Additional Report for Acoustic Signals in 1999

Acoustic Signals in 1999-2000 from Unknown Source

Volcano Islands, Japan


Index of Monthly Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

11/1999 (BGVN 24:11) Hydro-acoustic signals of a possible eruption detected

12/1999 (BGVN 24:12) Spectra of hydrophone-detected tremor

05/2000 (BGVN 25:05) After 6 months of quiescence, tremor returns

06/2003 (BGVN 28:06) Hydroacoustic signals detected during 1998-2002 in the Volcano Islands


Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

11/1999 (BGVN 24:11) Hydro-acoustic signals of a possible eruption detected

Robert Dziak at the NOAA/Pacific Marine Environmental Laboratory in Newport, Oregon reported that the large-aperture hydrophone array deployed throughout the north Pacific Ocean basin has been detecting extremely loud, tremor-like signals since May 1998. The best preliminary estimates of the signal sources lie ~1,000 km S of Honshu Island, Japan along the Volcano Island chain (astride the Bonin trench, figure 1).

Figure 1. A sketch map of part of the island of Honshu, and some of the historically active volcanoes of the Izu, Marianas, and Bonin arcs. Data from the NOAA large-aperture hydrophone array indicates a submarine volcano has been erupting in the area within the box shown. Two possible candidate sources for the eruption discussed in the text are Fukutoku-okanoba and Kita-Iwo-jima (Funka-asane). Courtesy of Robert Dziak and Yasuo Otani.

Dziak believes these tremors to be volcanic in origin. The signals are characterized by a high amplitude fundamental around 10 Hz and the next three harmonics (20, 30, and 40 Hz). Typically signals appear as discrete packets lasting 4-5 minutes, with a brief ~30 second quiescence period, followed by the beginning of the next signal packet. For the duration of each signal packet, the spectral peaks typically increase monotonically by 5-10 Hz while maintaining their harmonic spacing. Similar distinctive characteristics have been previously identified in volcanic tremor records from both seismic and airborne acoustic measurements at Arenal Volcano in Costa Rica (Garces et al., 1998) and at Pavlof Volcano, Alaska (Garces and Hansen, 1998).

Unfortunately, the source of these signals is outside the optimum coverage area for the NOAA array, so the estimated locations are not accurate; the best preliminary estimates place the signal source in a box at 22-27°N and 138-141°E that lies W of the Bonin arc (figure 1).

The tremor has been occurring intermittently since May 1998, and was still being recorded as of late December 1999. During this period, intense tremor activity was recorded on 30 different days. The signals have for the most part been occurring continuously (with quiet times ranging from several days to several weeks) since first detected. Specific periods of peak amplitude and duration in 1998 and 1999 are presented in table 1. Signals measured on 10-12 December 1999 were the loudest yet detected.

Table 1. Dates of the strongest hydro-acoustic signals registered on the NOAA large-aperture hydrophone array compared to observation dates of discolored seawater over Fukutoku-okanoba and the Funka-asane vent of Kita-Iwo-jima. Hydro-acoustic data courtesy of R. Dziak; seawater observations courtesy of Yasuo Otani, Japan Maritime Safety Agency and Japan Meteorological Agency.

      Periods of peak           Discolored Seawater
       tremors from        Fukutoku        Kita-Iwo-jima
    hydro-acoustic data    -okanoba        (Funka-asane)

      16-18 Aug 1998          --                 --
      31 Aug 1998             --                 --
          --              16 Oct 1998            --
          --              28 Oct 1998            --
          --              14 Dec 1998            --
          --              16 Dec 1998            --
          --              12-13 Jan 1999         --
      22 Apr 1999             --                 --
      20-27 Aug 1999          --                 --
          --                  --             07 Sep 1999
          --              08 Sep 1999            --
      10-11 Oct 1999          --                 --
          --              22 Nov 1999            --
          --              10-12 Dec 1999         --

Yasuo Otani of the Hydrographic Department of Japan has provided subsequent information (courtesy of Yukio Hayakawa) regarding periods of discolored sea water seen over Fukutoku-okanoba (24.3°N, 141.5°E). The latter is a known volcanic area located S of Iwo-Jima (24.75°N, 141.33°E) on the fringes of the area delineated above by Dziak. These dates are also presented in the second column of table 1; however, there does not appear to be an obvious correlation between the two data sets. On the other hand, what is not yet known is the density of visual observations, in effect, the number of observations of these sites when surface discolorations were absent. Without such details, trying to correlate the two data sets could be biased by sampling density.

Japan Meteorological Agency reports provided one other case of sea surface discoloration, at Funka-asane, but this lone observation also failed to show any temporal correlation and has the same limitations of sampling bias mentioned above. Funka-asane, a submarine vent ~2 km NW of Kita-Iwo-jima (25.43°N, 141.23°E), is just E of the preliminary box delineated by the acoustical data.

Olivier Hyvernaud at the Geophysical Laboratory in Tahiti had found no evidence of volcanic T-waves from the region in question through the end of 1999.

The area of the preliminary box is large, and could include many other volcanic centers. Given all of the uncertainty, anyone having possibly related data or comments is urged to contact Robert Dziak or the Smithsonian's Global Volcanism Network.

References. Garces, M.A., Hagerty, M.T., Schwartz, S.Y., 1998, Magma acoustics and time-varying melt properties at Arenal Volcano, Costa Rica: Geophysical Research Letters, v. 25, no. 13, p. 2293-6.

Garces, M.A., Hansen, R.A., 1998, Wave form analysis of seismoacoustic signals radiated during the fall 1996 eruption of Pavlof volcano, Alaska: Geophysical Research Letters, v. 25, no.7, p. 1051- 4.

Information Contact: Robert P. Dziak, Oregon State University/NOAA, Hatfield Marine Science Center, 2115 SE OSU Drive, Newport, OR 97365 USA (Email: dziak@pmel.noaa.gov, URL: http://newport.pmel. noaa.gov/); Yasuo Otani, Coastal Surveys and Cartography Division, Hydrographic Department, Maritime Safety Agency, 3-1 Tsukiji, 5-Chome, Chuo-ku, Tokyo 104-0045, Japan (Email: otani@cue.jhd.go.jp); Olivier Hyvernaud, Laboratoire de Géophysique, BP 640 Pamatai, Tahiti, French Polynesia (Email: hyvernaud@labogeo.pf).

12/1999 (BGVN 24:12) Spectra of hydrophone-detected tremor

Robert Dziak at the NOAA/Pacific Marine Environmental Laboratory noted that loud acoustic signals continue to be detected from an unknown submarine source in or adjacent to the Volcano Islands of Japan (see sketch map in BGVN 24:11). Spectra and representative time series appear in figures 2, 3, and 4. These data were collected near the Galapagos Islands by a NOAA hydrophone (at 8°S, 95°W) roughly 14,000 km from the estimated source.

Figure 2. Hydrophone record showing the acoustic signals detected from an inferred submarine eruption along the Bonin arc. At this time, the exact location of the volcano is uncertain. The diagram shows a ~ 2,500-second interval of the time-series amplitude (bottom) and corresponding frequency spectra (from consecutive 1-second data windows) received on a hydrophone at 10°S, 95°W. The tremor signals typically last 2-5 minutes; they display a 10 Hz fundamental with 20, 30, and 40 Hz overtones. The signals were recorded on 21 March 1999 (2100-22:00 GMT), after propagating across the Pacific through the oceanic-sound (SOFAR) channel. Similar signals were detected at hydrophone arrays in the E and NE Pacific and at remote hydrophones maintained by NOAA located near the Galapagos Islands. Courtesy of R. Dziak.
Figure 3. This diagram shows an enlargement of the hydrophone record at 10°S, 95°W from an inferred submarine eruption along the Bonin arc. The diagram shows a ~ 1,000-second interval of the time-series amplitude (bottom) and the corresponding frequency spectra (top). For this interval, only the 20- and 30-Hz overtones are readily apparent (above). The signals shown arrived at the hydrophone at 2100-2200 GMT on 21 March 1999. Courtesy of R. Dziak.
Figure 4. A spectral plot of hydrophone amplitude versus frequency from an unconfirmed submarine eruption along the Bonin arc. The spectra were produced from one of the 2-minute signal packets shown in figures 3 and 4. The plot shows a fundamental frequency centered near 10 Hz, and lower-amplitude harmonics at 20, 30, and 40 Hz. Courtesy of R. Dziak.

The extremely high-amplitude tremor signals have been detected since May 1998, and are clearly recorded on hydrophone arrays in the N and E Pacific, and at a NOAA array in the equatorial area near the Galapagos Islands. The signals are larger, for example, than those from the 1993 submarine eruption at Soccoro Island (BGVN 18:01), and consist of a high-amplitude 10-Hz fundamental and three harmonics at 20, 30, and 40 Hz . The signals are unlikely to have come from any easily envisioned synthetic or biological sources (such as whales) because the fundamental wavelengths are substantial, seemingly too long to have been generated by these types of sources.

In general, the hydrophones are deployed within the ocean-sound (SOFAR) channel. The sound channel is a region of low acoustic velocity, and therefore acts much like a waveguide, allowing sound waves in the ocean to propagate over long distances with little loss in signal strength. However, the signal can be small or even absent at some stations due to bathymetry shadowing or other effects.

According to Olivier Hyvernaud, after careful comparison with the hydroacoustical data, the signals were recognized in seismic records at Tahiti, where the French Polynesian Network commonly records the converted seismic waves of ocean-acoustic signals that propagate past the islands. However, the signals have not been recognized on seismic instruments at Iwo Jima. Although it may seem surprising that Iwo Jima would lack a signal, Dziak notes that the difference could be explained by the physics of the situation. Acoustic waves propagating through an ocean-sound channel (2-D) would undergo little attenuation. In contrast, the seismic waves propagating through the Earth's crust (3-D) undergo much greater attenuation with distance. The Bulletin continues to present hydro-acoustical data so that others may compare them with records from local instruments.

The inferred source area of these acoustic signals (see BGVN 24:11) was derived by combining the arrival times of correlated signals throughout the Pacific basin with detailed ocean sound-speed models. Ocean sound-speed is a complex function of temperature, salinity, and pressure (depth). The models are a result of 30 years of direct sampling of these ocean parameters and account for seasonal variations. Unfortunately, the source of these signals is well to the W of the hydrophone arrays; consequently, the source location is not well-constrained. The estimated source area comprises over ~1.4 x 105 km2, and the area of uncertainty (the "box" shown on the sketch map) could extend far enough east to include the known active volcanic areas such as the Bonin arc. It is hoped that as more ancillary information becomes available, it will be possible to derive a better estimate of the source location.

On the topic of volcanism along the volcanic front of the Bonin arc, Yasuo Otani of the Japanese Maritime Safety Agency noted that there are daily commercial air flights to Guam (~1,300 km S of Iwo jima).

Otani conveyed the latest observations at the two known active vents near Iwo Jima. At Fukutoku-okanoba on 25 January observers saw very small changes in water color; on 26 January they noted somewhat larger-scale changes. Around the same time, 25-26 January, Funka-asane was also the scene of discolored water.

Both Fukutoku-okanoba and Funka-asane are sufficiently shallow that the vent's flux changes can be easily seen from the surface. In fact, hydrothermal emanations and small-scale eruptions gain much attention from fishermen, who keep an eye on various local eruptive sites because they believe some eruptive phases affect fishing.

Otani expressed doubt of far-traveled geophysical signals from either of these two sources near Iwo jima. Instead, he noted, enormous acoustical noise must eminate from breakwater construction on the margin of the Bonin Islands (Ogasawara Islands), ~200 km NE of Iwo Jima, where a large-scale blasting and other heavy moving has been occurring.

Information Contacts: Robert P. Dziak, Oregon State University/NOAA, Hatfield Marine Science Center, 2115 SE OSU Drive, Newport, OR 97365 USA (Email: dziak@pmel.noaa.gov, URL: http://newport.pmel. noaa.gov/); Yasuo Otani, Coastal Surveys and Cartography Division, Hydrographic Department, Maritime Safety Agency, 3-1 Tsukiji, 5-Chome, Chuo-ku, Tokyo 104-0045, Japan (Email: otani@cue.jhd.go.jp).

05/2000 (BGVN 25:05) After 6 months of quiescence, tremor returns

Robert Dziak at the NOAA/Pacific Marine Environmental Laboratory in Newport, Oregon reported that 10 Hz band-limited tremor was detected from the Volcano Islands area after a 6 month hiatus (BGVN 24:11 and 24:12). The current episode of signals began at 0800 UTC on 13 June, but were loudest at 0100-0200 UTC on 14 June. The tremor tracks to presumed submarine volcanism at an uncertain volcano.

Information Contact: Robert P. Dziak, Oregon State University/NOAA, Hatfield Marine Science Center, 2115 SE OSU Drive, Newport, OR 97365 USA (Email: dziak@pmel.noaa.gov, URL: http://newport.pmel.noaa.gov/).

06/2003 (BGVN 28:06) Hydroacoustic signals detected during 1998-2002 in the Volcano Islands

Robert Dziak and Christopher Fox at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory (NOAA PMEL), reported that a continuous series of low-frequency, long-duration signals were recorded beginning on 18 April 1998, and then during the next 3.3 years by omnidirectional hydrophones deployed throughout the Pacific basin (see BGVN 24:11, 24:12, and 25:05). These hydroacoustic signals were detected 21 different times from April 1998 through December 1999. After a 6-month hiatus, the signals were detected 26 more times from June 2000 through August 2001 (figure 5). The authors concluded that the signals came from a source in the Volcano Islands.

Figure 5. Maps showing location of hydrophones and islands of the Izu-Bonin and Volcano Islands groups in the western Pacific S of Japan, and source area of hydroacoustic signals during April 1998-August 2001 from Dziak and Fox (2002). (a) Approximate locations (shaded dots) of SOSUS and NOAA eastern equatorial hydrophones arrays; shaded bar shows the location of the French Polynesian Seismic Network (RSP). (b) Location of the major volcanoes in the Izu and Volcano Islands groups. (c) Location of the tremor waveforms (crossed error bars) estimated using 16 individual hydrophone stations positioned off the W coast of the United States, Hawaii, western Aleutians, and along the East Pacific Rise near the equator. The location error bars indicate the 68% confidence interval. See Dziak and Fox (2002) for additional details. Courtesy of Robert Dziak.

Dziak and Fox (2002) reported that "The character of the acoustic signals recorded from the Volcano Islands resembles tremor recorded during episodes of magmatic activity at subaerial volcanoes, suggesting that a significant magmatic, and potentially eruptive, process took place in the Volcano Islands between April 1998 and August 2001 and may occur again. To the authors' knowledge, the character of the Volcano Islands harmonic tremor with a 10-Hz fundamental and multiple overtones has not been previously recorded from a submarine volcano. Additionally, detection of harmonic tremor at teleseismic distances (>30°) is a rare occurrence for either a subaerial or submarine volcano."

Matt Fowler (Oregon State University) provided the following information on harmonic tremors from S of Japan from October 2000 to September 2002; all the tremor signals are from the same general area (figure 5) and have roughly the same frequency-time characteristics. Data through August 2001 were also reported by Dziak and Fox (2002). Possible tremors were detected on 18 and 28-31 October, 5, 9, 13, and 19 November 2000. Definite tremors were identified on 17, 22, and 30 December 2000, 13 January 2001, and 15-20 February 2001. During 21-23 February 2001 the tremors became well defined. Over the next few months tremors were only detected during 18-22 March, on 17, 21, and 24 April, and on 16 and 21 June. After another quiet interval, tremor signals were again recorded on 12, 14, 19, 22, and 30 July, and 8-10, 20, and 29 August 2001; tremors on 20 August were "excessively loud." No tremors were detected again until 26 February 2002, followed by a quiet interval until 16-19 and 31 March. Activity increased again during April-May 2002 with tremors recorded on 1, 2, 18, 20, 22, and 23 April, and 2-3, 13-16, and 21 May; "exceptionally loud" tremors occurred on 2-3 May. Additional tremors were detected on 9 June, 6-13 August, 15 August ("exceptionally loud"), and 19 September 2002.

Reference. Dziak, R.P., and Fox, C.G., 2002, Evidence of harmonic tremor from a submarine volcano detected across the Pacific Ocean basin: Journal of Geophysical Research, v. 107, no. B5, p. ESE 1-1 - 1-12.

Information Contacts: Robert P. Dziak and Matt Fowler, Cooperative Institute for Marine Resource Studies, Hatfield Marine Science Center, Oregon State University/NOAA, 2115 SE OSU Drive, Newport, OR 97365 USA (Email: Robert.p.dziak@noaa. gov, URL: http://www.pmel. noaa.gov/).