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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 Corporation2. University of Hawaii, Manoa

Sponsored by the DoD Nuclear Arms Control Technology ProgramUnder

Contract DASG60-03-C-0009, US Army Space and Missile Defense Command

Infrasound Technology WorkshopUniversity of California, San Diego

October 27-30, 2003

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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

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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

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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 OrigID2003/02/22 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

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Waveform Data (Stations used by IDC)

I10CA

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

0.6 – 4.5 Hz

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No Obvious Detection at Higher Frequencies (0.6-4.5 Hz)

I10CA I34MN I59US

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No Obvious Detection at Higher Frequencies (0.6-4.5 Hz)

I59H1

I59H2

I59H3

I59H4

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Signal Observed in 0.1-0.7 Hz Microbarom Band

REB BAZ = 296º REB BAZ = 81º REB BAZ = 304º

I10CA I34MN I59US

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Distribution of Back Azimuths

I10CA I34MN

I59US

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Detections on Additional IMS Stations

I53US I57US I24FR

Clear Detection Clear Detection Null Detection

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Detections on Non-IMS Stations

Clear Detection Noisy Detection Null Detection

NVIAR TXIAR DLIAR

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Summary of Detection Azimuths

ObservedDistance (km) BAZ (°) BAZ (°)

I59US 3982 303 297I53US 4399 244 235I34MN 5138 82 81NVIAR 6424 291 283I57US 6791 295 284SGAR 6866 294 NullPDIAR 6918 293 302I24FR 7437 322 NullI10CA 7493 298 283DLIAR 7507 298 NullTXIAR 8084 301 294

PredictedStation

• 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

REB Event

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Probable Cause of Observed Signals

Microbarom energy from storms in the North Pacific is the most likely source of the REB event, based on:

a) coherent signals are observed in the 0.1 - 0.7 Hz frequency band for all detections;

b) 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;

c) 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.

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.

35.5 N, 167.6 E

Significant Wave Height

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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/PeriodFrequency band of the signalSNRComment from the analyst on the signal duration

and signal quality

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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

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IDC Automatic Locations, Feb 21-22, 2003

Blue - I only events, Green – I and S events, Red – REB event (I only)

Storm Progression

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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

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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