Understanding the physical processes causing surface creep on faults.

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Faults have been knownto slip aseismicallyor creep at the surface since the 1960s when creep was observed along the San Andreas Fault following the 1966 Parkfield Earthquake 1. The San Andreas Fault has since been observed to be creeping along a 150km long sectionbetween San Juan Bautista and Parkfield. Since the discovery of creep along this section of the San Andreas Fault, creepmeters and alignment arrays have been installed to monitor the creep2. Surface creep has been observed on many other fault systems around the world. Creephas been observed on other large strike-slipfaults such as the North Anatolian Fault near Ismetpasa, Turkey 3and the Philippine Fault on the Island of Leyte 4. Strike-slip faults are not the only type of faults that have surface creep, with creep observed on reverse faults in Taiwan 5,6.Creep on faults has been observed to occur in bursts known as creep events; these can be identified on the recordings oncreepmeters(Figure 1).Creep events have variations in slip characteristics (slip rate, duration,and slip) as a result ofphysical processesthat causes creep events. Despite the observation of surface creep since the 1960s,thephysical processby which creep is produced is not yet known. Many models have been proposed for creep including,viscous rheology 7, rate-and state-friction 8, size-limited friction 9and shear-induced dilatancy 10. Each modelhas its own merits.However,the debate over which one process produces surface creep is still ongoing. Another fundamental question still tobeanswered is howmuch of the fault is involved in creep events are.Given creep eventsare recorded at point locations using creepmeters, we do not know how much of the fault is creeping during these events.By using complementary creepmeters, creep propagation along the Calaveras Faulthas been observed 11. By investigating creep propagation, thesize extent of these creep events maybe uncovered leading to a new understanding of the role surface creep plays in the wider earthquake cycle. AimsDetermine how big creep eventsare.Discriminate between physical models proposed for creep events.Implementand developa time-lapse imaging-based technique for monitoring surface creep.