Eos, Vol. 90, No. 41, 13 October 2009 VOLUME 90 NUMBER 41 13 OCTOBER 2009 EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 361–372 Grand Challenges for Seismology in slow events that occur surprisingly regu- larly, accompanied by low-amplitude seis- mic tremor (Figure 1). There are many spe- PAGES 361–362 environment, discovering and mapping nat- ciﬁc issues remaining to be addressed, such ural resources, contributing to national and as the relationship of this episodic slip and Seismology is the study of the propaga- international security, and understanding tremor to major earthquakes, and achiev- tion of elastic waves, the sources that gener- the dynamic processes in the interior of the ing a detailed physical understanding of the ate them, and the structures through which Earth. This article provides a summary of nonlinear processes by which faults slip is a they propagate. It also is a fundamental, high- the 10 grand challenges. major challenge. resolution tool for exploring the interior of How does the near- surface environment the Earth from crust to core, as well as other Specific Challenges affect natural hazards and resources? The bodies in the solar system. A remarkable location and severity of many natural haz- diversity of multidisciplinary societal applica- How do faults slip? The steady motions ards are strongly inﬂuenced by near- surface tions of seismology has emerged, including of the tectonic plates build up stresses that material properties. Determining Earth’s his- hydrocarbon and resource exploration, earth- are relieved mainly through slip on faults. tory of natural climate change relies in part quake detection and hazard assessment, Recent observations have revealed a rich on seismic imaging of shallow sedimen- nuclear test monitoring and treaty veriﬁca- spectrum of fault behavior, ranging from tary deposits that record and respond to tion, volcano and tsunami warning systems, steady sliding with little apparent resis- climate variations. Near- surface processes and aquifer characterization. New directions tance to earthquakes that can slide at super- affect water, energy, and mineral resources in seismology are evolving that are relevant shear velocities (faster than the speed of at depths of meters to a few kilometers. to climate and environmental change, such S waves in the rocks) and that can emit Detailed knowledge of the Earth’s near sur- as resolving ﬁne- scale seismic stratigraphy, shock waves that may cause exceptionally face is therefore a crucial part of managing a monitoring carbon sequestration, detect- damaging ground motions. Only in the past sustainable environment for civilization. One ing sudden movements of glaciers and ice decade has it been discovered that major of the most important challenges for seis- sheets, mapping the internal ﬁne structure of parts of some fault systems slip repeatedly mology is to understand how strong ground the ocean, and reconstructing the twentieth- century history of global storm activity from ocean-generated seismic noise. The broad scope of seismological research positions the discipline to contrib- ute signiﬁcantly to the U.S. National Science Foundation (NSF) Directorate for Geosci- ences’ emphases on dynamic Earth pro- cesses and climate change in the 2010 bud- get request to Congress. The seismological community, through a community workshop and writing commit- tee process sponsored by NSF, earlier this year identiﬁed 10 grand challenges for seis- mology at the forefront of research on Earth systems. The resulting document, “Seismo- logical grand challenges in understanding Earth’s dynamic systems,” was published in early 2009 and is available online at http:// www.iris.edu/hq/lrsps. The report is directed at a broad readership, including researchers and educators in other disciplines as well as a general academic and government audi- ence. The report includes a number of side- bars illustrating recent discoveries and appli- cations that are appropriate for classroom and other broad use. Fig. 1. Location of migrating tremor during a 2- to 3-week episode of slow slip on the Cascadia The grand challenges, which are framed subduction zone. Most of the relative plate motion in the slow slip area is accommodated by by fundamental research issues, encompass similar slip events that repeat approximately every 14 months. Plate boundary slip in the “locked mitigating natural hazards, monitoring the zone” to the west of the contours of partial locking occurs during great earthquakes such as the Mw ~9 Cascadia megathrust earthquake in 1700. Locking refers to the percentage of slip BY D. W. FORSYTH, T. LAY, R. C. ASTER, between plates that occurs in stick-slip events as opposed to gradual, nearly continuous creep. AND B. ROMANOWICZ Image courtesy of A. Wech and K. Creager. Eos, Vol. 90, No. 41, 13 October 2009 motions are produced by earthquakes and how much water is stored in the mantle core? The thermal evolution of the Earth, the and to translate this understanding into (which may amount to more than ﬁve ocean driving forces of plate tectonics, and the gen- improved hazard maps. Nonlinear responses volumes) and whether changes in mineral- eration of the magnetic ﬁeld all involve con- to shaking, such as soil liquefaction, and the ogical phase lead to greater concentrations vective ﬂow in the mantle and core. Improv- complex pattern of strong ground motions of water in the mantle transition zone and ing the seismological resolution of deep can be predicted with comprehensive three- cause regions of partial melt near the global structure as data accumulate and as new dimensional (3- D) modeling of potential 410-kilometer- deep discontinuity. analysis methods are developed will help earthquakes and knowledge of soil proper- How do magmas ascend and erupt? Seis- reveal the patterns of ﬂow. Recent observa- ties, an undertaking that straddles the inter- mological monitoring is one of the primary tional studies, combined with mineral phys- face between seismology and earthquake ways of forecasting or predicting volcanic ics experiment and theory, have shown that engineering. eruptions. An increase in microearthquake large- scale chemical heterogeneity is pres- What is the relationship between stress activity and harmonic tremor, or changes in ent in the mantle and that the interaction of and strain in the lithosphere? Plate tecton- seismic velocity as moving magma changes compositional and thermal buoyancy must ics provides the kinematic framework for the shape of the volcano and fractures the be considered in modeling convective pro- describing rates of deformation, but it does surrounding rock, often precedes eruptions cesses. The large- scale 3-D elastic struc- not quantitatively account for how plates by several days, providing some warning ture of the mantle is now fairly well known, move and deform. Rheology describes the of an eruption. Current eruption prediction but where detailed studies provide higher linkage between the forces (stresses) and methods are primarily empirically based, resolution, pronounced sharp or short- the resulting deformation (strains). Motions however, because magma plumbing systems wavelength features are found. This suggests and strains now are precisely measured with are poorly known. A major challenge is to that small- scale convection plays a critical satellite imaging and networks of Global improve scientiﬁc understanding and predic- role in the dynamics of Earth’s deep interior. Positioning System receivers, strainmeters, tion capabilities through better determina- How are Earth’s internal boundaries seismometers, and tiltmeters, but the caus- tion of the physical changes that accompany affected by dynamics? Internal boundaries ative stresses only can be inferred. Meeting eruptions, including improved imaging of in Earth (and other planets) are associated the grand challenge of understanding the the interior of volcanic systems and quanti- with the primary compositional layering that stress distribution and the temporally and tative characterization of magma migration resulted from the chemical differentiation spatially dependent rheology is necessary and eruption processes. of the planet and with mineralogical phase to unraveling how some earthquakes trigger What is the lithosphere- asthenosphere changes controlled by pressure and tem- other earthquakes thousands of kilometers boundary? The lithosphere is Earth’s perature variations. These boundaries may away or how, for instance, the great Sumatra mechanically strong outer shell that makes be deﬂected by convective processes, thus earthquakes of 26 December 2004 (seismic up the tectonic plates, underlain by the providing clues to the location and inten- moment magnitude Mw = 9.3) and 28 March weak asthenosphere, which ﬂows and sity of upwelling and downwelling. Because 2005 (Mw = 8.7) were coupled. deforms to accommodate plate motions. The changes in rheology, composition, and den- How do processes in the ocean and atmo- lithosphere often is thought of as the ther- sity occur across the boundaries, they can in sphere interact with the solid Earth? Ocean mal boundary layer between the cold sur- turn exert a strong inﬂuence on the pattern storms, bolides, tornadoes, and glacier face of Earth and the planet’s hot interior, of convection. The challenge for seismology calving all generate signals that are readily but recent studies have shown that there is is to map these boundaries, including their detected by seismometers and atmospheric often a sharp seismic discontinuity at the 3- D topography and sharpness, which are infrasound recorders. The multidisciplinary base of the lithosphere inconsistent with a key clues to quantifying their mineralogical, topic of how processes in the ocean and simple gradual thermal transition. Changes thermal, and compositional nature and to atmosphere couple into seismic waves and in composition, volatile content, and anisot- interpreting the dynamic processes that con- how these waves can be used to monitor ropy of the mantle, and perhaps the pres- trol these variations. the global environment is one of the high- ence of melt, may play roles in creating the priority challenges. Recently, it was estab- discontinuity. Lithosphere- scale seismol- Requirements to Meet the Challenges lished that the Earth’s long-period “hum” of ogy is being revolutionized by new data free oscillations continuously excited at peri- from large- scale seismometer deployments, The report “Seismological grand chal- ods of hundreds of seconds is generated by such as the USArray component of the NSF- lenges in understanding Earth’s dynamic midlatitude winter storms through an as yet funded EarthScope project, and by new systems” describes the detailed seismologi- poorly understood mechanism. On the other analysis techniques. However, many chal- cal approaches and practical requirements end of the seismic frequency scale, active lenges to understanding the evolution and needed to make progress in attacking each sources used in seismic proﬁling, such as structure of the lithosphere and the astheno- of the 10 grand challenges. A number of in routine imaging of subseaﬂoor structure, sphere remain. common themes emerge in terms of these can detect layering and mixing in the water How do plate boundary systems evolve? approaches and needs. For example, increas- column itself. The images’ unprecedented Most earthquakes and volcanoes occur at ingly massive data sets, inversions for 3-D and horizontal resolution can help with under- plate boundaries. Most of the deformation 4-D multiscale models, and realistic simula- standing internal waves, turbulent mixing, and volcanic activity at plate boundaries in tions incorporating as much of the physics and ocean circulation. the oceans may take place in a zone only a as possible require enormous computational Where are water and hydrocarbons hid- few hundred meters across at the surface, capabilities. Thus, collaborative efforts to den beneath the surface? Seismological tech- yet plate boundary systems may be hun- increase access to state- of-the-art computing niques have long been used to map aqui- dreds of kilometers wide in the continents. are essential. Another such theme is seismol- fers and explore for hydrocarbon resources. The geometry of these diffuse boundaries ogy’s long tradition of open access to all data Modern exploration seismology methodolo- changes with time, and the areas within sets and storage of these data sets in perpe- gies, including 4-D (time lapse) mapping, the boundary system that are most active tuity; this approach needs to be encouraged routinely are used to monitor the extraction may shift. Coordinated seismological, geo- and supported globally. Networks of perma- and movement of hydrocarbons in real time detic, geomorphological, deep drilling, and nent, broadband, real-time observatories on land and at sea. Similar approaches now geological studies are needed to meet the form a backbone of national and worldwide are being applied to investigate the poten- challenge of determining what controls the monitoring efforts, and these networks need tial for carbon dioxide sequestration, and location, width, and activity of dynamically to be maintained, upgraded, and, where pos- these approaches will be critical for manag- evolving plate boundaries. sible, expanded to the oceans. ing these efforts. Looking deeper, there is How do thermal and compositional varia- An essential need for several of the great interest at present in deducing where tions control convection in the mantle and grand challenges is the availability of large Eos, Vol. 90, No. 41, 13 October 2009 pools of portable instruments for seismo- the academic community. Further, the new seismology and other disciplines need to logical investigations of continental and seagoing R/V Marcus G. Langseth (owned be fostered and strengthened. Progress on oceanic environments at higher resolution by NSF and operated by Lamont- Doherty the seismological grand challenges noted than that afforded by the current global Earth Observatory of Columbia University) here, and on the many societal applica- network of permanent stations. The pools needs to be fully supported in a way that tions of seismology, hinges on improved of three- component, short- period, and makes its 3-D imaging capabilities more interdisciplinary interactions and commu- broadband sensors need to be expanded, readily accessible to investigators. nications, in addition to the shared, practi- in the oceans and on land, for the next There is a common need for the devel- cal requirements described above. generation of 3-D and 4-D imaging efforts opment and coordination of advanced —DONALD W. FORSYTH, Brown University, Provi- of crustal, lithospheric, and deep mantle data products to make the results of seis- dence, R. I.; E-mail: firstname.lastname@example.org; and core structure. mological research more accessible to THORNE LAY, University of California, Santa Cruz; In addition, a new facility should be the public and to Earth scientists in other RICHARD C. ASTER, New Mexico Institute of Mining established to make controlled seismic disciplines. Finally, strong synergisms and Technology, Socorro; and BARBARA ROMANO- sources for land studies more available to within the Earth science arena between WICZ, University of California, Berkeley opportunity to use the full range of seismic Probing the Hawaiian Hot Spot With New techniques that have been applied success- Broadband Ocean Bottom Instruments fully in land- deployed experiments. Body wave and surface wave tomographic imag- PAGES 362–363 a multidisciplinary program whose center- ing as well as receiver function and compli- piece is a large network of four- component ance analyses will provide new constraints The Hawaiian hot spot is regarded as the broadband ocean bottom seismometers on elastic and anelastic seismic structure textbook example of the product of a deep- (OBSs) and three- component portable and major discontinuities from crustal rooted mantle plume [Wilson, 1963; Morgan, broadband land stations (Figure 1). Occupy- depths into the lower mantle. The analysis 1971]. Its isolated location, far from any plate ing a total of 82 sites and having an overall of shear wave splitting and surface wave azi- boundary, should provide an opportunity aperture of more than 1000 kilometers, this muthal anisotropy will help reveal mantle to test most basic hypotheses on the nature experiment is one of the ﬁrst large- scale, fabric and ﬂow patterns. of plume-plate interaction and related mag- long- term deployments of the new broad- Now, about 18 months after the last matism [e.g., Ribe and Christensen, 1999]. band OBSs in the U.S. National Science OBSs were recovered from the ocean Yet the lack of crucial geophysical data has Foundation–supported national OBS Instru- ﬂoor, the high overall return and quality sustained a debate about whether Hawaii’s ment Pool (OBSIP). PLUME is providing an of PLUME data allow for the production of volcanism is plume-related or is instead the consequence of more shallow processes, such as the progressive fracturing of the plate in response to extensional stresses [Turcotte and Oxburgh, 1973]. In the plume model for Hawaii’s volca- nism, hot material is expected to ascend near vertically within the more viscous sur- rounding mantle before ponding and spread- ing laterally beneath the rigid lithosphere. Mantle convection in general, and the fast moving Paciﬁc plate in particular, shear and tilt the rising plume. The plume top is dragged downstream by the plate, and this dragged material may give rise to an elongated bathymetric swell [Davies, 1988; Olson, 1990; Sleep, 1990; Phipps Morgan et al., 1995]. However, identifying the domi- nant cause of the swell remains elusive, and proposed mechanisms include thermal reju- venation, dynamic support, compositional buoyancy, and mechanical erosion (see Li et al.  for a summary). There is also considerable debate about the continuity of the plume within the mantle, how discrete islands are formed, and how a deep-rooted plume interacts with the mantle transition Fig. 1. Site locations of the two deployment phases of the Hawaiian Plume-Lithosphere Under- zone [e.g., van Keken and Gable, 1995]. sea Mantle Experiment (PLUME). Also shown are sites of permanent stations of global seismic networks relevant to this study. Station KIP (Kipapa, Oahu) is jointly operated by the French Geo- scope program and the U.S. Geological Survey (USGS); POHA (Pohakuloa, Hawaii) is operated Seismic Imaging of Hawaiian Mantle by USGS; and MAUI is operated by the German Geo-ForschungsNetz (GEOFON). USGS station MIDW (Midway; see Figure S1 in the electronic supplement) is not shown. Phase 1 operated from Seismic imaging can help distinguish January 2005 through January 2006, and phase 2 operated from April 2006 through June 2007. among plausible models, but the deploy- Two sites with unrecovered ocean bottom seismometers (OBSs) from phase 1 and six sites from ment of seismic stations that has been lim- phase 2 were visited by Woods Hole Oceanographic Institution’s remotely operated vehicle (ROV) ited to the nearly aligned Hawaiian Islands Jason in November 2007.The OBSs at sites 57 and 59 were recovered at that time. Four sites has so far led to incomplete images of the with ﬁve lost OBSs (sites 9; 42, with two OBSs; 52; and 72) remain unvisited. Open numbered crust and mantle beneath and around circles mark instruments with a loss of differential pressure gauge (“no DPG”) and/or vertical- Hawaii. The Hawaiian Plume- Lithosphere component seismometer data (“no Z”). During the ﬁrst deployment cruise, 11 dredge hauls were Undersea Mantle Experiment (PLUME) is performed at six locations to retrieve fossil corals and deep-rift volcanic rocks.