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Acoustic Monitoring of Transient Volcanic and Tectonic Events

Acoustic Monitoring of Transient Volcanic and Tectonic Events. Ocean seismicity can be placed into two categories: Magmatic: Earthquakes associated with volcanic activity on mid-ocean ridges (dike emplacement), hotspots, or island-arc volcanoes (this lecture)

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Acoustic Monitoring of Transient Volcanic and Tectonic Events

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  1. Acoustic Monitoring of Transient Volcanic and Tectonic Events Ocean seismicity can be placed into two categories: Magmatic:Earthquakes associated with volcanic activity on mid-ocean ridges (dike emplacement), hotspots, or island-arc volcanoes (this lecture) Tectonic: Earthquakes associated with movement on faults (next lecture) Both can perturb hydrothermal systems, generate new vent fields, and expel biological material from deep within the crust Examples of volcanic seismicity: • Juan de Fuca Ridge (medium 6 cm/yr) and Axial hotspot using seafloor hydrophones (SOSUS) • East Pacific Rise (fast 10-15 cm/yr) using moored hydrophones • Mid-Atlantic Ridge (slow ~ 3 cm/yr) “ • Marianas and Volcano Island-arc volcanoes “

  2. NOAA Acoustic Project: • Use hydrophones to monitor unobserved seafloor volcanic activity in Pacific and Atlantic Oceans, alert research communities • Began in 1991 in NE Pacific – 7 major seafloor eruption & magmatic events on Juan de Fuca Ridge system. • In 1993, first real-time observation of a seafloor spreading episode – fundamental process for creation of ocean crust.

  3. Histograms of earthquake Counts from major magma dike emplacement episodes On the Juan de Fuca Ridge All episodes similar in time duration and peak event rates

  4. CoAxial Segment eruption seismicity - June1993

  5. Time-distance of epicenters tracks movement of magma dike through ocean crust Decrease in rise-time of T-wave signal packet indicates shallowing of dike

  6. Northern Gorda Ridge Segment eruption seismicity: February 1996

  7. Example earthquake swarm: Axial Volcano, Juan de Fuca Ridge, January 1998 Earthquakes begin in summit caldera, migrate 60 km down rift-zone over a 2 day period. In situ instruments detect water- temperature anomalies, seafloor subsidence, and are buried in lava! Evidence of eruption at summit and lateral injection of magma down the rift zone at speeds of 0.2 - 1.0 m/s Earthquake swarm accounted for minimum of ~4.5 yrs of plate motion across ridge.

  8. Earthquake locations displayed through time track the sub-seafloor movement of magma Steady increase in number and size of earthquake swarms at Axial Volcano during 8 yrs prior to eruption.

  9. Rumbleometer data • Pressure data

  10. Rumbleometer data • Temperature data

  11. Rumbleometer data • The rumbleometer was the first “eyewitness” to a submarine eruption • First direct information on eruption duration, effusion rates, lava flow emplacement, volcano deflation, and magma reservoir depth • “In the fields of observation, chance favors the prepared mind” - Louis Pasteur

  12. Hydrophone Mooring

  13. East Pacific Rise Hydrophones • Deployed along fast-spreading (10 cm/yr) East Pacific Rise since May 1996 • Annual recoveries and processing time, observe earthquakes 1-2 yrs after occurred • 56,072 earthquakes located in 6 years • Detected 7 volcanic earthquake swarms

  14. Events detected along EPR prior to deployment of autonomous hydro- phone array

  15. Mid-Atlantic Ridge Hydrophones • Two arrays deployed along slow-spreading (2 cm/yr) Mid-Atlantic Ridge since February 1999 and May 2002 • Southern array recovered on yearly schedule, funded by NSF through 2007 • Northern array collaborative experiment with French researchers (U. Brest-Occidental), funded by OE for 2002-2003 • 11,062 earthquakes located in 3 years • Detected 1 volcanic swarm to date – first detected on deep-ocean portion of MAR

  16. One volcanic event detected along MAR prior to deployment of auto- nomous hydrophone array

  17. Lucky Strike Segment:Seamount & Vent Fields ~2.1 cm/yr Lava Lake Scheirer et al, MGR, 2000 Escartin et al, JGR, 2001

  18. Lucky Strike Earthquake Swarm • Began on March 16, 2001 • Largest earthquake swarm ever recorded on MAR (128 on hydrophones, 30 on land seismic networks) • Earthquakes locate along seamount and north rift valley, accompanied by volcanic tremor • Increased microbial activity and hydrothermal venting observed 3-12 mo. afterward • Interpreted as magma intrusion event

  19. Modified Omori distribution • Lucky Strike Swarm does not follow tectonic decay rate 1/tp • Suggests volcanic component to earthquake swarm

  20. Histogram of Earthquake activity at Lucky Strike • Majority of hydrophone earthquakes at beginning of Lucky Strike swarm - few teleseismic events • Small events at onset, large at end of swarm • Initial intrusion, followed by segment-scale normal faulting?

  21. Earthquake-Tremor Spectra

  22. Was this a magmatic earthquake swarm? Characteristics of Lucky Strike swarm similar to plume producing seafloor eruptions on the Juan de Fuca Ridge (5.5 cm/yr): • Vigorous earthquake swarm (30 events/hr) • Omori distribution non-tectonic • Possible intrusion tremor • Small earthquake swarms at Krafla (Iceland) and Axial Volcano (JdFR) previously interpreted as magma intrusions beneath volcano summit In contrast to previous mid-ocean ridge eruptions: - Short duration swarm (hours not weeks) - No along-axis propagation ofearthquakes

  23. In situ observations of Lucky Strike hydrothermal vents since earthquake swarm • June–July 2001 ROV Victor(D. Desbruyeres, pers. Com.) • Increase in bacterial floc and mats east of Lava Lake, near High-Temp vent Y3 since 1999. • Comparable in size (lesser extent) to 1991 EPR (9N) eruption. • July 2002 Submersible MIR (T. Shank, pers. Com.) • Increase in diffuse venting along sides/base of black smoker mounds since last observed in 1997. No evidence yet of recent lava flows, hydrothermal vents still intact

  24. Summary – Lucky Strike • Interpret Lucky Strike earthquake swarm as magmatic event • Consistent with intrusion of magma into shallow crust - Possibly without eruption of lava onto seafloor • Most significant L.S. earthquake swarm in 25+ yrs • Isolated event or precursor to increased activity? • Lucky Strike area good candidate for long-term monitoring (MOMAR) efforts

  25. Summary: • Monitoring of seafloor earthquake/volcanic activity has broad impacts (e.g.): - Destruction/creation of seafloor and hydrothermal ecosystems - Seismic/volcanic hazard for coastal communities - Unobserved volcanic eruptions effect accuracy of climate change models - Increase to ocean noise effects fisheries and marine mammal populations • Future goals: - Develop project assets into an integrated, global observing system - Can be applied to a variety of ocean research issues (ocean noise, fisheries, marine mammal and climate change). • Integrated models of global ocean processes requires development of alternative acoustic monitoring technologies: - Need for Real-time hydrophone communication (buoys) - Cabled hydrophone arrays

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