Submarine Volcanic Eruption Research Articles

Monitoring the volcanic unrest of El Hierro (Canary Islands) before the onset of the 2011–2012 submarine eruption

On 10 October 2011, a submarine volcanic eruption started 2 km south from El Hierro Island (Spain). Since July 2011 a dense multiparametric monitoring network was deployed all over the island by Instituto Geográfico Nacional (IGN). By the time the eruption started, almost 10000 earthquakes had been located and the deformation analyses showed a maximum deformation of more than 5 cm. Earthquake migration from the north to the south of the island and acceleration of seismicity are in good correlation with changes in the deformation pattern as well as with some anomalies in geochemical and geomagnetic parameters. An earthquake of local magnitude 4.3 at 12 km depth (8 October 2011) and shallower seismicity a day after, preceded the onset of the eruption. This is the first time that a volcanic eruption is fully monitored in the Canary Islands. Data recorded during this unrest episode at El Hierro will contribute to understand reawakening of volcanic activity in this region and others of similar characteristics.

The 2011 submarine volcanic eruption in El Hierro (Canary Islands)

Forty years after the Teneguía Volcano (La Palma, 1971), a submarine eruption took place off the town of La Restinga, south of El Hierro, the smallest and youngest island of the Canarian Archipelago. Precursors allowed an early detection of the event and its approximate location, suggesting it was submarine. Uncertainties derived from insufficient scientific information available to the authorities during the eruption, leading to disproportionate civil protection measures, which had an impact on the island's economy—based primarily on tourism—while residents experienced extra fear and distress.

Active submarine eruption of boninite in the northeastern Lau Basin

Subduction of oceanic crust and the formation of volcanic arcs above the subduction zone are important components in Earth’s geological and geochemical cycles. Subduction consumes and recycles material from the oceanic plates, releasing fluids and gases that enhance magmatic activity, feed hydrothermal systems, generate ore deposits and nurture chemosynthetic biological communities. Among the first lavas to erupt at the surface from a nascent subduction zone are a type classified as boninites. These lavas contain information about the early stages of subduction, yet because most subduction systems on Earth are old and well-established, boninite lavas have previously only been observed in the ancient geological record. Here we observe and sample an active boninite eruption occurring at 1,200 m depth at the West Mata submarine volcano in the northeast Lau Basin, southwest Pacific Ocean. We find that large volumes of H2O, CO2 and sulphur are emitted, which we suggest are derived from the subducting slab. These volatiles drive explosive eruptions that fragment rocks and generate abundant incandescent magma-skinned bubbles and pillow lavas. The eruption has been ongoing for at least 2.5 years and we conclude that this boninite eruption is a multi-year, low-mass-transfer-rate eruption. Thus the Lau Basin may provide an important site for the long-term study of submarine volcanic eruptions related to the early stages of subduction. Boninite lavas are erupted during the early stages of subduction, however they have previously been found only in the ancient geological record. Discovery of an active boninite eruption shows that abundant volatile gases derived from the subducting slab drive this violent eruptive activity, even in the deep sea.

Earthquake swarm associated with volcanic eruption, CuraCoa Reef Area, Northern Tonga, July 1973

A submarine volcanic eruption near Curagoa Reef, first observed on 12 J u l y 1973 (U.T.) f r om Tafalii Island, northern Tonga, was associated with an e a r t h q u a k e swarm in t h e same area. The swarm began about 03'1 on I I J u l y and died away gradually about 07'' on 13 J u l y , ft comprised 504 events of magnitude Mi, „2= 3.4, the largest being of magnitude Ml = 5.1. The swarm area for these comparatively low magnitude earthquakes was abnormally large The v a r i a t i o n in r a t e of e a r t h q u a k e occurrence during t h e s w a rm shows two distinct stages, one similar to t h a t in a foresliock sequence, and t h e other like t h a t in a n a f t e r s h o c k sequence, with t h e largest events occurring between t h e two. The average value of b, defining t h e f r e q u e n c y - m a g n i t u d e relationship, was high: 1.77 ± 0.15. Furthermore, this p a r a m e t e r varied during the course of t h e swarm, a decrease in t h e b value f r om 1.8 to 1.1 being followed by a sharp increase to 2.5 a f t e r t h e largest earthquakes and principal volcanic eruption. Values of seismic moment were obtained from A B for 118 e a r t h q u a k es which were well recorded by long-period seismographs. Other source charact e r i s t i c s were determined from the difference between t h e l o g a r i t hm of seismic moment and the local magnitude. The source sizes were found to he u n u s u a l l y large. There was a t i m e variation of source properties during the swarm. The v a r i a t i o n of t h e displacement shows an inverse correlation with t h e variation of the coefficient b. The swarm produced very d i s t i n c t i v e T phases, well recorded at a hydrophone near Wake Island and at seismograph stations s i t u a t e d on t h e oceanic side of the Tonga Trench: these were not recorded at seismograph stations within the island arcs.

内蒙狼山新元古代酸性火山岩的发现及其地质意义

Bedded acid volcanic rocks, which preserve blasto-glomeroporphyritic or porphyritic crystals consisting of quartz and albite with clear evidence of alterations and deformations, from the Mesoproterozoic Zha’ertaishan Group in the southwestern section of Langshan, Inner Mongolia were analyzed. Zircons from two acid volcanic rock samples, A8-0 and A14-5, were separated. The zircons exhibited an igneous morphology, oscillatory zonings and relatively high Th/U ratios, and they were mostly prismatic or short prisms either colorless or brown in color, suggesting a magmatic origin. SHRIMP U-Pb dating was carried out and the 20 measurement points obtained from sample A8-0 yielded a weighted average 206 Pb/ 238 U age of 805±5 Ma. Twenty measurements were also taken on the A14-5 sample, but five points (3.1, 6.1, 7.1, 20.1 and 21.1) were discarded because of their higher 206 Pb/ 238 U ages (867.0 ± 10, 829.1 ± 8.0, 849.7 ± 9.8, 839.2 ± 9.7 and 830.7 ± 9.3 Ma, respectively). The remaining 15 points yielded a weighted average 206 Pb/ 238 U age of 814 ± 4.6 Ma. These ages are interpreted as the timing of an acid volcanic eruption and thus represent a Neoproterozoic (805–814 Ma) submarine volcanic eruption in the southwestern section of Langshan, Inner Mongolia. Geochemical data show that the acid volcanic rock is enriched in light rare earth and high field strength elements, but depleted in heavy rare earth and lithophile elements that have well-developed Eu anomalies similar to those of the Neoproterozoic volcanic rocks. These rocks are comparatively depleted in high field strength elements Th, Nb, Ta, Hf, Zr, and from the western Yangtze block and Ciwu, Zhejiang, revealing that the acidic magma had erupted in. We suggest that this submarine volcanic eruption is probably related to the breakup of Rodinia in Langshan, Inner Mongolia. This discovery, together with the ages of 814–872 Ma reported for the neighboring terranes of Jinningian deformed granites in the Alax area and the age of 827 Ma for the ultramafic rock (ultramafite) in Jinchuan, as well as the recent report of magma activities during the Neoproterozoic in the North China Platform and adjacent areas, is of significance help in understanding the Rodinia evolution in the western section of Langshan, Inner Mongolia.

Construction of the Galápagos platform by large submarine volcanic terraces

New multibeam bathymetric and side‐scan sonar data from the southwestern edge of the Galápagos platform reveal the presence of ∼60 large, stepped submarine terraces between depths of 800 m and 3500 m. These terraces are unique features, as none are known from any other archipelago that share this geomorphic form or size. The terraces slope seaward at <2° and are surrounded by escarpments that average ∼300 m in height with average slopes of 24°. The stepped morphology, fine‐scale features, and sinuous planform continuity of terrace edges indicate that each terrace results from a sequence of major submarine volcanic eruptions, similar in extent to young deep‐water (>3000 m) lava flow fields west of Fernandina and Isabela Islands. The terraces are formed of thick sequences of lava flows that coalesce to form the foundation of the Galápagos platform, on which the subaerial central volcanoes are built. The compositions of basalts dredged from the submarine terraces indicate that most lavas are chemically similar to subaerial lavas erupted from Sierra Negra volcano on southern Isabela Island. There are no regular major element, trace element, or isotopic variations in the submarine lavas as a function of depth, relative stratigraphic position, or geographic location along the southwest margin of the platform. We hypothesize that magma supply at the western edge of the Galápagos hot spot, which is influenced by both plume and mid‐ocean ridge magmatic processes, leads to episodic eruption of large lava flows. These large lava flows coalesce to form the archipelagic apron upon which the island volcanoes are built.

Interrelationships Between Vent Fluid Chemistry, Temperature, Seismic Activity, and Biological Community Structure at a Mussel-Dominated, Deep-Sea Hydrothermal Vent Along the East Pacific Rise

In April 1991, submarine volcanic eruptions initiated the formation of numerous hydrothermal vents between 945# and 952#N along the crest of the East Pacific Rise (EPR). Dramatic changes in biological community structure and vent fluid chemistry have been documented throughout this region since the eruptive event. By April 2004, mussels (Bathymodiolus thermophilus) dominated the faunal assemblages at several of the vent sites formed during of after the 1991 eruptions, whereas other habitats within the region were dominated by the vestimentiferan Riftia pachyptila. In the present paper, we build upon the extensive data sets obtained at these sites over the past decade and describe a manipulative experiment (conducted at 949.94#N; 10414.43#W on the EPR) designed to assess interrelationships between vent fluid chemistry, temperature, biological community structure, and seismic activity. To this end, in situ voltammetric systems and thermal probes were used to measure H2S/HS - and temperature over time in a denuded region of an extensive mussel bed in which an exclusion cage was placed to inhibit the subsequent migration of mussels into the denuded area. Fluid samples were taken from the same locations to characterize the associated microbial constituents. Basalt blocks, which were placed in the cage in April 2004 and subsequently recovered in April 2005, were colonized by more than 25 different species of invertebrates, including numerous vestimentiferans and remarkably few mussels. Recorded temporal changes in vent fluid chemistry and temperature regimes, when coupled with microbiological characterization of the vent fluids and seismic activity data obtained from ocean bottom seismometers, shed considerable light on factors controlling biological community structure in these hydrothermal ecosystems.

A Submarine Volcano Is Caught in the Act

Ocean-bottom seismometers have recorded the seismic activity associated with a submarine volcanic eruption, revealing important differences from eruptions on land.

A Refined Historical Record of Volcanic Eruptions around Taiwan: Tectonic Implications in the Arc-continent Collision Area

The historical record of volcanic eruptions is extremely important for natural hazard reduction, especially in densely inhabited areas. In concert with clarification of contradictory points in the historical eruptions' record, we have modified the locality, eruption size, and timing of submarine volcanic eruptions around Taiwan. Three out of five submarine eruption records (catalog numbers 0801-04, 0801-05 and 0802-01) in northern offshore Taiwan can be clearly traced. These events indicate that volcanic activity in this area is still in progress; and hence a program for volcanic hazard reduction should be seriously considered in northern Taiwan. Two events (catalog numbers 0801-03 and 0801-02) shown to have occurred in the southeast offshore region of Taiwan appear to be related to the Eurasian Continental Plate's eastwardly subduction under the Philippine Sea Plate. However, the new seismic wave travel time tomography of the subduction zone will be needed to clarify this. The refined results shall contribute to the Smithsonian Institution's Global Volcanism Program and the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) for editing the next version of the Catalog of Active Volcanoes of the World.

Volcanic eruptions on mid‐ocean ridges: New evidence from the superfast spreading East Pacific Rise, 17°–19°S

Side‐scan sonar, submersible observations and sampling of lava flows from the East Pacific Rise, 17°–19°S constrain the character and variability of submarine volcanic eruptions along mid‐ocean ridges. Nine separate lava sequences were mapped using relative age and lithological contrasts among recovered samples. Axial lengths activated during eruptive episodes range from ∼1 to >18 km; individual flow field areas vary from <1 to >19 km2. Estimated erupted volumes range from <1 to >200 × 106 m3. The largest unit is the chemically uniform Animal Farm lava near 18°37′S. The youngest lava is the Aldo‐Kihi flow field, 17°24′–34′S, probably erupted in the early 1990s from a fissure system extending >18 km along axis. Near 18°33′S two distinct lava compositions with uniform sediment cover were recovered from lava that buries older faulted terrain. The boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector, consistent with magmas from two separate reservoirs being erupted in the same event. Chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments. A comparison of our results to global data on submarine mid‐ocean ridge eruptions suggests consistent dependencies of erupted volume, activated fissure lengths, and chemical heterogeneity with spreading rate, consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths.

Evidence of harmonic tremor from a submarine volcano detected across the Pacific Ocean basin

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 magma‐gas mixture within a large chamber/conduit near the surface of a shallow (<1 km) submarine volcanic edifice, allowing for efficient release of acoustic energy into the ocean sound channel. During periods of peak energy radiation the tremor could be clearly detected on hydrophones in the western Aleutians, Hawaii, west coast of the United States, and eastern equatorial Pacific and was detected by the French Polynesian seismic network, covering a maximum distance of ∼14,000 km.

Direct observation of a submarine volcanic eruption from a sea-floor instrument caught in a lava flow.

Our understanding of submarine volcanic eruptions has improved substantially in the past decade owing to the recent ability to remotely detect such events and to then respond rapidly with synoptic surveys and sampling at the eruption site. But these data are necessarily limited to observations after the event. In contrast, the 1998 eruption of Axial volcano on the Juan de Fuca ridge was monitored by in situ sea-floor instruments. One of these instruments, which measured bottom pressure as a proxy for vertical deformation of the sea floor, was overrun and entrapped by the 1998 lava flow. The instrument survived-being insulated from the molten lava by the solidified crust-and was later recovered. The data serendipitously recorded by this instrument reveal the duration, character and effusion rate of a sheet flow eruption on a mid-ocean ridge, and document over three metres of lava-flow inflation and subsequent drain-back. After the brief two-hour eruption, the instrument also measured gradual subsidence of 1.4 metres over the next several days, reflecting deflation of the entire volcano summit as magma moved into the adjacent rift zone. These findings are consistent with our understanding of submarine lava effusion, as previously inferred from seafloor observations, terrestrial analogues, and laboratory simulations.

Characteristics and origin of the Middle Proterozoic Dongshuichang chambersite deposit, Jixian, Tianjin, China

The Jixian-type boron–manganese (B–Mn) deposits include both chambersite and rhodochrosite deposits, from which Dongshuichang is economically the best chambersite deposit. Although the ore bodies of the region are relatively small, they contain a rare B–Mn ore that is scientifically significant. Chambersite (Mn 3B 7O 13Cl) is a rare mineral, discovered as a mineral in 1957 and as an ore in 1971 at the Dongshuichang deposit near Jixian, Tianjin City. The deposit forms a monocline with a southwest dip and covers an area of 2 km 2. In the mine area, the Gaoyuzhuang Formation, the upper member of the Middle Proterozoic Changcheng System, consists mainly of stromatolitic and muddy dolostone with B–Mn-bearing siliceous shale and dark-gray dolostone. Individual ore lenses range in length from 1 to 4 m with a mean thickness of 0.25 m. The ores can be divided into high-grade and low-grade, the former is composed of spindle and granular chambersite aggregates and the latter of spheroidal chambersite in dark-gray micritic dolostone. The center of spheroids is commonly composed of micritic and columnar chambersite, dolomite, clay minerals, and organic matter, around which columnar chambersite grew, forming a sunflower shape. The organic-carbon content in high-grade ores is 0.63%; in low-grade ores, 0.83%; and in overlying black siliceous shale, 3.75%. Total rare earth element (REE) contents of two chambersite ore samples are 29 and 35 ppm, whereas that of rhodochrosite ore is 22 ppm. The chondrite-normalized REE patterns of chambersite and rhodochrosite ores are similar, with small negative Ce and Eu anomalies and a small positive Gd anomaly. The δ 13 C PDB values of two rhodochrosite ore samples are −4.3‰ and −7.3‰, indicating that the carbon source of Mn-carbonate minerals was mainly seawater bicarbonate with a small contribution from the degradation of organic carbon. The source of the Dongshuichang ore components was mainly submarine volcanic eruptions and a small amount from the weathering of pre-Proterozoic boron deposits. The chambersite precipitated under conditions of high salinity, high B, Mn, and Mg concentrations, and weak alkalinity. Low-oxygen conditions might be favorable for precipitation of chambersite.

Time-series temperature measurements at high-temperature hydrothermal vents, East Pacific Rise 9°49′–51′N: evidence for monitoring a crustal cracking event

Temperature measurements of hydrothermal vent fluids provide an important indicator of the physical and chemical state of mid-ocean ridge crest hydrothermal and magmatic systems. Changes in vent fluid temperature and chemistry can have dramatic effects on biological communities that inhabit these unique ecosystems. In an attempt to understand temporal variability of ridge crest hydrothermal activity as it relates to geological processes at the ridge axis, six high-temperature hydrothermal vents on the East Pacific Rise crest between 9°49′N and 9°51′N were instrumented and sampled repeatedly during five years following a submarine volcanic eruption in 1991. Bio9 vent, located on the floor of the axial trough near 9°50.2′N, has the most complete record of fluid temperatures from 1991 to 1997, including a continuous temperature record of nearly three years (1994–1997). Bio9 vent fluids were 368°C in 1991, increased to an estimated temperature ≥388°C after a second volcanic event in 1992, and thereafter declined over the next ∼2 years reaching a temperature of 365°C in December 1993. Continuous temperature records and point measurements made by Alvin's thermocouple probe show Bio9 vent fluids were stable for ∼15 months at 365±1°C, until March 26, 1995. On March 26, an abrupt 7°C increase occurred over a period of eight days at this vent, and a maximum temperature of 372±1°C persisted for 14 days. The vent fluid cooled gradually over ∼3.5 months to 366±1°C, and for several months at the end of the recording period the temperature increased a few degrees. A continuous record of fluid temperature at this vent between November 1995 and November 1997 shows a 5±1°C increase for the two-year period. The abrupt temperature increase at Bio9 vent, and coincident changes in faunal community structure, and geochemistry of vent fluids from this area suggest that a crustal event occurred, either in the form of a cracking front in the crust or intrusion of a small dike. Based on the results of a microseismicity experiment conducted around the Bio9 vent in 1995 [Sohn et al., Trans. Am. Geophys. Union 78 (1997) F647; Sohn et al., Nature (in press)], and the identification of a small earthquake swarm which occurred on March 22, 1995 we conclude that the temperature anomaly measured at Bio9 four days following the swarm was caused by a cracking front penetrating into hot crustal rocks beneath the vent.

Early Devonian Kess-Kess carbonate mud mounds of the eastern Anti-Atlas (Morocco), and their relation to submarine hydrothermal venting

ABSTRACT Spectacular conical Early Devonian carbonate buildups up to 55 m high that crop out in the eastern Anti-Atlas of southern Morocco are microbially mediated carbonate mud mounds that were surfaced by soft-bottom communities dominated by small tabulate corals. They formed on the Hamar Laghdad elevation, which was created by a submarine volcanic eruption, and were associated with a network of synsedimentary radial and tangential faults that originated by uplift of the intrusive laccolithic body underlying the Kess-Kess Formation. These faults served as conduits for the migration of hydrothermal fluids to the sea floor. Most mounds developed over cross-points of radial and tangential faults. Vents were episodically active until the Famennian, but extensive vent carbonate production occurred only during the Emsian. Preliminary geochemical results document that mud-mound carbonates and calcite cements in neptunian dikes precipitated from brines comprising a mixture of hydrothermal fluids and seawater. In addition, carbon isotope compositions (13C as low as -18o/oo PDB) suggest a contribution from thermogenic methane derived presumably from underlying basaltic intrusives. Aerobic bacterial oxidation of methane is favored as the main process driving carbonate precipitation in, and rapid lithification of, the mounds.

SeaBeam depth changes associated with recent lava flows, Coaxial Segment, Juan De Fuca Ridge: Evidence for multiple eruptions between 1981–1993

After a swarm of earthquakes was detected on the CoAxial segment of the Juan de Fuca Ridge in June‐July 1993, the area was resurveyed with SeaBeam multibeam sonar to search for depth changes associated with a submarine volcanic eruption. Quantitative comparison of the 1993 SeaBeam survey with surveys in 1981/82 and 1991 shows one area of seafloor depth change (up to 29 m) between 1991–93 exactly where a pristine lava flow was discovered. In addition, two other depth anomalies (up to 37 m and 20 m) are identified between 1981–91, evidence that other recent eruptions have occurred along this spreading ridge segment.

Economics, volcanoes, and Phanerozoic revolutions

Two intervals of the Phanerozoic stand out as times of biosphere-scale revolution in the sense that biogeochemical cycles came under increased control by organisms. These are the early Paleozoic (extending from just before the Cambrian to the Middle Ordovician, a duration of about 100 m.y.), characterized by the appearance of predators, burrowers, and mineralized skeletons, and by the subsequent diversification of planktonic animals and suspension-feeders; and the later Mesozoic (latest Triassic to mid-Cretaceous, a duration of somewhat more than 100 m.y.), marked by a great diversification of predators and burrowers and by the rise of mineralized planktonic protists. This paper explores the economic conditions that make such revolutions possible.I argue that opportunities for innovation and diversification are enhanced when raw materials and energy are supplied at increasing rates, or when organisms gain greater access to these commodities through rising temperatures and higher metabolic rates. Greater per capita availability of resources enables populations to grow; lessens or alters ecological constraints on functional improvement; makes possible the evolution of high metabolic rates (large incomes), which in turn permit improvement in each of several otherwise incompatible functions; and favors the establishment and spread of daughter species arising through founder speciation. Reductions in productivity reinforce adaptational constraints and may bring about extinctions.Massive submarine volcanism, together with its associated phenomena of warming, sea-level rise, and widening of warm-weather zones, is proposed to be the chief extrinsic trigger for the Phanerozoic revolutions. The later Mesozoic was characterized by continental rifting, which accompanied massive submarine volcanic eruptions that produced large quantities of nutrients and carbon dioxide. This activity began in the Late Triassic and peaked in the mid- to Late Cretaceous. The Early Cambrian was also a time of rifting and may likewise have been marked by large-scale submarine volcanism. Continental and explosive volcanism, weathering, and upwelling are other potential means for increasing evolutionary opportunity, but their effects are either local or linked directly or indirectly with cooling. Intense chemical weathering in the Early Cambrian, however, may have contributed to the early Paleozoic revolution.The extrinsic stimulus was greatly amplified through positive feedback by the evolution of higher metabolic rates and other means for acquiring, trading, retaining, and recycling resources more rapidly and from a wider range of environments. Because these novelties usually require a high and predictable supply of resources, their evolution is more likely when extrinsically controlled supplies increase rather than when per capita availability is low.In the view adopted here, the microevolutionary and microeconomic market forces of competition and natural selection operate against a backdrop of macroeconomic supply and demand. Resources are under both extrinsic and intrinsic control. Positive and negative feedbacks link processes at the micro- and macroeconomic levels. This view complements the genealogical and hierarchical conception of evolution by emphasizing that the pattern of descent is influenced by resources and by market forces operating at all scales of space and time.

T Waves Associated with Submarine Volcanic Eruptions in the Marianas Observed by Ocean Bottom Seismographs.

In September and December of 1989, swarms of T waves were observed by ocean bottom seismographs (BSOBS) off the Boso Peninsula of Japan, by a SOFAR hydrophone array near Wake Island, and by a station at Rangiroa. Using arrival times observed by these networks, the source location of the T waves was determined to be in the northern Marianas, where submarine eruptions had been observed frequently in recent years.The T waves were not accompanied by corresponding body waves. The T waves had shorter duration (from a few to ten seconds) and higher prominent frequency than those of tectonic earthquakes which also occurred in the northern Marianas. The amplitude variation of the T waves observed by BSOBS showed a different pattern from those from tectonic earthquakes. This is a manifestation of a difference in the spectral contents between them. The spectra of these T waves showed conspicuous harmonic peaks. These characteristics are similar to those of volcanic T waves as previously reported. Therefore we concluded that they are excited by submarine volcanic eruptions.From the detailed analysis of the T wave events, this volcanic activity is considered to proceed through three stages: the first which was not so explosive, the second which was composed of intermittent explosive eruptions, and the third which was small and similar to the first. The observed fundamental frequencies of the T waves varied with time. This could be caused by changes in the eruption site or in the size of the area where reverberations occurred.

The rupture process of the 1989 offshore ito earthquakes preceding a submarine volcanic eruption

An earthquake swarm at the eastern offshore region of Ito, the Izu peninsula, started on June 30, 1989, and became more active on and after July 4. The largest earthquake among the swarm (MJMA= 5.5) occurred at 1109 JST (0209 UT) on July 9, 1989, and a second significant event followed 45 s later. A remarkable volcanic tremor started on July 11, and a submarine eruption occurred just above the source region of the largest earthquake on July 13. In this paper we present a new inversion method to derive the source time function of rupture process. The source time function is divided into several sections following an automatic algorithm and is interpolated with a cubic spline function. The parameters, which are the values and first derivatives at knots of the spline function, are determined by minimizing a squared error between the observed and synthetic seismograms. To demonstrate the performance of this method, some numerical experiments (simulations) are carried out on near‐field seismic records. Applying this method to analysis of the two large earthquakes, the detailed rupture processes are obtained. A heterogeneous crustal structure near the volcanic area affects the rupture processes of these earthquakes. The large stress drop due to the largest earthquake just beneath the submarine eruption point would assist a magma ascending and may have triggered the eruption 4 days after these earthquakes.

Fallout of Pyroclastic Debris from Submarine Volcanic Eruptions

Volcanic fallout deposits on land, being widespread and accessible for study, have received much attention and have revealed a great deal about subaerial eruption mechanisms. In contrast, virtually nothing is known about equivalent deposits produced by submarine volcanoes, despite the probable abundance of such material in today's oceans and in accreted volcanic arc terrains. Many submarine deposits may form by the fallout of debris to the sea floor downcurrent from the umbrella region of submarine eruption columns. Experiments on water-saturated pumice and pieces of rock (lithics) show that particles settling to the sea floor at terminal velocities of 10 to 50 centimeters per second will display conspicuous bimodality of particle diameters: pieces of pumice may be five to ten times as large as codeposited lithic fragments. Similar material, erupted into the air and deposited on land, displays less well-developed bimodality; pumice diameters are generally two to three times as large as associated lithics. Submarine fallout deposits are therefore distinctive and may be used to indicate a subaqueous origin for some of the great thicknesses of nonfossiliferous volcanic debris contained in ancient volcanic terrains worldwide whose environment of deposition has been uncertain.