SUMMARYRecently developed three-dimensional global seismic-velocity models have demonstrated location improvements through independent regional and teleseismic travel-time calibration. Concurrently, a large set of high quality ground truth (GT) events with location accuracies 10 km or better (GT0-GT10) has been collected for Europe, the Medite rranean, North Africa, the Middle East, and Western Eurasia. In this study, we validate event location improvements using this new data set by applying the regional and teleseismic modelbased travel-time calibrations (independently and jointly) to demons trate that significant improvements can be achieved using 3D global models for locating small events with sparse network data. Besides relocating events using all station arrivals, a subset of the GT events was also relocated using controlled station geom etries generated from a "constrained bootstrapping" technique. The advantages of this approach include: (1) generating simulated sparse networks (Simulated Sparse Network Bulletin or SSNB), (2) increasing the statistical power of the tests, (3) reducing the effect of correlated errors to ensure valid 90% error ellipse coverage statistics, and (4) measuring location bias due to un-modeled three-dim ensional (3D) Earth structures.With respect to the GT events, we compared event relocations, with and without t raveltime calibrations, considering statistics of mislocation, error ellipse area, 90% coverage, origin time bias, origin time errors, and misfit. Relocations of more than 1000 GT0-GT10 reference events show significant reductions in location bias and uncertainty. Pn and/or P calibration reduces mislocation for between 60% and 70% of the events. Joint regional Pn and teleseismic P travel-time calibration provided the largest location improvements, and approximately achieved GT5 accuracy levels. Due to correlated 3 errors, calibrated event locations using large numbers of stations have deficient 90% error ellipse coverage. However, the coverages derived from the model errors are appropriate for sparse regional and teleseismic networks. This validation effort demonstrates that the global model-based travel-time calibrations of Pn and teleseismic P travel-time reduce both location bias and uncertainty over wide areas.

Lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction. In this study we inferred possible strain geometries in the mantle beneath the eastern Aleutians by mapping seismic anisotropy with shear wave splitting. We analyzed S phases of local events recorded by the Shumagin Seismic Network and obtained splitting parameters (fast direction ϕ and splitting time δt) and their 95% confidence regions. To avoid complexity from converted phases, data were restricted to paths at incidence angles less than 32° and to events deeper than 50 km. The S phases sample the forearc and back arc of the Shumagin segment from 159.7°W to 163.8°W at depths up to 216 km. In our data set of 84 local events, 44 of 127 S phases yielded constrained splitting parameters with consistent fast directions that are approximately parallel to the volcanic arc (roughly 60°). The remaining 83 S phases produced null measurements, and the fast directions inferred from these phase polarizations agree with the fast‐axis alignment in the constrained measurements. Splitting times vary from 0.1 to 0.35 s and show an overall increase with hypocentral depth. Observed splitting parameters are well matched by predicted values for olivine‐rich mantle wedge models with ∼1% SV anisotropy where the olivine b axis is arc‐orthogonal and the a axis is vertical (no arc‐parallel strain) or arcparallel (no vertical strain). These results are consistent with mantle wedge strain models in which arc‐normal compression is accompanied by arc‐parallel or vertical shearing or extension.

The northern piedmont of the Tianshan Mountains consists of three rows of Cenozoic EW‐striking fold and reverse fault zones, with en échelon right‐lateral steps. The southernmost row involves sediments as young as lower Pleistocene, but there is no evidence of activity along this row during the last 30,000 years. The central row is composed of three linear anticlines (Houerguos, Manas, and Tugulu) and associated reverse faults. The northernmost row includes the Dushanzi, Halaande, and Anjihai anticlines and their associated reverse faults. Abundant fault scarps and folds of late Pleistocene to Holocene river terraces across the anticlines within the central and the northernmost rows indicate recent folding and reverse faulting. We divide the northern piedmont into the Dushanzi and Manas fold and reverse fault zone. In the Dushanzi zone, we excavated 15 trenches across scarps controlled by reverse faults and back thrusts. By comparing 11 trench logs among the 15 trenches, we identify three paleoearthquakes since 13,000 years B.P. The first event occurred between 11,300 and 13,300 years B.P., and the second and the third events occurred 6300–8400 and 3000–5000 years B.P., respectively. Considering the uncertainties of the data, the average recurrence interval for large earthquakes in the Dushanzi zone is about 4000 years. A large earthquake along this zone is expected in the near future because the elapsed time since the last surface‐rupturing event is already 3000–4000 years. Three large trenches and several small trenches excavated across the fault scarps along the Manas fold and reverse fault zone reveal four events. The first, second, and third events occurred at 18,000–13,000 years B.P., 11,300–10,500 years B.P., and 6900–3600 years B.P., respectively. The fourth, the latest one, is the 1906 M = 7.7 Manas earthquake. Field investigation suggests that the 1906 Manas earthquake occurred along a blind thrust fault. This earthquake formed three discontinuous zones of fresh surface ruptures, the longest of which is only 8 km long along the eastern segment of the Tugulu reverse fault, and was associated with a zone of uplift 130 km long related to the Manas earthquake. The average recurrence interval along this zone is probably 5000–6000 years. Therefore it is unlikely that a large earthquake will occur along this zone in the near future because the Manas earthquake occurred only 89 years ago.

Geomorphic study on Wujiahe segment of Serteng piedmont fault, Inner Mongolia is made. Through analysis of the available data in combination with the results of predecessors' studies it can be obtained that average vertical displacement rate is 0.48-0.75 mm/a along the Wujiahe segment since the late Pleistocene (14.450~22.340 ka BP) and 0.56--0.88 mm/a since the early-middle Holocene (5.570--8.830 ka BP). Analyzing paleoseismic phenomena revealed in the excavated 5 trenches in combination with the results of predecessors' studies of paleoearthquakes on the fault, we determine five paleoseismic events on the Wujiahe segment of Serteng piedmont fault since 27.0 ka BP and the recurrence interval to be about 4.300~4.400 ka. A cluster of paleoearthquakes occurred probably during 8.000-9.000 ka BP and two paleoseismic events in 10.000-20.000 ka BP may be missed. A comparison between height of fault scarps and sum of displacement caused by paleoseismic events revealed in trenches, and recurrence interval of paleoseismic events obtained from average displacement rate along the fault and the dislocation by one event suggest that three paleoseismic events are absent in Alagaitu trench. Two paleoseismic events may be absent on the whole active fault segment.

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To constrain the orientation and extent of seismic velocity anisotropy in the Kuril‐Kamchatka subduction zone, we have developed a technique that isolates anisotropy at shallow depths by differencing splitting observed in teleseismic shear‐wave phases. Olivine a‐axis orientations inferred from 29 sS‐S phase pairs and two local S phases vary between the major tectonic components of the trench/back‐arc system. Although the sS paths do not uniquely constrain a single model of anisotropy, a‐axes are consistent with finite strain patterns predicted by plate convergence and back‐arc spreading and shearing.

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