1. THE ANALYSIS OF THE INFRASOUND SIGNALS FROM MAY 12 EARTHQUAKE WENCHUAN CHINA Wang Xiaohang Signal Processing Department North China Institute of Computing Technology

2. May 12 2008, 14:28, a strong motion earthquake shocked the area of Wenchuan Sichauan China. This disaster caused a terrible death and property destruction at the local areas.

3. This is our array and epicenter Array Epicenter The distance between our array and epicenter is about 1500km.

4. During two hours following the moment when the earthquake occurred, our experimental infrasound array recorded two series of infrasound signals. first second

5. The waveform of the raw data of The first series of signals recorded at our infrasound array about 400s after the earthquake occurred The waveform of the filtered data of the first series of signals recorded at our infrasound array about 400s after the earthquake occurred. (0.2Hz-1Hz)

6. The waveform of the raw data of the second series of signals recorded at our infrasound array about 5000s after the earthquake occurred The waveform of the filtered data of the second series of signals recorded at our infrasound array about 5000s after the earthquake occurred. (0.1Hz-0.3Hz) A strong local noise. It is not the signal generated by the earthquake

7. An infrasound array far away from epicenter of a strong motion earthquake might receive infrasound signals generated by three different mechanisms. The infrasound waves generates by different mechanisms are named separately: Local infrasound Local infrasound Diffracted infrasound Diffracted infrasound Epicentral infrasound Epicentral infrasound

8. Local Infrasound Local infrasound is a ground-coupled air waves witch is generated in the geographical area of the infrasonic array, caused by surface motions as the seismic waves pass through this area via the inner or the surface of the earth.  With speeds much greater than the speeds of sound in air  Earlier arrive than other type of infrasound  Propagate almost vertically upward  More plentiful frequency  Unstable azimuth

9. Diffracted Infrasound Diffracted infrasound is generated in the atmosphere when seismic surface waves travel through regions of high mountains acting as large acoustical diffracters.  With the speeds of sound in air  Arrive after the local and before the epicentral infrasound  Including more parallel propagating component to the surface of earth

10. Epicentral Infrasound Epicentral infrasound is generated at epicenter by the strong ground motion. It propagate to infrasound array via atmosphere at normal atmospheric acoustic velocities.  With the speeds of sound in air  Last arrive  In a direction almost parallel to the earth’s surface  Lower frequency

11. When saying “epicenter” and “local”, we don’t means talking about two points. “Epicenter” and “local” are two areas with fuzzy boundary. The earthquake infrasound from different mechanisms are intergrading classed gradually.

12. The second series of infrasound signals High correlation section Arrival and duration time after earthquake:5110s-5200s Peak-to-Peak amplitude:100mPa Frequency of main power: 0.1Hz-0.3Hz Azimuth:~ 241° Wave speed:278m-311m/s Max-mean correlation coefficient:>0.9

13. The first series of infrasound signals High correlation section S P Arrival and duration time after earthquake:410s-600s Peak-to-Peak amplitude:300mPa Frequency of main power: 0.2Hz-1.0Hz Azimuth:345 ° -7° Wave speed:300m-480m/s Max-mean correlation coefficient:>0.9

14. The computed source direction is basically north, almost at the opposite of the epicenter. The variation of the direction have a tendency of changing form west to east. The curve of the variational azimuth of the first series of the infrasound signal

15. Beijing downtown Mountains We can attribute this phenomenon to the terrain about our array in Beijing

16. conclusion The two series of signals recorded at our experimental infrasound array after the earthquake were both caused by the earthquake. The time of the first arrival tallied with the time spend by seismic waves traveling from the epicenter to the array. The waveform had a near-field seismic wave’s appearance. We can distinguish the arrival of P waves and S waves. The signals were the diffracted infrasound emission of the north-west mountains near Beijing. They were secondary source infrasound. The variation of the azimuth revealed that the seismic waves travel from south-west to north-east along the mountains. The time of the second arrival tallied with the time spend by acoustic waves traveling from the epicenter to the array, so did the azimuth. These arrivals were the epicenter infrasound, maybe, including diffracted infrasound from some places near the epicenter.

17. Infrasound Monitoring is such an art that dependent on observations. It needs us to keep long term observation and accumulate our knowledge and skill while observing. An well understand to the characteristic of infrasound from various typical events, to analyse the mechanisms of generating infrasound, is very helpful to identification of different infrasonic signals and boost up our capability of nuclear explosion monitoring.