Joydeep Bhattacharyya 1 , Claus Hetzer 2 ,Milton Garces 2 , Victoria Oancea 1

1. Description and Analysis of Infrasound Signals Recorded from the North Pacific Event of February 22, 2003 Joydeep Bhattacharyya1, Claus Hetzer2,Milton Garces2, Victoria Oancea1 1.Science Applications International Corporation 2.University of Hawaii, Manoa Sponsored by the DoD Nuclear Arms Control Technology Program Under Contract DASG60-03-C-0009, US Army Space and Missile Defense Command Infrasound Technology Workshop University of California, San Diego October 27-30, 2003

2. Outline • Event information • Infrasound waveforms and detections • Infrasound detections at additional stations • Probable cause of signals • Other automatic locations in same time period and general location • Conclusions

3. Infrasound Event of February 22, 2003 • The sole infrasound-only event listed in the Reviewed Event Bulletin (REB) of the IDC during the first half of 2003 • Initial automatic location was based on automatic detections at two stations (I34MN and I59US) • These detections were revised by the IDC analysts, who added a third signal recorded on I10CA

4. Event Location EVENT 1574799 NORTH PACIFIC OCEAN Date Time Err RMS Latitude Longitude Smaj Smin Az Depth Err Ndef Nsta Gap mdist Mdist Qual Author OrigID 2003/02/22 00:11:03.84 573.1 88.61 35.4927 167.5908 279. 230.9 30 0.0f 3 3 199 35.81 67.38 m i uk IDC_REB 1590029

5. Waveform Data (Stations used by IDC) I10CA 0.6 – 4.5 Hz I34MN I59US • No clear arrival at REB arrival time at frequencies > microbarom • However, relatively low-frequency signals (0.1-0.6 Hz), which persist over a long period of time (e.g., hours) are observed

6. No Obvious Detection at Higher Frequencies (0.6-4.5 Hz) I59US I10CA I34MN

7. No Obvious Detection at Higher Frequencies (0.6-4.5 Hz) I59H1 I59H2 I59H3 I59H4

8. Signal Observed in 0.1-0.7 Hz Microbarom Band REB BAZ = 296º REB BAZ = 81º REB BAZ = 304º I10CA I34MN I59US

9. Distribution of Back Azimuths I59US I10CA I34MN

10. Detections on Additional IMS Stations I53US I57US I24FR Clear Detection Clear Detection Null Detection

11. Detections on Non-IMS Stations NVIAR TXIAR DLIAR Clear Detection Noisy Detection Null Detection

12. Summary of Detection Azimuths REB Event • Detections generally point to one of two storms (discussed in next slide) • Some detections point towards south of larger storm • Azimuth deviations: 1° - 15°, not correlated with distance

13. Probable Cause of Observed Signals Significant Wave Height • Microbarom energy from storms in the North Pacific is the most likely source of the REB event, based on: • coherent signals are observed in the 0.1 - 0.7 Hz frequency band for all detections; • back-azimuths estimated from the coherent signals point to one or both of the two separate storms present in the North Pacific during the time of the REB event; • the characteristics of the infrasound signals (e.g., correlation across array, back-azimuth estimates, frequency content) persist for several hours both before and after the time of the REB event. 35.5 N, 167.6 E Though the location of the REB event corresponds closely to the larger of the two storms, our analysis suggests that one or more stations may detect signals from both storms.

14. Analysis by IDC • Arrival times were changed significantly from SEL3 to REB • For the SEL3 arrivals • Dominant frequency of  0.16 Hz (from Infra_features table) • Signal duration of a few tens of seconds • For external evaluation of such long-duration signals the following information would be helpful: • Amplitude/Period • Frequency band of the signal • SNR • Comment from the analyst on the signal duration and signal quality

15. Other IDC Automatic Locations • The IDC automatic event lists (SEL3) for February 21-22 include an unusually high number (average is less than 1/day) of infrasound-based locations • A total of 21 events, all located in the North Pacific using automatic detections from I59US and I34MN, are reported over the 20 hours starting at 1700 on February 21 • There is a clear progression of these events towards the northeast with time (next figure) • This progression matches the general motion of the storm

16. IDC Automatic Locations, Feb 21-22, 2003 Storm Progression Blue - I only events, Green – I and S events, Red – REB event (I only)

17. Predictions of Infrasonic Source Pressure • Infrasonic source pressure at 0.2 Hz derived from the WW3 wave model • Three distinct high-pressure regions (in red) • The infrasound-only IDC events cluster in the three regions • Most observations converge on a region south of the REB location • I34MN passes through all three regions and is therefore less sensitive to the actual location

18. Conclusions • Routine infrasound processing at the IDC detected an event on February 22, 2003 • The infrasound event was probably due to storm swells in the North Pacific • We detected signals from the storms at seven infrasound stations, five of which are part of the IMS • This hypothesis is supported by the location and time progression of an unusually large number of IDC automatic locations based on infrasound detections • Additional event parameters from the IDC would significantly improve our understanding of such sources of infrasound signals