S51E-01 INVITED 08:05h
IRIS Controlled Source Seismic Experiments: Continental Structure, Instrumentation, and Education
The controlled-source seismology program of IRIS/PASSCAL has made major contributions to the study of continental structure and evolution. It has also undergone major developments in seismic instrumentation. The first PASSCAL experiments (1984/85) targeted the Basin and Range Province and the Ouachita orogenic belt. The Basin and Range study provided remarkably clear images of this thin, highly-extended crust, while the Ouachita experiment tested competing hypotheses for the deep structure of this Paleozoic orogen. However, both of these projects were limited by a lack of seismic instruments. The situation improved in the late 1980's with the benefit of a mixed array of 600 seismic recorders from the USGS, Stanford, and the Geological Survey of Canada. The resolution achieved with these instruments was revolutionary. Results include the imaging of such remarkable features as crustal-scale duplexes in the Brooks Range compressional orogen of northern Alaska, and of crustal "core complexes" in the extended crust of southwest Arizona. The 3-channel PASSCAL Jr. instrument was developed, leading to experiments in which $\sim$1000 instruments were deployed, including three-component recording. This complex mix of instruments served the community well for several years, but required large, complex instrument centers and lots of technical support. With input from PASSCAL and the international community, a newly designed, compact instrument (the Texan) was finalized in the spring of 1998, and the first 200 instruments was delivered to the Univ. of Texas-El Paso in late 1998. The present instrument pool of Texans exceeds 1,400 and these have been used on such projects as the high-resolution imaging of the Los Angeles and San Fernando basins (LARSE I and II experiments), where active thrust faults have been imaged. Controlled-source seismic experiments are now very numerous. During calendar year 2004 alone, portable Texan instruments have traveled from Venezuela to Denmark, Romania, Hungary, Nevada, Chesapeake Bay (VA.), the Dead Sea rift, and back to Venezuela. The scientific targets of these experiments range in scale from shallow environmental depths to the entire lithosphere. In this talk we highlight many of the key scientific achievements of IRIS/PASSCAL controlled source experiments, with an emphasis on new insights into the structure, composition, and evolution of the continental crust. We also briefly describe the experiences of the hundreds of students who have participated in past field work. Many of these students are now pursuing active careers in seismology and are using the PASSCAL facility as young professionals.
S51E-02 08:25h
IRIS and the Rise of Receiver Functions
The development and popularization of the use of receiver functions to model crustal and upper mantle structure over the last 20 years has been driven in a large part by the availability of digital broadband data collected through IRIS programs. Receiver function analysis has become a standard method in passive-source PASSCAL experiments while the structure beneath most GSN stations has been examined using this method. Over the last 20 years, the method has evolved from the detailed analysis of single station receiver functions along two paths: The use of the method as a routine tool for estimation of bulk structural properties (such as thickness and poisson's ratio) and the application of imaging techniques to dense arrays of passive recorders. This paper extends the first of these paths to take advantage of another feature of the IRIS program: the increasing availability of real-time data and access tools that can easily obtain real-time data without human interaction. These capabilities offer the possibility of automating the use of receiver functions in reconnaissance mode for bulk property estimation. These automatically-generated bulk properties are termed "Receiver Reference Models" or RRM. We outline the RRM method and the IRIS data access tool (SOD, Standing Order for Data) that facilitate this approach. The resulting system will be routinely applied to the telemetered data from the EarthScope USArray stations to produce bulk property estimations that can be made available to geoscientists and educators for a wide spectrum of potential applications.
http://www.seis.sc.edu/EARS
S51E-03 08:40h
Numerical Invesigation of Migration Imaging Resolution
The resolution of seismic migration imaging resolution is often given as some function of the data aperture, the size of the Fresnel zone at a given location, the frequency of the data used in the migration and the illumination of the target region. However, other issues limit image resolution or distort the image. For example, the velocity model is generally not reliably known and variations of the model from the true structure can have a significant impact on image quality, as well as reduce the image focus. Another factor that limits resolution is the type of propagator used in the migration imaging. To compare the resolution of images obtained when using ray-based Kirchhoff migration with that obtained from wave equation migration, we developed a heterogeneous model which can be numerically simulated. Migrations obtained for the numerical dataset show the improvements that can be obtained when using wave equation imaging.
S51E-04 08:55h
Interpretation of Images from Joint Body-Wave Tomography
Joint body-wave tomography exploiting the arrival times of both P and S waves can provide high-resolution images of the bulk-sound and shear wavespeed for more than half of the mantle. The use of source-and station pairs for which both P and S arrivals have been picked provides strong quality control on the data and ensures comparable coverage of P and S paths. Current and past subduction is generally marked by distinct fast shear wavespeed anomalies with a variable bulk-sound speed component, which in the upper mantle appears to have significant dependence on the age of the subducted lithosphere. Orogenic belts generally show an anti-correlation of bulk-sound and shear wavespeed heterogeneity indicating that the S waves are more strongly affected than the P waves. However, there are intriguing features to be found elsewhere in the 3-D structure, such as the anti-correlation of bulk-sound and shear wavespeed features in some parts of the transition zone, e.g., near western Australia, and the rather strong signature of deep penetration for both the Tethys and Farallon slabs in shear wavespeed but not in bulk-sound. Insight into character of the images for both wavespeeds can be found from the application of finite strain theory for the increment from the hydrostatic state of the reference model in a second order Birch-Murnaghan development.
S51E-05 INVITED 09:30h
20 year IRIS: impact on seismological research at home and abroad
Abstract: The positive impact of IRIS, through its programs (GSN, PASSCAL, DMS, EO) and its workshops, on seismological research and community building can hardly be overestimated. The Data Management System has been very successful in bringing data to users for research and education anywhere in the world; it enables routine, and in many cases real time, analysis of massive amounts of waveform data for a spectacularly diverse range of studies. (I will give examples of surface wave tomography and inverse scattering studies of the core mantle boundary.) The support that PASSCAL provides for the planning and execution of field campaigns allows seismologists to shift attention from operational issues to exciting science, and the required data dissemination through DMS does not only result in tremendously valuable data sets but also contributes to community building through (international) collaboration. Europe, Australia, and Asia also have rich histories of network and portable array seismometry, and in many areas the cumulative station density exceeds that of North America (even, perhaps, with USArray). Moreover, in some cases, such as the use of temporary, roving arrays of broad band seismometers, activities overseas may have preceded and inspired developments in the US. However, the absence of effective central systems for management and dissemination of quality-controlled data has left many unique historical and regional data sets underutilized. This situation is changing, however. As an example I will mention the NERIES initiative to build a better infrastructure for seismological research and education in Europe. Apart from providing an example, through international collaboration IRIS can continue to play an important role in the improvement of the global seismological infrastructure.
S51E-06 INVITED 09:50h
The IRIS DMC: 16 Years of Managing Global, National and Regional Seismological Data
The IRIS GSN began recording data in 1988 and at that time the IRIS DMC was nearly non-existent. The GSN would eventually generate roughly 500 gigabytes of data per year and we anticipated that hundreds of requests would likely be serviced per year. While these figures seem small by today's standards, in 1988 they were formidable. Currently the IRIS DMC archives more than 16 terabytes of data per year and this year we will service more than 100,000 requests for data. The DMC has evolved in a manner that is closely linked to the demands of the seismological and earth science community. We have developed a diverse set of request tools, data management systems and tools that assist users in getting seismological data in the form they need, and in the time frames required by their research. One of the most significant accomplishments of the DMC over the past ten years has been the integration of seismological data from a vast array of permanent and temporary networks at global, national and regional scales. In the early days of IRIS, the DMC's holdings were very much dominated by GSN data and while to this day the GSN data lie at the heart of the DMC, the holdings have been significantly improved by adding data from other sources. Data from IRIS PASSCAL experiments, international partners in the Federation of Digital Seismographic Broadband Networks (FDSN), USGS national and regional networks, and data from a variety of other networks now routinely flow into the IRIS DMC. These data are seamlessly integrated with data from the GSN making the combined data holdings of far greater scientific value. This talk will provide a summary of the current data holdings, access techniques and usage of the IRIS DMC focusing on the tremendous contributions of the GSN.
http://www.iris.edu
S51E-07 INVITED 10:05h
Techniques for Handling Large Data Sets in Global Tomography
Largely due to the success of the IRIS consortium, it is now possible to easily obtain in excess of 250 high quality three component recordings for moderately sized earthquakes (a total of roughly 100,000 recordings per year). Of course, with the advent of US Array, this number will continue to grow. Many techniques have been developed to handle such data streams but usually in a regional context. Here, we describe a development of a cross-correlation technique combined with a waveform clustering algorithm to semi-automatically measure relative arrival times. This method has already been applied to body waves and here we give an extension to measure the relative group arrival times of globally distributed surface waves. The method works with band-pass filtered envelope functions (as in the standard group velocity analysis) but measures the relative arrival times in a fixed frequency band of all waveforms for an event. The clustering provides an automatic method of grouping similar waveforms and of removing bad waveforms. The method is fast and accurate and leads to very large datasets. For example, application to 50 second Rayleigh waves results in a dataset of over 200,000 relative group arrival times with over 25,000 measurements per year in recent years. Such datasets are capable of giving fine-scale resolution of near-surface structure over much of the globe.