Effects of the 2011 Tohoku Earthquake on VLBI Geodetic Measurements

The VLBI antenna TSUKUB32 at Tsukuba, Japan regularly observes in 24-hour observing sessions once per week with the R1 operational network and on additional days with other networks on a more irregular basis. Further, the antenna is an endpoint of the single-baseline, 1-hour Intensive sessions observed on the weekends for determination of UT1. TSUKUB32 returned to normal operational observing 25 days after the earthquake. The antenna is 160 km west and 240 km south of the epicenter (about the same distance west of the plate subduction boundary). We looked at the transient behavior of the TSUKUB32 position time series following the earthquake and found that significant deformation is continuing. The eastward rate as of July 2011, 4 months after the earthquake, is 20 cm/yr greater than the long-term rate prior to the earthquake. The VLBI series agrees with the corresponding JPL GPS series (M. B. Heflin, http://sideshow.jpl.nasa.gov/mbh/series.html, 2011) measured by the co-located GPS antenna TSUK. The coseismic UEN displacement at Tsukuba was approximately (-90 mm, 550 mm, 50 mm). We examined the effect of the variation of TSUKUB32 position on EOP estimates and specifically how best to correct its position for estimation of UT1 in the Intensive experiments. For this purpose and to provide operational UT1, the IVS scheduled a series of weekend Intensive sessions observing on the Kokee-Wettzell baseline immediately before each of the two Tsukuba-Wettzell Intensive sessions. Comparisons between UT1 estimates from these pairs of sessions were used in validating a model for the post-seismic displacement of TSUKUB32.