Tadahiro Sato

Results from parallel observations of superconducting and absolute gravimeters and GPS at the Hsinchu station of Global Geodynamics Project, Taiwan

few mm a � 1 of horizontal and vertical motions around HS. The calibration factor and driftingrateofT48are � 75.96±0.07mGalV � 1 and0.2±0.7mGala � 1 (1mGal=10 � 8 ms � 2 ). Both the SG and absolute gravity records contain trends of about 2–3 mGal a � 1 . The ocean tide gravity effects (OTGEs) were estimated from NAO.99b, FES2004, and CSR4.0, and their amplitudes agree with the SG observations at the submicroGal level, but their phasesdifferfromtheobservationsupto10.TheNewtonianeffectofoceantidecontributes 20% to the total OTGE at HS, and it is larger at islands in the Taiwan Strait. The inelastic body tide model of Dehant et al. (1999) is more consistent with the SG observations than the elastic model. Modeled gravity-atmosphere admittances based on an exponential distribution of air mass explain well the observed admittances. The average gravityatmosphere admittance during typhoons is 30% larger than that in a nontyphoon time. A list of coseismic gravity changes from T48 caused by earthquakes over 2006–2007 is given for potential studies of fault parameters. The modeled effects of atmospheric pressure, groundwater, soil moisture, and polar motion explain the FG5 observed gravity trend to 1.1 mGal a � 1 . Seasonally, the groundwater-induced gravity change contributes the most

Gravity and uplift rates observed in southeast Alaska and their comparison with GIA model predictions

gravity change rates (unit: mGal/yr, 1 mGal = 10 8 ms 2 ) over the 6 sites are estimated to be 4.50 0.76 and 4.30 0.92 by only using our data and also using the 1987 data, respectively. We computed the uplift and gravity rates predicted by ice load models for three different time intervals: Last Glacial Maximum (LGM), Little Ice Age (LIA) and Present-Day (PD). Except for 1–2 examples, the predictions recover the observed rates within the observation errors. We also estimated the viscous portion of the ratio (unit: mGal/mm) of the observed gravity rate to the uplift rate by correcting for the effects of the Present-Day Ice Mass Change (PDIMC). Two PDIMC models are compared, which are called here as UAF05 and UAF07. Mean ratios are estimated to be 0.205 0.089 and 0.183 0.052 for the cases using UAF05 and UAF07, respectively. The predicted mean ratios are 0.166 0.001 and 0.171 0.002 for the cases using both the LGA and LIA ice models and only using the LIA ice model, respectively. We have confirmed that our AG and GPS observations detect the rates and ratios reflecting an early stage of viscoelastic relaxation mainly due to the unloading effects after the LIA.

Southern Ocean mass variation studies using GRACE and satellite altimetry

The Southern Ocean is a major link between the world oceans via complicated processes associated with the melting and accumulation of the vast Antarctic ice sheets and the surrounding sea ice. The Southern Ocean sea level is poorly observed except from recent near-polar orbiting space geodetic satellites. In this study, the Southern Ocean mass variations at the seasonal scale are compared using three independent data sets: (1) the Gravity Recovery And Climate Recovery Experiment (GRACE) observed ocean bottom pressure (OBP), (2) steric-corrected satellite altimetry (ENVISAT) and, (3) the Estimating the Circulation and Climate of the Ocean (ECCO) model OBP data. The height difference between sea level derived from altimetry and steric sea level contains the vertical displacement of the Earth surface due to elastic loading. Here we provide a formulation of this loading term which has not been considered previously in other studies and demonstrate that it is not negligible, especially for regional studies. In this study, we first conduct a global comparison using steric-corrected JASON-1 altimetry with GRACE to validate our technique and to compare with recent studies. The global ocean mass variation comparison shows excellent agreement with high correlation (∼0.81) and with discrepancies at 3–5 mm RMS. However, the discrepancies in the Southern Ocean are much larger at 12–17 mm RMS. The mis-modeling of geocenter variations and the second degree zonal harmonic (J2) degrade the accuracy of GRACE-derived mass variations, and the choice of ocean temperature data sets and neglecting the loading correction on altimetry affect the OBP comparisons between GRACE and altimetry. This study indicates that the satellite observations (GRACE and ENVISAT) are capable of providing an improved constraint of oceanic mass variations in the Southern Ocean.

Polar Motion Effect on Gravity Observed with a Superconducting Gravimeter at Syowa Station, Antarctica

Polar motion effect on gravity was analyzed using the 2 years data obtained with a superconducting gravimeter at Syowa Station, Antarctica. From the analysis, we obtained the amplitude factor (δ-factor) of 1.198±0.035 and a time lag about 20 days against to the gravity changes predicted from the IERS (International Earth Rotation Service) polar motion data. Our analysis using simulation data suggests that the results for least-squares fitting are affected by the accuracy of the correction for step-like changes in the observed gravity data.

Sea level and gravity variations after the 2004 Sumatra Earthquake observed at Syowa Station, Antarctica

The Indian Ocean Tsunami reached Syowa Station, Antarctica, in approximately 12.5 hours after the 2004 Sumatra-Andaman Earthquake. We have analyzed the tsunami records of the tide gauge, including the superconducting gravimeter (SG) at the station. The synthetic tsunami and the induced gravity variations were calculated in order to compare with observations. It was found that the gravity effects of the tsunami exhibited an amplitude of microGal (10−8 m/s2); obtained from the Syowa SG. Furthermore, the effects of the tsunami on the Earth’s free oscillation records of the SG were subtracted by applying a transfer function method, using the tide gauge records as input. The improvement of S/N at frequencies of 0.3 mHz is remarkable.