关键词: 于田M_W6.9地震, 震前热异常, ZS法, RST法, 定性与定量对比

Abstract:

The reliability of anomaly extracting methods is crucial for pre-seismic thermal anomalies research. However, there is a lack of relevant researches. We compared two commonly used anomaly extracting methods, Z-score(ZS)and Robust satellite technology(RST)method, taking the 2014 Yutian earthquake as a typical example and the 2008 Wenchuan earthquake as a validation. The four aspects of extracted results are compared qualitatively and quantitatively, including the extraction effect, sensitivity to slight change, suppression of background information and indication of seismic information in the actual earthquake case. Moreover, the extracted results of validation case are used to validate the reliability of typical case results. Many intermittent anomalies in surface temperature and outgoing longwave radiation appeared before Yutian earthquake. The frequency of anomalies increases with the proximity of earthquake. The spatial distribution of surface temperature and outgoing longwave radiation anomalies gradually concentrated around the fault zone at the same time. The largest surface temperature and outgoing longwave radiation anomalies occurred one month before Yutian earthquake. The difference between the extraction results of ZS and RST method is mainly manifested in the frequency and amplitude of anomaly changes. The frequency and amplitude of anomaly changes obtained by RST method are higher than those obtained by ZS method. To further explore the reason for these differences, we further evaluate the two methods quantitatively by combining the data of two non-seismic years before and after Yutian earthquake respectively. The sensitivity of anomaly extraction method represents its ability to identify the slight changes of thermal parameters caused by the seismogenic process. The two methods are sensitive to slight changes, but the RST method is better than ZS method. The background information represents normal variation in surface temperature and outgoing longwave radiation caused by non-seismic factors. Suppression of background information determines the accuracy of extraction results. The comparison results show that both methods have certain suppression effect to background information, but the ZS method is better. The spatial distribution of pre-seismic thermal anomalies is an important index for predicting earthquake information(e.g. time of occurrence and location of epicenter). The results of quantitative comparison through normalized distance index show that for surface temperature data, ZS method is slightly better than RST method in indicating the location of epicenter. However, RST method is better for outgoing longwave radiation data. The maximum value of normalized distance index of ZS method occurred closer to the origin time of earthquake. We used the same quantitative evaluation method for the validation earthquake case. The verification results show that in addition to the sensitivity to anomaly changes, the comparison results of the two earthquake examples are similar in terms of the ability to suppress background information and indicate earthquake information. The difference is that ZS method has a better ability to suppress background information and RST method is better in indicating earthquake epicenter in the verification earthquake example. The main reason for the difference in extraction effect between the two methods is that the RST method averages the ground feature classification, and the difference between the observed value and the average value of the classification makes the RST method have a certain amplification effect on the weak signal. The difference between the typical earthquake case and the verification earthquake case is mainly due to the different complexity of the object types in the regions. Based on the above research results, we believe that ZS method and RST method have certain ability to extract pre-seismic anomalies. However, comparatively speaking, the RST method also has a good effect on the extraction of anomalies caused by other factors, and there is uncertainty in the ground feature classification. We believe that ZS method is a more appropriate and simple anomaly extraction method in the general seismic anomaly change extraction.

Key words: Yutian M_W6.9 earthquake, pre-seismic thermal anomalies, ZS method, RST method, qualitative and quantitative comparison