We have studied differences in ground motion and fault rupture characteristics between surface rupture and buried rupture earthquakes. We found that the ground motion generated by buried rupture in the period range around 1 second is on average 1.5 times larger than the average empirical relationship. In contrast, ground motion from earthquakes that rupture the surface is 1.5 times smaller in the same period range. This phenomenon is considered to be caused by differences in fault rupture process between the two types of earthquakes. To examine possible reasons of the above effect we analyzed source slip distribution data derived from waveform inversions, and divided them into two groups: surface rupture and buried rupture earthquakes. It was found that the large slip areas (asperities) of surface rupture earthquakes are concentrated in the depth range shallower than about 5 km. In contrast, large slip areas of buried rupture earthquakes are spread over the depth deeper than 5 km. We also found that the total rupture area of buried rupture earthquakes is 1.5 times smaller than that of surface rupture earthquakes having the same seismic moment, and that deep asperities have about 3 times larger effective stress drops and 2 times higher slip velocities than shallow asperities. These observations are verified by numerical simulations using stochastic Green's function method.

View full abstract

The Indonesian gravity field is neither accurately nor comprehensively determined, especially due to inadequacy of land gravity data. This study deals with determination of Indonesian land gravity and proposes the solution to data unavailability by means of a simulation technique. The simulation was carried out by combining short wavelength topographic effects from GTOPO30 and long wavelength information from EGM96. The simulated result was then compared with the observed gravity data. Over Java, Sumatra and Sulawesi islands, using three methods commonly used on the computation of topographic effect; topography, isostatic and RTM (Residual Terrain Model), it was estimated that error propagation by the GTOPO30 into the simulated gravity is about 4.5 to 11.7 mgal, with the RTM method was affected less than others. It was also shown that the simulated gravity from the RTM method gave the best agreement with STD (Standard Deviation) differences of 17 to 42 mgal compared to the observed data. This result was achieved after applying optimal RTM parameters over the Indonesian area: a reference field of 25′-27.5′ and density of 2-2.2 gr/cm³. Compared to STD differences between EGM96 and observed data, that between the simulated gravity and observed data improved by 2.5-7 mgal, and gave more detailed gravity features, especially over areas of high topography.

View full abstract

The current GPS satellite constellation provides limited availability and reliability for a country like Japan where mountainous terrain and urban canyons do not allow a clear skyline to the horizon. At present, the Japanese Quasi-Zenith Satellite System (QZSS) is under investigation through a government-private sector cooperation. QZSS is considered a multi-mission satellite system, as it is able to provide communication, broadcasting and positioning services for mobile users in a specified region with high elevation angle. The performance of a Global Navigation Satellite System (GNSS) can be quantified by availability, accuracy, reliability and integrity. This paper focuses on availability, accuracy and reliability of GPS with and without augmentation using QZSS. The availability, accuracy and reliability of GPS only and augmented GPS using QZSS in the Asia-Pacific and Australian area is studied by software simulation. The simulation results are described by the number of visible satellites as a measure of availability, geometric dilution of precision as a measure of accuracy and minimal detectable bias, and bias-tonoise rate as a measure of reliability, with spatial and temporal variations. It is shown that QZSS does not only improve the availability and accuracy of GPS positioning, but also enhances the reliability of GPS positioning in Japan and its neighboring area.

View full abstract

On 21 September 1999 (Taiwan local time), a major earthquake measuring M7.3 occurred near the town of Chi-Chi in central Taiwan. After the Chi-Chi earthquake, geomagnetic data recorded by a network of 8 stations equipped with continuous recording systems was analyzed. The results revealed that the total geomagnetic intensity of the Liyutan station, about 8 kilometers from the northern end of the Chelungpu fault (considered to be related to the earthquake), fluctuated significantly for more than a month prior to the earthquake. The fluctuation features continued and then stopped after the Chia-Yi earthquake (M6.2) occurred near the southern end of the Chelungpu fault on 22 October 1999. The variation of intensity reached 200 nTs. Geomagnetic fluctuations were also found at the Tsengwen station, located about 42 kilometers from the southern end of the Chelungpu fault and 30 kilometers from the Chia-Yi earthquake. These geomagnetic disruptions with highly anomalous amplitudes associated with the Chi-Chi and Chia-Yi earthquakes appear to have been the result of the accumulation and release of crustal stress that led to the subsequently severe surface rupture at the time of the earthquakes.

View full abstract

The quiet-time geomagnetic variations at high latitudes have not been systematically studied so far. Here we present quiet-time variation results from more than two years of continuous magnetic observations in Fennoscandia and Svalbard using the IMAGE magnetometer network. The CHAMP CO2 model is found to yield an excellent secular variation correction with a simple linear trend. The effect of the magnetospheric ring current on the quiettime field values for each component is presented with the coefficients for the expected linear correlation with the DST index. A general trend for these coefficients is found for the CGM latitudes from 54 to 68 degrees. In this area the diurnal variation is well in accordance with the expected seasonal Sq behaviour. North of the Fennoscandian mainland the DST coefficients and the diurnal variations show an unexpected behaviour and additional current systems are presented as a likely cause. An objective, automated baseline method used in this study is also introduced as a method useful for various applications.

View full abstract

The accretion rate of micrometeorites in the last glacial period was estimated from the concentrations of micrometeorites in the blue ice around the Yamato Mts. in Antarctica. The samples from this study were collected from the five sampling points (M03, K02, K11, J09 and J10) in the blue ice. The blue ice was melted and filtered, and the micrometeorites were handpicked from the collected “glacial sands”. The weight of the micrometeorites in the blue ice was estimated from the abundance of recovered micrometeorites and the solar noble gas concentrations in the “residue” after handpicking. The age of the blue ice from the K area was estimated to be 27-33 kyr before present based on oxygen isotope data. The estimated accretion rate to the whole Earth ranges from 5300×103kg/a to 16000×103kg/a. However, the lower end of this range probably represents lower limits due to possible loss of solar noble gases during long residence in the glacier ice. Hence, we estimate that the accretion rate of micrometeorites 27-33 kyr before present to be in the range between (11000 ± 6600) × 103kg/a and (16000 ± 9100) × 103kg/a. These results, as well as the other estimates, suggest that the accretion rate of micrometeorites in the last glacial period was comparable to that in the present.

View full abstract

Study of the lunar gravity anomaly has not been straightforward since direct tracking data of lunar satellites are available only at its nearside. In such a case, direct inversion of the line-of-sight acceleration data into surface mass istribution has several merits, e.g., (1) high resolution can be attained without relying on artificial constraints, (2) short computation time by estimating regional parameter sets stepwise. After confirming the validity of the method using synthesized data, we processed the line-of-sight acceleration data of the Lunar Prospector extended low-altitude mission. The obtained gravity anomaly map of the lunar nearside has resolution as high as 0.8°×0.8°, equivalent to 225th degree/order of spherical harmonics, with less spurious signatures than past studies. To take advantage of the high resolution, we calculate mass deficits for 92 medium-sized craters (50-300 km in diameter), and confirmed that they are nearly proportional to 2.5 power of crater diameter.

View full abstract