Published
Estimation of the date and magnitude of impending massive earthquakes using the integration of precursors obtainable from remote sensing data
Estimación del momento y la magnitud de los terremotos masivos inminentes a través de la integración de precursores obtenidos a través de información de detección remota
DOI:
https://doi.org/10.15446/esrj.v28n4.105079Keywords:
Earthquake, Anomaly detection, Remote sensing, Support vector machine, Random forest (en)terremoto, detección de anomalías, detección remota, máquina de vectores de soporte, bosque aleatorio (es)
A single precursor is not usually an accurate, precise, and adequate measure to predict earthquake parameters. Therefore, it is more appropriate to combine multiple precursors and exploit parameters extracted from them to reduce the uncertainty of the prediction. The assumption in this study is based on the fact that most Earthquakes happen in active fault zones. The study is about the estimation of Earthquake parameters such as date and magnitude. In this study, remote sensing observations (such as electron and ion density, electron temperature, Total Electron Content (TEC), Land Surface Temperature (LST), Sea Surface Temperature (SST), Aerosol Optical Depth (AOD), and Surface Latent Heat Flux (SLHF)) in different modalities acquired several days before impending earthquakes have been investigated to extract earthquake parameters. In this study, three methods: median, support vector regression (SVR), and random forest (RF) have been used to detect anomalies. Then, by estimating the amount of anomaly deviation from the normal state, the magnitude of the impending earthquake is estimated. The final earthquake parameters (such as date and magnitude) can be obtained by integrating the earthquake parameters extracted from different earthquake precursors using the mean square error (MSE) method.
Un solo precursor no es usualmente una medida exacta, precisa y adecuada para predecir los parámetros de un terremoto. Además, es más apropiado combinar múltiples precursores e identificar sus propios parámetros para reducir la incertidumbre de la predicción. El supuesto de este trabajo está basado en que la mayoría de terremotos ocurren en zonas de fallas activas. Este estudio se basa en la estimación de los parámetros de terremoto como momento y magnitud. En este trabajo se investigaron las observaciones de detección remota en diferentes modalidades (tales como densidad de iones y electrones, temperatura de electrones, contenido total de electrones, temperatura de la superficie terrestre, temperatura de la superficie marina, profundidad óptica del aerosol y flujo de calor latente), adquiridas varios días antes de los terremotos inminentes, para extraer los parámetros del terremoto. Se usaron tres métodos para detectar las anomalías: mediana, regresión de vectores de soporte, y bosque aleatorio. Luego se estimó la magnitud de los terremotos inminentes al estimar la medida de la desviación de la anomalía. Los parámetros finales del terremoto (como momento y magnitud) se pueden obtener al integrar los parámetros de terremoto extraídos de diferentes precursores de terremoto al usar el método del error cuadrático medio.
Recibido: 2 de octubre de 2022; Aceptado: 10 de diciembre de 2024
ABSTRACT
A single precursor is not usually an accurate, precise, and adequate measure to predict earthquake parameters. Therefore, it is more appropriate to combine multiple precursors and exploit parameters extracted from them to reduce the uncertainty of the prediction. The assumption in this study is based on the fact that most Earthquakes happen in active fault zones. The study is about the estimation of Earthquake parameters such as date and magnitude. In this study, remote sensing observations (such as electron and ion density, electron temperature, Total Electron Content (TEC), Land Surface Temperature (LST), Sea Surface Temperature (SST), Aerosol Optical Depth (AOD), and Surface Latent Heat Flux (SLHF)) in different modalities acquired several days before impending earthquakes have been investigated to extract earthquake parameters. In this study, three methods: median, support vector regression (SVR), and random forest (RF) have been used to detect anomalies. Then, by estimating the amount of anomaly deviation from the normal state, the magnitude of the impending earthquake is estimated. The final earthquake parameters (such as date and magnitude) can be obtained by integrating the earthquake parameters extracted from different earthquake precursors using the mean square error (MSE) method.
Keywords
Earthquake, anomaly detection, remote sensing, support vector machine, random forest.RESUMEN
Un solo precursor no es usualmente una medida exacta, precisa y adecuada para predecir los parámetros de un terremoto. Además, es más apropiado combinar múltiples precursores e identificar sus propios parámetros para reducir la incertidumbre de la predicción. El supuesto de este trabajo está basado en que la mayoría de terremotos ocurren en zonas de fallas activas. Este estudio se basa en la estimación de los parámetros de terremoto como momento y magnitud. En este trabajo se investigaron las observaciones de detección remota en diferentes modalidades (tales como densidad de iones y electrones, temperatura de electrones, contenido total de electrones, temperatura de la superficie terrestre, temperatura de la superficie marina, profundidad óptica del aerosol y flujo de calor latente), adquiridas varios días antes de los terremotos inminentes, para extraer los parámetros del terremoto. Se usaron tres métodos para detectar las anomalías: mediana, regresión de vectores de soporte, y bosque aleatorio. Luego se estimó la magnitud de los terremotos inminentes al estimar la medida de la desviación de la anomalía. Los parámetros finales del terremoto (como momento y magnitud) se pueden obtener al integrar los parámetros de terremoto extraídos de diferentes precursores de terremoto al usar el método del error cuadrático medio.
Palabras clave
terremoto, detección de anomalías, detección remota, máquina de vectores de soporte, bosque aleatorio.1: Introduction
When an earthquake is happening, energy transmission is generated due to the destructive effects of the earthquake on the environment. The occurrence of these changes before and/or after the earthquake may have various physical and chemical effects on the lithosphere, atmosphere, and ionosphere, making the earthquake more accurately predictable. The abnormal variations in lithospheric, atmospheric, and ionosphere parameters are taken as "earthquake precursors". They serve as alarms for impending earthquakes. Many studies have been carried out on earthquake predictions using precursors in the lithosphere, atmosphere, and ionosphere. The problem arises when some of these major abnormalities do not appear during an earthquake. There are several studies based on the observation of the seismic Lithosphere Atmosphere Ionosphere Coupling (LAIC) anomalies which confirm that the anomalies begin several days before the earthquake and remain a few days after. None of the earthquake precursors can be used alone as an accurate and independent parameter for estimating earthquake parameters without generating some level of uncertainty. Hence, it is necessary to integrate different types of earthquake predictors or precursors. By integrating a variety of earthquake parameters extracted from different precursors, a more accurate and suitable estimation of the final earthquake parameters may be obtained.
Recent advances in remote sensing and Earth observation technology have facilitated monitoring the ionosphere, the atmosphere, and the Earth's surface using various sensors. Nowadays, researchers investigate the factors and indications of earthquakes in more practical and efficient ways. The most important earthquake precursors relevant to ionospheric anomalies recently studied are changes in ion density, ion temperature, electron density, and electron temperature provided by DEMETER satellite data (Berthelier et al., 2006; Lebreton et al., 2006; Parrot et al., 2006; Li and Parrot 2012; Li and Parrot 2013; Tao et al., 2017; Ibanga et al., 2018; Li and Parrot 2018). Ionospheric anomaly studies also include changes in total electron content (TEC) obtained from global positioning receivers (GPS) (Liu et al., 2004; Akhoondzadeh 2013; Tao et al., 2017; Akhoondzadeh et al., 2019). Regarding the Earth's surface, another useful precursor is thermal anomaly obtainable from land surface temperature (LST) (Ouzounov and Freund 2004; Ouzounov et al., 2006; Tronin 2006; Panda et al., 2007; Saraf et al., 2008; Blackett et al., 2011; Zoran 2012; Akhoondzadeh 2013; Bhardwaj et al., 2017a; Bhardwaj et al., 2017b; Chen et al., 2020; Jiao and Shan 2021), and from sea surface temperature (SST) (Dziak et al., 2003; Ouzounov et al., 2006; Freund et al., 2009). Other useful precursors are outgoing longwave radiation (OLR) (Ouzounov et al., 2007; Rawat et al., 2011; Eleftheriou et al., 2016), surface latent heat flux (SLHF) (Dey and Singh 2003; Cervone et al., 2004; Cervone et al., 2006; Pulinets et al., 2006; Pulinets and Ouzounov 2011; Zhang et al., 2013; MansouriDaneshvar et al., 2014; Qin et al., 2014), and atmospheric anomalies in the form of aerosol optical depth (AOD) (Freund et al., 2009; Akhoondzadeh, 2015; Ganguly, 2016; Akhoondzadeh et al., 2018; Akhoondzadeh et al., 2019).
2: Data
According to the objective of this study, the data used have been chosen from multiple sources, which are as follows:
DEMETER data
The French microsatellite DEMETER was launched in June 2004, and its scientific mission stopped on December 9, 2010. The data provided by DEMETER is used to investigate ionospheric disturbances due to seismic activity (Parrot et al., 2006). The DEMETER satellite collected its ionospheric parameters related to seismic activities using five sensors. The sensors are Instrument Champ Eletrique (ICE), Instrument Magnetic Search Coil (IMSC), Instrument Detecteur de Partcules (IDP), Instrument Analyseur Plasma (IAP), and Instrument Sonde de Longmuir (ISL). In this study, ion density (cm-3) and ion temperature (K), as well as electron density (cm-3) and electron temperature (K) data were collected from ISL and IAP sensors. DEMETER satellite data is available via: http://demeter.cnrs-orleans.fr/.
TEC data
The most popular product to analyse the ionosphere state is the global ionosphere maps (GIM) of the Total Electron content (TEC) provided by NASA (National Aeronautics and Space Administration) in the IONEX format. The GIM-TEC covers ±87.5 of latitude and ±180 of longitude with a spatial resolution of 2.5 and 5.0, respectively, and a cadence of 2h. In this study, the TEC variations according to the closest node to the epicentre of the earthquakes have been analysed. The GIM-TEC map is obtained from the website https://cddis.nasa.gov/archive/gnss/products/ionex/.
MODIS data
Two products of Moderate Resolution Imaging Spectroradiometer (MODIS) satellite, i.e., Land Surface Temperature (LST) and Aerosol Optical Depth (AOD) data were used in this study. Both the day/night-time LST images provided by NASA (http://modis.gsfc.nasa.gov/data) were processed. The MODIS Terra and Aqua daily level-3 aerosol product, which is produced by the Dark Target and Deep Blue algorithms and is called "Aerosol Optical Depth at 550 nm", is available via: https://giovanni.gsfc.nasa.gov/giovanni/.
AVHRR data
Two products of AVHRR (Advanced Very High Resolution Radiometer) including Sea Surface Temperature (SST) and Surface Latent Heat Flux (SLHF) data have been used in this study. Sea surface temperature (SST) anomaly can be related to near coastal seismic activity (Ouzounov and Freund, 2004), but conditions and currents can strongly affect SST. Due to a large thermal inertia of the seawater, its temperature changes more slowly; therefore, in the case of the SST anomalies, some mechanisms of LST anomalies are not applicable (Jiao et al., 2018). The NOAA 0.25° daily Optimum Interpolation Sea Surface Temperature (OISST) is an analysis constructed by combining observations from different platforms (satellites, ships, buoys) on a regular global grid. A spatially complete SST map is produced by interpolating to fill in gaps. The SST products are available via: https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html.
SLHF is (Wm-2) the heat flux absorbed or released by the phase transition (i.e., condensation, evaporation, and melting) of water from the Earth's surface to the atmosphere (Jiao et al., 2018). SLHF is one of the important components of Earth's surface energy budget, which is mainly affected by the atmospheric relative humidity, wind speed, surface temperature, and season (Jiao et al., 2018). Due to the underground fluid movement and the interaction among the underground, surface, and atmosphere, the SLHF anomaly that occurs prior to earthquakes is considered (Alvan et al., 2013). The SLHF products are available via: https://psl.noaa.gov/data/gridded/data.ncep.reanalysis.html.
Geomagnetic indices
The ionospheric parameters measured by satellite are mainly influenced by the geomagnetic storms and geomagnetic field disturbances, particularly in the equatorial and polar regions. However, in the case of an impending Earthquake, it may be affected further more in the form of anomaly. Such anomalies should also be removed. In order to distinguish anomalies caused by seismic activity from anomalies created by geomagnetic and solar activities, the geomagnetic and solar indices i.e. Dst, Kp, Ap, and F10.7 acquired from Space Physics Data Facility (SPDF) have been utilized in this study. In conditions where the quiet solar geomagnetic is established (i.e. Kp < 2.5, -20 nT < Dst < 20 nT and F10.7 < 120 SFU) the situation will be regarded as normal. However, if unusual changes are seen in the ionospheric data, it can be considered due to seismic activity. The geomagnetic and solar indices are available through NOAA (https://www.ngdc.noaa.gov/stp/stp.html).
3: Method
Anomaly detection method
Anomaly detection is the process of identifying unexpected items or events in data sets, which differ from the norm. Anomaly detection has two basic assumptions: 1) anomalies only occur very rarely in the data, and 2) their features differ from the normal instances significantly. In order to detect anomalies in each set of time series data from remotely sensed precursors, identification of the normal behaviour of the phenomenon is necessary.
3.1a: Normal behavior modeling
In order to model time series behaviour, two common machine learning methods namely the Support Vector Regression (SVR) and Random Forest (RF) have been used in this study.
In SVR-based regression solutions, the input vectors are mapped into a higher-dimensional feature space, then by employing a linear regression in the feature space, the input vectors are separated as far apart as possible. In this study, which is operating in a large space, a kernel function is used (Khosravi et al., 2017).
Random forest algorithm which is a nonparametric machine learning ensemble method, is an extended version of CART (Classification and Regression Trees) model proposed by Breiman (2001). It is based on the information combination method in which a large number of decision trees are generated and then the results of all the trees are combined for prediction (Cutler et al. 2007). The RF is the multitude of regression trees which performs based on the variance of the data (Liaw and Wiener, 2002).
3.1b: Anomaly detection
In order to detect anomalies, a reasonable range for regular variations in the time series data should be specified. Signals with normal behaviour fluctuate inside both the upper and lower bounds. Signals outside the boundaries will be detected as anomalies. In this study, the median and interquartile method is used to determine the upper and lower bounds (Liu et al., 2004).
The upper and lower bounds are determined using the following Eq. (1):
Where xlow, xhigh, m and iqr are respectively the lower bound, upper bound, median value and interquartile range of x. If the signal lies within the range of the upper and lower bounds, the behaviour of the signal is considered as normal. To determine the intensity of abnormal behaviour of the signal, the parameter Dx as deviation of x is calculated by Eq. (2):
If the absolute value of the parameter Dx is greater than k (i.e. |Dx| > k), the behaviour of the parameter is regarded as anomalous. Also, the percentage of parameter deviation from the natural state can be calculated using the Eq. (3) (Saradjian and Akhoondzadeh, 2011):
Preliminary parameters estimation
The Earthquake parameters are preliminarily estimated for each individual precursor. The Dx value obtained from the previous step is quite suitable parameter for calculating Earthquake magnitude. Saradjian and Akhoondzadeh (2011) showed the relationship between this parameter and the Earthquake magnitude that can be extracted from Table 1. Although the selection of Dx and its correspondence with magnitude ranges was investigated previously in another study (Saradjian and Akhoondzadeh, 2011), but it was investigated again in this study and verified.
Table 1.: Earthquake magnitude estimation Saradjian and Akhoondzadeh, 2011
Also, according to the day when the anomaly is observed, an impending Earthquake's approximate date can be estimated. Based on observations so far, as an average, a 15-day interval in ionospheric and atmospheric precursors, and as an average, a 16-day interval in thermal precursors from the anomaly observation till earthquake day is considered (Saradjian and Akhoondzadeh, 2011). Although the periods of 15 or 16 days were estimated previously by other researchers (Ouzounov and Freund, 2004; Pulinets et al., 2006; Jiao et al., 2018; Saradjian and Akhoondzadeh, 2011), but it was investigated again in this study and verified.
Finalizing parameters estimation method
After the earthquake parameters (i.e. date and magnitude) are estimated through various precursors using the preliminarily Earthquake parameters estimation, the final value of parameters of the earthquake can be estimated by combining their results using MSE method. In MSE method, the date and magnitude of an earthquake is calculated by Eq. (4) (Wackerly et. al., 2008):
where V and M are variance and median for the predicted upper and lower limits for the date and magnitude of the earthquake are calculated separately for all precursors. Finally, any parameter that has a minimum value of MSE is considered as the final parameter of the earthquake. This equation is applied when MSE is used as an estimator. Since input to MSE is not a physical quantity the difference in the type of precursors has no effect.
4: Case Studies and Results
Four major earthquakes with Magnitude Mw > 6 have been investigated in this study. These earthquakes occurred in Samoa Islands, Sichuan (China), Kermanshah and Bam (Iran). The characteristics of these earthquakes have been presented in Table 2. The ionospheric parameters obtained from the DEMETER has been studied and analysed over the relevant periods of time for each earthquake for areas selected according to Dobrovolsky Formula R = 100.43M which relates the size of affected area to the magnitude of the earthquake (Dobrovolsky et al., 1979). The rest of the time series data for other precursors have been acquired for the relevant periods of time for areas of about 5x5 degrees in size around each epicentre. All time series data were provided for the period of 100 days and in some cases more than 100 days.
Table 2.: Characteristics of earthquakes investigated in this study http://earthquake.usgs.gov/
Kermanshah earthquake
In the case study of Kermanshah Earthquake, all-time series data were provided for the period of 1 August to 27 November 2017. The TEC anomaly associated with Kermanshah earthquake was observed on November 4 (Table 3). By observing this anomaly, it can be concluded that an earthquake with magnitude ranging from 7 to 8 Mw between November 5 and 19, 2017 would have happened (Figure 1d).
Figure 1: Results of TEC Analysis using median for Kermanshah earthquake. The earthquake time is indicated by an asterisk. (a) TEC variations, (b) DTEC, (c) Detected anomalies without considering the solar-magnetic indices, (d) Detected anomalies with considering the solar-magnetic indices.
By investigating the Aqua night-time LST (°C) anomalies, the maximum value of 3.04°C October 28 indicates an earthquake between October 29 and November 13 with Mw > 8 would have happened (Figure 2a).
The anomaly observed in the AOD data obtained from the Aqua sensor, with a maximum of 452.78% on October 30, 2017, indicates an impending earthquake with Mw > 8 between October 31 and November 15 (Figure 2b). Also, the estimated earthquake magnitude for the observed anomaly on November 2, 2017 related to the AOD precursor obtained from the Terra sensor would be greater than 8 Mw (Figure 2c).
By investigating the changes in SLHF, a sharp increase of 126.97%, 213.64% have been observed on 3 and 4 October, respectively (Figure 2d). Due to these anomalies, it can be predicted that an earthquake with magnitude more than 8 Mw will occur in the region. Another sharp increase of 204.36% has been observed on 30 October, it can be indicating an earthquake with Mw > 8 between October 31 and November 15 would have happened.
The results obtained by SVR and Random Forest methods can be found in tables 4 and 5, respectively.
In the case study of Kermanshah, the earthquake parameters deduced based on median from the different precursors using the MSE method indicate that an earthquake would occur between November 1 and 16 with a magnitude more than 8 Mw (Table 15). Also by using MSE method for the obtained results from both SVR and RF methods, the predicted magnitude of earthquake will be, respectively, from 7 to 8 Mw between November 1 and 16, 2017 and from 7 to 8 Mw between October 29 and November 16, 2017 (Table 15).
Table 3.: List of anomalies obtained from different precursors of Kermanshah earthquake using Median method
Table 4.: List of anomalies obtained from different precursors of Kermanshah earthquake using SVR method
Table 5.: List of anomalies obtained from different precursors of Kermanshah earthquake using Random Forest method
Figure 2.: Results of (a) Night-time LST data (Aqua, MODIS), (b) AOD (Aqua, MODIS), (c) AOD (Terra, MODIS), (d) SLHF (NOAA), analysis using median method for Kermanshah earthquake. The earthquake day is displayed as a vertical dotted line. The green horizontal lines indicate the higher and lower bounds. The blue line indicates the median value.
Samoa earthquake
For the Samoa earthquake, the time series data of precursors were provided for the period of 1 June to 15 October 2009. TEC anomaly fluctuations began on September 18. The estimated magnitude of the Earthquake for this anomaly would have more than 8 Mw happening between September 19 and October 3. Anomalies have also been observed on September 25 and 28, 2009 implying that an Earthquake with magnitude greater than Mw = 8.0 would be expected to happen (Figure 3d).
The data on the changes in total ion density, electron and ion density and electron temperature recorded by DEMETER IAP and ISL sensors for Samoa Earthquake are given in Table 6. The data were recorded when the satellite's orbits were near the epicentre of the Earthquake (i.e. areas selected according to Dobrovolsky Formula R = 100.43M which relates the extend of affected area to the magnitude of the earthquake). With an increase in total ion density observed around 10:30 local time on 25 September, 2009, it was expected that an Earthquake with magnitude between 7 and 8 Mw would occur between 26 September and 10 October, 2009 (Figure 4a). Also, the changes in electron density showed unusual behaviour. These changes peaked around 10:30 am local time and exceeded 9.77% upper bound on September 25, 2009 (Figure 4b). The ion density anomaly was observed around 10:30 local time on September 25, 2009. For Dx = 2.61, the magnitude of the impending Earthquake is estimated to be between 7 and 8 Mw that would have happened between September 26 and October 10, 2009 (Figure 4c). An unusual increase in total ion density was observed around 22:30 local time on September 24, 2009 (Figure 4d). For this anomaly, an Earthquake with magnitude greater than 8 Mw would be expected to happen between September 25 and October 9.
By investigating changes in SST, there was sharp increase of 8.03% on September 6, 2009 (Figure 4g). The irregularity seen on September 6 indicates that an Earthquake with magnitude between 7 to 8 would have occurred between September 7 and 22.
The AOD obtained from the Aqua sensor shows a maximum anomaly (Dx = 2.96) on September 21, 2009 indicating that an Earthquake with magnitude ranging from 7 to 8 Mw would have happened between September 22 and October 7, 2009 (Figure 4h).
The characteristics of detected anomalies using SVR and Random Forest methods are shown in Table 7 and 8, respectively.
In order to finalize each Earthquake's date and magnitude parameters using the parameters obtained from different precursors, the MSE fusion method has been applied. In the case study of Samoa, according to the MSE method, it was predicted that an Earthquake with magnitude between 7 and 8 Mw would occur between September 25 and October 9, 2009 indicated by both Median and SVR results (Table 15). Based on Random Forest method, the parameters would happen between September 20 and October 4, 2009, with magnitude between 7 and 8 Mw.
Figure 3.: Results of TEC Analysis using median for Samoa earthquake. The earthquake time is indicated by an asterisk. (a) TEC variations, (b) DTEC, (c) Detected anomalies without considering the solar-magnetic indices, (d) Detected anomalies with considering the solar-magnetic indices.
Table 6.: List of anomalies obtained from different precursors of the Samoa Earthquake using Median method
Table 7.: List of anomalies obtained from different precursors of the Samoa earthquake using SVR method
Table 8.: List of anomalies obtained from different precursors of the Samoa earthquake using Random Forest method
Figure 4.: Results of (a) Daytime total ion density data (IAP, DEMETER), (b) Daytime electron density data (ISL, DEMETER), (c) Daytime ion density data (ISL, DEMETER), (d) Night-time total density data (IAP, DEMETER), (e) Night-time electron density data (ISL, DEMETER), (f) Night-time ion density data (ISL, DEMETER), (g) SST (NOAA), (h) AOD data (Aqua, MODIS), analysis using median method for Samoa earthquake.
Sichuan earthquake
In the case study of Sichuan Earthquake, all time series data were provided for the period of 1 February to 27 May 2008. Some intense anomalies related to TEC data have been observed on April 5 (2 UTC), and May 9 (14 UTC) (Figure 5d). These strong anomalies indicate an impending Earthquake with magnitude between 7 and 8 Mw.
The variations of the various parameters extracted from the DEMETER experimental data over the Sichuan region have been presented in Table 9. A sudden and unusual change in total ion density has been observed three days prior to the Earthquake around 22:30 local time (Figure 6a). This means that an Earthquake with magnitude ranging from 7 to 8 Mw would have occurred between May 3 and 7, 2008. An anomaly has also been observed on May 3, 2008 in electron density around 10:30 local time which implies an impending earthquake as strong as 7 to 8 Mw between 4 April and 18 May (Figure 6b). An unusual decrease have been observed in ion temperature around 10:30 local time on May 3, 2008 (Figure 6c). The characteristics of other detected anomalies are seen in Table 9.
The Tables 10 and 11 show the result of anomaly detection by SVR and Random Forest methods, respectively.
By using the MSE method and the combination of the earthquake parameters obtained from different precursors, it was predicted that an earthquake 7 < Mw < 8 would have happened between 8 and 22 May 2008 (Table 15). Also, the MSE method for Random Forest and SVR methods indicate that an Earthquake with the magnitude between 7 to 8 Mw would have been occurred between 3 and 17 May, 2008 (Table 15).
Figure 5.: Results of TEC Analysis using median for Sichuan earthquake. The earthquake time is indicated by an asterisk. (a) TEC variations, (b) DTEC, (c) Detected anomalies without considering the solar-magnetic indices, (d) Detected anomalies with considering the solar-magnetic indices.
Table 9.: List of anomalies obtained from different precursors of the Sichuan Earthquake using Median method
Table 10.: List of anomalies obtained from different precursors of the Sichuan Earthquake using SVR method
Figure 6.: Results of (a) Night-time total ion density data (IAP, DEMETER), (b) Daytime electron density data (ISL, DEMETER), (c) Daytime ion temperature data (IAP, DEMETER), (d) Night-time LST data (Aqua, MODIS), (e) AOD (Aqua, MODIS), (f) SLHF (NOAA), analysis using median method for Sichuan earthquake.
Table 11.: List of anomalies obtained from different precursors of the Sichuan Earthquake using Random Forest method
Bam earthquake
For Bam earthquake, the time series data of precursors derived during the period of 1 September 2003 to 10 January 2004. The TEC signal fluctuation exceeded the defined bounds on November 8, 2003. This anomaly indicates that an Earthquake with the magnitude between 6 and 7 Mw would have happened (Figure 7d).
By investigating the Aqua night-time LST (°C) anomalies, the minimum value of -1.91°C December 18 indicates an earthquake between December 19 and January 3 with 6 < Mw < 7 would have happened. Also, a sudden decrease in temperature was observed in the night-time LST data (Aqua sensor) on December 13 and 14, which indicates that an Earthquake with magnitude ranging from 7 to 8 Mw would have been occurred. By investigating the day-time LST (Terra and Aqua Sensors) fluctuations, extreme decreases in temperature on December 12 and 13, 2003. Moreover, by observing the maximum values of 2.39 and 2.12 on December 6 and 7, respectively, it can be concluded that an Earthquake with magnitude between 7 and 8 Mw would have happened between December 7 and 23. Unusual change in day time LST data (Terra-MODIS sensor) with the minimum value (Dx = -3.71) occurred on November 30. Due to this anomaly, an Earthquake with Mw > 8 would have been occurred (Figure 8a to 8d).
Unusual AOD changes on December 12, 2003, with values of 4.76 and 3.75 respectively for the Aqua and Terra sensors indicate an Earthquake with a magnitude greater than 8 Mw would have been occurred between December 13 and 28 (Figure 8e & f). The characteristics of the SLHF detected anomalies are seen in Table 12.
The anomalies obtained by SVR and Random Forest methods, respectively, are shown in tables 13 and 14.
By combining the predicted parameters obtained from different predictors using the MSE method, it is predicted that an Earthquake would occur between December 13 and 28, 2003, for median anomaly detection method. The combination of anomalies obtained from the SVR and Random Forest by using MSE method predicts an Earthquake between December 13 and 28, 2003. The magnitude of this Earthquake is estimated to be 7 < Mw < 8 for median, SVR and Random Forest methods (Table 15).
Table 12.: List of anomalies obtained from different precursors of Bam Earthquake using Median method
Table 13.: List of anomalies obtained from different precursors of the Bam earthquake using SVR method
Figure 7.: Results of TEC Analysis using median for Bam earthquake. The earthquake time is indicated by an asterisk. (a) TEC variations, (b) DTEC, (c) Detected anomalies without considering the solar-magnetic indices, (d) Detected anomalies with considering the solar-magnetic indices.
Table 14.: List of anomalies obtained from different precursors of the Bam Earthquake using Random Forest method
Figure 8.: Results of (a) Daytime LST data (Terra, MODIS), (b) Night-time LST data (Terra, MODIS), (c) Daytime LST data (Aqua, MODIS), (d) Night-time LST data (Aqua, MODIS), (e) AOD data (Aqua, MODIS), (f) AOD (Terra, MODIS), (g) SLHF (NOAA), analysis using median method for Bam earthquake.
Table 15.: The registered and estimated earthquake parameters related to the case studies
5: Conclusions
Assuming that estimation of Earthquake parameters using each predictor individually is accompanied by some uncertainties, this study considered integrating the capabilities of different earthquake parameters extracted from some of the same earthquake predictors to better estimate earthquake parameters. By using the combination of precursors in this study, the uncertainties in estimating earthquake parameters have been removed implicitly. To identify the anomalous states that may be associated with impending earthquakes, variations of different earthquake precursors have been analysed for four earthquakes by using Median, SVR and Random Forest methods. For each precursor, the date and magnitude were estimated according to the earthquake signals. By integrating the earthquake parameters obtained from all precursors, the final earthquake parameters were estimated more accurately.
Since different precursors have been used to analyse the final earthquake parameters, therefore, for all earthquakes, the estimated earthquake parameters for each earthquake are close to the actually recorded parameters. This can lead to accurate estimation of earthquake parameters with respect to the number and variety of earthquake precursors. Based on the results, it seems that methods such as SVR and Random Forest dealing with nonlinear and complex behaviours of time series are more sensitive than the Median method. Therefore, it can be accounted that these methods are suitable tools for detecting anomalies in nonlinear time series related to changes in seismic precursors.
Since various factors can cause unusual behaviour in different ionospheric, atmospheric or lithospheric parameters, more careful studies should be conducted to distinguish the anomalies caused by the daily changes from anomalies due to seismic activities.
Acknowledgments
The authors would like to acknowledge LPC2E for the DEMETER data, NASA Jet Propulsion Laboratory for the TEC data and NOAA for the geomagnetic indices, SST and SLHF data. Also analyses and visualizations used in this study were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC.
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