Download Using Ambient Noise to “See” into the Crust.

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.