GEONET Data Research Articles

A large scale of apparent sudden movements in Japan detected by high-rate GPS after the 2011 Tohoku Mw9.0 earthquake: Physical signals or unidentified artifacts?

A moment magnitude Mw9.0 earthquake hit northeastern Japan at 14:46:18 (Japan Standard Time), March 11, 2011. We have obtained 1 s precise point positioning solutions for 1198 GEONET stations. Although GPS position time series have been routinely investigated and used as waveforms for dynamic inversion of earthquakes, we focus on exploring the spatial displacement features of GEONET stations for this earthquake. A movie inspection of high-rate GPS waveforms leads us to find that 76.21% of the GEONET stations in the Japanese islands subsided suddenly within 1 s between 14:59:45 and 14:59:46, Japan local time, with an average displacement of -,2.43, 2.83 and -,4.75 mm in the east, north and vertical components, respectively, about 15 min after the 2011 Tohoku earthquake. We have performed different types of independent tests, namely measurement error analysis, processing the GEONET data with a different software system, a statistical hypothesis testing under a simple assumption of sign distributions, the test computation of the displacement field outside of the Japanese islands and an independent test with the Japanese strong motion borehole network KiK-net, to see whether these sudden movements actually occurred. The first four independent tests are passed almost without any doubt, and the direction of the average sudden displacements is roughly consistent tectonically with the direction of subduction of the Pacific plate. Because there are only 78 KiK-net borehole stations available for an independent seismic test, the KiK-net results are marginally consistent with those of GEONET. In the daily seismological and geophysical practice, one may then conclude that the sudden movement within the second is real after passing these five independent tests. However, a further epoch-by-epoch check pinpoints a few more seconds with even a higher probability of sudden displacement from the 20-min three-component high-rate GPS waveforms after the main shock, or more precisely, the seconds between 14:59:04 and 14:59:05, 15:01:04 and 15:01:05, and 15:03:39 and 15:03:40 with 80.80, 84.14 and 85.89% of the GEONET stations simultaneously moving upward, southward and westward, respectively. Although these probabilities are very high, it may hardly be imagined that a large scale of sudden movements could occur repeatedly between 14:59:04 and 15:03:40. The high-rate GPS results imply that some detected sudden movements after the earthquake could be unidentified artifacts of GPS data processing, though we cannot rule out the possibility that the detected sudden movements in Japan after the 2011 Tohoku Mw9.0 earthquake are real physical signals.

EVALUATION OF RELATIVE GEOMETRIC ACCURACY OF TERRASAR-X BY PIXEL MATCHING METHODOLOGY

Abstract. Recently, high-resolution commercial SAR satellites with several meters of resolutions are widely utilized for various applications and disaster monitoring is one of the commonly applied areas. The information about the flooding situation and ground displacement was rapidly announced to the public after the Great East Japan Earthquake 2011. One of the studies reported the displacement in Tohoku region by the pixel matching methodology using both pre- and post- event TerraSAR-X data, and the validated accuracy was about 30 cm at the GEONET reference points. In order to discuss the spatial distribution of the displacement, we need to evaluate the relative accuracy of the displacement in addition to the absolute accuracy. In the previous studies, our study team evaluated the absolute 2D geo-location accuracy of the TerraSAR-X ortho-rectified EEC product for both flat and mountain areas. Therefore, the purpose of the current study was to evaluate the spatial and temporal relative geo-location accuracies of the product by considering the displacement of the fixed point as the relative geo-location accuracy. Firstly, by utilizing TerraSAR-X StripMap dataset, the pixel matching method for estimating the displacement with sub-pixel level was developed. Secondly, the validity of the method was confirmed by comparing with GEONET data. We confirmed that the accuracy of the displacement for X and Y direction was in agreement with the previous studies. Subsequently, the methodology was applied to 20 pairs of data set for areas of Tokyo Ota-ku and Kawasaki-shi, and the displacement of each pair was evaluated. It was revealed that the time series displacement rate had the seasonal trend and seemed to be related to atmospheric delay.

EVALUATION OF RELATIVE GEOMETRIC ACCURACY OF TERRASAR-X BY PIXEL MATCHING METHODOLOGY

Recently, high-resolution commercial SAR satellites with several meters of resolutions are widely utilized for various applications and disaster monitoring is one of the commonly applied areas. The information about the flooding situation and ground displacement was rapidly announced to the public after the Great East Japan Earthquake 2011. One of the studies reported the displacement in Tohoku region by the pixel matching methodology using both pre- and post- event TerraSAR-X data, and the validated accuracy was about 30 cm at the GEONET reference points. In order to discuss the spatial distribution of the displacement, we need to evaluate the relative accuracy of the displacement in addition to the absolute accuracy. In the previous studies, our study team evaluated the absolute 2D geo-location accuracy of the TerraSAR-X ortho-rectified EEC product for both flat and mountain areas. Therefore, the purpose of the current study was to evaluate the spatial and temporal relative geo-location accuracies of the product by considering the displacement of the fixed point as the relative geo-location accuracy. Firstly, by utilizing TerraSAR-X StripMap dataset, the pixel matching method for estimating the displacement with sub-pixel level was developed. Secondly, the validity of the method was confirmed by comparing with GEONET data. We confirmed that the accuracy of the displacement for X and Y direction was in agreement with the previous studies. Subsequently, the methodology was applied to 20 pairs of data set for areas of Tokyo Ota-ku and Kawasaki-shi, and the displacement of each pair was evaluated. It was revealed that the time series displacement rate had the seasonal trend and seemed to be related to atmospheric delay.

Geodetic accuracy observations of regional land deformations caused by the 2011 Tohoku Earthquake using SAR interferometry and GEONET data

Geodetic accuracy observations of regional land deformations caused by the 2011 Tohoku Earthquake using SAR interferometry and GEONET data

Crustal deformation associated with the 2011 Shinmoe-dake eruption as observed by tiltmeters and GPS

The National Research Institute for Earth Science and Disaster Prevention (NIED) developed volcano observation stations at the Kirishima volcanic group in 2010. The stations observed remarkable crustal deformation and seismic tremors associated with the Shinmoe-dake eruption in 2011. The major eruptive activity began with sub-Plinian eruptions (January 26) before changing to explosive eruptions and continuous lava effusion into the summit crater (from January 28). The observation data combined with GEONET data of GSI indicated a magma chamber located about 7 km to the northwest of Shinmoe-dake at about 10 km depth. The tiltmeter data also quantified detailed temporal volumetric changes of the magma chamber due to the continuous eruptions. The synchronized tilt changes with the eruptions clearly show that the erupted magma was supplied from the magma chamber; nevertheless, the stations did not detect clear precursory tilt changes and earthquakes showing ascent of magma from the magma chamber just before the major eruptions. The lack of clear precursors suggests that magma had been stored in a conduit connecting the crater and the magma chamber prior to the beginning of the sub-Plinian eruptions.

Local deformations around volcanoes associated with the 2011 off the Pacific coast of Tohoku earthquake

An Mw 9.0 great earthquake struck northeast Japan on 11 March 2011, causing crustal deformation of 1 to 2 m in volcanic areas. We applied synthetic aperture radar interferometry using Phased Array type L‐band Synthetic Aperture Radar on the ALOS satellite to investigate local deformation around volcanoes associated with the earthquake. We estimated the fault‐slip distribution along the trough based on obtained interferograms and crustal deformation from GEONET, and the estimated fault model explained the observed coseismic deformations well. The residual suggested local deformation around the Akita‐Komagatake, Kurikoma, Zao, Azuma, and Nasu volcanoes. Their deformations were obtained from independent synthetic aperture radar pairs. Additionally, deformations in the Kurikoma, Zao, and Azuma volcanoes were confirmed by GEONET data (no GEONET stations are located in the deformation areas of other volcanoes). These facts indicate that the obtained local deformations must be actual deformations, rather than noise. Our hypothesis is that coseismic extensional deformation concentrates in the soft medium under a volcano (e.g., magma and its surrounding rock) and that this deformation has caused local deformation with subsidence. To validate this hypothesis, we carried out numerical experiments using the finite element method for a soft medium under a volcano. Distributions of observed local deformations stretch in the direction of the minor principal axis of strain, and such distributions were obtained in this experiment, indicating that the hypothesis is plausible. Conceivably, such a concentration of coseismic deformation in a magma chamber may induce a volcanic eruption.

Seismic and geodetic evidence for the existence of hot materials beneath the Wakayama swarm activity, southwestern Japan

A remarkable seismic swarm area exists amid the Wakayama Plain on the western coast of the Kii Peninsula, in southwestern Japan, close to the Median Tectonic Line (MTL). The hypocenters of these swarm earthquakes are very shallow with a median of 6 km. This is exceptionally shallow for seismic activity in plain areas where the seismogenic layer is usually deep compared to that in mountainous regions ( Takayama et al., 2006). Also, the b-value below 8 km in the eastern part of this swarm region is anomalously high. In addition to these seismic features, GEONET data and leveling survey data obtained by the Geospatial Information Authority of Japan indicate that land around the swarm region are expanded and upheaved. Areas of the dilatation and upheaval almost overlap and their centers correspond to the site where the high b-value anomaly is observed. These facts strongly suggest that there are hot materials beneath the source region. In the Kii Peninsula acid volcanic rocks outcrop widely, and the Wakayama swarm area is located on the southern side of the MTL in the metamorphic belt below which the relatively young hot Philippine Sea plate is subducting and dehydration from a surpentinized forearc mantle is expected to occur. Based on the seismic and geodetic observations and the tectonic background of the area in which the swarm region is situated, we propose a hypothesis that a diorite or a serpentine diapir with plentiful hot fluids might be intruding beneath the source area.

Crustal deformation map for the 2011 off the Pacific coast of Tohoku Earthquake, detected by InSAR analysis combined with GEONET data

Applying interferometric SAR (InSAR) analysis using ALOS/PALSAR data to a megathrust earthquake (Mw 9.0) that ruptured off-shore of Tohoku, Japan, on 11 March, 2011, we succeeded in mapping a ground displacement distributed over ~600 km range in the north-south orientation and ~200 km range in the east-west orientation. A relative slant range lengthening of ~4 m that is the largest displacement ever observed in Japan on InSAR records was detected at the tip of Oshika peninsula, consistent with GPS observations. The interferograms, calculated using quick but less accurate orbit data, are contaminated by long-wavelength noise, preventing us from measuring the correct ground displacement. We demonstrate that an InSAR analysis incorporating GEONET data effectively reduces noise and enables the prompt and accurate mapping of the ground displacement. Additionally, a clear coseismic signal due to an inland earthquake in northern Ibaraki on 19 March, 2011 (Mj 6.1) was also detected. Our preferred fault model assuming two segments shows a nearly pure normal fault motion with a west-dipping fault plane. The ACFF calculation suggests that the Mw 9.0 main shock promoted the Mj 6.1 event.

The Darfield (Canterbury) earthquake of September 2010

The Darfield moment magnitude (Mw) 7.1 earthquake of September 2010 is the first heavily damaging earthquake to strike New Zealand since the surface wave magnitude (MS) 7.8 Hawkes Bay earthquake in 1931. Although the earthquake has a clear strike-slip surface expression characterised by the Greendale Fault, seismological evidence suggests it is a complex event beginning as a reverse faulting earthquake. Evidence for complexity of the mainshock includes a well constrained epicentre north of the surface fault trace, high near-source vertical accelerations, first-motion and regional moment tensor focal mechanisms which differ from teleseismic solutions, and a complex aftershock pattern. The earthquake and aftershock sequence were very well recorded by the GeoNet sensor networks in the region, and provide an exceptional dataset for understanding the earthquake rupture process and reducing damage from future earthquakes. This was the most significant test of GeoNet since its inception in 2001, and the first such New Zealand event in the “internet age”. GeoNet data proved important for the response and the interaction with emergency management, media and the public. The GeoNet website sustained continued heavy load over the weeks and months following the earthquake but continued to deliver timely information because of significant improvements carried out as the aftershock sequence continued.

Medium-Scale Traveling Ionospheric Disturbances and Plasma Bubbles Observed by an All-Sky Airglow Imager at Yonaguni, Japan

We report on nighttime airglow imaging observations of the low latitude ionosphere by means of a 630-m all-sky imager installed in March 2006 at Yonaguni, Japan (24.5°N, 123.0°E; 14.6°N geomagnetic), about 100 km east of Taiwan. The imager detected medium-scale traveling ionospheric disturbances (MSTIDs) for about 7 hours on the night of 26 May 2006. A dense GPS network in Japan also observed the same MSTID event on this night. The imager and GEONET data indicate that most of the MSTIDs propagated southwestward from the north of Japan to the south of Yonaguni and Taiwan over 4000 km, with a southern limit of 19°N (geomagnetic latitude 9°N) or lower On the night of 10 November 2006, the imager observed two weak emission bands that were embedded on the F-region anomaly crest to the south of Yonaguni. The simultaneous electron density profiles from the FORMOSAT-3/COSMIC mission demonstrate that the weak emission bands are due to density depletions in equatorial plasma bubbles. These case studies suggest that the Yonaguni imager in collaboration with other instruments is very suitable for the study of ionospheric disturbances in and around the northern F-region anomaly crest.

Prompt derivation of TEC from GEONET data for space weather monitoring in Japan

Continuous monitoring of ionospheric conditions is essential to monitoring and forecasting space weather. The worldwide use of global navigation satellite systems like the Gobal Positioning System (GPS) makes it possible to continuously monitor the total electron content (TEC) of the ionosphere and plasmasphere up to a height of about 20,000 km. We have developed a system for deriving the TEC from GEONET data rapidly and we use the TEC distribution over Japan in the daily operations of the Space Weather Forecast Center at NICT (RWC Tokyo of ISES). Using instrumental biases from a few days before enables us to drastically shorten the processing time for deriving TEC. The latest TEC values (with a delay of about 1 h) are obtained every 3 h, and most of the values are within 2 TEC units of the actual TEC. We have found our system for deriving TEC rapidly to be useful for continuously monitoring the progress of ionospheric storms under any ionospheric conditions, even those under which the usual ionosonde observations are unable to obtain F-region profiles.

Postseismic deformation following the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) derived by GPS observation

AbstractWe constructed a temporary GPS network around the aftershock area of the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) in order to investigate the characteristics of its postseismic deformation. Our GPS network data, as well as the GEONET data, were analyzed using Bernese GPS software. We detected notable postseismic deformation in horizontal components close to the fault plane. The observed maximum displacement was 5.6 cm at the GNKI site on Genkaijima Island. A logarithmic law was adapted to the coordinate time series data, revealing decay times from 2 to 23 days, similar to those obtained for the 2003 Tokachi-oki Earthquake (M8.0). The amount of postseismic slips on the fault was assessed using the coseismic fault model proposed by the Geographical Survey Institute (GSI). We derived an optimum fault model of postseismic slip on the shallow (less than 3 km depth) portion of the fault. Our findings indicate that postseismic slip occurred only in shallow parts of the coseismic fault.

Relation of High Strain Rate Zone Calculated from GEONET data and Distribution of Active Faults in Northeastern Japan Arc

Relation of High Strain Rate Zone Calculated from GEONET data and Distribution of Active Faults in Northeastern Japan Arc

Determination of GPS receiver differential biases by neural network parameter estimation method

The dual‐frequency signals of GPS can be used to measure the total electron content (TEC). The differential instrumental biases inherent in GPS satellite and receivers are considered as the main sources of error, and they must be removed for an accurate estimation of TEC. We aim at developing an effective method to solve the difficulties involved in the TEC measurement; there are only a few usable ground receivers, especially in lower‐latitude areas near the geomagnetic equator where large ionospheric variability exists. For this purpose a new parameter estimation method based on a residual minimization training neural network is applied to determination of the GPS receiver biases. The alternative method is realized by making use of the excellent features of neural networks to approximate a wide range of mapping functions, for which the network training is carried out by minimizing squared residuals of integral equation. To determine receiver biases (unknown parameters), we used additional “neural networks,” each of which consists of only one neuron without an input channel. It is assumed that satellite biases have already been determined by applying the least squares method to the GEONET data gathered by a large number of receivers. Various cases of observation data for different seasons, different local times, and different geographic locations of the receiver as well as the cases of model data are analyzed, and it is confirmed that the method is very effective for a small number of receivers located in the lower‐latitude areas.

Comparative Study of Rate of Zonal Shortening Calculated from GEONET data and Distribution of Active Faults in Northeastern Japan Arc

Comparative Study of Rate of Zonal Shortening Calculated from GEONET data and Distribution of Active Faults in Northeastern Japan Arc

Dry Biases of Humidity Measurements from the Vaisala RS80-A and Meisei RS2-91 Radiosondes and from Ground-Based GPS

During the Tsukuba GPS (Global Positioning System) Dense Network Campaign Observations in the autumn of 2000, we performed 3 hourly upper air soundings using two types of radiosondes. One was the Meisei RS2-91, operational sonde of the Japan Meteorological Agency (JMA) made by Meisei Electric Co. Ltd. with an independent thin-film capacitive sensor, and the other was Vaisala RS80-15G with A-type Humicap humidity sensor (RS80-A hereafter). It was found that the Precipitable Water Vapor (PWV) measured by Vaisala RS80-A were clearly smaller (dry bias) by 3-4 mm (about 4-6%) than those by JMA RS2-91 and those analyzed from GPS observations. The dry bias error of Vaisala RS80-A was also confirmed with dual sonde balloon flights equipped with both Vaisala RS80-A and JMA RS2-91 instruments, and near simultaneous flights of two types of radiosondes. It was found that the dry bias error was reduced by the new tight protective cap for the humidity sensor, but substantial amount of dry bias error still remained. We further compared humidity measurements by the two radiosondes to a chilled-mirror dew-point hygrometer using a two-pressure type humidity calibration chamber. The experiment was conducted under room temperature conditions. It showed that relative humidity as measured by the Vaisala RS80- A was about 5 to 15% smaller than that of the dew-point hygrometer. The bias was larger in high humidity conditions. Although the dry bias of Vaisala RS80-A radiosondes has been pointed out in several other studies, the dry bias revealed in this study is much larger than previously reported, indicating that a problem still exists in RS80-A humidity measurements taken in moist air conditions such as Japan. The RS2-91 showed only small dry bias relative to the dew-point hygrometer (less than 4%). However, the Aerological Observatory of the JMA reported that they improved the RS2-91 humidity sensor since 1999 (Shibue et al. 2000). RS2-91 sondes manufactured before and in 1999 had a dry bias with the same order of magnitude as the Vaisala RS80-A. There have been several reports (e.g., Ohtani et al. 2000) that the PWV derived by GEONET (Japanese nationwide GPS array) and other campaign GPS observations agreed well with those by JMA or Vaisala radiosondes. The implication of the dry bias errors of the radiosonde to the previous GPS studies is that the past GPS analysis in Japan produced a drier atmosphere than actual conditions. It is recommended that the past GPS data, including the GEONET data, be reanalyzed using the most advanced GPS analysis methods.

Calibration of antenna-radome and monument-multipath effect of GEONET—Part 2: Evaluation of the phase map by GEONET data

Phase maps for GSI/GEONET (Geographical Survey Institute/GPS Earth Observation NETwork) monuments and antennas obtained in Hatanaka et al. (2001) are evaluated by applying them to the analysis of GEONET. We used the same strategy as the GEONET routine analysis except for the antenna phase model. The coordinate solutions change by more than 10 cm in height when we apply the new phase maps. A scale change of up to 20 ppb is also observed for one of the sub-networks. The height bias is not constant but changing daily and seasonably, which implies that seasonal variations in the GPS solution are related to mis-modeling of the phase characteristics and that other environmental or geometric factors are coupled to the phase mis-modeling. Two more checks are done by comparing tropospheric delay estimates, and by conducting elevation angle cut off tests. Both tests show dramatic improvement when the new phase maps are applied, as compared to applying the standard antenna phase maps. It is concluded from this experiment with almost 1000 GEONET sites, that monument/antenna specific phase characteristic calibrations are essential for any application of GPS to achieve the highest accuracy for Earth science applications.