A damage model for the absence of significant precursory seismicity

Acoustic emissions prior to rupture indicate precursory damage. Laboratory studies of frictional sliding on model faults feature accelerating rates of acoustic emissions prior to rupture. Precursory seismic emissions are not generally observed prior to earthquakes. To address the problem of precursory damage we consider failure in a fiber-bundle model. We observe a clearly defined nucleation phase followed by a catastrophic rupture. The fibers are hypothesized to represent asperities on a fault. Two limiting behaviors are equal load sharing (stress from a failed fiber is transferred equally to all surviving fibers) and local load sharing (stress from a failed fiber is transferred to adjacent fibers). We show that precursory damage in the nucleation phase is greatly reduced in the local-load sharing limit. We argue that laboratory experiments on fracture involve near-uniform load sharing whereas actual faults involve local load sharing. When one asperity fails on a fault the force carried by the asperity is transferred to adjacent asperities. We argue that this explains the absence of a well defined nucleation phase prior to an earthquake.