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Using Ambient Noise to “See” into the Crust Ensing, J. X., van Wijk, K., Spörli, K. B. Introduction Research Questions Seismometer Orientation Previously, it has been difficult to produce a high resolution model of the subsurface structure of Auckland due to its low seismicity. However, we recently used ocean-wave-generated seismicnoise to obtain sixteen 1D shear-wave velocity models for the subsurface of the Auckland Volcanic Field (Ensing, 2015). This study used vertical component data from a network of 12 seismometers, including the University of Auckland seismometer, RBAZ, and was the first to use ocean-noise to infer the structure of the Auckland subsurface. A higher resolution 3D shear-wave-velocity model will not only improve our understanding of the mechanisms and risk in the Auckland Volcanic Field but also shed light on the structure of crust. Some of the questions that may be answered with such further work are: How thick is the crust in Auckland? Is the crustal thickness uniform? What is the connection between the Dun Mountain belt and the Murihiku terrane at depth? Can we estimate the strike and dip of the Maitai terrane at depth? Can the ultramafic body within the Dun Mountain belt that causes the large Takapuna Gravity anomaly be imaged in more detail? What is the configuration of the interface between basement rocks and Neogene cover sequences? What other geological and structural features may AVF be imaged? What is the connection between these structures and volcanism in the AVF? The cross-correlation functions are most useful when we know the orientation of the components. By convention, when seismometers are installed at the surface the horizontal components are oriented north and east. However, when a borehole seismometer is lowered into the borehole, there is little control over the orientation of the horizontal components (H1 and H2 in Figure 4). This means that the orientation of the horizontal components of 9 of the seismometers in our network are unknown. Previous Results Even at this stage we were able to distinguish different seismic velocity environments both at shallow and deeper levels that reflects the geology at the surface (Figure 1), and deeper the Maitai Terrane (Figure 2), a dominant feature that diagonally crosses the region and causes the Junction Magnetic anomaly (JMA). We also and assembled the 1D models into a reasonable firstpass tomographic model. 3 Components and Cross-Terms The Waiatarua seismometer (WTAZ) has only a vertical component. All the other seismometers in our network have three orthogonal components: one vertical and two horizontal. In our earlier work we cross-correlated only the vertical components, capturing the vertical signal of Rayleigh waves. Rayleigh waves, however, have particle motion in two dimensions, vertical and radial. The advantage of cross-correlating between vertical and radial components is that it preferentially filters out signal that is out of line with the pair of seismometers, yielding a more accurate impulse response (van Wijk et al. 2011). Our first results demonstrate that this is a promising method to improve our understanding of the 3D crustal structure of the Auckland Volcanic Field. We are now undertaking to do so using more data from more pairs of seismometers. This should greatly improve the amount of structural detail that can be detected. In a heterogenous media, it’s also likely that there will be some Rayleigh wave signal on transverse components too. This signal can be captured by cross-correlating both all components and crosscomponents. This also captures Love wave signal; mostly on the transverse-tranverse cross correlations. Figure 3 shows some of these signals emerging in the cross-correlation functions for the Army Bay (ABAZ) and Whangaparoa (AWAZ) seismometers. Normalised Amplitude Figure 1. Shallow structure (<1.5km depth). Time Lag (s) Figure 2. Deeper structures (> 1.5 km depth). Body signals and ambient seismic noise records can be analyzed to estimate the orientations. Once the orientations are know, the data can be rotated and we can retrieve all the crosscorrelation functions in the most useful orientations. Figure 3. The cross-correlation functions of all three components and their cross terms from 60 days seismic data on the ABAZ and AWAZ seismometers. Figure 3. Diagramatic representation of preferred orientation of horizontal seismometer components (N, E), the reality of uncertain orientation on borehole seismometer (H1, H2), and the orientation data needs to rotated into (Radial, Transverse) to compute all 9 CCFs for each pair of seismometers. 3D S-wave Models and Crustal Structure In this project, interrogation of the subsurface will involve the same three stages as used in Ensing (2015) but with some alterations: 1. Cross-Correlation of all components and their cross-terms. 2. Multiple Filter Techniques 3. Inversion of surface wave group and phase velocities for shear-wave velocities directly to 2D (Guo et al. 2013) or 3D (Fang et al. 2015) models Equipped with significantly more data (and more reliable data), and more sophisticated tomographical techniques the we can obtain a more robust, higher resolution model that will penetrate deeper than that of our earlier work Ensing (2015), revealing greater structural detail on the crust in the AVF. Contact References J. X. Ensing University of Auckland Email: [email protected] Website: unidirectory.auckland.ac.nz/profile/jens755 www.physics.auckland.ac.nz/research/pal/josiah-ensing/ 1. Ensing, J. X. (2015). Ambient Seismic Noise Tomography in the Auckland Volcanic Field. (Master’s thesis, University of Auckland). 2. van Wijk, K., Mikesell, T. D., Schulte‐Pelkum, V., & Stachnik, J. (2011). Estimating the Rayleigh‐wave impulse response between seismic stations with the cross terms of the Green tensor. Geophysical Research Letters, 38(16). 3. Guo, Z., Gao, X., Shi, H., & Wang, W. (2013). Crustal and uppermost mantle S-wave velocity structure beneath the Japanese islands from seismic ambient noise tomography. Geophysical Journal International, ggs121. 4. Fang, H., Yao, H., Zhang, H., Huang, Y. C., & van der Hilst, R. D. (2015). Direct inversion of surface wave dispersion for threedimensional shallow crustal structure based on ray tracing: methodology and application. Geophysical Journal International, 201(3), 1251-1263.