SUMMARY Estimating reliable depths for shallow seismic sources is important in both seismo‐tectonic studies and in seismic discrimination studies. Surface wave excitation is sensitive to source depth, especially at intermediate and short‐periods, owing to the approximate exponential decay of surface wave displacements with depth. A new method is presented here to retrieve earthquake source parameters from regional and teleseismic intermediate period (100–15 s) fundamental‐mode surface wave recordings. This method makes use of advances in mapping global dispersion to allow higher frequency surface wave recordings at regional and teleseismic distances to be used with more confidence than in previous studies and hence improve the resolution of depth estimates. Synthetic amplitude spectra are generated using surface wave theory combined with a great circle path approximation, and a grid of double–couple sources are compared with the data. Source parameters producing the best‐fitting amplitude spectra are identified by minimizing the least‐squares misfit in logarithmic amplitude space. The F‐test is used to search the solution space for statistically acceptable parameters and the ranges of these variables are used to place constraints on the best‐fitting source. Estimates of focal mechanism, depth and scalar seismic moment are determined for 20 small to moderate sized (4.3 ≤Mw≤ 6.4) earthquakes. These earthquakes are situated across a wide range of geographic and tectonic locations and describe a range of faulting styles over the depth range 4–29 km. For the larger earthquakes, comparisons with other studies are favourable, however existing source determination procedures, such as the CMT technique, cannot be performed for the smaller events. By reducing the magnitude threshold at which robust source parameters can be determined, the accuracy, especially at shallow depths, of seismo‐tectonic studies, seismic hazard assessments, and seismic discrimination investigations can be improved by the application of this methodology.
1The existence of earthquakes within continental lithospheric man-2 tle remains a highly controversial topic. Here, we present a detailed set 3 of seismological analyses confirming the occurrence of a mantle earth-4 quake beneath the Wind River Range of central Wyoming. Combining 5 regional waveform inversion with the analysis of the delay and rela-6 tive amplitudes of teleseismically-observed depth phases, we demon-7 strate that the 2013 Wind River earthquake -a M W 4.7 highly-oblique 8 thrust-faulting event -occurred at 75 ± 8km, well beneath the base of 9 the crust. The magnitude, mechanism, and location of this earthquake 10 suggest that it represents simple brittle failure at relatively high tem-11 peratures within the mantle lithosphere, as a result of tectonic, rather 12 than magmatic, processes. and Molnar, 1983; Wong and Chapman, 1990; Zhu and Helmberger, 1996; Zhu and Helmberger, 1996;Chen and Yang, 2004; Priest-38 ley et al., 2008; Craig et al., 2012), although the precise location of deep Here, we present a comprehensive seismological study of an earthquake (Table S1).and Molnar
The northeastern margin of the Tibetan Plateau, which includes the Qiangtang and Songpan-Ganzi terranes as well as the Kunlun Shan and the Qaidam Basin, continues to deform in response to the ongoing India-Eurasia collision. To test competing hypotheses concerning the mechanisms for this deformation, we assembled a high-quality data set of approximately 14 000 P-and 4000 S-wave arrival times from earthquakes at teleseismic distances from the International Deep Profiling of Tibet and the Himalaya, Phase IV broad-band seismometer deployments. We analyse these arrival times to determine tomographic images of P-and S-wave velocities in the upper mantle beneath this part of the plateau. To account for the effects of major heterogeneity in crustal and uppermost mantle wave velocities in Tibet, we use recent surface wave models to construct a starting model for our teleseismic body wave inversion. We compare the results from our model with those from simpler starting models, and find that while the reduction in residuals and results for deep structure are similar between models, the results for shallow structure are different. Checkerboard tests indicate that features of ∼125 km length scale are reliably imaged throughout the study region. Using synthetic tests, we show that the best recovery is below ∼300 km, and that broad variations in shallow structure can also be recovered. We also find that significant smearing can occur, especially at the edges of the model. We observe a shallow dipping seismically fast structure at depths of ∼140-240 km, which dies out gradually between 33 • N and 35 • N. Based on the lateral continuity of this structure (from the surface waves) we interpret it as Indian lithosphere. Alternatively, the entire area could be thickened by pure shear, or the northern part could be an underthrust Lhasa Terrane lithospheric slab with only the southern part from India. We see a deep fast wave velocity anomaly (below 300 km), that is consistent with receiver function observations of a thickened transition zone and could be a fragment of oceanic lithosphere. In NE Tibet, it appears to be disconnected from faster wave velocities above (i.e. it is not downwelling or subducting here). Our models corroborate results of previous work which imaged a relatively slow wave velocity region below the Kunlun Shan and northern Songpan-Ganzi Terrane, which is difficult to reconcile with the hypothesis of southward-directed continental subduction at the 1724
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