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SAPSE - The Southern Alps Passive Seismic Experiment

T. V. McEvilly, Robert A. Uhrhammer

Subsections


Conception

SAPSE was conceived at the NSF-NZ science workshop in 1993. It was clear that the optimal scientific strategy needed to include distributed passive seismic stations, to enable both interpretation of the local seismicity and to understand the 2-D transect results in terms of broader-scale 3-D structure of the crust and upper mantle. The passive experiment was run to include the active profiling to obtain the most comprehensive data set. The onland explosions and offshore airgun sources are well-recorded by the distributed array since the South Island crust has low attenuation and active sources are visible to 250-300 km. The SAPSE South Island array is in turn within a broader passive network that will examine large-scale lithospheric heterogeneity and anisotropy surrounding the plate boundary. The goal of SAPSE is to obtain a comprehensive view of seismicity and 3-D variations in lithospheric structure in order to understand how deformation in this obliquely convergent plate boundary is accommodated aling the Alpine fault and within the Southern Alps.

The SAPSE Array

The SAPSE array consisted of interspersed broadband (26) and short-period (14) temporary 3-component stations plus permanent (17) New Zealand stations, see Figure 17.1, operated between November 1995 and April 1996, with 7 stations distributed remaining throughout the southern winter to enhance the regional network. The broadband stations are from IRIS PASSCAL and have been deployed by the University of California, Berkeley (South Island network) and the State University of New York, Binghamton (regional network). The short-period recorders are from New Zealand, Institute of Geological and Nuclear Sciences (IGNS), and have been deployed by IGNS and Otago University, through Earth and Ocean Sciences Research (EOS). The stations are broadly distributed, but centrally weighted towards the central Alpine fault and thew transect region. The network data volume was a bit more that 1.4 Gb per day in SEG-Y format, requiring strict discipline in data reduction procedures. This is the first "direct-to-DMC" PASSCAL experiment, and UCB scientists constructed software to produce the requisite mini-seed formatted data in the field.


  
Figure 17.1: SAPSE Station Deployment.
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Data Processing

During the past year we have discovered numerous problems with the PASSCAL and IRIS DMC software which significantly hindered our progress in processing and analyzing the data. As a consequence, we are currently reprocessing the entire SAPSE data set from the raw data tapes into the mini-seed format required by the IRIS DMC for archival and distribution. This includes the data from the original South Island Passive Seismic Experiment (SIPSE) and the IGNS short-period data (converted from AH data format on CD-ROM's) as well as the SAPSE broadband data. This task is being done on a workstation, equipped with tape stackers, that is on loan from the IRIS DMC.

Analysis

We have read phase data from 800+ earthquakes of magnitude 3 and larger recorded by the SAPSE instruments. The events were from the magnitude 6.2 Cass earthquake sequence, along the Alpine fault zone, and distributed throughout the South Island. We have also done a moment tensor analysis of the largest events in the Cass sequence. The moment tensor solution for the Mw 6.2 Cass mainshock, using data from four of the SAPSE broadband stations, is shown in Figure 17.2.


  
Figure 17.2: Cass Mainshock Moment Tensor Solution.
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The South Island Project Working Group presented a progress report at the Fall AGU meeting, (Stern et al., 1997). A number of seismic techniques were used along two 600 km long main transects to study the structural elements of the obliquely compressional Southern Alps orogen, (Davey et al., 1998). A direct image of the crust beneath the Alps has been obtained from a CDP stack of onshore explosions into 420 portable instruments. East of the Alpine fault a strongly reflective zone in the lower crust is interpreted as the base of the Pacific Plate and this appears to dip westward and it reaches a maximum depth of 45 km near the Alpine fault. Multichannel seismic data on each side of the South Island provide information on the thickness and nature of the crust entering the orogen. To the east, there are two discrete zones of reflectivity beneath the Canterbury shelf. We tentatively propose the upper zone to mark the top of the old oceanic crust, and the lower zone, the base of the Pacific Plate crust at about 25 km depth. To the west, Australian plate crust is up to 30 km thick close to the Alpine fault and appears to thin markedly offshore. Sediments are up to 2 km thick and a prominent Pliocene(?) reflection dips towards the east and onlaps basement. Post Pliocene units thicken landward suggesting loading of the Australian crust during convergence.

References

Stern, T. A., P. E. Wannamaker, D. Eberhart-Phillips, D. Okaya, F. J. Davey, T. McEvilly, R. Uhrhammer, H. Anderson, F. Wu, G. R. Jiracek, G. Caldwell, N. Christensen, and P. Molnar, Mountain building and active deformation studied in New Zealand, EOS Trans, AGU ,32, 329,335-336, 1997.

Davey, F. J., T. Henyey, W. S. Holbrook, D. Okaya, T. A. Stern, A. Melhuish, S. Henrys, H. Anderson, D. Eberhart-Phillips, T. McEvilly, R. Uhrhammer, F. Wu, G. R. Jiracek, P. E. Wannamaker, G. Caldwell, and N. Christensen, Preliminary results from a geophysical study across a modern, continent-continent collisional plate boundary; the Southern Alps, New Zealand, Tectonophysics ,288, 221-235, 1998.


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Next: Asymptotic and non-asymptotic approaches Up: Ongoing Research Previous: The MOISE experiment: Monterey

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