Abstract:Outgoing Long-wave Radiation （OLR）,also known as thermal radiant flux density,refers to the energy density of electromagnetic waves emitted by the earth-atmosphere system into outer space.OLR is the broadband （5～50 μm） information that forms after reprocessing of the telemetry data of a single infrared band.The dataset adopted in this study is the evening data of the National Oceanic and Atmospheric Administration （NOAA） OLR satellite networking with a resolution of 1 ° × 1 ° in which the unit is W/m2.Such information appears on the NOAA Website at ftp：//ftp.cpc.noaa.gov.In this study,the eddy method and time-frequency analysis are used to examine the temporal and spatial distribution characteristics of Outgoing Longwave Radiation （OLR） in the epicenter area of the Minxian-Zhangxian MS6.6 earthquake （102.50° E-105.50° E,33.50°N-37.50° N） determined by infrared remote sensing.Results indicate that an enhancement of the monthly average of the eddy field of the OLR did not appear in the corresponding period between 2009 and 2012.The time series curve of the OLR eddy monthly average,in the epicenter grid points （103.50°E,35.50°N）,（104.50°E,34.50°N） of the earthquake,showed was no significant seasonal variation in the OLR eddy value.Generally,the eddy value fluctuated randomly near the average value line.Prior to the earthquake,however,the value showed abnormal and distinct short-term changes.To examine these change characteristics,the short-time Fourier transform method was used to determine the time-frequency analysis of the daily eddy value of the grid points （103.50°E,35.50°N）.The results indicate that prior to the earthquake,an unusual increase in energy density occurred.During the abnormal period,the characteristics of the spectral structure changed.The low-frequency components were principal in the early stage,whereas the radio-frequency component became the principal part during the later stage.Previous research identified an abnormal increase in OLR information of the nearby epicenter prior to the Wenchuan MS8.0 earthquake.The Lushan MS7.0 earthquake occurred near the Guan County-An County fault zone in the southern region of the Longmenshan Mountains following the Wenchuan earthquake.Prior to the former earthquake,an abnormal two-center high-value area was present in the epicenter area that was similar to that occurring prior to the latter earthquake.The present study identified an additional abnormal increase in the OLR eddy in southeastern Gansu,which was the epicenter area of the Minxian-Zhangxian MS6.6 earthquake.This example proves that prior to some earthquakes,the OLR eddy increases abnormally.Therefore,OLR has very broad application prospects in earthquake prediction research.
