Evidence of harmonic tremor from a submarine ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B5, 2085, 10.1029/2001JB000177, 2002

Evidence of harmonic tremor from a submarine volcano detected across the Pacific Ocean basin Robert P. Dziak Cooperative Institute for Marine Resource Studies, Hatfield Marine Science Center, Oregon State University/National Oceanic and Atmospheric Administration, Newport, Oregon, USA

Christopher G. Fox National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory, Hatfield Marine Science Center, Newport, Oregon, USA Received 24 January 2001; revised 20 October 2001; accepted 25 October 2001; published 2 May 2002.

[1] Since the 1950s seismic activity associated with submarine volcanic eruptions has been detected throughout the Pacific Ocean basin using hydroacoustic methods. However, narrowband harmonic tremor with multiple overtones typically associated with subaerial volcanic eruptions has not commonly been observed. This paper presents a series of recently recorded hydroacoustic signals that originated in the Volcano Islands arc south of Japan, a region that frequently experiences submarine eruptions. The signals are characterized by a narrowband, long-duration, high-amplitude fundamental centered at 10 Hz with three harmonics at 20, 30, and 40 Hz and are consistent with harmonic tremor signals observed using traditional seismic methods at active subaerial volcanoes throughout the world. These hydroacoustic signals were detected 21 different times over the period from April 1998 through December 1999, typically lasting from several days to several weeks. After a 6-month hiatus, the signals were detected 26 more times from June 2000 through August 2001. The signals are qualitatively interpreted as a result of resonance of a magmagas mixture within a large chamber/conduit near the surface of a shallow ( 4 earthquake) within a source chamber every 2 – 5 min for the last 3.3 years, with short intervals of rest that last from several days to several weeks. An artificial origin for the Volcano Islands signals seems unlikely. [13] Cetaceans are known to generate low-frequency acoustic waves, with overtones, that propagate through the ocean-sound channel [Stafford et al., 1998]. Figure 8 shows the spectrogram of a South Pacific Blue Whale which generates some of the lowestfrequency signals ever detected from a marine organism. It is doubtful that the tremor signals were produced by cetaceans because (1) the tremor signals have a lower fundamental frequency than what is typically observed from even the largest whales (>15 Hz), (2) individual whale vocalizations typically are much shorter duration (20 min) T wave codas, but these codas are still much shorter than the tremor episodes and do not exhibit the band-limited, harmonic character of the individual tremor signals. Furthermore, the magnitude 7 earthquakes are located 100 – 300 km north of the tremor location ellipse. In summary, it seems unlikely that any of the NEIC detected seismicity was associated with the tremor signals. [15] Additionally, the Izu, Volcano, and Marianas Islands south of Japan have been extremely volcanically active during the past several centuries [Simkin and Siebert, 1994]. The only documented volcanic eruption along this 2500-km-long island arc during the time when the tremor signals were detected occurred on the island of Miyake-jima, located 200 km south of Tokyo and roughly 750 km north of the tremor location ellipse (Figure 1b). The eruption began on 27 June 2000, continued until 10 September and resulted in several thousand earthquakes, collapse of the island’s summit caldera, and phreatic/ash eruptions (Bulletin of the Global Volcanism Network, 2000). During the first day of the eruption the earthquake epicenters moved from beneath the caldera to west of the island [Sakai et al., 2000]. Discolored seawater was observed above the offshore earthquake swarm in water a few tens of meters deep, indicating that a submarine eruption had occurred. This submarine eruption produced a small basaltic andesite flow, but this was just the precursor to large phreatomagmatic eruptions and collapse of the caldera and that occurred onshore at the summit of the volcano on 8 July [Nakada and Fujii, 2000]. Of the several hundred earthquakes produced by this eruption and detected by SOSUS and equatorial hydrophone arrays, 432 were well recorded on fewer than three hydrophones so that their acoustic locations could be estimated. Miyake-jima is located at 34.08N, 139.53E, while the earthquake locations determined from the hydrophone arrays range from 32 to 36N and 136 to 140E. This large distribution, similar to the distribution in tremor signal locations along the Volcano Islands, is again a result of being well outside the hydrophone arrays. However, these earthquakes are located with sufficient accuracy that the events can be clearly associated with Miyake-jima. It is unlikely that any of the Miyake-jima seismicity was associated with the tremor signals since Miyakejima is far from the tremor ellipse, and there were no tremor signals during the Miyake-jima earthquakes. Moreover, although there were reports of very long period seismic signals attributed to vaporization of hydrothermal fluids [Kumagai et al., 2000], there

DZIAK AND FOX: HARMONIC TREMOR DETECTED ACROSS PACIFIC OCEAN Table 1. Dates of Detected Tremor Signals and Observed Discolored Seawater Discolored Seawaterb Signal Detection Acoustic Source Amplitude,a dB Fukutoku-Okanoba Funka-asane (Kita-Iwo Jima) 18 – 20 April 1998 15 May 1998 23 – 28 May 1998 15 – 20 June 1998 9 – 10 July 1998 16 – 18 Aug. 1998 31 Aug. 1998 15 – 17 Oct. 1998 28 Oct. 1998 12 – 17 Nov. 1998 12 – 14 Jan. 1999 25 – 28 Feb. 1999 21 March 1999 22 April 1999 3 – 11 June 1999 20 – 27 Aug. 1999

218.2 216.5 218.3 214.2 213.3 220.8 221.3 220.9 217.3 218.2

224.8 217.8 225.3 223.2 218.4 221.6

16 Oct. 1998 28 Oct. 1998 14 Dec. 1998 16 Dec. 1998 12 – 13 Jan. 1999

7 Sept. 1999 8 Sept. 1999 20 – 29 Sept. 1999 4 Oct. 1999 10 – 11 Oct. 1999 22 – 23 Nov. 1999 10 – 12 Dec. 1999 13 – 14 June 2000 4 – 8 Sept. 2000 17 Sept. 2000 24 – 25 Sept. 2000 5 Oct. 2000 7 – 12 Oct. 2000 17 Oct. 2000 27 Oct. 2000 4 Nov. 2000 12 Nov. 2000 21 Dec. 2000 26 Dec. 2000 29 – 31 Dec. 2000 13 Jan. 2001 18 – 20 Feb. 2001 22 – 23 Feb. 2001 18 – 22 March 2001 17 April 2001 24 April 2001 16 June 2001 21 June 2001 12 – 14 July 2001 19 July 2001 22 July 2001 30 July 2001 7 Aug. 2001

217.7 211.3 222.5 216.8 225.4 220.6 218.7 214.2 216.7 216.2 224.3 212.6 214.3 216.7 213.9 223.5 217.1 220.1 214.6 223.9 218.5 221.6 213.2 215.4 217.2 212.9 221.8 218.3 219.8 215.6 218.7

22 Nov. 1999 10 – 12 Dec. 1999

pffiffiffiffiffiffi Decibels (dB) are relative to 1 m Pa / Hz at 1 m. Amplitudes are in bold when 220 dB, or roughly the acoustic equivalent of a moment magnitude 4.5 earthquake. b Dates when discolored water was not observed may mean that visual observations were not performed or are not available. a

was no evidence of harmonic tremor during the Miyake-jima eruption.

5. Discussion and Interpretation [16] Seismic and acoustic signals emanating from active volcanoes have been the focus of intense study since the early half of the 20th century [Omori, 1912]. An extensive literature exists on the theoretical models of the origin of volcanic tremor; the last 20 years

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of effort has been neatly summarized by Chouet [1996]. Typically, earthquakes and explosions from volcanoes produce transient, broadband seismic and acoustic waves with clear arrivals. Harmonic tremor, on the other hand, is a sustained low-frequency ground vibration which exhibits evenly spaced spectral peaks. Harmonic tremor has been observed at many active subaerial volcanoes around the world [McNutt, 1989] and has generally been attributed either to reverberation within the solid volcanic strata or to the resonance of volcanic conduits or fluid-filled cavities that are under the effect of a pressure transient acting upon a part of the conduit or cavity wall [Chouet, 1996]. As discussed by Chouet [1992], the fluid-filled crack (cavity) model can explain the radiation of resonant seismic waves, with the resonance frequency and harmonic spacing being a complex function of the geometry of the cavity and of the mechanical properties of the two media (inside and outside the crack) controlled by the stiffness of the crack and the impedance contrast at the crack boundary. Both of these parameters depend critically on the velocity of sound in the fluid filling the crack which is strongly influenced by the presence of impurities, most notably gas bubbles. In the Volcano Islands case the gas-laden fluid would be a magma, with a high volatile content, either in a duct or a chamber. In Chouet’s [1992] model the relative amplitudes of the fundamental and overtones are controlled by the specific excitation condition, which is the location and duration of the pressure transient on the surface of the crack. A sustained resonance of the source associated with a large impedance contrast sharpens the definition of the spectral peaks [Chouet, 1996]. Thus a possible source for the Volcano Islands signals is resonance of a magma-gas mixture within either a large chamber or conduit near the surface of a submarine volcanic edifice. The acoustic resonance of the mixture occurs due to unsteady fluid flow from either degassing of the magma body or perhaps flow of the magma through a conduit. The unsteady flow gives rise to pressure oscillations in the magma conduit and produces the tremor signals. If the submarine volcano generating the tremor is in relatively shallow water (depths 30) distances is a rare occurrence for either a subaerial or submarine volcano. The ability of hydroacoustic methods to detect massive, yet otherwise unobserved, volcanic activity throughout the world’s oceans may have a great impact on our understanding of the contribution of seafloor volcanism to the global pace of volcanic activity and may lead to a better understanding of the effect of seafloor eruptions on a wide range of phenomena such as ocean-temperature variations and global climate change. [20] Acknowledgments. The authors would like to thank Steve McNutt, Rick Wunderman, Olivier Hyvernaud, Jacques Talandier, Haru Matsumoto, and Kate Stafford for providing their much needed input and insights to this manuscript. Special thanks to Paul Will and Matt Fowler for data processing assistance and to the personnel of Naval Facility, Whidbey Island, for key operational assistance. Funding and facilities provided by the NOAA VENTS program, PMEL contribution 2289, and the National Ocean Partnership Program’s Ocean Acoustic Observatory Federation.

References Figure 8. Hydrophone record and spectrogram of a series of South Pacific Blue Whale vocalizations and one earthquake to illustrate the contrast between these signals and the volcanic tremor. The record is from a NOAA autonomous hydrophone located at 8S and 110W in the eastern equatorial Pacific Ocean (station 5 in Figure 1). Whale calls are typically higher frequency, last a smaller amount of time (10 s), and occur more rapidly (10 – 20 s) than the tremor signals. Earthquakes (the signal centered at 100 s) typically exhibit a wide range of frequencies (1 – 40 Hz) and are isolated events that do not continually repeat at a consistent time interval. See color version of this figure at back of this issue.

eruption. The Volcano Islands are silicic arc volcanoes which generally produce more violent, explosive eruptions than the basaltic eruptions observed along a typical mid-ocean ridge. Explosions would provide shock (pressure) waves needed to excite acoustic resonance of the magma chamber (as well as unsteady fluid flow), which then leads to the generation of harmonic tremor and overtones. Furthermore, the depth of a seafloor volcano may play a significant role in the ability to detect the tremor at longrange hydroacoustically. Talandier and Okal [1987] propose that the detection of hydroacoustic volcanic tremor requires sources shallower than 780 m, based on the maximum depth of observed underwater volcanism [Staudigal and Schminke, 1984], and noted that volcanic tremor has not been detected from the entire southern and eastern segments of the East Pacific Rise (typically >1000 m deep) despite adequate monitoring by the RSP for the past 25 years. There are also no reports of harmonic tremor along the Mid-Atlantic Ridge where there is evidence of deep-water (1700 m) basaltic eruptions [Fouquet et al., 1998], although this may be because extensive civilian hydroacoustic monitoring of the Atlantic Ocean began in earnest just within the last decade [e.g., Nishimura and Conlon, 1994; Smith et al., 2002]. Much of the Volcano Islands group, including the two candidate volcanic sources, is well above the 780-m limit.

6. Summary [19] 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 our knowledge, the character of the Volcano Islands harmonic tremor with a 10-Hz fundamental and multiple

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DZIAK AND FOX: HARMONIC TREMOR DETECTED ACROSS PACIFIC OCEAN Kumagai, H., T. Ohminato, M. Nakano, M. Ooi, A. Kubo, and H. Inoue, Source mechanism of very-long-period seismic signals associated with the 2000 volcanic activity at Miyake Island, Japan (abstract), Eos Trans. AGU, 81(48), Fall Meet. Suppl., Abstract V52A-09, 2000. McNutt, S., Volcanic tremor from around the world, Bull. N. M. Bur. Mines Miner. Res., 131, 183 pp., 1989. Medwin, H., and C. S. Clay, Fundamentals of Acoustical Oceanography, 712 pp., Academic, New York, 1988. Nakada, S., and T. Fujii, Sequence and interpretation of caldera-forming event at Miyake-jima Volcano, Japan (abstract), Eos Trans. AGU, 81(48), Fall Meet. Suppl., Abstract V52A-02, 2000. Nishimura, C., and D. Conlon, IUSS dual use: Monitoring whales and earthquakes using SOSUS, Mar. Technol. Soc. J., 27, 13 – 21, 1994. Norris, R. A., and D. N. Hart, Confirmation of SOFAR hydrophone detection of submarine eruptions, J. Geophys. Res., 75, 2144 – 2147, 1970. Norris, R. A., and R. H. Johnson, Submarine volcanic eruptions recently located in the Pacific by SOFAR hydrophones, J. Geophys. Res., 88, 650 – 660, 1969. Okal, E. A., and J. Talandier, T waves from the great 1994 Bolivian deep earthquake in relation to channeling of S wave energy up the slab, J. Geophys Res., 102, 27,421 – 27,437, 1997. Omori, F., The eruptions and earthquakes of the Asama-Yama, Bull. Imp. Earthquake Invest. Comm. Tokyo, 6, 1912. Sakai, S., T. Yamada, S. Ide, H. Shiobara, T. Urabe, N. Hirata, T. Kanazawa, A. Nishizawa, G. Fujie, and H. Mikada, Hypocenter distribution in and around the Izu volcanic islands with pop-up and real-time OBS’s data (abstract), Eos Trans. AGU, Fall Meet. Suppl., Abstract V52A-01, 2000. Sandwell, D. T., and W. H. F. Smith, Marine gravity anomaly from Geosat and ERS 1 satellite altimetry, J. Geophys. Res., 102, 10,039 – 10,054, 1997. Simkin, T., and L. Siebert, Volcanoes of the World, 349 pp., Geosci. Press, Tucson, Ariz., 1994. Slack, P. D., C. G. Fox, and R. P. Dziak, P wave detection thresholds, Pn velocity estimates, and T wave location uncertainty from oceanic hydrophones, J. Geophys. Res., 104, 13,061 – 13,072, 1999.

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R. P. Dziak, Oregon State University/NOAA, Cooperative Institute for Marine Resource Studies, Hatfield Marine Science Center, Newport, OR 97365, USA. ([email protected]) C. G. Fox, NOAA Pacific Marine Environmental Laboratory, Hatfield Marine Science Center, Newport, OR 97365, USA. ([email protected])

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B5, 10.1029/2001JB000177, 2002

Figure 1. Maps showing location of hydrophones and islands of the Izu-Bonin and Volcano Islands groups in the western Pacific south of Japan. (a) Approximate locations (shaded dots) of SOSUS and NOAA eastern equatorial hydrophone arrays. Shaded bar shows the location of the French Polynesian Seismic Network (RSP). Numbers show locations of hydrophones used in Figures 3 and 5. (b) Location of the major volcanoes in the Izu and Volcano Islands groups. (c) Location of the tremor waveforms (open dots) estimated using 16 individual hydrophone stations positioned off the west 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, estimated from the covariance matrix of the nonlinear regression location algorithm [Slack et al., 1999]. Red dots show earthquakes located by the National Earthquake Information Center during the time tremor signals were detected. The bathymetry is from Sandwell and Smith [1997].

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Figure 3. Diagram showing the time series (black and white) and frequency spectra (derived from continuous 1-min time windows) of hydrophone data during a time of peak tremor energy (2100 UT on 10 December 1999). The sample rate of the hydrophones is 128 Hz, with corresponding frequency spectra of 0 – 64 Hz. Spectra are plotted with a 0.5-Hz resolution. Hydrophones 1 – 4 are from North Pacific SOSUS arrays. Hydrophone 5 is from the NOAA eastern equatorial Pacific hydrophone array. The tremor is characterized by a high-amplitude fundamental followed by an ensemble of three harmonics that change in frequency through time but maintain their harmonic spacing. In this example, individual signal packets have durations of 5 min. Red arrows show the beginning and end times of the signal used to produce the frequency spectra in Figure 4.

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Figure 5. Diagram showing the time series and frequency spectra (derived from continuous 1-min time windows) of hydrophone data during a time of peak tremor energy at 0130 UT on 16 October 1998. The sample rate, frequency range, and hydrophones displayed are the same as in Figure 3. The signals in this example have durations of 2 min, about half as long as those shown in Figure 3. Red arrows show beginning and end times of the signal used to produce the frequency spectra in Figure 6. Four small earthquakes (impulsive, broadband signals) from the Aleutian Trench are present on the station 1 hydrophone 5 min into the record.

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Figure 8. Hydrophone record and spectrogram of a series of South Pacific Blue Whale vocalizations and one earthquake to illustrate the contrast between these signals and the volcanic tremor. The record is from a NOAA autonomous hydrophone located at 8S and 110W in the eastern equatorial Pacific Ocean (station 5 in Figure 1). Whale calls are typically higher frequency, last a smaller amount of time (10 s), and occur more rapidly (10 – 20 s) than the tremor signals. Earthquakes (the signal centered at 100 s) typically exhibit a wide range of frequencies (1 – 40 Hz) and are isolated events that do not continually repeat at a consistent time interval.

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