REGIONAL TRAVEL-TIME AND LOCATION UNCERTAINTY ASSESSMENT ALONG THE TETHYAN MARGIN USING A NEW THREE-DIMENSIONAL P-VELOCITY MODEL

We have completed a new, comprehensive three-dimensional model of S-velocity in a broad region extending from the western Mediterranean Sea to the Hindu Kush and encompassing Northern Africa, the Arabian Peninsula, and the Middle East. Our joint inversion is an integration of regional waveform constraints, surface-wave group velocity measurements, teleseismic P and S arrival times, and receiver functions. The data offer complementary sensitivity to crust and mantle structures and are jointly inverted to image the complexity of this tectonically diverse area. The S-wave model is now converted to a P-wave model such that we can compute finite difference travel-times through the new model to evaluate P-wave arrival times from ‘ground-truth’ events. The travel times are well predicted by the Joint model at several stations, and the Joint model outperforms the WENA1.0 model, as measured by a higher variance reduction at stations AJM, CFTV, ELL, FRU, KHO, MFP, NIL, RYD, and SVE. The Joint model seems to predict the P-wave times particularly well in the eastern part of the model and over portions of northern Africa. We now use P-wave travel-time correction surfaces with our location algorithm to account for three-dimensional P-velocity structure and to improve the location estimates of several GT5 and better events that have been regionally recorded throughout the Middle East and North Africa. OBJECTIVES Our primary objective is developing new 3-D Sand P-velocity models for the Middle East and Mediterranean region, including North Africa, southern Europe, and Arabia that 1) are resolved in aseismic regions, 2) are resolved throughout the upper mantle (to 660 km), 3) resolve laterally varying crustal thickness, 4) contain laterally varying vertical velocity gradients, 5) are simultaneously compatible with multiple data sets, 6) utilize several recent, unique waveform data sets, and 7) include uncertainties of the model parameters. These features would increase the model’s ability to predict and calibrate regional travel times and waveforms, thereby providing improved event locations, focal mechanisms, and other event discriminants. After completing the S-wave model we now convert it to a 3-D P-velocity model, using teleseismic P-arrival times. We test the P-wave model’s ability to predict regional P travel times and to improve location estimates of small regionally recorded events. RESEARCH ACCOMPLISHED The study region is centered around the Africa-Arabia-Eurasia triple junction and extends west to the Africa-Eurasia-North America junction at the Azores, and east to the Arabia-Eurasia-Indian Plate junction. The interaction of these five major tectonic plates with each other and with several microplates within an area of one quarter of the Earth’s circumference yields this region rich with tectonic complexity. We plan to capture various renditions of this structurally complicated part of the world in one S-velocity model through the joint inversion of several different types of seismic data simultaneously; the new model will refine our understanding of the structure and tectonics in this region of the Earth. The data types we combine are constraints from independent studies on the depth to the Moho, fundamental-mode Rayleigh-wave group velocity measurements, waveform fits of regional S and Rayleigh waves, and arrival times of teleseismic S and SKS waves. We convert the resulting S-velocity model to a P-velocity model with over 2.9 million teleseismic P arrival times. Then, we validate our model by performing travel-time prediction with a dataset of ground truth events. Regional Waveform Fits We fit over 5,600 available waveforms from the Lawrence Livermore National Laboratory (LLNL) database and MIDSEA dataset (Marone et al., 2004) which sample the Mediterranean region, North Africa, the Middle East, Pakistan, and Afghanistan using the non-linear inversion procedure employed by partitioned waveform inversion. We utilize events with magnitude larger than 4.0 and seismograms with epicentral distance from 5o to 50o. The great-circle wave paths for these seismograms are shown in Figure 1(a). We have estimated path-averaged S-velocity structures for these paths using the same starting S-velocity model but a different crustal thickness (in 5-km increments) for each path, based on a priori reported estimates (Marone et al., 2004). Teleseismic S and SKS Arrival Times We obtained S and SKS phase arrival time data from two different sources. Both arrival times are adjusted for topography and Earth’s ellipticity before inversion. First, we used high-quality relative arrival times of teleseismic S and SKS waves (Benoit et al., 2006; Park et al., 2007; Schmid et al., 2004) that are measured on broadband seismograms with use of multi-channel cross-correlation (MCCC; VanDecar and Crosson, 1990). Benoit et al. (2006), Park et al. (2007), and Schmid et al. (2004) used seismograms recorded in the Mediterranean, Ethiopia, and Saudi Arabia, respectively. Moreover, we measured additional relative delay times at Turkey and central Asia. The number of S phase relative arrival times is over 5900 with epicentral distance of 30o – 90o and the number of SKS phase is over 1400 with distance of 87o – 140o; over 7300 in total. Second, we obtained over 223,000 S phase arrival time data from the reprocessed ISC database (Engdahl et al., 1998) from 1964 to 2007. We illustrate location of stations and events for each of the two types of arrival-time data in Figure 1(b). (a) Waveform fits (b) Arrival times (c) Group velocities (d) Moho depth constraints Figure 1. Datasets used for the joint inversion. (a) Ray path coverage for regional waveform fits. Stations are illustrated as red triangles, and events as yellow circles. (b) Events and stations used for teleseismic S and SKS arrival time estimation. Cyan circles and blue triangles represent events and stations used for relative delay time estimation with MCCC, respectively. Yellow circles and red triangles are events and stations from the reprocessed ISC catalogue. (c) Great-circle wave paths for 45 s period Rayleigh waves. Stations are illustrated as red triangles, and events as yellow circles. (d) Map of the Moho depth distribution acquired from literatures. Artificial point constraints of 10 km depth are placed to the Atlantic and Indian Oceans where measurements are absent. Surface Wave Group Velocities We measure over 105,000 group velocities of fundamental-mode Rayleigh waves recorded at MIDSEA and other stations in the region and used them to update previous group velocity maps (Pasyanos, 2005). Figure 1(c) shows the great-circle wave paths for which we included fundamental-mode Rayleigh wave group velocities in our joint inversion. Compared to other data types we include, lateral coverage is best for this group velocity data set, though vertical coverage is provided mainly by the teleseismic arrival times and regional S and Rayleigh waveforms. The period for group velocities ranges from 7 to 100 sec. Crustal Thickness Constraints While the fundamental-mode Rayleigh-wave group velocities and regional S and Rayleigh waveforms have significant sensitivity to Moho depth, they cannot uniquely resolve it. We therefore include independent estimates of crustal thickness as point constraints in the joint inversion and thus compile such measurements from a large number of published studies. A partial list of these studies is provided in Marone et al. (2003) and part of these points stem from the database used for CRUST5.1 (Mooney et al., 2001). We have added new estimates of Moho depth from more recent studies and interactively resolved or removed conflicting data and outliers from the data set. Over 4,700 Moho depth constraints are mapped in Figure 1(d). For the oceans we use a constraint of 10 km for Moho depth, but only for points also covered by data from our other data sets.