Slow-Slip Events in Northern California with 8-Month and 6-Year Periods

TitleSlow-Slip Events in Northern California with 8-Month and 6-Year Periods
Publication TypeConference Proceedings
Year of Conference2018
AuthorsDwivedi, A, Herring, T
Conference NameAGU Fall Meeting 2018
Date Published12/2018
PublisherAmerican Geophysical Union
Conference LocationWashington, DC

Slow-slip events in the Cascadia subduction are well established with strong correlations between seismic tremor and geodetically measured surface deformation signals that last 1-2 weeks and have repeat times of ~14 months. Less attention has been focused on the Gorda-North America section of the subduction zone which spans southern Oregon and northern California. We present results here for GPS sites in northern California obtained from the analysis of time series produced by the Plate Boundary Observatory (PBO) and made available through UNAVCO. We analyzed sites in northern California with data spanning January 2009 to June 2018. We estimated linear trends, annual sinusoids, and offsets in the time series at times of antenna changes and earthquakes (for sites near the epicenters of off-shore earthquakes in 2010 and 2014). The north and east position residuals to these fits were analyzed with a principal component analysis (PCA). Two distinct, quasiperiodic signals can be seen in the first two principal components. One of these signals is similar to northern Cascadia, but with a period of ~8 months, with the other signal having a much longer, almost sinusoidal, variation with a period of ~6 years. To better isolate these signals, we generate one data set where we smooth the time series residuals with a 100-day Gaussian filter and another data set is generated by differencing the 100-day Gaussian smoothed results from ~1-day Gaussian smoothed residuals. The spatial pattern of first principal component from the two sets of smoothed results are similar with the largest eigenvectors centered on 40.5°N latitude and 123°W longitude. The long period signal is distributed over a larger geographical area. Inversion for fault planes that can explain the spatial patterns yields fault planes that lie near the subduction zone interface with a strike of ~-17° and a dip of ~17° with uncertainties of 2-3° in each component. The motion on the fault planes is almost all up-dip. Within the uncertainties of the inversions both the fault planes are similar except the larger size of the fault plane for the longer period signal. As validation of the results, we show that the vertical signals predicted from the fault plane inversions can be seen in many of the GPS sites for both the long period and short period signals.