Analyzing Slip Events Along The Cascadia Margin Using An Improved Sub-Daily GPS Analysis Strategy

A GPS analysis strategy that reduces the noise level of GPS-based sub-daily strain measurements and improves sub-daily resolution of positions and baseline estimates is used in order to study slow slip events along the Cascadia margin. These improvements are accomplished by reducing the key sources of sub-daily noise from path delays caused by reflections and refractions of the GPS signal near the receiver (multipath), and from tropospheric delays. Errors due to poorly determined tropospheric path delays are mitigated by using the tropospheric parameters estimated in static positioning runs as fixed values. The multipath effects are treated as periodic errors and are mitigated by a modified sidereal filter applied to the phase prior to processing. Together, these improve the sub-daily noise levels of station positions by a factor of ~2-5 and the baseline strain estimates by up to a factor of ~10 depending on baseline length and the strain signal period. Using these strategies we examine the along-strike slip migration during episodic slip and tremor accounted for the 2007-2009 period. We will present comparisons of the sub-daily GPS deformation during the slip with episodic seismic tremor. We will also present comparisons of regional sub-daily strain estimated from GPS measurements, with local borehole strain-meter data.