| 摘要(英) |
Recently, seismo-electromagnetic anomalies and related phenomena before large earthquakes have intensively studied. Among these existing studies, the seismo-ionospheric precursors (SIPs) of the GPS total electron content (TEC) is the most promising one. In this dissertation, to search SIPs, the global ionosphere map (GIM) of the TEC is employed, while both temporal and spatial analyses are conducted. In a certain region, to see if there is a forthcoming earthquake, a time series of the TEC at a fixed location (TEC variation, a point, zero-dimensional, monitoring) of the region is examined. At each time point, the previous 15-day running data are used to compute the median and upper/lower quartile constructing a reference for detecting anomalies on the observation (16th) day. When the observed TEC exceeds its upper or lower bounds, it can declare that a positive or negative anomaly is detected. To see if the detected anomaly is a temporal SIP, TEC anomalies associated with a large amount of earthquakes in the same region are statistically investigated by z test to find the characteristics of the temporal SIP, which are the polarity of positive/negative, the appearing local time period, the leading day, etc. When the detected anomaly meets the associated characteristics during the magnetic quiet, it can be considered to be a temporal precursor. Here, the TEC along the monitoring point longitude (Latitude-Time-TEC (LTT), a line, one-dimensional, monitoring) is also employed to confirm the SIP detection. On the other hand, the ionosphere can be easily disturbed by solar radiations, magnetic storms, etc. and reveals both positive and negative anomalies. To discriminate effects of the local (such as, earthquakes) from those of the global (such as, solar disturbances, magnetic storms), the global distributions of anomalies in the GIM TEC (i.e. Latitude-Longitude-TEC (LLT), two-dimensional, monitoring) and the associated gradient are examined. If anomalies frequently and persistently appear near the monitoring region, where the temporal SIP is detected, it can declare the spatial precursors being detected.
The temporal and spatial SIPs of the GIM TEC associated with the 12 January 2010 Mw7.0 Haiti Earthquake, the 16 October 1999 Mw7.1 Hector Mine Earthquake, the 26 December 2004 Mw9.1 Sumatra Earthquake, and the 11 March 2011 Mw9.0 Tohoku Earthquake are investigated. Results show that the temporal SIPs are the positive anomalies appearing 1 day before the Haiti Earthquake, and 3 days before the Tohoku Earthquake, but the negative anomalies occurring 5 days before the Sumatra Earthquake. The temporal SIP of the Hector Mine Earthquake is unclear, which seems having dual polarities of positive and negative appearing 7-5 days before the earthquake. Likewise, the LTT plots reveal that the TEC significantly enhances at the epicenter- and conjugate-latitude 1 day before the Haiti Earthquake, the TECs at the equatorial ionization anomaly (EIA) region considerably increase 3 days before the Tohoku Earthquake, while the TEC of the northern EIA extensively reduces 5 days before the Sumatra Earthquake. The LLT map further show that the TEC anomalies of 30-day extreme minimum and maximum frequently, persistently, and specifically appear at 1000-1600km southeast from the epicenter, 5 days before the Sumatra Earthquake, and at 800 km northeast from the epicenter, 4-3 days before the Tohoku Earthquake, respectively. There is no obvious gradient anomaly can be found of these two devastating earthquakes. On the other hand, for the Mw7.0 Haiti Earthquake, the TEC anomalies of 30-day extreme maximum persistently and specifically appear at 1000-2700km northwest from the epicenter, 1 day before, while anomalies of the associated westward gradient persistently appear at 900km northeast from the epicenter. For the Mw7.1 Hector Mine Earthquake, anomalies of the southward gradient continuously appear at 1300km west from the epicenter, 6-5 days before. It seems that the gradient anomalies are more sensitive to these two relatively smaller earthquakes. In general, the distances of the associated anomaly from the epicenter of the four earthquakes are approximate thousands km. However, comparing the earthquake preparation area, the anomalies are in fact very close to the epicenter of the Sumatra Earthquake and the Tohoku Earthquake. Finally, it can be seen that the anomalies often appear concurrently over the epicenter and the conjugate points, which indicates that the seismo-generated electric field are essential. |
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