New Candidate Ultralow-Velocity Zone Locations from Highly Anomalous SPdKS Waveforms

Ultralow-velocity zones (ULVZs) at the core–mantle boundary (CMB) represent some of the most preternatural features in Earth’s mantle. These zones most likely contain partial melt, extremely high iron content ferropericlase, or combinations of both. We analyzed a new collection of 58,155 carefully processed and quality-controlled broadband recordings of the seismic phase SPdKS in the epicentral distance range from 106° to 115°. These data sample 56.9% of the CMB by surface area. From these recordings we searched for the most anomalous seismic waveforms that are indicative of ULVZ presence. We used a Bayesian approach to identify the regions of the CMB that have the highest probability of containing ULVZs, thereby identifying sixteen regions of interest. Of these regions, we corroborate well-known ULVZ existence beneath the South China Sea, southwest Pacific, the Samoa hotspot, the southwestern US/northern Mexico, and Iceland. We find good evidence for new ULVZs beneath North Africa, East Asia, and north of Papua New Guinea. We provide further evidence for ULVZs in regions where some evidence has been hinted at before beneath the Philippine Sea, the Pacific Northwest, and the Amazon Basin. Additional evidence is shown for potential ULVZs at the base of the Caroline, San Felix and Galapagos hotspots.

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Post-earthquake Prioritization of Bridge Inspections

Earthquake Spectra ◽

10.1193/1.2428313 ◽

2007 ◽

Vol 23 (1) ◽

pp. 131-146 ◽

Cited By ~ 15

Author(s):

R. T. Ranf ◽

M. O. Eberhard ◽

S. Malone

Keyword(s):

Ground Motion ◽

Pacific Northwest ◽

Epicentral Distance ◽

Fragility Curves ◽

Washington State ◽

Motion Processing ◽

The Pacific ◽

Bridge Damage ◽

Washington State Department ◽

Prioritization Strategy

Bridge damage reports from the 2001 Nisqually earthquake were correlated with estimates of ground-motion intensity at each bridge site (obtained from ShakeMaps) and with bridge properties listed in the Washington State Bridge Inventory. Of the ground-motion parameters considered, the percentage of bridges damaged correlated best with the spectral acceleration at a period of 0.3 s. Bridges constructed before the 1940s, movable bridges, and older trusses were particularly vulnerable. These bridge types were underestimated by the HAZUS procedure, which categorizes movable bridges and older trusses as “other” bridges. An inspection prioritization strategy was developed that combines ShakeMaps, the bridge inventory and newly developed fragility curves. For the Nisqually earthquake, this prioritization strategy would have made it possible to identify 80% of the moderately damaged bridges by inspecting only 481 (14%) of the 3,407 bridges within the boundaries of the ShakeMap. To identify these bridges using a prioritization strategy based solely on epicentral distance, it would have been necessary to inspect 1,447 (42%) bridges. To help the Washington State Department of Transportation (WSDOT) rapidly identify damaged bridges, the prioritization procedure has been incorporated within the Pacific Northwest Seismic Network (PNSN) ground-motion processing and notification software.

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Local travel-time curves and their geologic implications for the Pacific Northwest states

Bulletin of the Seismological Society of America ◽

10.1785/bssa0550030587 ◽

1965 ◽

Vol 55 (3) ◽

pp. 587-607 ◽

Cited By ~ 2

Author(s):

Peter Dehlinger ◽

E. F. Chiburis ◽

M. M. Collver

Keyword(s):

Travel Time ◽

Pacific Northwest ◽

Epicentral Distance ◽

Gravity Data ◽

Normal Velocity ◽

Low Velocity ◽

P Waves ◽

The Pacific ◽

Low Velocity Layer ◽

Velocity Layer

Abstract Travel-time curves were constructed for the Pacific Northwest states based on recordings of recent local earthquakes. Average velocities of Pn and S waves were found to be 4 per cent lower in the region west of the Cascade Mountains than they are to the east of the Cascades, while velocities of P* and P¯ waves are essentially the same in the two provinces. West of the Cascades the velocities obtained are 7.67 km/sec for Pn, 4.37 for Sn, 6.61 for P*, and 5.48 for P¯. East of the Cascades they are 7.96 km/sec for Pn, 4.56 for Sn, 6.60 for P*, and 5.53 for P¯. Intercept times indicate that the crust (above the Mohorovicic discontinuity) is 5 to 10 km thinner to the west than east of the Cascades. Pn velocities are found to be independent of epicentral distance to distances of 1000 km, implying that the corresponding mantle materials and densities are uniform down to the low-velocity layer in each province. In both provinces Poisson's ratio in the mantle is 0.26. From the travel-time curves and gravity data, it appears that the section above the low-velocity layer east of the Cascades is approximately typical or “normal,” while the corresponding mantle west of the Cascades consists of a different or anomalous material. The likelihood of crystal alignment, temperature anomalies, and normal velocity-density relations in the upper mantle sections are discussed.

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