Real-time Monitoring of Slip • Triangles: Tremor Activity – Average envelope seismogram amplitudes in tremor passband – Active Tremor = black triangles • Arrows: GPS Short-term Velocity – 50 -day GPS site velocities after long -term velocity is removed – Slow Slip = large arrows pointing towards trench
January-February 2007
June-July-August 2007
April-May-June 2008
NVT Location Approach • Filtered envelope waveforms • STA/LTA to find NVT bursts • Pick relative times of good S/N arrivals – By hand and/or automated • 1 D inversion for locations • Analyze for patterns in space and time
NVT Migration in Southern Cascadia 2005 -2006: Hand-picked vs. Automated
NVT Migration Along All of Cascadia 2007 -2008 (Automated) • Still preliminary, but find long migrating episodes that overlap in time despite being separated by >100 km • Three 2007 -2008 events are more interrelated alongstrike than all events in previous few years
NVT Locations So Far • Still plenty of work for us to do in the northern half • Beginning to see a persistent gap at ORCA border • NVT band primarily between 30 -40 km slab contours • Matches some aspects of slab curvature
Integrating ETS with Earth Structure and Geology ETS Periodicity Brudzinski & Allen, 2007 Surface Wave Crust Structure Body Wave Mantle Structure Arc volcanism Schmidt et al. , 2008