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Ionospheric VLF Electric Field Spectrum Disturbance Before YuShu Earthquake with Ms7.1

Ionospheric VLF Electric Field Spectrum Disturbance Before YuShu Earthquake with Ms7.1. YAO Li, CHEN HuaRan, HE YuFei Institute of geophysics, China earthquake administration Email: liyao@spaceweather.ac.cn 2010-09-22. contents. Information of Yushu earthquake Data :

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Ionospheric VLF Electric Field Spectrum Disturbance Before YuShu Earthquake with Ms7.1

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  1. Ionospheric VLF Electric Field Spectrum Disturbance Before YuShu Earthquake with Ms7.1 YAO Li, CHEN HuaRan, HE YuFei Institute of geophysics, China earthquake administration Email: liyao@spaceweather.ac.cn 2010-09-22

  2. contents • Information of Yushu earthquake • Data : VLF(20-20K Hz) electric field spectrum of DEMETER satellite • DEMETER observations : 1.Ionosphere disturbance during 20-10 k HZ 2.Signal to noise ratio variance in 10k-20k Hz • Conclusions

  3. Information of Yushu earthquake • date: 13 Apr. 2010. • Area: YUSHU, QINGHAI province, China • M: 7.1 • Lat. and long.[deg]: 33.1 N, 96.7 E • Depth[km]: 10

  4. Data VLF Electric Field spectrum data: • Frequency range: [20 – 20k Hz]. • Frequency resolution: 19.5 Hz • Time resolution: 2s • Lt: 10:30, 22:30: only the data for nighttime orbits are used

  5. DEMETER observations • Date: 9 Apr. 2010 • Half orbit: 30880_1 • Two black dotted vertical lines:1000 km from epicenter • The horizontal lines: the signals from the VLF transmitters in the 10k–20k Hz around China • Drop of VLF power spectrum is observed in 50-200 Hz frequency region marked by the black ellipse.

  6. (1)VLF electric field variancein 20-10k Hz Median and inter-quartile range(IQR) Method : • Higher bounds: xhigh,i=Mi+IQRi • Lower bounds: xlow,i=Mi-IQRi • Positive anomaly: observation values which are higher than higher bounds • Negative anomaly: observation values which are lower than Lower bounds

  7. neighbourhood orbits Closest orbit MM/DD Closest orbits analysis Fig. (a) represents the closest DEMETER revisited orbit from YuShu earthquake epicenter: • Background orbits: 01/08-03/01 • Assumed disturb orbits: 03/14-04/09

  8. Anomalous variance relate to YuShu earthquake 04/09 • X-axis:±10°in latitude from the epicenter • Black curve: median value(M) • Red curve: higher and lower bounds(M ±IQR) • Green line: observation value of assumed disturb orbit • Blue region: nagetive or positive anomaly • Negative anomalous variance appears on 9 Apr. and 27 Mar. (the top two panels) • Anomalous position: southward (equator side) of the epicenter • The orbit time is closer that of the EQ, the anomalous region is larger 03/27 03/14

  9. neighbourhood orbits Closest orbits neighbourhood orbits analysis Fig. (b) represents the five DEMETER revisited orbits which lies in ±5°from epicenter in longitude, which are marked by L2, L1, Epi, R1, R2

  10. Neighbourhood orbits analysis L2 • Panels from top to bottom represent the L2, L1, Epi, R1, R2, respectively • Obvious anomaly can be seen in 0°-2° region at L1, Epi, R1 orbits in the southward of the epicenter • the maximum anomaly appeared at the eastward of the epicenter L1 Epi R1 R2

  11. 2010/02/01-2010/03/05 A(f0) A(f+) A(f-) (2)the average electric field spectrum before YuShu earthquake in 10k-20k Hz Area: Long(deg):70-140E Lat(deg):10-50N Dotted blue lines: Frequency of Transmitters • SNR=2A(f0)/[A(f+)+A(f-)] • f0: transmitter frequency • f+,f-: high and low bounds • Of broadening band • A: amplitude spectrum density • df: the width of frequency band f0=19.8k Hz ，df=500 Hz f0=17.8k Hz ，df=200 Hz f0=11.9k Hz ，df=100 Hz

  12. Characteristics of VLF transmitters around China

  13. the SNR distribution in three re-visited periods before YuShu earthquake _NWC (19.8k Hz) Revisited period : • T1: 0306-0318 • T2: 0319-0331 • T3: 0331-0413 T1 T2 T3 • Blue circled area: • 500 Km from epicenter • A decreasing trend appears • from T1 to T3 in 2010 • SNR first drops • from T1 to T2 and then • recovers from T2 to T3 • in 2009 2010 2009

  14. the SNR distribution of three re-visited periods before YuShu earthquake _ JP (17.8k Hz) Revisited period : • T1: 0306-0318 • T2: 0319-0331 • T3: 0331-0413 • The SNR variances are similar as those of NWC T1 T2 T3 2010 2009

  15. the SNR distribution of three re-visited periods before YuShu earthquake _RA (11.9k Hz) Revisited period : • T1: 0306-0318 • T2: 0319-0331 • T3: 0331-0413 • The SNR variances are similar as those of NWC again. T1 T2 T3 2010 2009

  16. results • Anomalous variance occurred in 0°-2°at eastward, westward,and southward of the epicenter about half month before the yushu earthquake during the frequency range of 50-200 Hz, and the maximum anomaly appeared at the eastward of the epicenter . • A drop of the VLF signal to noise ratio appeared in 1-25 days prior to earthquake. • These ionospheric anomalies with quite geomagnetic conditions indicate that the abnormal variances probably relate to earthquake preparation processes.

  17. Thank you

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