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Pre-seismic ionospheric anomalies detected before the 2016 Taiwan earthquake
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On Feb. 5 2016 (UTC), an earthquake with moment magnitude 6.4 occurred in southern Taiwan, known as the 2016 (Southern) Taiwan earthquake. In this study, evidences of seismic earthquake precursors for this earthquake event are investigated. Results show that ionospheric anomalies in Total Electric Content (TEC) can be observed before the earthquake. These anomalies are obtained by processing TEC data, where such TEC data are calculated from phase delays of signals observed at densely arranged ground-based stations in Taiwan for Global Navigation Satellite Systems. In this paper, it is shown how pre-seismic activity caused by this earthquake can be observed in terms of TEC in the ionosphere, and that one such anomalies were detected within one hour before the event. keypoints:
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... Some TEC anomalies were observed within one day after the earthquakes due to acoustic resonance of the aftershocks. Goto et al. (2018) investigated the TEC variation related to the February 5, 2016 (UTC) Tainan earthquake with ML = 6.6. The TEC data were calculated using the phase delays of the signals observed from the densely arranged groundbased stations in Taiwan for GNSS. ...
... (6) Their results demonstrated that the TEC precursors were mostly located over the epicenter. Moreover, TEC data processing was complicated, which could require considerable examination time to sufficiently determine the earthquake TEC precursors instead of short-term prediction, e.g., within 1 h before an earthquake (Goto et al., 2018). Therefore, the aim of this study is to identify the ionospheric TEC precursors, which are generally detectable, using a type of mathematical tool without the disadvantage of the previous methods so that the research method and the data acquisition should be not as expensive as possible and the data processing is not complicated thereby making it a better survey method dealing with TEC precursors. ...
Kernel-Based Two-dimensional principal component analysis (K2DPCA), a nonlinear method, was performed to examine ionospheric 2D total electron content (TEC) variations obtained from the NASA Global Differential GPS (GDGPS) network, which consisted of low spatial resolution ionospheric TEC data, to detect TEC precursors prior to the China Ludian earthquake at 08:30:13 UT on 03 August 2014 (M = 6.1). Simultaneously, Two-dimensional principal component analysis (2DPCA), a linear method, was performed to examine data related to the same earthquake for comparison purposes. Results have shown that two TEC precursors were observed three days prior to this earthquake. The characteristics of all principal eigenvalues were considered to determine the TEC precursors of the earthquake in this study. This study aimed to differentiate the previous studies of Lin about 2DPCA using only the characteristic of the first principal eigenvalue to determine the TEC precursors of the earthquake. However, some other TEC precursors could be lost in processes that do not analyze characteristics of other principal eigenvalues. Therefore, this research can be considered as a full analysis that examined the detailed surrounding TEC precursors of the earthquake and could be treated as a new study scope with 2DPCA and K2DPCA. The TEC precursor during 06:15 to 06:20 (UT) on 01 August 2014 localized near the epicenter was more intense compared with the characteristic of the first largest principal eigenvalue. However, another localized TEC precursor, which occurred from 06:15 to 06:20 (UT) on 01 August 2014 at a site west of the epicenter, was found to be more intense. The more plausible reason for the two TEC precursors could be the density variances caused by radon-gas release due to the fine crack stress transfer from the rocks according to Coulomb stress transfer theory as applied to a far TEC precursor. The durations of both TEC precursors were at least 5 min. This research could be treated as a full and detailed examination of surrounding TEC precursors. K2DPCA was better at identifying the TEC precursors.
Recent observations suggested that ionospheric anomalies appear immediately before large earthquakes with moment magnitudes (Mw) of 8.2 or more. Do similar phenomena precede smaller earthquakes? Here we answer this question by analyzing vertical total electron contents (VTEC) observed near the epicenters before and after 32 earthquakes with Mw7.0-8.0 using data from nearby Global Navigation Satellite System (GNSS) stations. To detect anomalies, we defined the reference curves to fit the observed VTEC, and considered the departure from the curves as anomalies. In estimating the reference curves, we excluded time windows, prescribed for individual earthquakes considering Mw, possibly affected by earthquakes. We validated the method using synthetic VTEC data assuming both pre-, co- and postseismic anomalies. Out of the 32 Mw7.0-8.0 earthquakes, 8 earthquakes showed possible preseismic anomalies starting 10-20 minutes before earthquakes. For earthquakes of this Mw range, we can observe preseismic ionospheric changes probably when the background VTEC is large, say 50 TECU or more.
This paper examines variations of the greatest plasma frequency in the ionosphere, foF2, recorded by the Chung-Li ionosonde (25.0° N, 121.1° E) before M>=6.0 earthquakes during 1994-1999. The 15-day running median and the associated inter-quartile range are utilized as the reference and the upper or lower bounds to monitor the ionospheric foF2 variations for finding seismo-ionospheric signatures (precursors) of the earthquakes. It is found that precursors, in the form of the recorded foF2 falling below its associated lower bound around 1200-1700 LT, appear 1-6 days prior to these earthquakes. On September 20, 1999 UT (September 21, Taiwan local time) a large Mw=7.7 earthquake struck central Taiwan near the small town of Chi-Chi. We analyzed the foF2 and found three clear precursors 1, 3, and 4 days prior to the Chi-Chi earthquake.
The 2011 March 11 Tohoku-Oki earthquake (Mw9.0) caused vast damages to the country. Large events beneath dense observation networks could bring breakthroughs to seismology and geodynamics, and here I report one such finding. The Japanese dense network of Global Positioning System (GPS) detected clear precursory positive anomaly of ionospheric total electron content (TEC) around the focal region. It started ∼40 minutes before the earthquake and reached nearly ten percent of the background TEC. It lasted until atmospheric waves arrived at the ionosphere. Similar preseismic TEC anomalies, with amplitudes dependent on magnitudes, were seen in the 2010 Chile earthquake (Mw8.8), and possibly in the 2004 Sumatra-Andaman (Mw9.2) and the 1994 Hokkaido-Toho-Oki (Mw8.3) earthquakes, but not in smaller earthquakes.
Growing evidence for ionospheric signatures of impending earthquakes comes from electron content measurements along slanted paths from GPS satellites to multiple ground stations located up to 500 km away from the epicenters. These slant total electron content (STEC) measurements deviate from the classic U-shape pattern, starting about 40 min to over an hour before major earthquakes. Unlike other naturally occurring STEC fluctuations at midlatitudes, we show here that these earthquake-induced deviations are simultaneous over a wide geographical area and do not propagate, thereby indicating a ground-based origin. Prior to the 11 March 2011 Tohoku-Oki earthquake (Mw 9.0), the deviations were as much as 10% of the undisturbed STEC. We argue that such deviations must be due to an electric field-forced rise or fall of the main ionosphere with little change in the vertical electron density profile. Hence, “apparent” is used in the title. We show how stress-related underground electric fields penetrate to 80 km altitude (above which penetration to the main ionosphere easily occurs) with magnitudes high enough to create STEC variations comparable to those observed. Since many thousands of GPS receivers exist worldwide, our theory suggests the possibility of early warning systems that could provide 10 to 20 min notice prior to large earthquakes, after allowing time for signal processing. This theory for prequake-induced STEC fluctuations also explains the ground-based ULF magnetic field data acquired by Fraser-Smith et al. 40 min prior to the Loma Prieta earthquake.
On April 15, 2016, the Kumamoto earthquake (Mw 7.3) occurred in Japan with no warning signals. Global Navigation Satellite System (GNSS) receivers provide useful information on disturbances in ionosphere by calculating the changes in Total Electron Content (TEC), which is the number of electrons in ionosphere. Here we show our recently proposed correlation analysis of TEC data which can detect the pre-seismic ionospheric anomalies from the public GNSS data. Our method detected the ionospheric anomaly several tens of minutes before the 2016 Kumamoto earthquake near its epicenter. Furthermore, we gave an indicator to distinguish between the pre-seismic TEC anomalies and the medium scale traveling ionospheric disturbances (MSTIDs) by calculating the anomalous area rates. These results support the hypothesis for existence of the preceding phenomena before large earthquakes.
We can observe the changes of Total Electron Content, TEC, in ionosphere by analyzing the data from GNSS satellites. Up to now, preseismic TEC anomalies have been reported in several papers. However, they are not so clear as coseismic TEC anomalies, and their analysis methods have some problems for practical earthquake prediction. One factor making it difficult to detect TEC anomalies is large noises in TEC data. Non-negligible TEC disturbances are caused by many natural mechanisms. To overcome this difficulty, we propose correlation analyses between one GNSS station and GNSS stations surrounding it. First, we model TEC time series over a few hours using polynomial functions of time. Second, we calculate prediction errors as the departure of the TEC time series from the models over time scale of a few minutes, and define it as the TEC anomaly. Third, we calculate the correlation between anomaly of one GNSS station and those at the surrounding stations. Although such a correlation method has long been used for radio communications, in particular for spread spectrum communications and very long baseline interferometry (VLBI) to increase SNR (signal to noise ratio), it has not been widely applied for TEC analysis. As a result of our method, we demonstrate that the correlation analysis can detect pre-seismic anomalies about one hour before the 2011 Tohoku-oki earthquake on March 11 (Mw 9.0), 20 minutes before the foreshock on March 9 and 40 minutes before the aftershock on April 7 (Mw 7.3).
Ionospheric electron enhancement was reported to have occurred ~40 min before the 2011 Tohoku-oki (Mw9.0) earthquake, Japan, by observing total electron content (TEC) with Global Navigation Satellite Systems receivers. Their reality has been repeatedly questioned due mainly to the ambiguity in the derivation of the reference TEC curves from which anomalies are defined. Here we propose a numerical approach, based on Akaike's information criterion, to detect positive breaks (sudden increase of TEC rate) in the vertical TEC time series without using reference curves. We demonstrate that such breaks are detected 25–80 min before the eight recent large earthquakes with moment magnitudes (Mw) of 8.2–9.2. The amounts of precursory rate changes were found to depend upon background TEC as well as Mw. The precursor times also showed Mw dependence, and the precursors of intraplate earthquakes tend to start earlier than interplate earthquakes. We also performed the same analyses during periods without earthquakes to evaluate the usefulness of TEC observations for short-term earthquake prediction.
The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15° × 0.15° in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.
Routine Faraday rotation observations of the VHF signal from the geostationary satellite ATS 3 made at Sagamore Hill (Massachusetts) revealed that an unusually large and rapid decay in the ionospheric total electron content (TEC) occurred near 1240 EST on May 14, 1973. The onset of the TEC disturbance occurred within 10 min of the launch of NASA's Skylab workshop by a Saturn 5 rocket. As the rocket moved at F region heighs, the burning second-stage engines passed within 150 km of the Sagamore Hill ray path to ATS 3. A detailed analysis of the aeronomic reactions initiated by the constituents of the exhaust field revealed that the F2 region plasma experienced a devastating loss process as the plume expanded. The specific mechanism involved was the rapid ion-atom interchange reactions between the ionospheric O(+) and the hydrogen and water vapor molecules in the plume, followed by dissociative recombination of the molecular ions.
Theoretical and observational results describing atmospheric gravity
wave (AGW) traveling ionospheric disturbance (TID) phenomena at high
latitudes are presented to identify and characterize dominant source
mechanisms responsible for the generation of AGWs at high latitudes.
Experimental evidence on the characteristics of TID/AGWs detected at
auroral latitudes is presented, and AGW propagation considerations
pertinent to equatorward motion of AGW/TID phenomena are examined. AGW
source phenomenology is described, with a semiquantitative assessment of
the relative importance of Joule heating, Lorentz forces, intense
particle precipitation and other mechanisms generating AGWs. Several
instances of F region electron density, temperature and plasma
periodicities accompanied by horizontal plasma velocities are presented,
which are consistent with theoretical AGW/TID models.