Periodic slow earthquakes in Cascadia constitute one of the most geophysically interesting signals detected to date with continuous GPS monitoring of subduction zones. However, the GPS measurements are limited in what they can resolve about the underlying physics of the transient creep process. In particular, the vertical deformation is poorly resolved with GPS; daily GPS solutions averaged over several days cannot constrain whether slip occurs in discrete, short-lived episodes predicted by heterogeneous slip regimes on faults. This Major Research Instrumentation grant supports installation of five long baseline tiltmeters in order to supplement ongoing GPS measurements within the Pacific Northwest Geodetic Array (PANGA). These instruments have almost three orders of magnitude higher spatial and temporal resolution than GPS, and will be able to capture fine scale vertical motion and any temporal variations in deformation that accompany the slow faulting episodes. The long baseline tiltmeters are to be deployed within the Northern Puget Sound region and co-located with existing PANGA GPS stations, where the slow events are known to be nucleating on average every 14 months, give or take 1 month. The first tiltmeter is expected to be deployed in the autumn of 2003, followed by four additional installations over the following two years. In conjunction with ongoing GPS measurements, the addition of the tiltmeters will increase our resolving power in location and duration of slip from ~50 km and several days to less than a km and tens of minutes, respectively.
