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Heterogeneous Lowermost Mantle Beneath the Pacific Ocean

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Heterogeneous Lowermost Mantle Beneath the Pacific Ocean fall 2009 featured science: Heterogeneous Lowermost Mantle Beneath the Pacific Ocean The seismic stations of the USArray Transportable Array (TA) record earthquakes from around the globe. Seismic waves are affected by the structure and composition along their travel path through the Earth, allowing us to deduce Earth structure between source and station. This article highlights the unique capabilities of the dense TA for deep Earth studies. Direct P and S waves recorded at ~90°-100° distance (1° = 111 km) from an earth- onSitenewsletter quake are sensitive to the structure near the core-mantle boundary (CMB), where these waves bottom and return to the surface (Figure 1). The TA is ideally situated to record waves at these distances from Fiji-Tonga, where the largest number of deep-focus earth- quakes originate. This permits the investigation of the lowermost mantle (referred to as From the National Science the D" region) beneath the central Pacific Ocean, roughly half way between the earth- quakes and TA stations. Foundation Earlier studies had established the presence of a large low shear velocity province It’s an exciting time to be at the National in the D” region beneath the Pacific. Recently, the TA enabled several discoveries of a Science Foundation and to succeed Kaye variety of fine-scale complexities. These include isolated and thin ultra-low velocity zones Shedlock in overseeing the EarthScope Program. (ULVZs), some tens of km (or less) thick with velocities reduced by 10% and more; direc- After a five-year effort, EarthScope is now pro- tional dependence of seismic wave speed that may be related to mineralogy, rheology, viding continuous deformation measurements and flow; and discontinuities in velocity that are consistent with the presence ofpost- across Alaska and the contiguous 48 states perovskite. The nature of these low in near real time. EarthScope scientists have velocity regions is still enigmatic; 80 developed high-resolution images of mantle they appear to have relatively o 90 “drips” under the Great Basin and Sierra Nevada; sharp boundaries and possibly o S 100 synoptic views of slow earthquakes and tremor consist of material with higher den- o in Cascadia; and direct measurements of the sity than the surrounding mantle, physical properties of an active earthquake which suggests a chemically dis- zone. Education and outreach activities are tinct origin. another EarthScope hallmark: training workshops for teachers, park rangers, and students; the We investigate this problem D’’ EarthScope Speaker Series; strong student par- using TA data. Figure 2, as an mantle core ticipation in USArray siting; and, as you can read, example, shows seismograms the onSite newsletter. from a Fiji earthquake organized according to direction (azimuth) Our challenge is: what's next? How do we from the earthquake. We observe continue EarthScope’s success and keep our broadening of the S waves for focus while expanding into new research areas? azimuths between ~42°-47°; such Natural growth is one way: over 100 research- broadening is consistent with ers have now received EarthScope funding, multi-pathing, the phenomena of and this year there are many sessions involving a seismic wave splitting into two EarthScope during the AGU Fall Meeting. We waves when traveling tangential can also encourage ourselves to look around to a sharp velocity contrast. and seek new ideas. The EarthScope Steering Comparison with global tomogra- Committee is working with the community to phy (Figure 1) reveals these waves update the EarthScope Science Plan, starting indeed propagated near a low Edge with the October 2009 workshop. I encour- velocity boundary. Such analyses, age you to participate actively in this process, along with travel-time studies of which will identify emerging research areas for various S phases traversing the EarthScope in the coming years. lowermost mantle, enable us to 2 map the northern edge of the You can also make your voice heard even 0 low-velocity material beneath the dVs more directly: e-mail me at [email protected] Pacific (thick black line in Figure 1). (%) -2 or call me at 703-292-4693. I welcome your thoughts, criticism, and suggestions for continuing Seismic imaging of deep Earth Figure 1: Ray path geometry of S waves (top). Rays EarthScope’s success. structure is important because penetrate deeper at larger distances, eventually bending the dynamics and evolution around the core for distances greater than 100°. An of the deep interior are likely earthquake that occurred deep beneath Fiji in 2007 closely linked to Earth’s outermost sampled the lowermost mantle beneath the Pacific Ocean (bottom globe). Shown are paths (green) from shell(s), including tectonic plates, the earthquake (star) to the TA stations (triangles) on their motions, and evolution. For top of lowermost mantle S wave velocity perturbations Greg Anderson example, subduction of dense, (tomography courtesy of Steve Grand, Univ. Texas, Austin), NSF EarthScope Program Director cold oceanic lithosphere into the where red and blue represent lower and higher speeds relative to an average model, respectively. The thick black lower mantle could convectively line denotes a sharp velocity boundary deduced from (continued on page 3) observed waveform and travel time anomalies. 2 featured science: Tectonic Block Motions in Southeast Alaska and Adjacent Canada Southeast Alaska and the adjacent por- to the Fairweather fault, is deforming, repre- coastal mountains in the world. tion of Canada (Figure 1) form an important sented in our model by two small blocks. The segment of the Pacific-North American plate Fairweather fault remains almost purely strike- The coastal regions of Alaska and boundary zone and mark the beginning of slip, but convergence occurs on faults closer Canada are actively deforming at least as the transition between a transform margin to the coast and offshore. One of these far south as the Queen Charlotte Islands. Our and subduction along the Aleutian mega- faults may have ruptured as part of the 1899 block model shows that active deformation thrust. The tectonics of this region are driven Yakutat Bay earthquake sequence. East of occurs throughout the entire coastal region by the ~50 mm/yr relative motion between the Fairweather fault, the Fairweather block of southeast Alaska. Work by Stephane the Pacific plate and North America, and rotates clockwise relative to North America, Mazzotti and others showed that the same is the Yakutat block’s collision with and accre- resulting in transpression along the Duke River true for the Queen Charlotte Islands region of tion to southern Alaska. Active deformation and Eastern Denali faults. There is a clear British Columbia, although that region moves extends significantly inland, even in the strain transfer from the coastal region east- independently of the Fairweather block. boundary’s transform segment, and effects ward into the Northern Cordillera, which also While the motion of coastal blocks fur- of the Yakutat block collision extend far to rotates clockwise relative to North America. ther south is much better known, the entire the northeast. Both the Fairweather fault and the Pacific coastal region may be mobile and The observed GPS velocities from Queen Charlotte fault have average right-lat- part of a continuous plate boundary zone campaign and continuous sites (Figure 1) eral slip rates of 44 ± 2 mm/yr, but the Queen system. Future data from several PBO sites show rapid motion along the coast and a Charlotte fault also displays transpression with located south of the Fairweather-Queen distinct rotational pattern for inland sites. We southward-increasing fault-normal motion Charlotte junction (Figure 2) will help pro- inverted the GPS data to estimate angular that reaches 16 ± 4 mm/yr near the Queen vide additional insights. However, the PBO velocities of several rigid blocks and derived Charlotte Islands. Relative motion between network is very sparse in the region, making a self-consistent set of fault slip rates from the Yakutat block and the Pacific plate is coastal Alaska and Canada a promising the block motions. The Yakutat block has a accommodated by left-lateral oblique slip on target for future EarthScope densification velocity of 51 ± 3 mm/yr towards N22 ± 3˚W an offshore fault such as the Transition fault and focused studies. ■ relative to North America, almost identical zone. The collision of the leading edge of the By Julie L. Elliott, Christopher F. Larsen, in rate to that of the Pacific plate but with a Yakutat block with southern Alaska results in Jeffrey T. Freymueller, and Roman J. Motyka, more westerly azimuth (Figure 2). The north- 45 mm/yr of shortening across the St. Elias Geophysical Institute, University of Alaska eastern edge of the Yakutat block, adjacent Range, which contains some of the steepest Fairbanks, Fairbanks, Alaska 62.0˚ 62.0˚ 200 km 25 mm/yr velocity 62.0˚ ED 62.0˚ DR SOAK NC ED 60.0˚ 60.0˚ 60.0˚ 60.0˚ Y FF Y TZ 58.0˚ FW 58.0˚ P 58.0˚ QC AK CAN 5 mm/yr velocity 56.0˚ 1 mm/yr uncertainty 56.0˚ P QC 50 mm/yr velocity 1 mm/yr uncertainty AM P -136.0˚ -134.0˚ -142.0˚ -138.0˚ -134.0˚ -130.0˚ Figure 1: Observed GPS velocities (green arrows) in southeast Alaska relative to North Figure 2: Block boundaries and predicted block motions (arrows). Colors denote America. For clarity, no velocity error ellipses are shown; the scale vector shows a blocks. Labeled faults are the Duke River (DR), the Eastern Denali (ED), the Fairweather typical error ellipse. Inset map box shows area covered on data map. AM is the (FF), the Queen Charlotte (QC), and the Transition fault zone (TZ). Labeled blocks are Aleutian Megathrust, P the Pacific plate, Y the Yakutat block, and QC the Queen Southern Alaska (SOAK), Pacific (P), Yakutat (Y), Fairweather (FW), Eastern Denali (ED), Charlotte fault.
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