Kazunari Nawa

Loading and Gravitational Effects of the 2004 Indian Ocean Tsunami at Syowa Station, Antarctica

The 2004 Indian Ocean tsunami reached Syowa Station, Antarctica, approximately 12.5 hr after the December Sumatra–Andaman earthquake. We have analyzed the tsunami signals recorded on ocean-bottom pressure gauges, broadband seismometers (sts-1), and a superconducting gravimeter (sg). We calculated the sea level variation, tilt, and gravity changes induced by the tsunami and compared these results to observations. From this comparison we confirmed the loading and gravity effects of the tsunamis on the sts-1 (horizontal components) and the sg records at Syowa Station. The magnitudes of these effects given as root mean square amplitudes are as follows: for the tilt effects obtained from 20-hr-long sts-1 records at frequencies in the range 0.3–0.6 mHz, 5 and 8 μ Gal (10 −8 m/sec 2 ) in the east–west and north–south directions, respectively; and for the gravity effect obtained from the sg records for the same time period of 20 hr at frequencies in the range 0.1–0.2 mHz, 0.2 μ Gal. By using detailed bathymetry around Syowa Station, the synthetic amplitudes similar to the observed were obtained, although the waveforms of synthetic and observation are not always consistent.

Sea level variation in seismic normal mode band observed with on‐ice GPS and on‐land SG at Syowa Station, Antarctica

[1]xa0We analyze sea level variation data acquired by a differential GPS and gravity data acquired by a superconducting gravimeter (SG) at Syowa Station, Antarctica, in an eight month period of 1998. At frequencies between 0.2 and 2.5 mHz in the seismic normal mode band we observe similar spectral peaks in both of the data sets. We also observe high coherence and zero phase between the two data sets at the frequencies of these peaks. The results of response analysis and simple mode calculation suggest that the observed peaks in the SG data are due to the effects of ocean water attraction and loading associated with sea level variation, a possible cause of which is the seiche in Lutzow-Holm Bay around the station. Applying a transfer function method to both of the data sets, we can reduce the background noise due to the oceanic effects in the SG data.

Effects of horizontal acceleration on the superconducting gravimeter CT #036 at Ishigakijima, Japan

In the gravity sensor of a superconducting gravimeter, a superconducting sphere as a test mass is levitated in a magnetic field. Such a sensor is susceptible to applied horizontal as well as vertical acceleration, because the translational degrees of freedom of the mass are not perfectly limited to the vertical direction. In the case of the superconducting gravimeter CT #036 installed at Ishigakijima, Japan, horizontal ground acceleration excited by the movements of a nearby VLBI antenna induces systematic step noise within the gravity recordings. We investigate this effect in terms of the static and dynamic properties of the gravity sensor using data from a collocated seismometer. It is shown that this effect can be effectively modeled by the coupling between the horizontal and vertical components in the gravity sensor. It is also found that the mechanical eigenfrequency for horizontal translation of the levitating sphere is approximately 3xa0Hz.

Combined Use of a Superconducting Gravimeter and Scintrex Gravimeters for Hydrological Correction of Precise Gravity Measurements: A Superhybrid Gravimetry

A variant of hybrid gravimetry using both a superconducting gravimeter and Scintrex gravimeters is proposed. One of the main factors limiting the accuracy of time lapse gravity measurements is the instrumental drift of spring-type gravimeters. Running the Scintrex CG-5 gravimeter in the nighttime on the same pier as the superconducting gravimeter allows us to model the long-term behavior of the former and to remove efficiently the effect of irregular drift on measured gravity. Initial tests performed at Ishigakijima, Japan, proved that accuracy of a few μGal level can be achieved with this method. This will help us precisely correct for the effect of underground water on superconducting gravimeters with 2-dimensional local gravity survey.