Spatial and temporal development of the early afterslip following the 2011 Tohoku-Oki earthquake using onshore GNSS and offshore pressure data

The information of temporal and spatial slip behavior of the early afterslip contain the important information for the frictional properties of the plate interface. Several studies conducted estimation of the early afterslip for the 2011 Tohoku-Oki earthquake (e.g. Munekane et al., 2011). The previous studies, however, only used onshore GNSS data. Based on these backgrounds, we investigated the spatial and temporal development of the early afterslip of the 2011 Tohoku-Oki earthquake using both of the onshore GNSS and offshore pressure data.

We processed the raw GNSS data by the kinematic PPP analysis. To remove the common-mode noise error, we applied the principal component analysis (PCA) to the estimated time series, and only extracted the 1^stand 3^rd mode of the principal components as the signal. For the offshore OBP data, ocean tides were accounted for by harmonic analyses using the BAYTAP-G model and removed from the OBP data. The effect of the motion of the sea-water layer was estimated by using global barotropic ocean model (Inazu et al., 2012) and removed. Finally, we applied the two-hour moving average to reduce the effect of tsunami propagation. To estimate the amount of the displacement stably, we firstly estimated the "common" time decay constant of logarithmic function by the fitting using the only onshore GNSS data. Secondly, we fixed the estimated common value for the time constant and then estimated again the amplitude of the logarithmic function for both of the GNSS and OBP data. We used 18-hour data and estimated afterslip distribution in the every two-hour.

We adopted the L1 regularization approach (LASSO , Tibshirani, 1996) for the estimation of the early afterslip distribution using both of the GNSS and OBP data. The estimated cumulative early afterslip distribution until 18-hour from the mainshock mainly concentrated to the down-dip extension of the coseismic slip while the small afterslip patches also confirmed in the shallower portion of the plate interface at Miyagi-Oki region. The estimated afterslip could basically explain the data. The variance reduction by estimated afterslip distribution was 78.4% and 36.4% for GNSS and OBP stations, respectively. We will describe the obtained results more in detail as well as the characteristic of the spatial and temporal development of the early afterslip.