 MPa m** one-half when the crack length is larger than about 40 m). Consequently, the large water-filled cracks considered for use in the extraction of geothermal heat might be expected to pop to the earth's surface. It is shown that a freely slipping horizontal joint (as well as a not so freely slipping joint) can stop the upward motion of a large liquid-filled crack. If the joint is tightly fitting, there is no limit to the length of a stopped crack. If slip on a joint results in the joint being propped open, the maximum crack length of a stopped crack is given by the crevasse expression: total crack length equals pi T/2( rho minus rho ')g, where T is the tectonic stress, g is the gravitational acceleration, and rho minus rho ' is the difference between the rock and liquid density. The results can also be applied to the problem of magma transport in the earth's crust.