Seismic Body Wave Attenuation Tomography of the Australian Continent Agus Abdulah Research School of Earth Sciences The Australian National University Annual Report 2005 February 2006 Table of Contents Abstract.............................................................................................................................. 2 Chapter 1 .......................................................................................................................... 4 Introduction....................................................................................................................... 4 Chapter 2 ......................................................................................................................... 10 Australian Setting and Previous Seismic Studies of Australia ................................... 10 2.1 Australian Setting........................................................................................................ 10 2.1.1 The Australian Continent ..................................................................................... 10 2.1.2 Surrounding Regions ........................................................................................... 13 2.2 Previous seismic Tomography and Seismic Attenuation Studies of Australia........... 15 2.2.1 Tomography Studies of Australia ........................................................................ 16 2.2.2 Seismic Attenuation Studies of Australia ............................................................ 21 Chapter 3 ......................................................................................................................... 24 Seismic Attenuation Tomography................................................................................. 24 3.1 Seismic Attenuation Measurement Theory................................................................. 24 3.2 Method of Attenuation Measurements........................................................................ 26 3.2.1 The Station Ratio Method .................................................................................... 27 3.2.2 The Wave Ratio Method....................................................................................... 28 3.3 Attenuation Measurement Technique ........................................................................ 29 3.4 Robust Attenuation Measurements of Australian Data ............................................. 33 3.4.1 Seismic Data ........................................................................................................... 33 * 3.4.2 Differential Attenuation (∆t SP) Representations ................................................... 33 3.5 Seismic Attenuation Tomographic System of Equations and Model Parameterizations ....................................................................................................... 33 3.6 Checkerboard Test ..................................................................................................... 40 Chapter 4 ......................................................................................................................... 42 Australian Seismic Wave Speeds, Attenuation, and Anisotropy Models .................. 42 4.1 Seismic Wave Speeds Models .................................................................................... 42 4.2 Seismic Attenuation Models....................................................................................... 43 4.3 Seismic Attenuation Anisotropy Model ..................................................................... 46 Future Work.................................................................................................................... 48 References ....................................................................................................................... 49 1 Abstract The strategic position of the Australian continent in the middle of the seismicity belt which extends from Indonesia, through New Guinea to Fiji, Tonga and New Zealand with the extensive deployment of portable broadband seismic stations across the Australian Continent and Tasmania since 1993 offers robust seismological data with a dense coverage at distances from 5° to 45°. P and S wave seismic traveltimes from nearly 4050 three-component seismic dataset from the record have been hand picked. The wave ratio method is then applied to estimate the spectral ratio between shear (SH and SV) and compressional waves. Seismic spectra are estimated using the Multitaper method with 512 points in window range of 30s to 45s and frequency range of 0.25Hz to 1.00Hz. Three-dimensional P and S wave speed tomography is conducted by inverting a kernel matrix obtained from a quasi three dimensional ray tracing which respect to P and S wave seismic traveltime residuals from the ak135 model. The study area from latitude 22° N to 65S° and longitude 78° to 189° and 0-1240km depth is discretized into 11100 cells with a cell size 3°x3° and depth range of 35 to 200km. Both P and S- wave speed information from the seismic wave speed tomography are then utilized as data input for the three-dimensional seismic attenuation tomography. In this inversion, it is assumed that QP=2.3QS. The seismic attenuation anisotropy in term of the ratio between seismic attenuation derived from SV and SH component is also presented. The major feature that is revealed from the both seismic wave and seismic attenuation studies is a strong contrast in deep structure between central Australia and the eastern seaboard. The Archaean and the Proterozoic rocks in the west and in the middle of the continent are associated with a high seismic wave speed anomaly and 2 low seismic attenuation and the Phanerozoic rocks and the presence of recent volcanism and region of high heat flow in the east are associated with low seismic wave speed anomaly and high seismic attenuation. The representation of seismic attenuation anisotropy suggests that in the region where seismic coverage is good, transverse component (SH) wave is less attenuated than radial component (SV). 3 Chapter 1 Introduction The strategic position of the Australian continent in the middle of seismicity belts to the north and east of the continent: the world’s greatest earthquake belt, the circum-Pacific belt (also called “The Ring of Fire”) which extends from Japan, Philippine Islands, New Guinea, the island groups of the Southwest Pacific, and to New Zealand and the Alpide belt which extends from Java to Sumatra and the mid- ocean ridge to the south of the continent provides a wealth of events at suitable distances to be used as probes into the seismic structure of the upper mantle. The extensive deployments of portable broadband seismic stations across the Australian Continent and Tasmania since 1993 offers robust seismological data with a dense coverage at distances from 5° to 45°. Over the last two decades, a wide range of studies has been used to gain information even on one-dimensional and three- dimensional structure in the mantle which exploits different aspects of seismograms. Studies on seismic tomography in which utilize thousands of seismic traveltimes picked from high quality body wave seismograms and utilize seismic waveform of large amplitude surfaces waves in the later part of the seismogram travel nearly horizontally has been conducted and successfully to delineate the major features of the geological structure beneath the Australian Continent. Study on seismic travel time tomography for the Australian region was pioneered by Widiyantoro & van der Hilst (1996) who produced tomographic images in a zone covering the southern Philippines, Malaysia, Indonesia, Papua New Guinea and northern Australia by linearized inversion of travel-time data of direct P phases and 4 the surface-reflected depth phases pP and pwP. The radially stratified iasp91 model was used as global reference for the seismic velocities and for the tracing of the ray paths. The images suggest that the lithospheric slab beneath the Sunda island arc penetrates to a depth of at least 1500 kilometers. The slab was imaged as a continuous feature from the surface to the lower mantle below Java, with a local deflection where the slab continues into the lower mantle. Subsequently, Gorbatov and Kennett (2001) have implemented a tomographic inversion with a parameterization which has smaller cells near the earth’s surface and larger cells at depth. Blocks of 50x50x50 km have been used in the upper mantle, and the cell size increased to 100x100x100 km blocks in the middle mantle. An iterative development with a 3-D ray tracing algorithm which exploits the 3-D structures recovered during the course of inversion have been used. It is reveals that, the tomographic image for the southwestern Pacific region in which the subduction zones below New Caledonia, the Solomon Islands, and Papua New Guinea shows seismic speed contrast with their surroundings. Effort in exploring the features of geological structure beneath the Australian Continent and surrounding regions is continued in the implementation of surface wave tomography to an abundant of seismic data recorded by temporary broadband seismometers which have been deployed in a numerous number of projects conducted by Research School of Earth Sciences, the Australian National University: SKIPPY [continent-wide,1993-1996], KIMBA [Kimberley Block, 1997,1998], QUOLL [southeast Australia, 1999], WACRATON [Western Australia, 2000-2001, 2002- 2003], TIGGER [Tasmania, 2001-2002], and TASMAL experiment [Gulf of