Coseismic and early postseismic slip of the 2011 Tohoku-Oki sequence deduced from the GNSS carrier phase to fault slip approach

Abstract:

Broadband fault slip monitoring is essential to correctly understand thorough process of earthquake occurrence cycle. However, current positioning analysis shows lower precision for some phenomena such as early afterslip, with time scale of hour to day. This is mainly due to difficulty of parameter separation. Therefore we investigate alternative approach, GNSS carrier phase to fault slip approach (PTS (Phase To Slip)). PTS directly relates carrier phase to fault slip via Green’s function and geometric transform into LOS direction. Then, it can truck relative change of all unknown from certain initial epoch simultaneously. Thus, we can expect that PTS has advantages in evaluation of separation accuracy and its improvement.

In this study, we tried to investigate the continuous estimation of coseismic and early postseismic slip of the 2011 Tohoku-Oki sequence using PTS. We used 1 Hz carrier phase data of 62 GEONET sites around eastern Japan. Analysis period is 05:00-07:00 (UTC) on 2011 March 11^th (Mainshock origin is 05:46). As a result, we obtained coseismic slip distribution of the mainshock and the largest aftershock in the off Ibaraki region, that well agree with the estimations based on normal positioning. Additionally, in time after the mainshock, the estimated slip time series show slow thrust slip around downdip edge of the major slip area of the mainshock. Mainly two slip areas were estimated below coastal region of Iwate, Miyagi and Fukushima. These areas show different temporal evolution of slip and maximally reach about 0.8m within 39 minutes after the mainshock. Equivalent total moment reached M_w7.55 within the same time window. Although these amounts seem to be clearly larger than estimations by previous studies, we conclude that these slip areas certainly indicate the early afterslip. Our results show capability of the PTS for broadband monitoring system which can accommodate co- and postseismic processes. In the presentation, we will discuss in more details of our method, results and discussion about their validity.