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Rayleigh and Love Wave Magnitudes 2010 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies RAYLEIGH AND LOVE WAVE MAGNITUDES Anastasia Stroujkova1, Jessie Bonner1, Robert Herrmann2, and Dale N. Anderson3 Weston Geophysical Corporation1, St. Louis University2, and Los Alamos National Laboratory3 Sponsored by the Air Force Research Laboratory Award No. FA8718-09-C-0012 Proposal No. BAA09-75 ABSTRACT We continue to study the Ms(VMAX) surface wave magnitude formula (Russell, 2006). During the past year, we have automated the method for both Rayleigh and Love waves, examined structural effects on the formula in the Middle East, applied the method to Italian earthquakes, and examined performance of the method at the United States Geological Survey National Earthquake Information Center (USGS NEIC). We are also in the process of incorporating a formal discriminant of Ms(VMAX):mb into the Event Classification Matrix (Anderson et al., 2007). We have applied the Ms (VMAX) analysis (Bonner et al., 2006) using both Love and Rayleigh waves to events in the Middle East, Italy, and the Yellow Sea/Korean Peninsula region. The Middle East dataset consists of approximately 120 events with reported body wave magnitudes (mb) between 3.8 and 5.6. We found a significant surface wave magnitude bias between the stations situated to the northwest and northeast from the area of Middle East. We attribute this bias to the surface wave scattering by the Caspian and the Black Seas and the Great Caucasus Mountains. As a part of this project we examined the validity of the Russell formula for Love waves using the Middle East dataset. We removed the attenuation correction term suggested by the Russell formula and examined the decrease in magnitude as a function of distance for both Rayleigh and Love waves. The attenuation coefficients calculated by fitting a linear regression to uncorrected Ms (VMAX) measurements are 0.0037 for the Rayleigh and 0.0042 for the Love waves. Application of the new corrections improves the residuals for the events used in the inversion; however it doesn’t improve the RMS residuals for the entire data set. We studied the relationship between Love and Rayleigh-wave magnitudes for earthquakes occurring in Italy, with a primary focus on the L'Aquila earthquake (6 April 2009 Mw 6.1) and its aftershocks. We have estimated Ms(VMAX) for 125 Italian earthquakes with 2.8 < Mw < 6.1 at distances ranging from 50 to 414 km. The network- averaged magnitudes show that most of the events (80%) had a Love-wave Ms(VMAX) that was larger (by 0.2 m.u. on average) than the Rayleigh-wave estimate. In addition, we observe larger interstation standard deviation for the Love-wave magnitudes (0.2 m.u.) than for Rayleigh waves (0.17 m.u). Residual Ms(VMAX) estimates (e.g., station minus network average) show no significant distance dependence on the magnitudes; however, there is a clear azimuthal effect on the Rayleigh-wave station residuals. Ms(VMAX) for Rayleigh waves is currently being tested in the automated USGS NEIC Hydra system. We will present results from the first four months of the test phase, showing comparisons of Ms(VMAX) with other surface wave magnitude formulas and to mb. 529 2010 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies OBJECTIVES Russell (2006) developed a time-domain method for measuring surface waves with minimum digital processing using zero-phase Butterworth filters. We refer to this technique as Ms(VMAX) for Variable-period, MAXimum amplitude magnitude estimates. The technique was implemented in Matlab (program EVALSURF, Bonner et al., 2006) to estimate variable-period (8 < T < 40 sec) Rayleigh-wave magnitudes for comparison to the historical formulas of Marshall and Basham (1972) and Rezapour and Pearce (1998). The original version of the program requires considerable analyst involvement for the magnitude picking. We recently extended application of the Ms(VMAX) technique to Love waves in attempt to improve seismic event screening using the properties of Rayleigh and Love waves. We are accomplishing this through the development of a Love-wave magnitude formula that is complementary to the Russell (2006) formula for Rayleigh waves RESEARCH ACCOMPLISHED Love and Rayleigh Wave Ms(VMAX) in the Middle East We computed Ms(VMAX) (Bonner et al., 2006) for over 120 seismic events located in the Middle East with reported body wave magnitudes (mb) between 3.8 and 5.6. The majority of the location and magnitude information (with a few exceptions) was obtained from the NEIC bulletin. The study area (Figure 1) is located in the zone of continental collision between Eurasian, African and Arabian plates. The complex tectonic setting of the region creates highly irregular velocity structure, with rapid changes between the areas with different crustal thicknesses and velocities. One of the world’s thickest sedimentary basins is located beneath the Caspian Sea. The South Caspian Basin and has a 15-25 km thick sedimentary layer overlying 10-15 km thick oceanic crust (Neprochnov 1968; Rezanov and Chamo, 1969). This basin forms a deep aseismic depression bounded to the south by the Alborz Mountains in Iran, to the east by the Turkmenistan lowlands and Kopet Dag Mountains of Iran, to the west by the Caucasus Mountains, and to the north by the Apsheron-Balkhan Sill (Priestley and Mangino, 1995). Figure 1. Map of the seismic events (red circles) and stations (blue triangles) used for Ms (VMAX) study. We have applied the Ms (VMAX) analysis to both Love and Rayleigh waves using the Russell (2006) formula: 8.1 1 T0 20 s aM b )log( += ( )()+∆ 0031.sinlog fc −−−∆ log66.043.0)log( (1) 2 T T 530 2010 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies The details of the processing used to estimate Ms(VMAX) are described in Bonner et al. (2006). Our initial hope is to be able to use the same formula for both surface wave types. The Ms(VMAX) computed using Love waves is greater than the magnitude for Rayleigh waves for the majority of the events of larger magnitudes (above mb~4). For smaller events, however, we observe a large number of events with the Rayleigh Ms(VMAX) exceeding the Love Ms(VMAX). This peculiarity could be caused by either reduced SNR for smaller magnitude events, or by source radiation effects (e.g., due to normal fault mechanisms). In addition regional differences in the wave attenuation and/or anisotropy could cause changes in the amplitudes for the rays traveling in different directions. Since the station coverage is not homogeneous, these propagation effects could potentially result in biases in Ms(VMAX) estimate for smaller events with limited sampling. Table 1. The average differences between Ms (VMAX) measured for each event at a single station and the mean value for all stations. These values represent biases in the magnitude measurements for individual stations. Bias in M (VMAX) (in m.u.) Station s Rayleigh Love ANTO -0.04 0.13 BFO -0.16 -0.17 BRVK 0.13 0.07 GNI -0.05 0.03 KIEV -0.09 0.02 KIV -0.17 -0.14 Figure 2. Map of the mean residuals of the station Ms (VMAX) estimate as shown in Figure 2: a) Ms (VMAX) estimated using Rayleigh waves; b) Love waves. 531 2010 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies Figure 3. Ray trajectories from selected events to station GNI, showing the influence of the Caspian Sea on Love wave dispersion. The panels surrounding the map show multiple filter analysis results for the Rayleigh waves recorded at the station GNI. 532 2010 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies The average Ms (VMAX) residuals for each station are shown in Figure 2 with the red circles corresponding to the stations with positive bias, and the blue circles corresponding to the negative bias. Note that the sign of the bias is predominantly negative toward the northwest from the region of study and predominantly positive to the northeast. Negative bias is likely an indication of relatively higher attenuation in that direction. The numerical values of the bias for some of the stations of the region are shown in Table 1. Note that the values of the bias for some stations (e.g., KIV, BFO) are comparable to the Ms (VMAX) mean value of the interstation standard deviation computed for the entire dataset (0.2 m.u.). Therefore, determining surface wave magnitudes using different subsets of stations may lead to significant bias, especially for smaller events for which only a limited number of magnitude measurements are available. To show the effect of the structure variations on the surface wave dispersion, we plotted the results of the multiple filter analysis (MFA), which we used to verify the nature of the phases. Figures 3 show the application of the MFA analysis to the Love waves, in which the dispersion of the trace (illustrated by the colored contours) is compared to the dispersion predicted from the averaged dispersion curve explained earlier (white squares with uncertainty bars connected by the dashed white line). The white dots on each figure show the theoretical (averaged) Love wave group velocities with their error bars. Figures 3 shows rays going trough to Station GNI from different events located to the east from the station, where some of the ray paths cross the Caspian Sea, and others do not. Notice strong dispersion and high velocities for the short periods (less than 20 s) corresponding to the ray paths coming from south and central Iran. The surface waves from events separated by the Caspian Sea, however, are considerably degraded. Notice lowering of the group velocity for short periods (less than 20 s) caused by a thick layer of the sediments.
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