Seismic Studies of the Tonga Subduction Zone and the Lau Back-arc Basin

This dissertation utilizes multiple techniques of seismic tomography and earthquake location to investigate the upper mantle structure and intermediate-depth seismicity of the Tonga subduction zone, the Lau back-arc basin, and adjacent regions. Data for these studies consist of broadband records from 49 ocean bottom seismographs and 17 island-based seismic stations deployed for one year in 2009-2010, as well as more limited earlier datasets. I conducted tomographic studies of Rayleigh wave velocity and body wave attenuation to examine the thermal variations and the distribution of partial melt in the mantle wedge. The shear-wave velocity structure is first determined using only teleseismic data with the two-plane-wave method, and then jointly inverted from the phase velocities of teleseismic and ambient-noise Rayleigh waves obtained from noise cross-correlation. Additionally, I determine the 3-D P and S wave attenuation structure from t* measurements using local and regional earthquakes. Tomographic results show extremely low velocity and high attenuation within the upper 80-km of the mantle beneath the Lau back-arc basin, suggesting perhaps the lowest shear-wave velocity (VSV = 3.6 km/s) and highest seismic attenuation (QP \u3c 35 and QS \u3c 25) known in the mantle. These anomalies require not only abnormally high temperature but also the existence of partial melt. The inferred melting regions align with the spreading centers at shallow depths of 20-70 km, but shift westwards away from the slab, indicating a passive decompression melting process governed by the mantle wedge flow rising from the west. Assuming that velocity anomalies reflect variations in mantle porosity filled with melt, the mantle porosity is reduced in areas of high mantle water content, implying that the melt segregation and extraction are significantly enhanced by the water released from the subducting slab. The low velocities and high attenuation beneath the northeastern Fiji Plateau and northern Lau Ridge suggest the missing lithospheric root in this region, where the active Taveuni Volcano exists. This, along with the low-velocity anomalies beneath the northwestern Lau Basin, are consistent with a second origin from the deep mantle in addition to the Samoan mantle plume. In order to investigate water-related slab processes, I precisely locate intermediate-depth earthquakes and associate them with the Global Centroid Moment Tensor solutions. These events form a double seismic zone with a separation of about 30 km in the northern part of the Tonga slab, with a downdip compressional upper plane and a downdip tensional lower plane. The lower termination of the double seismic zone correlates with the convergence rate, extending to 300 km in places, and is consistently deeper than in Japan and other slabs worldwide. Similar trends have been found for a tripe of seismicity at depths of 200-300 km. These observations indicate that the depth of intermediate-depth seismicity is primarily influenced by temperature, implying the importance of the thermally controlled processes, such as serpentine dehydration and fluid-related embrittlement. However, the disappearance of the double seismic zone towards to the south coincides with the change in slab curvature, suggesting that the stress states rather than dehydration reactions control the activity of the lower plane

Oceanic plateau of the Hawaiian mantle plume head subducted to the uppermost lower mantle.

The Hawaiian-Emperor seamount chain that includes the Hawaiian volcanoes was created by the Hawaiian mantle plume. Although the mantle plume hypothesis predicts an oceanic plateau produced by massive decompression melting during the initiation stage of the Hawaiian hot spot, the fate of this plateau is unclear. We discovered a megameter-scale portion of thickened oceanic crust in the uppermost lower mantle west of the Sea of Okhotsk by stacking seismic waveforms of SS precursors. We propose that this thick crust represents a major part of the oceanic plateau that was created by the Hawaiian plume head ~100 million years ago and subducted 20 million to 30 million years ago. Our discovery provides temporal and spatial clues of the early history of the Hawaiian plume for future plate reconstructions

Oceanic plateau of the Hawaiian mantle plume head subducted to the uppermost lower mantle.

The Hawaiian-Emperor seamount chain that includes the Hawaiian volcanoes was created by the Hawaiian mantle plume. Although the mantle plume hypothesis predicts an oceanic plateau produced by massive decompression melting during the initiation stage of the Hawaiian hot spot, the fate of this plateau is unclear. We discovered a megameter-scale portion of thickened oceanic crust in the uppermost lower mantle west of the Sea of Okhotsk by stacking seismic waveforms of SS precursors. We propose that this thick crust represents a major part of the oceanic plateau that was created by the Hawaiian plume head ~100 million years ago and subducted 20 million to 30 million years ago. Our discovery provides temporal and spatial clues of the early history of the Hawaiian plume for future plate reconstructions

Under the Surface: Pressure-Induced Planetary-Scale Waves, Volcanic Lightning, and Gaseous Clouds Caused by the Submarine Eruption of Hunga Tonga-Hunga Ha’apai Volcano Provide an Excellent Research Opportunity

We present a narrative of the eruptive events culminating in the cataclysmic 15 January 2022 eruption of Hunga Tonga-Hunga Ha’apai Volcano by synthesizing diverse preliminary seismic, volcanological, sound wave, and lightning data available within the first few weeks after the eruption occurred. The first hour of eruptive activity produced fast-propagating tsunami waves, long-period seismic waves, loud audible sound waves, infrasonic waves, exceptionally intense volcanic lightning and an unsteady volcanic plume that transiently reached—at 58 km—the Earth’s mesosphere. Energetic seismic signals were recorded worldwide and the globally stacked seismogram showed episodic seismic events within the most intense periods of phreatoplinian activity, and they correlated well with the infrasound pressure waveform recorded in Fiji. Gravity wave signals were strong enough to be observed over the entire planet in just the first few hours, with some circling the Earth multiple times subsequently. These large-amplitude, long-wavelength atmospheric disturbances come from the Earth’s atmosphere being forced by the magmatic mixture of tephra, melt and gasses emitted by the unsteady but quasi-continuous eruption from 0402±1—1800 UTC on 15 January 2022. Atmospheric forcing lasted much longer than rupturing from large earthquakes recorded on modern instruments, producing a type of shock wave that originated from the interaction between compressed air and ambient (wavy) sea surface. This scenario differs from conventional ideas of earthquake slip, landslides, or caldera collapse-generated tsunami waves because of the enormous (∼1000x) volumetric change due to the supercritical nature of volatiles associated with the hot, volatile-rich phreatoplinian plume. The time series of plume altitude can be translated to volumetric discharge and mass flow rate. For an eruption duration of ∼12 hours, the eruptive volume and mass are estimated at 1.9 km3 and ∼2,900 Tg, respectively, corresponding to a VEI of 5-6 for this event. The high frequency and intensity of lightning was enhanced by the production of fine ash due to magma—seawater interaction with concomitant high charge per unit mass and the high pre-eruptive concentration of dissolved volatiles. Analysis of lightning flash frequencies provides a rapid metric for plume activity and eruption magnitude. Many aspects of this eruption await further investigation by multidisciplinary teams. It represents a golden opportunity for fundamental research regarding the complex, non-linear behavior of high energetic volcanic eruptions and attendant phenomena, with critical implications for hazard mitigation, volcano forecasting, and first-response efforts in future disasters

Under the Surface: Pressure-Induced Planetary-Scale Waves, Volcanic Lightning, and Gaseous Clouds Caused by the Submarine Eruption of Hunga Tonga-Hunga Ha’apai Volcano Provide an Excellent Research Opportunity

We present a narrative of the eruptive events culminating in the cataclysmic 15 January 2022 eruption of Hunga Tonga-Hunga Ha’apai Volcano by synthesizing diverse preliminary seismic, volcanological, sound wave, and lightning data available within the first few weeks after the eruption occurred. The first hour of eruptive activity produced fast-propagating tsunami waves, long-period seismic waves, loud audible sound waves, infrasonic waves, exceptionally intense volcanic lightning and an unsteady volcanic plume that transiently reached—at 58 km—the Earth’s mesosphere. Energetic seismic signals were recorded worldwide and the globally stacked seismogram showed episodic seismic events within the most intense periods of phreatoplinian activity, and they correlated well with the infrasound pressure waveform recorded in Fiji. Gravity wave signals were strong enough to be observed over the entire planet in just the first few hours, with some circling the Earth multiple times subsequently. These large-amplitude, long-wavelength atmospheric disturbances come from the Earth’s atmosphere being forced by the magmatic mixture of tephra, melt and gasses emitted by the unsteady but quasi-continuous eruption from 0402±1—1800 UTC on 15 January 2022. Atmospheric forcing lasted much longer than rupturing from large earthquakes recorded on modern instruments, producing a type of shock wave that originated from the interaction between compressed air and ambient (wavy) sea surface. This scenario differs from conventional ideas of earthquake slip, landslides, or caldera collapse-generated tsunami waves because of the enormous (∼1000x) volumetric change due to the supercritical nature of volatiles associated with the hot, volatile-rich phreatoplinian plume. The time series of plume altitude can be translated to volumetric discharge and mass flow rate. For an eruption duration of ∼12 hours, the eruptive volume and mass are estimated at 1.9 km3 and ∼2,900 Tg, respectively, corresponding to a VEI of 5-6 for this event. The high frequency and intensity of lightning was enhanced by the production of fine ash due to magma—seawater interaction with concomitant high charge per unit mass and the high pre-eruptive concentration of dissolved volatiles. Analysis of lightning flash frequencies provides a rapid metric for plume activity and eruption magnitude. Many aspects of this eruption await further investigation by multidisciplinary teams. It represents a golden opportunity for fundamental research regarding the complex, non-linear behavior of high energetic volcanic eruptions and attendant phenomena, with critical implications for hazard mitigation, volcano forecasting, and first-response efforts in future disasters

Slab to back-arc to arc: fluid and melt pathways through the mantle wedge beneath the Lesser Antilles

Volatiles expelled from subducted plates promote melting of the overlying warm mantle, feeding arc volcanism. However, debates continue over the factors controlling melt generation and transport, and how these determine the placement of volcanoes. To broaden our synoptic view of these fundamental mantlewedge processes, we image seismic attenuation beneath the Lesser Antilles arc, an end-member system that slowly subducts old, tectonized lithosphere. Punctuated anomalies with high ratios of bulk-to-shear attenuation (Qκ-1/Qμ-1 &gt; 0.6) and VP/VS (&gt;1.83) lie 40 km above the slab, representing expelled fluids that are retained in a cold boundary layer, transporting fluids toward the back-arc. The strongest attenuation (1000/QS ∼ 20), characterizing melt in warm mantle, lies beneath the back-arc, revealing how back-arc mantle feeds arc volcanoes. Melt ponds under the upper plate and percolates toward the arc along structures from earlier back-arc spreading, demonstrating how slab dehydration, upper-plate properties, past tectonics, and resulting melt pathways collectively condition volcanism.</p

Slab to back-arc to arc: Fluid and melt pathways through the mantle wedge beneath the Lesser Antilles

Volatiles expelled from subducted plates promote melting of the overlying warm mantle, feeding arc volcanism. However, debates continue over the factors controlling melt generation and transport, and how these determine the placement of volcanoes. To broaden our synoptic view of these fundamental mantle wedge processes, we image seismic attenuation beneath the Lesser Antilles arc, an end-member system that slowly subducts old, tectonized lithosphere. Punctuated anomalies with high ratios of bulk-to-shear attenuation (Qκ−1/Qμ−1 > 0.6) and VP/VS (>1.83) lie 40 km above the slab, representing expelled fluids that are retained in a cold boundary layer, transporting fluids toward the back-arc. The strongest attenuation (1000/QS ~ 20), characterizing melt in warm mantle, lies beneath the back-arc, revealing how back-arc mantle feeds arc volcanoes. Melt ponds under the upper plate and percolates toward the arc along structures from earlier back-arc spreading, demonstrating how slab dehydration, upper-plate properties, past tectonics, and resulting melt pathways collectively condition volcanism