,-.,f.,-,. - - UCRL-JC-128102 PREPRINT Rayleigh Wave Group Velocity Dispersion Across Northern Africa, Southern Europe and the Middle East D. E. McNamara W. R. Walter This paper was prepared for submittal to the 19th Seismic Research Symposium on Monitoring a Comprehensive Test Ban Treaty Orlando, FL September 23-25,1997 July 15,1997
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Rayleigh Wave Group Velocity Dispersion Across Northern Africa, Southern Europe and the Middle East ABSTRACT Daniel E. McNamara, and William R. Walter Lawrence Livermore National Laboratory Shannon Hazier University of Colorado This report presents preliminary results from a large scale study of surface wave group velocity dispersion throughout Northern Afkica, the Mediterranean, Southern Europe and the Middle East. Our goal is to better define the 3D lithospheric shear-wave veloeity structure within this region by improving the resolution of global surface wave tomographic studies. We hope to accomplish this goal by incorporating I@onal &@ at relatively short periods (< 40 see), into the regionrdization of lateral veloeity variation. Due to the sparse distributions of stations and earthquakes throughout the region (Figure 1) we have relied on data recorded at both teleseismic and regioml distances. Also, to date we have concentrated on Rayleigh wave group velocity measurements since valuable measurements can be made without Imowledgeof the source. In order to obtain Rayleigh wave group veloeity throughout the region, vertical component teleseismic and regional seismograms were gathered from broadband, 3-component, digital MEDNET, GEOSCOPE and IRIS stations plus the portable PASSCAL deployment in Saudi Arabia. Figure 1 shows the distribution of earthquakes (black circles) and broadband digital seismic stations (white triangles) throughout southern Europe, the middle east and northern Africa used in this study. The most seismicly active regions of northern Africaare the Atlas mountains of Morocco and Algeria as well as the Red Searegion to the east. Significant seismicity also occurs in the Meditenanean, southern Europe and throughout the high mountains and plateaus of the rniddleeast. To date, over 1300 seismograms have betm analyzed to determine the individual group velocities of 10-150 second Rayleigh waves. Travel times, for each period, are then inverted in a backprojection tomographic method in order to determine the lateral group velocity variation throughout the region. These results are preliminary however, Rayleigh wave group velocity maps for a range of periods (10-95 see) are presented and initial inteqwetations are discussed. Significant lateral group velocity variationis apparent at all periods. In general, shorter periods (10-45 see) are sensitive to crustal structure as seen by the reactivelylow velocities associated with large sedimentary features (eastern Arabian shield, Persian Gulf, Eastern Mediterranean, Caspian Sea). At longer periods (50-95 see), Rayleigh waves are most sensitive to topography on the Moho and upper mantle shearwave velocity structure. This is observed in the group velocity maps as low velocities associated with features as the Zagros Mountains, Iranian Plateau and the red Sea. Analysis will eventually be expanded to include Love wave group and phase velocity, Knowledge of the lateral variation of group velocity and phase veloeity will allow us to invert for shear veloeity at each grid point. A detailed regionalization of shear-wave velocity will potentially lower the threshold for Ms determinations and improve event location capabilities throughout the region. Better Ms estimates and locations will improve our ability to reliably monitor the Comprehensive Test Ban Treaty (CTBT). This work is preliminary and performed under the auspices of the U.S. Department of Energy at the Lawrence Livermore National Laboratory under contract number W-7405 -ENG-48. Key Words: Middle Eas~ North Africa, Rayleigh waves, group velocity, dispersion
OBJECTIVE The purpose of this research is to improve lithospheric shear wave velocity models for northern Aiiica and the middle east. This type of study is usefbl to CTBT monitoring efforts because improved shear velocity models will allow a lower threshold for Ms determinations and will improve event location capabilities throughout the region. In order to obtain our objective we first estimate the lateral variation of Rayleigh and Love wave group and phase velocity using a backprojection tomography technique. Second we invert the group and phase velocities at each grid point within the tomogram for shear-velocity structure. This work is on-going and to date, we have concentrated on the Rayleigh wave group velocity component of this study. RESEARCH ACCOMPLISHED Our procedure to accomplish the stated objectives has include the following steps: (1) waveform acquisition and dispersion measurement (2) backprojection data selection (3) backprojection and group velocity map construction (4) resolution analysis and (5) preliminary interpretations. Brief discussions of our procedural steps are presented below. Obtaining Rayleigh Wave Group Velocity Dispersion Curves To obtain the Rayleigh Wave dispersion curve a narrow-band-gaussian filter is applied to the broadband vertical component, displacement seismogram over many different periods (Herrrnann, 1973; Taylor and Patton, 1986) (Figure 2). The maximum amplitude at each period is picked on the envelope f%nction and the arrival time corresponding to this maximum amplitude is used to compute the Rayleigh wave group velocity (Figure 3). BackProjection of Travel Times for Lateral Group Veloci~ Variation The Rayleigh wave travel time, for a given period, is expressed simply by: (1) t=ds t is the total travel time, D is source to receiver distance, s is slowness profile as a fimction of depti (l/v). For estimating lateral group velocity variations, the sampling region is gridded into a single layer and the slowness for each grid cell is determined. The travel time equation then becomes: (2) t= Sdisi. where di is the distance the ray travels in cell I and Siis the slowness in cell i. The individual cell group velocity is calculated using a backprojection algorithm in the following sequence of actions. First, residuals are calculated from the mean fit to the travel time data for a given period. Secondly, using(2), slownessesare estimated as the weighted mean of the apparent slownesses, tld, of all rays traversing each cell. The factor used to weight the individual slownesses is the product of the total ray path length within a
particular cell. Between iterations, the model is smoothed in order to minimize extreme slowness estimates, due to poor sampling. Smoothing done by averaging each cell value with the surrounding eight cells. Using this new smoothed group velocity model, new residuals are calculated and the process is repeated until convergence is reached. Usually about 40-50 iterations were required for convergence. Convergence is declared when the rrns residual improvement is less than 10/0between iterations. Data Selection Waveforms used in this study were obtained from numerous network sources in many different tectonic environments. Earthquake dislocation is likely to be the largest source of error since station coverage is sparse throughout large portions of the region. For this reason we have applied strict selection criteria to the earthquakes used in this study order to assure that only high-quality Rayleigh wave travel times are used in the back-projection. Only PDE events with mb >4.0 (most >4.5), distance less than 6000 km and located using at least 10 stations were included. To eliminate potential errors in the group velocity measurement process, such as cycle skipping, only travel times from high-quality (relatively continuous) dispersion curves were used. Qualitative assessments were made at the time of calculation and were used to eliminate spurious travel times. Also, tmvel time residuals resulting in a velocity deviation greater than 20 /0 from the data set mean were eliminated (Figure 4). Afler Applying the data selection criteria above, the remaining travel time residuals are likely due to lateral shear velocity variations within the lithopshere. The remaining residuals are then inverted in a backprojection tomography to determine the lateral variation of Rayleigh wave group velocity. Figure 5 shows one example 1019 tmvel times for 20 second Raleigh waves that met the criteria discussed above. Resolution A first-order, qualitative, measure of data set resolution can be obtained by inspecting the ray path distribution throughout the sampling region (Figure 5). Though ray path density is important, azimuthal sampling is most significant. For a more quantitative assessment, resolution is best investigated with synthetic travel times computed through laterally varying checkerboard test velocity models. Using the 20 second period as an example we compute the Rayleigh wave travel time for each path (Figure 5) through a model with 10Oxl0 checkers that vary in velocity by 5 /0about a mean of 2.95 lcmh. The synthetic travel times are then inverted using the backprojection scheme described above. Our ability to reproduce the input model determines the resolution of the data set. As demonstrated in Figure 6, we are able to resolve the amplitude (5Yo) of the 10 anomalies throughout much of the Middle East, Mediterranean, Saudi Arabia and the northern most Africa, Resolution is significantly diminished throughout central Africa due to a lack of crossing ray paths (Figure 5). Preliminary Results Before inteqxetation of Rayleigh wave group velocity it is usefid to understand how well the backprojection models explain the lateral variation of travel times. Travel time residual reduction for tie 20 second period data set is shown in Figure 7 as a series of histograms. With each successive iteratio~ residuals are reduced as tie spike around O
increases. This suggests that the resulting laterally varying velocity model sufficiently explains the residuals in the 20 second period data set. Iterations were allowed to continue untii the rrns residual improvemen~ between iterations, was less than 1Yo. Generally 40-50 iterations were reqtied to satisfj this criterion. Our results are preliminary and will eventually include additional data however, Rayleigh wave group velocity maps for a range of periods (10-95 see) are presented and initial interpretations are discussed. We observe significant lateral group velocity variation at all periods. In general, shorter periods (1O-45 see) are sensitive @ crustal structure as seen by the reactively low velocities associated with large sedimentary features (Eastern Arabian shield Persian Gulfi Eastern Mediterramq Caspian Sea) (Figure 8). At the middle periods presented (-50 see), Rayleigh waves are most sensitive to crud thickness and topography on the Moho. This is observed in the group velocity maps as low velocities associated with features such as the Zagros Mountains and Iranian Plateau. At the longest periods presented (85-95 see) sensitivity is greatest to lateral variations of the upper mantle shear velocity. Low group velocity is seen associated with the warm upper mantle beneath the Red Sea (Figure 8e). CONCLUSION AND RECOMMENDATIONS We find that Rayleigh wave group velocity models, for periods ranging from 10-100 see, vary laterally across the region and diverge from global models obtained using longer periods. For this reason it is important to continue utilizing regional data to more accumtely determine the lateral variation of shear-wave velocity across the region. We intend to do this by including additional raypaths in the backprojection procedure and expanding our analysis to Rayleigh phase and Love wave group and phase velocity. References Herrmann, R. B., Some aspects of band-pass filtering of surface waves, Bull. Seism. Sot. Am., 63,663-671, 1973. Taylor, S. R., and H. J. Patton, Shear-velocity structure from regionalized surface-wave dispersion in the Basin and Range, Geophys. Res. Lett., 13,30-33, 1986.
Figure 1: Map of Northern Afica, the Meditemmean, Southern Europe, the middle East and the Arabian Peninsula. Also shown are the recording stations (triangles) and the distribution of earthquakes used in the Rayleigh wave group velocity backprojection tomograpy. Tunisian earthquake, 92164191643 (mb=5.3) and the GEOSCOPE station TAM are shown to illustrate Rayleigh wave group velocity measurement examples in Figures 2 and 3.. seconds from origin time Figure 2: Rayleigh wave from earthquake 92164191643 (mb=5.3) recorded at TAM narrow band filtered for a range of periods (10-11 Osee). The Rayleigh wave travel time is taken as the maximum amplitude with the group velocity window of 2.0-5.0 klllh. ioglo(period) Figure 3: Contoured peak amplitudes for event 92164191643 at station TAM.
.. t 6000 km Oistance cut-off :. $si 3J :/ E :...... G :...... ~ ;...:, :. ~:. :%.1. ;.~::,y.:... &?ooo.: +.? :... ~..+,:@. :.. z.: 1000. :.... :..+,.#=a:.~ Figure 4: (A) Maximum amplitude travel time picks for 131720 second period Rayleigh waves from 572 earthquakes recorded at31 stations. Individual apparent velocities were constrained to be less than 200/0of the average group velocity for the entire data set (2.95 km/s). (B) 101920 second period Rayleigh wave travel times that passed the data selection criteria shown with a reducing velocity of 2.95 kds. Remaining residuals are due to lateral velocity variation... 1 I 2000 4000 60011 omnce (km) I Figure 5: Ray paths for 101920 second Rayleigh waves that met the travel time selection criteria and used in the backprojection procedure to determine the lateral distribution of 20 Rayleigh wave group velocity. Triangles are the recording stations and circles are earthquakes.
Velocity Perturbation (from a mean of 2.95 km/s) Figure 6: Inversion results from input velocity models used to investigate data set resolution. Input model velocity anomalies alternate in a checkerboard pattern. Each checker is a 10Ox10 square with a velocity perturbation of 5 /0 from the mean. This is an example of the 20 second period data set. Iteration --.1 ---1o 30 30 43 ) Residual (see) Figure 7:Histograms of residuals for several iterations. Convergence is reached at iteration 43 and residuals are significantly reduced as shown by the spike around zero. This is an example of the 20 second period data set.
.. 0 Group Velocity (km/see) 3.002~0 2.95539 2.94975 2.94569 2.94222 2.93898 2.93573 2.93226 2.92820 2.92257 2.85848 Figure 8: Rayleigh wave gtoupvelocity maps (A) 10 second period (B) 20 second period.
Figure 8: Rayleigh wave group velocity maps (C)45 second period (D) 55 second period.
.,.. 3.67675 3.81607 3.80957 3.80488 3.80088 3.79714 3.79339 3.78939 3.78470 3.77820 3&358(5 K * w A ~1- \B -yd v \ y Figure 8: Rayleigh wave group velocity maps (E) 85 second period(f) 95 second period.
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