Advances in the understanding of the crustal structure through passive and active seismological methodologies
Active and passive seismological methods are largely employed for characterizing the crustal structure in tectonic or volcanic settings, from the near-surface down to several kilometers of depth and at a global scale.
Active seismic methods (mainly reflection and refraction seismic) have shown to be particularly effective in providing images of the crust, in terms of velocities, seismic tomography, reflection coefficient, and seismic attributes. Although they are commonly used for mineral prospecting purposes, these techniques also provide a fundamental tool for studying the structural and stratigraphic patterns in different geological settings. Nonetheless, active seismic methods show several issues and limitations, mainly due to the cost and availability of the instruments, the difficulties in exploring remote areas, and the loss in resolution with depth.
In this perspective, a fruitful synergy can arise from the combination of active and passive seismic methods, which use earthquakes or ambient noise as a source. For instance, passive seismic is fundamental to detect seismogenic crustal regions, and their attitude to release seismic energy with frequent low-energy earthquakes or few strong events, by studying the b-value of the Gutenberg & Richter Frequency-Magnitude Distribution. Such information could be compared to some extent with the seismo-stratigraphic and structural model inferred from the analysis of active seismic data, for a deeper understanding of the crustal structure.
As a final issue, other geophysical data (e.g. gravimetric, magnetic, or geo-electric) could also provide further useful information, to better constrain the interpretation of seismological data.
Contributions to the session may include challenging applications, where the joint inversion and interpretation of both active and passive seismic data, corroborated by the results deriving from other methodologies, are employed to shed light on not-straightforward complexities in different geological contexts.
The Semail ophiolite, a thick thrust sheet of Late Cretaceous oceanic crust and upper mantle, was obducted onto the previously rifted Arabian continental margin in the Late Cretaceous, and now forms part of the United Arab Emirates (UAE)-Oman mountain belt. A deep foreland basin along the west and SW margin of the mountains developed during the obduction process, as a result of flexure due to loading of the ophiolite and underlying thrust sheets. Structural and compositional complexities (e.g., presence of thick sand dunes, relatively shallow high-velocity and dense ophiolite structure) have made geophysical imaging of the sub-ophiolite and mid-lower crustal structure particularly challenging.
A combination of active and passive-source seismic techniques, potential field modelling and surface geological mapping are used to constrain the stratigraphy, velocity structure and crustal thickness beneath the UAE-Oman mountains and its bounding basins. Depth-migrated multichannel seismic-reflection profile data are integrated in the modeling of traveltimes from long offset reflections and refractions, which are used to resolve the crustal thickness and velocity structure along two E-W onshore/offshore transects in the UAE. Additionally, we apply receiver function and virtual deep seismic sounding methods to distant earthquake data recorded along the two transects to image crustal thickness variations. Seismic and geological constraints from the transects have been finally used to model gravity and magnetic anomaly data along two coincident profiles.
Geophysical methods define the Semail ophiolite as a high-velocity, high density, > 15 km thick body dipping to the east. The western limit of the ophiolite is defined onshore by the Semail thrust while the eastern limit extends several km offshore, where it is defined seismically by a ~40–45° normal fault. Emplacement of the ophiolite has probably flexed down a previously rifted continental margin, thus contributing to subsidence of flanking sedimentary basins. The new crustal thickness model presented in this work provides evidence that a crustal root is present beneath the Semail ophiolite, suggesting that folding and thrusting during the obduction process may have thickened the pre-existing crust by 16 km.
How to cite:
Pilia, S., Ali, M., Searle, M., Watts, A., Keats, B., and Ambrose, T.: An integrated geophysical approach for imaging of the Semail ophiolite, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5048, https://doi.org/10.5194/egusphere-egu22-5048, 2022.
The Southeast Asia (SEA) region is tectonically very active as it accommodates the northward movement of the Indo-Australian plate in the south and the westward movement of the Philippine Sea plate in the east. Borneo and Sulawesi are located in the centre of SEA, which is our area of interest. Borneo has an intraplate setting, while Sulawesi is situated above several microplate boundaries. For that reason, Sulawesi is seismically and volcanically more active than Borneo. The tectonic link and evolution between the two islands are not well understood as we are missing some fundamental knowledge, such as the variations in their crustal thickness and structure. This includes the provenance of their respective lithosphere, which may have Eurasian and/or East Gondwana origin.
Here, we show the results obtained from the receiver function (RF) study on seismic stations in the region to have a better understanding of the crust and mantle lithosphere beneath the two islands. The RF study includes H-k stacking, time-depth migration of the RF and inversion to estimate crustal thickness and the shear speed variation with depth. The finding from this study shows that the crust in Sulawesi is much more complex than that of Borneo. The crustal thickness gradually changes throughout Borneo, with northern Borneo having an overall thicker crust than other parts of the island. In Sulawesi, the crustal thickness is much more varied across small distances, especially along the northern and southern arms of the island.
We also show some results from the Virtual Deep Seismic Sounding (VDSS) method, which we only applied to the seismic stations in northern Borneo. We used VDSS on Northern Borneo to learn more about its complex tectonic history, such as the two subduction episodes and a continent-continent collision in a recent geological time scale. Our finding reveals a band of alternating thick and thin crust striking NE-SW in this region, which we believed resulted from extensional tectonics related to the Sulu Sea basin opening in the Miocene.
How to cite:
Linang, H. T., Gilligan, A., Jenkins, J., Pilia, S., Greenfield, T., Rawlinson, N., Supendi, P., Tongkul, F., and Widiyantoro, S.: Variation of crustal thickness in Borneo and Sulawesi, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12205, https://doi.org/10.5194/egusphere-egu22-12205, 2022.
A blanket of sedimentary and regolith material covers approximately three-quarters of the Australian continent, obscuring the crustal geology below and potential mineral resources within. Sedimentary basins also trap seismic energy increasing seismic hazard and generating noisy seismograms that make determining deeper crustal and lithospheric structure more challenging. The most fundamental question that can first be asked in addressing these challenges is how thick are the sediments? Borehole drilling and active seismic experiments provide excellent constraints, but they are limited in geographical coverage due to their expense, especially when operating in remote areas. On the other hand, passive-seismic deployments are relatively low-cost and portable, providing a practical alternative for initial surveys. Here we utilize receiver functions obtained for both temporary and permanent seismic stations in South Australia, covering regions with a diverse sediment distribution. We present a straightforward method to determine the basement depth based on the arrival time of the P-converted-to-S phase generated at the boundary between the crustal basement and sedimentary strata above. Utilizing the available borehole data, we establish a simple predictive relationship between Ps arrival time and the basement depth, which could then be applied to other sedimentary basins with some consideration. The method is found to work best for Phanerozoic sediments and offers a way to determine the sediment-basement interface in unexplored areas requiring only temporary seismic stations deployed for < 6 months.