Ryu Ohtani

Comparisons of GPS‐derived precipitable water vapors with radiosonde observations in Japan

We investigated the accuracy and features of precipitable water vapor (PWV) obtained from the Japanese Global Positioning System (GPS) network. We compared PWVs estimated from the Geographical Survey Institute (GSI) routine geodetic analysis using the Bernese software with those derived from the nearest 10 radiosonde stations over the Japanese islands. The comparisons for a half year showed that the agreement of the GPS-derived PWV was 3.7 mm in terms of rms difference, with a standard deviation of 2.6 mm and a mean bias of −2.7 mm, which is less precise than those obtained in central North America. This may reflect the high temporal and spatial variability of water vapor over the Japanese islands, causing differences between PWVs if GPS and radiosonde launch stations are not colocated. We also found systematic biases in the GPS-derived PWV. The absolute value of the mean bias at 1200 UT was systematically larger than that at 0000 UT. Their difference reached as much as 2 mm. Moreover, the mean biases tended to increase as PWV increased. We also confirmed that there were similar systematic biases in the PWV obtained from our GPS analysis using the GIPSY software with high temporal resolution. This fact indicates that the systematic biases depend neither on analysis software nor on temporal resolution of PWV estimation. We analyzed GPS data collected at the Tsukuba station using GIPSY for 2 years and showed a possibility that the systematic biases also found at the Tsukuba station were attributed to ocean tidal loading and seasonal variation of the mapping function both of which were not taken into account in the GPS analysis with the routine analysis using Bernese and our analysis using GIPSY. These results suggest the importance of implementing them in the analysis for accurate estimates of absolute value of PWV from the Japanese GPS network.

Network strain filter: A new tool for monitoring and detecting transient deformation signals in GPS arrays

[1] We have developed a tool to detect transient deformation signals from large‐scale (principally GPS) geodetic arrays, referred to as a Network Strain Filter (NSF). The strategy is to extract spatially and temporally coherent signals by analyzing data from entire geodetic networks simultaneously. The NSF models GPS displacement time series as a sum of contributions from secular motion, transient displacements, site‐specific local benchmark motion, reference frame errors, and white noise. Transient displacements are represented by a spatial wavelet basis with temporally varying coefficients that are estimated with a Kalman filter. A temporal smoothing parameter is also estimated online by the filter. The problem is regularized in the spatial domain by minimizing a smoothing (Laplacian) norm of the transient strain rate field. To test the performance of the NSF, we carried out numerical tests using the Southern California Integrated GPS Network station distribution and a 3 year long synthetic transient in a 6 year time series. We demonstrate that the NSF can identify the transient signal, even when the colored noise amplitude is comparable to that of transient signal. Application of the method to actual GPS data from the Japanese GPS network (GEONET) on the Boso Peninsula also shows that the NSF can detect transient motions resulting from aseismic fault slip.

Groundwater level changes related to the ground shaking of the Noto Hanto Earthquake in 2007

The Geological Survey of Japan, AIST has been monitoring groundwater using the observation network of about 40 stations in and around the Kinki and Tokai districts for earthquake prediction research. The Noto Hanto Earthquake in 2007 (MJMA 6.9) occurred in the northwestern part of the Noto Hanto Peninsula, Japan on March 25, 2007. The epicentral distance to the nearest and furthest observation well is about 100 km and 400 km, respectively. Changes in groundwater level related to the Noto Hanto Earthquake were detected at many stations. Most of these were changes in the long-term trend and/or short-term oscillation; there were a few steplike changes. The coseismic static volumetric strain changes calculated from the fault model ranged between 10−9 and 10−10 at most of the observation stations. These changes were therefore considered to be groundwater changes caused by the ground shaking because groundwater level sensitivity to crustal volumetric strain is a few centimeters to 10−8 strain at the most. We compared the coseismic groundwater level changes for the Noto Hanto Earthquake with those for five recent large earthquakes and obtained rough characteristics of the effect of the ground shaking on the groundwater level at each of the observation stations.

Preliminary results from permanent GPS array by the Geological Survey of Japan in conjunction with groundwater-level observations

A regional continuous Global Positioning System (GPS) array, consisting of uniform antenna type, has been established by the Geological Survey of Japan (GSJ) mainly around the Kansai district. Geodetic results for 2.5 years data are as follows. In comparison with the solutions obtained from the Geographical Survey Institute (GSI) continuous GPS array which employs different type of antenna from the GSJ’s array, it was found that the agreements between the daily positions were about 10 mm in horizontal and 15 to 20 mm in vertical components. This result indicates that the influence of the antenna type difference is at this level. The GSJ’s GPS stations around the Kansai district were found to have a common annual variation in the vertical component, whose amplitude was different from site to site. There were small correlations with in situ measurements of groundwater-level variation, except for the two stations whose annual variations of the GPS vertical component were the largest. The vertical displacements due to elastic deformation by groundwater-pressure changes in the aquifers were estimated using a simple model, but they were too small to account for the observed variations.

Estimating the Mitigation Effect of Tokai Earthquake Measures on Housing Damage : A Counterfactual Approach

This paper evaluates the mitigation effect of Tokai earthquake measures on housing damage using a counterfactual approach. It focuses on those measures that stimulate ex-ante investment in disaster prevention in the supposedly affected area, including earthquake-proof retrofitting and improved housing construction; the effect of the measures on housing losses is estimated monetarily. The study compares factual disaster damage computed using a real distribution of houses with counterfactual damage to a hypothetical housing distribution that would occur if the measures were not implemented. The key findings are: (i) the disaster mitigation effects of Tokai earthquake measures on housing amount to approximately JPY 18 billion (USD 0.18 billion) for Yamanashi Prefecture and JPY 0.26 trillion (USD 2.6 billion) for Shizuoka Prefecture, which would be at the centre of the event; (ii) a before-after comparison biases estimates of the mitigation effect; and (iii) statistically, the measures do not mitigate the housing damage predicted for an earthquake in Tokai.