APPARENT STRESSES
SUMMARY

In summary, we note that analysis of the kinetic energy of seismic waves suggests that the associated apparent stresses (taua) do not exceed 5 MPa. Similarly, analysis of long-term frictional heating and the predicted and observed effects on heat flow from conduction theory suggest that the average frictional resistance taur does not exceed about 10 MPa. Thus, according to equation 5, the long-term average combined earthquake stress tau-bar probably does not exceed about 15 MPa, and, of course, it could be much less. The initial stress tau1, or "fault strength," would be greater by half the stress drop (fig. 10.2; Lachenbruch and Sass, 1980, eq. 41a), no more than another 5 MPa, for an upper limit of 20 MPa. The major assumptions in this analysis are (1) that heat transfer is exclusively by conduction - that is, no significant heat is transferred by moving ground water; (2) that the fault geometry can be represented by the usual simple conventions (see figs. 10.2, 10.3, and 10.4); and (3) that an earthquake's energy is converted exclusively to seismic waves and heat - that is, no appreciable energy is consumed by creating new surfaces, phases, or chemical combinations (Lachenbruch and Sass, 1973, 1980). We shall discuss these points later.