Date of Award
Doctor of Philosophy in Oceanography
Volcanoes are one of the most prominent geological features on earth and on extraterrestrial planets, such as Mars. Shield volcanoes are one of the major types of volcanoes and a better understanding of their structures and activities could help decipher the interaction of mantle plumes and the tectonic plates, and mitigate future volcanic hazards for local communities. A large number of shield volcanoes were formed along the Hawaiian-Emperor seamount chain, where the Pacific plate moves relative to the Hawaiian mantle plume. At the easternmost end of the seamount chain, the Island of Hawai’i consists of five shield volcanoes, two of which, Kīlauea and Mauna Loa, are the most recent formed and active today. Starting late April, 2018, Kīlauea showed precursory events, indicating an eruption approaching. On May 3 (Hawaii local time), the eruption happened with the first fissure opening in Leilani Estates, and on the second day, a Mw 6.9 earthquake hit the south flank of Kīlauea. In total, 24 fissures opened during the eruption and a volume of 0.8 km3 lava poured out in the Leilani Estates. This volume is considerably large and the caldera of Kīlauea collapsed during the eruption, which is the first caldera collapse for onshore Hawaiian volcanoes. This unprecedented event attracted the global attention from the science community and general public, which also opened new windows to look into the dynamics and structures of Kīlauea and other nearby Hawaiian volcanoes.
In manuscript 1, I report a rapid-response ocean-bottom-seismometer (OBS) array deployed during the 2018 Kīlauea eruption. I was on the cruise to recover the OBSs. The array aims to capture the occurrence of offshore seismicity, including the aftershocks of the Mw 6.9 main shock and the volcano-tectonic earthquakes happened during the lower rift zone eruption. The general data quality of the array is analyzed and preliminary earthquake events captured by the array are presented.
In manuscript 2, I used most of the publicly available seismic data during the eruption period, including temporary arrays, to build a more complete earthquake catalog during the eruption. We used a short-time-average/long-time-average (STA/LTA) method to identify potential earthquakes. The detections were associated into events and automatically picked with P- and S-wave arrivals, which were used to locate the events in a three-dimensional velocity model. After re-examining these earthquake events, their coda/duration magnitudes were determined. The resulting half-year catalog contains 375,736 events with one of the highest daily earthquake numbers ever reported (6,128 on June 21st, 2018). A great number of events were recorded during the caldera collapses, from its beginning until its rapid ending. The catalog also contains abundant events near the Pu’u’ōō vent and in the lower East Rift Zone, where an increase of seismicity in the mid-July and August indicated a step-up in magma intrusion after the eruption.
In manuscript 3, I extracted and analyzed the empirical Green’s Functions (EGFs) from the ambient noise data on the Island of Hawai’i. The EGFs in different years are similar at low frequencies (0.1-0.4 Hz), but very different at high frequencies (0.4-1.0 Hz): Only the EGFs after the 2018 Kīlauea eruption show clear P waves. Grid search reveals a strong noise source near the Kīlauea summit before the eruption, which contaminated the EGFs but became silent after the eruption. Modeling of the P waves identifies the direct arrival and post-critical reflections from two velocity discontinuities at 4.7 and 7.2 km depth beneath the island, which we interpret as the base of volcanic edifices and deposits and the boundary between basaltic dikes and gabbros, respectively.
In manuscript 4, I used the Rayleigh wave part of the EGFs to perform surface wave tomography on the Island of Hawai’i, to study its shallow volcano-tectonic structures. The EGFs contained fundamental mode and first higher mode Rayleigh waves. The different modes were separated with a new algorithm and their group velocities were measured. Using the group arrival times, we inverted for two-dimensional group velocity maps, which provide, for the first time, a full coverage of the Island of Hawai’i. From the group velocity maps, we inverted for a three-dimensional shear wave velocity model, which shows strong lateral variations and yields new insights into the structure and growth of the volcanoes on the island: Kīlauea’s East Rift Zone has prominent high velocities at all depths, whereas the current rift zones of Mauna Loa are characterized by intermediate to high velocities only at depths greater than 1 km below ground surface, which may be attributed to their relatively short history and less developed state. The flanks of the volcanoes, some cut by fault zones, displayed low velocities at a range of depths, generally interpreted as consisting of extrusive rocks, which could be further shattered by faulting.
Wei, Xiaozhuo, "Seismological Investigation of the Hawaiian Volcanoes Following the 2018 Kilauea Eruption" (2023). Open Access Dissertations. Paper 1540.