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GLOBAL MANTLE STRUCTURE 2006 IRIS 5-YEAR PROPOSAL Study of Earth’s Layered Structure on a Global Scale Using Broadband Seismic Datasets Stephen S. Gao, Kelly H. Liu • Kansas State University Whether the Earthʼs lower mantle is layered or has steady velocity gradients has important implications on various models related to the dynamics of the Earthʼs deep interior. We are being funded by the National Science Foundation to systematically search for discontinuities in the Earthʼs lower mantle. Networks/experiments that contributed to the project include GDSN, GEOSCOPE, US Advanced National Seismic Systems, Southern African Seismic Experiment, and Northern and Southern California Seismic Networks and a few smaller-scale portable experiments. About 80% of the seismograms were obtained from the IRIS Data Management Center. We have manually checked all the seismograms and have converted them into P-to-S receiver functions (RFs). About 60,000 Figure 1. Distribution of ray-piercing points of P-to-S converted phase at 1000 depth. high-quality radial receiver functions are moveout-corrected and stacked to image possible sharp velocity discontinuities in the lower mantle. Stacking of ray-piercing points beneath caps of different sizes is being finalized. Figures 1, 2, and 3 show preliminary results beneath the largest cap (i.e., the whole earth). Possible discontinuities at depths around 850, 900, 1100, 1220, and 1600 km are observable. For each observed seismogram, we are generating a synthetic seismogram using the real focal mechanism and station-event geometry. We will then convert the synthetics into receiver functions and stack them for the purpose of identifying multiples and other non P-to-S phases originated from known discontinuities. Figure 2. Binned and stacked receiver functions for the 0.02-2.0 Hz band. Receiver functions are binned according to their focal-depth-corrected epicentral distances into 1 deg bins and those in the same bins are then stacked. Figure 3. Depth phase images for the entire earth for a sequence of frequency bands. Since early this year, the same data set has also been used to map crustal thickness and Vp/Vs beneath the stations by stacking hundreds of high-quality P-to-S converted phases at the Moho recorded by each of the stations. The main scientific objective of the NSF-funded project is to answer the critical question of whether continental formation processes have been the same over geologic times, or have been changing temporally. This project will also produce a brief description of tectonic history and geologic setting in the vicinity of each of the stations. 160
2006 IRIS 5-YEAR PROPOSAL GLOBAL MANTLE STRUCTURE Probing the Earth’s Interior by Stacking Stacked Short-Period Seismic Array Data Sebastian Rost, Michael Thorne • Arizona State University The short-period teleseismic wave field contains a great deal of information about the small-scale structure of Earthʼs interior. Due to the low signal-to-noise ratio of short-period arrivals, it is challenging to use this information in global seismological studies. Seismological arrays and networks can be used to increase the signal-to-noise ratio of seismic arrivals. By stacking seismic traces that are recorded at an array, the coherent part of the seismic wave field can be enhanced while suppressing incoherent noise. One array processing method often used is Nth-root slant-stacking. The result of this stacking method shows seismic energy as slowness (incidence angle) v. time (a vespagram). This enables the identification of coherent arrivals by travel-time and slowness that otherwise might be below the noise level. We use the information from a large number of vespagrams to map the short-period vertical seismic wave field. Approximately 500 recordings from the small aperture Yellowknife array (YKA) located in northern Canada were used to calculate vespagrams. Data were obtained through the NetDC from IRIS. In order to use the information from the vespagrams we condense the information from slowness-time space into a new, multidimensional time-series, by moving a sliding time window across the vespagrams. The highest stacked energy in the time window (i.e. the stack for the slowness that yields the highest amplitude), is used to construct a new time series. From the vespagram we are able to obtain information about traveltime, slowness, amplitude, polarity and backazimuth. The figure shows the new time series constructed in this manner, showing stacked energy. Displays containing slowness, polarity, and back-azimuth information can also be plotted. The energy displays allow the detection of several major seismic phases. Future research will focus on subtle coherent arrivals (e.g. the scattered energy arriving before the phase PP in the distance range 95° to 105°) using a larger number of array recordings. a) Stacked image for ~500 earthquakes recorded at YKA. Each line represents one new time series calculated from vespagrams. Stacked seismic phases are sorted by epicentral distance. Larger stack energy is shown in yellow. Zero energy is red. Several sharp arrivals can be identified. b) Travel-time predictions for major seismic phases in panel a) are labeled. 161
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Cornerstone Facilities for Seismolo
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Cornerstone Facilities for Seismolo
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2006 IRIS 5-YEAR PROPOSAL ACCOMPLIS
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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2006 IRIS 5-YEAR PROPOSAL EARTHQUAK
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2006 IRIS 5-YEAR PROPOSAL MONITORIN
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2006 IRIS 5-YEAR PROPOSAL SIGNALS A
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2006 IRIS 5-YEAR PROPOSAL EDUCATION
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