GSA TODAY • Declassified Satellite Photos, P

Vol. 5, No. 7 July 1995

INSIDE GSA TODAY • Declassified Satellite Photos, p. 139 • GSA Employment Service, p. 144 A Publication of the Geological Society of America • 1995 GSA Awards, p. 144

Normal-Mode Splitting Observations from the Great 1994 Bolivia and Kuril Islands Earthquakes: Constraints on the Structure of the Mantle and Inner Core Jeroen Tromp, Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138

ABSTRACT of Earth and contains information Body-wave seismologists who study tional dependence such that waves about its density and its elastic and the inner core have recognized for travel faster along the rotation axis On June 9, 1994, a magnitude anelastic structure. The effect of more than 10 years that compres- than in the equatorial plane; such 8.3 earthquake struck ~650 km Earth’s rotation, ellipticity, and lat- sional waves traversing the inner a directional dependence of wave below Earth’s surface in Bolivia. Four eral heterogeneity is to distort the core along a trajectory parallel to speed is called anisotropy. I confirm months later, on October 4, a second shapes of the resonance peaks; this Earth’s rotation axis arrive faster that both inner-core travel-time large earthquake of similar magni- phenomenon is referred to as split- than waves traveling in the equato- anomalies and the splitting of most tude occurred >60 km below the ting. The details of the splitting of a rial plane. In 1986, Morelli et al. and anomalous modes can be explained Kuril Islands. Both events were given resonance peak are determined Woodhouse et al. put forward the in terms of inner-core anisotropy. recorded by more than 80 digital by the mode’s sensitivity as a func- hypothesis that the inner-core com- instruments distributed around the tion of depth. By analyzing the fine pressional speed exhibits a direc- Splitting continued on p. 140 globe. For comparison, the last earth- structure of a large number of reso- quake of comparable magnitude was nance peaks, global seismologists the 1970 Colombia event, which attempt to improve our knowledge was recorded by just one digital about Earth’s three-dimensional seismometer. Analog data from 60 structure. additional stations had to be hand- Splitting observations for digitized. The Colombia earthquake mantle-sensitive modes are generally provided the basis for the first high- quite well explained by current resolution radial Earth models; one shear-speed models of the mantle. can imagine the wealth of informa- Observations of compressional tion contained in the numerous modes should help constrain the digital recordings of the Bolivia scaling relation between shear and and Kuril Islands events. compressional speeds, which in turn An amplitude spectrum of a time will tell us about thermal vs. chemi- series recorded after a big earthquake cal heterogeneities in Earth’s mantle. contains hundreds of easily identifi- A collection of core-sensitive able resonance peaks. Each reso- normal modes is split much more nance peak corresponds to a particu- than expected from Earth’s rotation, lar normal mode or free oscillation ellipticity, and mantle heterogeneity.

Figure 2. Splitting observations and predictions for mantle modes. In the left column the sensi- C tivity of a normal mode to perturbations in compressional speed α, shear speed β, and density ρ is shown as a function of depth. Labels indicate the radii of the inner-core boundary (ICB), core- mantle boundary (CMB), and the 670 km discontinuity (670). The center column shows the observed splitting function, which is a function of latitude and longitude. The splitting function Figure 1. A: Amplitude spectrum obtained by Fourier trans- may be regarded as a local radial average of Earth’s three-dimensional structure. The manner in forming an 80-hour-long time series recorded at Tucson, Ari- which a mode averages Earth’s structure is determined by its sensitivity kernels, shown in the left zona, after the June 9, 1994, Bolivia earthquake. B: Vertical column. For example, spheroidal mode 1S4 sees the entire mantle, whereas 2S8 is predominantly component of the surface displacement of the five spherical sensitive to the upper mantle; this difference in sensitivity is reflected in the observed splitting Earth singlets that compose an angular degree-two multiplet, functions. Blue shades correspond on average to fast velocities, whereas red colors reflect slow such as 3S2. In a spherically symmetric Earth model, all five velocities. The pattern of lateral heterogeneity consists of all even spherical harmonics up to singlets oscillate with the same frequency. In a laterally hetero- degree 6, corresponding to a total of 28 model parameters. Notice the distinct ring of fast veloc- geneous Earth model, the singlet eigenfrequencies are “split,” ities surrounding a relatively slow region beneath the Pacific Ocean. The splitting function pre- such that each singlet oscillates with its own individual eigen- dicted by mantle model SKS12WM13 (X. F. Liu and A. M. Dziewonski) is shown in the right col- frequency. In that case, the l = 2 singlet displacements are a linear combination of the five spher- umn. Multiplets 1S4, 1S7, and 2S8 are predominantly sensitive to shear-speed perturbations and ical Earth singlet displacements shown. C: The spheroidal multiplet 1S4 consists of 2 × 4 + 1 = 9 are quite well predicted by shear velocity model SKS12WM13. Perturbations in compressional singlets with azimuthal orders m = –4,–3,–2,–1,0,1,2,3,4. In this figure, all nine singlets that speed and density are obtained by scaling shear-speed model SKS12WM13. As shown by the constitute the spectral peak labeled 1S4 in Figure 1A have been “stripped”; the values of the sensitivity kernels in the left column, mode 4S3 is predominantly sensitive to perturbations in azimuthal order m for each strip are shown on the right. The linear splitting of this multiplet as compressional speed throughout the mantle, and mode 0S5 is sensitive to perturbations in com- a function of the azimuthal order m is characteristic of modes that are predominantly split by pressional speed in the upper mantle, which exhibits large lateral variations in speed. Neverthe- Earth’s rotation. (Courtesy of Guy Masters.) less, SKS12WM13 predicts the observed splitting reasonably well, as shown in the right column.

GSA Today Now on the World Wide Web widely used PostScript page-description language is the foundation on which Acrobat was created. The file- Issues of this publication are now available electronic access to be a useful advantage. name extension used for documents in the unique electronically, in full color, on the World Wide Web Each monthly issue posted on the WWW is a sepa- Acrobat format is “PDF.” (WWW)—even before the printed copies are off the rate electronic file created in “Portable Document For- Although the GSA Today files on the WWW cannot press. This news should be especially welcome to mat” (PDF), an electronic file format that combines be altered, you can download and view them, search those living outside the continental United States, high-quality graphics and electronically searchable text the text, and print them on DOS PCs, Windows PCs, who often have to wait several weeks for delivery. in the same document. The technology is called Acro- But even domestic subscribers may find fast, bat, from Adobe Systems, Inc., whose well known and Web continued on p. 138 Splitting continued from p. 137 and spheroidal modes. Toroidal modes can be observed only on the horizontal INTRODUCTION components of a seismometer, whereas spheroidal modes can be observed on From the point of view of a global all three components. Because of sensi- seismologist, Earth is a large musical tivity to tilting, the horizontal compo- instrument that is played by earth- nents of a seismometer tend to be quakes. What we can learn about Earth much noisier than the vertical compo- by listening to its music depends on nent, which is why the analysis of what notes the earthquakes play. If I toroidal splitting is still in its infancy. can make an analogy between Earth At high frequencies, toroidal modes and a piano, the music of the Bolivia correspond to horizontally polarized and Kuril Islands earthquakes is equiva- shear waves, whereas spheroidal modes lent to that of sitting down on a correspond to a combination of com- piano’s keyboard. These two large pressional waves and transversely earthquakes were each recorded by polarized shear waves. In the context more than 80 long-period seismome- of surface waves, toroidal modes ters all over the world. Every note of correspond to Love waves, whereas Earth manifests itself as a resonance spheroidal modes correspond to peak in amplitude spectra obtained Rayleigh waves. For a good introduc- from the recordings of this event. tion to normal-mode seismology, see Most notes that are readily detected Lay and Wallace (1995). have periods of five minutes or more To identify a particular free oscilla- and may be observed for as long as a tion, each mode is labeled by three week after the earthquake. In an ampli- unique integers: an overtone number n, tude spectrum obtained by Fourier an angular degree l, and an azimuthal transforming a time series of the order m. For every value of l there are Bolivia earthquake recorded in Tucson, 2l + 1 associated values of m: m = –l, … , Arizona (Fig. 1), several isolated reso- m = 0, … , m = l. A multiplet nTl nance peaks are readily identified, and Figure 3. Observed and predicted splitting functions for several anomalously split normal (toroidal modes) or S (spheroidal some, like the one labeled S , are visi- n l modes. Notice that a large zonal degree-two contribution—i.e., a banded signal parallel to the 1 4 modes) is the collection of all 2l + 1 bly split. Splitting is a manifestation of equator—is missing in the predicted splitting functions, but that the remaining signal is reason- free oscillations with the same quan- Earth’s rotation, ellipticity of figure, ably well predicted by mantle model SKS12WM13. All anomalously split modes are sensitive to tum numbers n and l; the 2l + 1 mem- and lateral heterogeneity. By analyzing structure in Earth’s inner core, which turns out to be the source of the anomalous signal. See the bers of a multiplet are called singlets the splitting of a large number of reso- Figure 2 caption for further details. and are denoted by T m or Sm. In Fig- nance peaks, we can learn something n l n l ure 1B the vertical component of the about Earth’s internal structure and surface displacement of the five spheri- dynamics. cal Earth singlets that compose an l = 2 every multiplet of a spherically sym- A convenient way to visualize multiplet, such as S , is displayed. On Normal-Mode 3 2 metric Earth. Any departure of Earth normal-mode splitting is provided by a spherically symmetric Earth model, Nomenclature from sphericity removes the degener- the splitting function, which was first all singlets within a given multiplet acy and causes the singlets to split, introduced by Giardini et al. (1987). Let us investigate the structure of have the same eigenfrequency ω ; n l such that each individual singlet has Basically, at a given location on the sur- a resonance peak in a bit more detail. we say that the singlets are 2l + 1 a resonance peak centered on its own face, a mode’s splitting function repre- A spherically symmetric Earth model degenerate. This implies that the reso- distinct eigenfrequency ωm. As a sents a local radial average of Earth’s such as PREM (Dziewonski and Ander- nance peaks of all 2l + 1 singlets within n l result, one can often identify several interior structure. By plotting the value son, 1981)—i.e., a model that is a func- a multiplet T or S are centered on n l n l peaks within the resonance peak of a of the splitting function everywhere tion of radius only—supports two dis- the same frequency ω . As a result, we n l particular multiplet. For example, in on the surface, we can visualize how tinct classes of normal modes: toroidal observe one single resonance peak for Figure 1A one can clearly see two peaks a certain mode averages Earth’s three- associated with spheroidal multiplet dimensional speed and density struc- 1S4. In Figure 1C all 2 × 4 + 1 = 9 singlet ture (Fig. 2). Red colors are used to resonance peaks that compose the mul- denote that the radial regions sampled tiplet 1S4 have been “stripped.” This by the normal mode are on average Ruptures multiplet is predominantly split as a slow, whereas blue colors indicate an result of Earth’s rotation, which causes average fast speed. of Major linear splitting as a function of the One disadvantage of considering azimuthal order m. the splitting of isolated resonance The width of a resonance peak peaks, such as those labeled 0S6, 3S2 and Earthquakes contains information about Earth’s 1S4 in Figure 1A, is that one can deter- shear and bulk attenuation, which mine only even heterogeneity—i.e., represent a measure of seismic energy heterogeneity that is symmetric upon and Active dissipation due to shearing and com- reflection through Earth’s center. To pression, respectively. Bulk attenuation determine both odd and even hetero- within Earth is poorly determined geneity, one must consider coupling Deformation because seismic dissipation tends to be between overlapping resonance peaks, dominated by shear. However, a certain such as those labeled 0S7- 2S3 and in Mongolia type of spheroidal free oscillation, 1S5- 2S4 in Figure 1A. This year, the called a radial mode, has a purely radial first coupled-mode inversions were by I. Baljinnyam displacement and is uniquely sensitive performed by Rosovsky and Ritzwoller to bulk attenuation. The Bolivia earth- (1995). and others, 1993 quake in particular excited several of In this paper I compare observa- Some of the largest known these radial modes; analysis of their tions of splitting functions for isolated intracontinental earth- spectral peaks should help to determine mantle- and inner-core sensitive a more detailed picture of Earth’s bulk spheroidal modes with predictions quakes have occurred in attenuation. based upon current shear-speed models Mongolia, and until now very little has been published about them, of Earth’s mantle. I subsequently invert especially in English. The deformation here is especially well preserved, Splitting Function core-sensitive traveltime and splitting observations for inner-core anisotropy. apparently because of the dry, cold climate. This volume presents Every normal mode “sees” the observations of recent faulting in Mongolia and its immediate surround- structure of Earth differently. Some MANTLE STRUCTURE ings, particularly evidence of surface faulting associated with major modes are predominantly sensitive to the shear-speed structure of the mantle, The splitting predictions in Fig- earthquakes. Summaries of deformation associated with all of the major and other modes see a combination of ure 2 are based upon the most recent earthquakes and several prehistoric earthquakes are given. A brief shear and compressional speeds. There Harvard model SKS12WM13 produced summary of the deep structure, regional topography, and geologic are observable modes that see all the by X. P. Liu and A. M. Dziewonski; the way into the inner core, whereas others long-wavelength structure of this history of western Mongolia allows the deformation patterns to be are confined to the crust. How a given model is similar to that of the latest discussed in the context of regional Asian deformation. normal mode samples the structure of Scripps model (Johnson et al., 1994). MWR181, 66 p., hardbound, indexed, ISBN 0-8137-1181-9, $37.50 Earth is determined by kernels, which SKS12WM13 is based upon an inver- describe a mode’s sensitivity to com- sion of traveltimes and waveforms, but 1-800-472-1988 • fax 303-447-1133 pressional speed, shear speed, and it contains no normal-mode informa- density as a function of depth (Fig. 2). tion. It is remarkable how well this By combining the information con- model predicts the observed splitting. tained in all observable modes we can As far as these modes are concerned, GSA Publication Sales • P.O. Box 9140 • Boulder, CO 80301 • 303-447-2020 improve our knowledge about Earth’s interior. Splitting continued on p. 141

140 GSA TODAY, July 1995 Splitting continued from p. 140 Masters and Gilbert. By the time of the Bolivia earthquake, a collection of the long-wavelength heterogeneity in about 20 such anomalously split modes the mantle is quite accurately repre- had been identified (Ritzwoller et al., sented by current tomographic shear- 1986, 1988; Giardini et al., 1988; Li et speed models. al., 1991; Widmer et al., 1992). The Compressional-speed and density Bolivia and Kuril Islands earthquakes perturbations throughout the mantle have more than doubled the number of are commonly obtained by scaling available spectra, and the quality of shear-speed perturbations. Compres- the splitting measurements is much sional-speed perturbations are roughly improved. The number of anomalously half and density perturbations roughly split modes will undoubtedly continue one-third the size of shear-speed pertur- to grow. bations. The splitting calculations pre- Compressional waves traveling sented in this paper are based upon through Earth’s inner core are called such scaling relations. Most of the PKIKP waves (Fig. 4). In 1983, Poupinet modes shown in Figure 2 are predomi- et al. reported that PKIKP waves travel nantly sensitive to perturbations in several seconds faster along a trajectory shear speed, except for 0S5 and in par- parallel to Earth’s rotation axis than in ticular 4S3, which are also sensitive to the equatorial plane. Three years later, perturbations in compressional speed. Morelli et al. (1986) and Woodhouse The splitting predictions for such et al. (1986) introduced the concept compressional modes are generally not of inner-core anisotropy as an explana- quite as good as for shear modes. Split- tion for both inner-core traveltime and ting observations from the Bolivia and normal-mode anomalies. In an aniso- Kuril Islands earthquakes, such as those tropic medium, seismic wave speeds shown in Figure 2, should help deter- exhibit a directional dependence. In mine better scaling relations among the case of the inner core, waves travel shear speed, compressional speed, and faster in a direction parallel to the rota- density throughout the mantle. If tion axis than perpendicular to it. Earth’s lateral heterogeneity is a mani- Although this concept appears to have festation of purely thermal phenomena been generally accepted as an explana- associated with convection, we may tion for the directional dependence of expect a relatively uniform scaling rela- PKIKP traveltimes (Shearer et al., 1988; tion throughout the mantle. Nonuni- Shearer and Toy, 1991; Creager, 1992; form scaling as a function of depth Song and Helmberger, 1993, 1994; would indicate potential chemical het- Vinnik et al., 1994; Shearer, 1994; Su erogeneities. It is likely that such chem- and Dziewonski, 1995), there has been ical heterogeneities exist not only in considerable debate about the level and the shallowest regions of Earth’s man- radial distribution of anisotropy, and tle, but also in its deepest part, D". doubts have been raised as to whether inner-core anisotropy can explain the INNER-CORE STRUCTURE anomalous splitting of all currently identified core-sensitive modes Earth’s inner core was discovered (Widmer et al., 1992). 50 years ago by Inge Lehmann (1936). In 1993 I demonstrated that the It has an approximate radius of 1221 anomalous splitting of most inner-core km and represents less than 1% of sensitive normal modes may be ex- Earth’s volume. Notwithstanding its plained in terms of cylindrical anisot- small size, it appears to be one of the ropy (Tromp, 1993, 1995). Even though more intriguing and unusual regions my analysis did not include PKIKP trav- within Earth. eltime observations, my anisotropic In a depiction (Fig. 3) of several inner-core model makes reasonable splitting functions for inner-core sensi- traveltime predictions (Tromp, 1993; tive normal mode, although some of the splitting pattern is reasonably well predicted by SKS12WM13, a large zonal Splitting continued on p. 148 degree-two contribution—i.e., a banded signal parallel to the equator—is missing. This type of anomalous splitting is characteristic of most normal modes that are sensitive to Earth’s inner core. The first observations of anomalous splitting were reported in 1981 by

Figure 5. PKIKP traveltime observations and predictions in eight epicentral distance bins. The circles represent the average traveltime anomalies determined by Su and Dziewonski (1995). The solid line is the traveltime predic- tion based upon the anisotropic inner-core Figure 4. A: Ray geometry of a compressional model displayed in Figure 7. The parameter ξ inner-core (PKIKP) wave. B: The parameter ξ represents the angle between the direction of denotes the angle between the inner-core leg the inner-core leg of the PKIKP wave and the of the PKIKP wave and the symmetry axis of symmetry axis of the anisotropy, as shown in the cylindrical anisotropy; this axis more or Figure 5B. This symmetry axis nearly coincides less coincides with Earth’s rotation axis. The with Earth’s rotation axis. When cos2ξ = 0, inner-core ray segment of a PKIKP wave is PKIKP waves travel in the equatorial plane, nearly a straight line because the speed in the and when cos2ξ = 1, they travel parallel to the inner core is relatively uniform. symmetry axis.

GSA TODAY, July 1995 141 als from disparate though complemen- The Active Tectonics Planning Initiative tary fields. The proposed concept lies somewhere between that of an individ- George H. Davis, University of Arizona ual principal investigator and a consor- tium. Its compelling difference acknowledges that, with some excep- tions, individual investigators make A National Science Foundation– tributes directly to the national goal of part moves, or why one part might greater scientific contributions if they supported planning initiative in active mitigating geologic hazards. [Another] move, requires systems knowledge. The enlarge their range of expertise by tectonics has now been completed. objective … is to foster and facilitate 1994 Northridge earthquake, a case in collaborating with a few other experts Recommendations are presented in interdisciplinary collaborations among point, occurred along a fault that does from complementary fields in both for- “Active Tectonics and Society: A Plan subdisciplines such as experimental not even breach the surface, and yet mulating and executing research plans. for Integrative Science” (1995). This and theoretical rock mechanics, field the natural disaster created by move- Small teams of investigators can work report is available through the World studies of active and Neogene faults ment along this fault was the most especially effectively on integrated Wide Web at http://www.muohio. and the rock products of faulting, geo- expensive in our nation’s history. active tectonics science, addressing edu/tectonics/activetectonics.html. dynamic modeling in areas of active understanding of the inner workings deformation, seismology and earth- THE PLAN of interconnected active tectonic pro- ORIGIN OF THE INITIATIVE quake mechanics, quantitative geomor- Draft versions of the active tec- cesses. Those of us involved in this phology and paleoseismology.” The planning effort in active tectonics science plan evolved between planning process hope that Active Tec- tonics grew from two major studies, RESPONSE September 1993 and April 1995, with tonics and Society (1995) can be used Active Tectonics (1986) and Solid-Earth input from the national geoscientific to gain broader and deeper support Sciences and Society (1993), both sup- A 20-person planning team (see community. The draft versions were from federal and state agencies whose ported by the National Research Coun- list) met in September 1993, and began made available for inspection and scientific and public policy mission cil. Active Tectonics advocated the drafting an active tectonics science comment on Mosaic. The plan was objectives can be supported effectively need to build a national basic science plan in response to the call from ACES. presented in poster sessions at national by active tectonics research. For exam- initiative in active tectonics, emphasiz- Their long-range objective was to pro- and regional meetings of the Geologi- ple, the NSF Division of Earth Sciences ing both the scientific and strategic duce an active tectonics science plan cal Society of America and of the Amer- has now established Active Tectonics as value of this research. Mitigation of for concerted, integrated efforts to ican Geophysical Union. Special sym- a special emphasis area. The planning hazards related to great earthquakes, understand the fundamental processes posia highlighted opportunities in document provides strong scientific explosive volcanism, and major land- underlying the full range of deforma- active tectonics research, including an and societal justification for enhanced slides were recurrent themes. Solid- tional processes that currently shape AGU session convened by Robert B. national funding of active tectonics Earth Sciences and Society identified Earth’s lithosphere and that threaten Smith in December 1993 and a GSA research. Research grant funding drawn active tectonics as one of eight priority life and property. A major emphasis symposium convened by J.-Bernard from sources nationwide would enable themes representing critical opportuni- emerged: To understand how single Minster and me in October 1994. The the community to investigate the fun- ties for research in the coming decades, tectonic processes work, and how these plan elaborates on five basic themes: damental processes underlying the especially as related to important soci- processes link together through time as Integration of geology and geophysics, development and evolution of tectonic etal issues involving such matters as parts of larger systems that ultimately relevance to mitigation of tectonic haz- systems, including individual defor- the global environment, natural lead to the formation of mountain sys- ards, capturing tectonic processes in mational mechanisms. resources, and natural hazards. In April tems and the evolution of the solid action, exploiting the geologic record, Regardless of funding source, 1993, NSF’s Advisory Committee for Earth. and harnessing new technologies. active tectonics research proposals Earth Sciences (ACES) recommended Strategic applications are clear. For might profitably share some distinctive active tectonics as one of five high-pri- example, as the past decade of earth- PROPOSED DIRECTION elements: high potential for meaning- ority initiatives. The ACES report said, quakes in California has so dramati- ful integration of multiple disciplines; Active Tectonics and Society (1995) “We recommend an increased empha- cally demonstrated to the public, cities demonstrated capability of measuring places a high premium on planning sis on interdisciplinary research on and towns in tectonically active regions dynamic-earth properties that are and on collaborative team efforts. Cur- active crustal deformation. The overall are built not on single faults, but on directly relevant to elucidation of spe- rent collective scientific approaches to scientific objective of better under- complex systems with hundreds of cific tectonic processes; harnessing of understanding active tectonic processes standing how and why crustal stresses potential “moving parts,” any one of new technologies (such as GPS, GIS, fail to fill one important niche: the for- ultimately result in faulting, seismicity, which might reveal itself through a mation of fully integrated partnerships and rapid geomorphic change con- sudden shift. Understanding why one among independent teams of individu- Tectonics continued on p. 149

Splitting continued from p. 141 pressional wave, and through the inner Liu and A. M. Dziewonski. The remain- of the inner core, whereas from 175° to core as a shear wave. This body-wave ing signal, which consists mainly of 180° they travel close to Earth’s center. Shearer, 1994; Su and Dziewonski, arrival, named PKJKP, would put valu- a large degree-two zonal pattern, is From their analysis, Su and Dziewonski 1995). able additional constraints on the third inverted simultaneously with PKIKP determined that the symmetry axis In this paper I simultaneously model parameter σ, but it has never traveltime measurements obtained by of the anisotropy is tilted relative to invert inner-core traveltime observa- been unambiguously observed because Su and Dziewonski (1995) for inner- Earth’s rotation axis such that the pole tions and anomalous splitting observa- the transmission coefficient at the core anisotropy. Su and Dziewonski of the symmetry axis is located at 80°N, tions from the Bolivia and Kuril Islands inner-core boundary is very small. Peter collected more than 310,000 PKIKP 160°E. Because of the relatively inaccu- earthquakes for inner-core anisotropy. Shearer recently pointed out to me that arrival times reported by the Interna- rate splitting corrections for mantle The simplest type of anisotropy that PKJKP has become the Holy Grail of tional Seismological Centre in eight structure, normal-mode data do not exhibits cylindrical symmetry about body-wave seismology. epicentral distance bins that corre- reliably constrain a tilting of the sym- Earth’s rotation axis is transverse The normal-mode data used in the spond to all regions within the inner metry axis. Therefore, for the purposes isotropy. Let ξ denote the angle inversion consist of splitting functions core. The longer the distance between of this analysis, I will take Su and between the inner-core ray segment for 16 anomalous modes similar to the the epicenter and the station, the Dziewonski’s orientation of the sym- of a body-wave trajectory and Earth’s ones shown in Figure 3. These splitting deeper the PKIKP waves penetrate into metry axis for granted and correct the rotation axis, as shown in Figure 4B. functions are corrected for mantle con- the inner core, as shown in Figure 4A. normal-mode observations for this tilt. The directional dependence of the tributions based upon shear-speed For example, from 120° to 130° PKIKP Their averaged traveltime anomalies compressional speed v in a transversely model SKS12WM13 produced by X. F. waves sample the upper few kilometers are represented by the circles in Fig- isotropic inner core may be expressed ure 5. In each epicentral distance bin as (Su and Dziewonski, 1995) the traveltime anomaly relative to Figure 6. Anisotropic inner-core model PREM plus SH12WM13 (Su and v = v (1 + εcos2ξ + γ sin2 2ξ), (1) 0 obtained by the joint inversion of anomalous Dziewonski, 1994) is plotted as a func- splitting functions and PKIKP traveltime where v0 is the unperturbed isotropic tion of the angle ξ between the ray compressional wave speed in spherical anomalies. The elastic response of an isotropic direction and the symmetry axis; when medium is determined by two elastic parame- Earth model PREM. I seek to determine cos2ξ = 0, PKIKP waves travel in the ters: the shear modulus µ and the bulk modu- ε 2ξ the behavior of the two parameters lus κ. A transversely isotropic medium, on the equatorial plane, whereas when cos γ and as a function of radius in the other hand, has an elastic response that is = 1, they travel parallel to the symme- inner core. Although PKIKP traveltime governed by five elastic parameters: A, C, F, L, try axis. The fact that the traveltime anomalies are solely determined by the and N (Love, 1927). The parameters A and C anomalies are negligible in the epicen- compressional-speed distribution in the govern the compressional speed of seismic tral distance range 120°–130° indicates inner core, and hence by the radial waves, whereas the parameters L and N gov- that the source of the signal in subse- dependence of the two parameters ε ern the shear speed. The fifth parameter F quent bins is located in the inner core, and γ, some normal modes have sensi- influences the speed of both compressional rather than the outer core or mantle. tivity to the shear-speed structure of and shear waves. PKIKP traveltimes constrain The 1 s (second) offset is an artifact due just two combinations of the five elastic the inner core; this shear-speed sensi- 1 1 1 to the spherical reference model that parameters: ε = ⁄2(C – A)⁄A0, and γ = – ⁄4( ⁄2A tivity will be represented by the radial 1 4 was used in the analysis. The small + ⁄2C – 2L – F)⁄A0, where A0 = κ + ⁄3µ, is dependence of a third parameter, determined by the bulk and shear moduli at the center of spherical Earth model PREM. Normal anomalies in the epicentral distance σ 1 which I will call . Theoretically, there modes constrain one additional combination of the five elastic parameters: σ = ⁄2(L – N)⁄A0. This range 120°–140° are in agreement with exists a body wave that travels through third combination corresponds to shear anisotropy in the inner core. The behavior of the three the mantle and outer core as a com- parameters ε, γ, and σ is shown as a function of inner-core radius. Splitting continued on p. 149

148 GSA TODAY, July 1995 Tectonics continued from p. 148 ACTIVE TECTONICS: SCIENCE AND SOCIETY remote sensing, innovative dating tech- A comprehensive understanding of niques, radar interferometry); designa- the process of Earth’s deformation, and tion of an actively deforming region effective transfer of this understanding or subregion where dynamic activities to society, will help mitigate the can be observed or measured; ability to human and economic costs associated integrate models of tectonic processes; with the inevitable tectonic events that and potential for transfer to proactive mark our planet. The active tectonics societal outreach in such arenas as planning effort is designed to foster the hazards and environmental and global formulation of basic research proposals change. that help explain the fundamental ori- COORDINATION gins of tectonic deformation. The geo- OF THE INITIATIVE logical and geophysical communities represent the nation’s strategic intellec- An Active Tectonics Coordinating tual resource in explaining the origins Committee has been established to of tectonic hazards. Active tectonics foster and enhance communication, science is an arena in which geologists coordination, and integration among and geophysicists can simultaneously scientists engaged in active tectonics carry out basic science and serve soci- research, education, and service to society’s needs. We expect that enhanced ety. Members are Kerry Sieh (California communication and integration Institute of Technology), chair; Susan among active tectonics scientists will Beck (University of Arizona), vice-chair result in breakthrough-quality basic sci- in charge of highlighting key scientific ence, more effective transfer of knowl- issues; Mark Brandon (Yale University), edge, and exceptional opportunities for coordinator of annual planning work- professional growth. A likely byproduct shops; Mike Bevis (University of is accelerated integration of geology Hawaii), coordinator of training work- and geophysics within college and uni- shops; Rick Allmendinger (Cornell Uni- versity curricula, thus preparing the versity), coordinator of international way for more effective earth science relations; Roy Dokka (Louisiana State education for the next generation. University), coordinator of societal relations; and Larry Mayer (Miami Univer- ACKNOWLEDGMENTS Membership of the Active Tectonics Planning Committee sity), coordinator of the Mosaic hub. I thank the members of the Active George H. Davis, Chair Eugene Humphreys Kerry Sieh OPPORTUNITIES Tectonics Planning Committee for the University of Arizona University of Oregon California Institute of TO GET INVOLVED wisdom, energy, and integrity with Michael B. Bevis Arch Johnston Technology which they carried out their work; Roy University of Hawaii Center for Earthquake Robert B. Smith If you are interested in becoming Dokka for joining me in the final writ- Ronald G. Blom Information, Memphis University of Utah involved in some aspect of working on ing efforts and for taking chief respon- Jet Propulsion Laboratory Larry Mayer John Sutter the initiative, please contact Kerry Sieh sibility for illustrations, design work, Mark Brandon Miami University U.S. Geological Survey, at [email protected]. There and layout of Active Tectonics and Yale University Thorne Lay Reston is a lot of work to go around, especially Society; Larry Mayer for, especially, William B. Bull University of California, Jan Tullis Santa Cruz Brown University during the launch year of this coopera- establishing Active Tectonics on University of Arizona tive initiative. The coordinating com- Mosaic; and Lori Stiles, University of Darrel Cowan Marcia McNutt Robert S. Yeats Massachusetts Institute of Oregon State University mittee seeks to establish a core group Arizona science reporter, for help in University of Washington Technology Mary Lou Zoback of 50 or more people distributed the preparation of this article. ■ Roy K. Dokka Louisiana State University J.-Bernard Minster U.S. Geological Survey, among several working committees, Scripps Institution of Menlo Park both to further develop the initiative Jonathan Fink Technology Arizona State University and to increase its impact.

Splitting continued from p. 148 lies in the epicentral distance range model that fits both the traveltime speed model, whereas the anomalously 175°–180° can be as large as 4 s. Such data and the anomalously split modes split modes are PKIKP-equivalent—i.e., the observations of Shearer (1994) and large traveltime anomalies were also reasonably well (Fig. 6) is quite similar compressional—modes), but it may Song and Helmberger (1994) and sug- observed by Vinnik et al. (1994) and to the model obtained by Su and also indicate that the inner-core gest relatively small anisotropy near suggest strong anisotropy near Earth’s Dziewonski (1995) by an inversion of anisotropy does not exhibit pure cylin- the ICB. Notice that traveltime anoma- center. The anisotropic inner-core PKIKP traveltimes. The fit to the travel- drical symmetry. The traveltime vari- time data is represented by the solid ance reduction relative to PREM plus line in Figure 5. The inner-core model SH12WM13 is 92%, and the normal- slightly overpredicts the traveltime mode variance reduction compared to anomalies in the epicentral distance a model incorporating only the effects range 130°–140°, and it slightly under- of rotation and ellipticity is 70%. It predicts the traveltime anomalies for should be noted that one anomalously the near polar paths in the epicentral split mode, 13S2, is poorly fit by the distance range 160°–173°. The observed inner core model displayed in Figure 6; and predicted traveltime anomalies in thus far, the splitting of this mode has the epicentral distance range 150°–153° never been explained satisfactorily in are in general agreement with differen- terms of inner-core anisotropy. tial traveltimes between rays turning in the liquid outer core, PKP(BC), and rays CONCLUSIONS turning in the solid inner core, PKIKP The 1994 Bolivia and Kuril Islands or PKP(DF) (Creager, 1992; Song and earthquakes have provided the seismo- Helmberger, 1993). Normal modes logical community with a large number have very little sensitivity to structure of high-quality splitting observations. near Earth’s center. Therefore, the large An analysis of some of Earth’s mantle- anisotropy in the center of the inner sensitive spheroidal free oscillations core is entirely constrained by the indicates that especially the splitting traveltime data in the epicentral dis- of shear-sensitive modes is very well tance bins 165°–173° and 173°–180°. explained by current tomographic Predicted splitting functions of several models of the mantle. The first anomalously split modes (Fig. 7) toroidal-mode splitting observations are based upon mantle model were reported by Tromp and Zanzerkia SKS12WM13, and the anisotropic (1995), and the first coupled-mode inner-core model shown in Figure 6. inversions for odd structure were Inner-core anisotropy provides the performed by Rosovsky and Ritzwoller large zonal degree-two signal that is (1995). missing in the predicted splitting in Both the anomalous splitting of Figure 7. Observed and predicted splitting functions of several anomalously split normal Figure 3. Clearly not all the remaining inner-core sensitive modes and anoma- modes. The splitting predictions are based upon mantle model SKS12WM13 (X. F. Liu and A. M. signal is explained. This unexplained Dziewonski) and the anisotropic inner-core model shown in Figure 6. See the caption of Figure 2 signal may reflect errors in the mantle for further details. model (the mantle model is a shear- Splitting continued on p. 150

GSA TODAY, July 1995 149 GSA SECTION MEETINGS — 1996 GSA ANNUAL MEETINGS

Call for Papers 1995 SOUTH-CENTRAL SECTION ROCKY MOUNTAIN SECTION New Orleans, Louisiana March 11–12, 1996 April 18–19, 1996 November 6–9 University of Texas, Austin, Texas Rapid City Civic Center, Rapid City, South Dakota Ernest N. Morial Abstract Deadline: Convention Center, November 20, 1995 Abstract Deadline: Hyatt Regency New Orleans January 5, 1996 Submit completed abstracts to: Call for Papers: April and June GSA Today William F. Mullican Submit completed abstracts to: Abstract Deadline: July 12 Bureau of Economic Geology Alvis L. Lisenbee Preregistration Deadline: September 29 University of Texas Deparment of Geology and Technical Program Schedule: September GSA Today University Station Box X Geological Engineering Austin, TX 78712 South Dakota School of Mines (512) 471-1534 and Technology EGISTRATION AND HOUSING MATERIALS [email protected] 501 East St. Joseph St. R Rapid City, SD 57701-3995 APPEAR IN JUNE GSA TODAY (605) 394-2463 REGISTER TODAY! SOUTHEASTERN SECTION CORDILLERAN SECTION March 14–15, 1996 April 22–24, 1996 Ramada Plaza Hotel, Jackson, Mississippi Red Lion Hotel at Lloyd Center, Portland, Oregon 1996 Abstract Deadline: November 15, 1995 Abstract Deadline: Denver, Colorado December 28, 1995 October 28–31 Submit completed abstracts to: Darrel Schmitz Submit completed abstracts to: Colorado Convention Center Department of Geosciences Richard Thoms Marriott City Center Mississippi State University Department of Geology General Chairs: Gregory S. Holden and Kenneth E. Kolm, Colorado School of Mines P.O. Box 5448 Portland State University Technical Program Chairs: John D. Humphrey and John E. Warme, Misssissippi State, MS 39762 P.O. Box 751 Colorado School of Mines (601) 325-2904 Portland, OR 97207-0751 Call for Field Trip Proposals: Please contact the Field Trip Chairs listed below. (503) 725-3379 Charles L. Pillmore, Ren A. Thompson U.S. Geological Survey, MS 913, P.O. Box 25046 NORTHEASTERN SECTION NORTH-CENTRAL SECTION Denver Federal Center, Denver, CO 80225 March 21–23, 1996 May 2–3, 1996 phones: Charles L. Pillmore, (303) 236-1240; Ren A. Thompson (303) 236-0929 Hyatt Regency, Buffalo, New York Iowa State University, Ames, Iowa

Abstract Deadline: Abstract Deadline: November 20, 1995 January 17, 1996 CALL FOR CONTINUING EDUCATION Submit completed abstracts to: Submit completed abstracts to: Charles E. Mitchell Kenneth E. Windom COURSE PROPOSALS Department of Geology Department of Geological and PROPOSALS DUE BY DECEMBER 1 University at Buffalo Atmospheric Sciences 876 Natural Science and Iowa State University The GSA Committee on Continuing Education invites those interested Mathematics Complex 253 Science I Building in proposing a GSA-sponsored or cosponsored course or workshop to contact Buffalo, NY 14260-3050 Ames, IA 50011-3210 GSA headquarters for proposal guidelines. Continuing Education courses may (716) 645-6800 (515) 294-2430 be conducted in conjunction with all GSA annual or section meetings. We are [email protected] [email protected] particularly interested in receiving proposals for the 1996 Denver Annual Meeting or the 1997 Salt Lake City Annual Meeting. ✁ Proposals must be received by December 1, 1995. Selection of courses for 1996 will be made by February 1, 1996. For those planning ahead, we will also 1996 ABSTRACT FORM REQUEST consider courses for 1997 at that time. To: GSA Abstracts Coordinator, P.O. Box 9140, Boulder, CO 80301-9140 For proposal guidelines or information contact: or E-mail: [email protected] Edna A. Collis Continuing Education Coordinator,GSA headquarters Please send _____ copies of the 1996 GSA abstract form. I understand 1-800-472-1988, ext. 134 • E-mail: [email protected] that the same form may be used for all 1996 GSA meetings — (the six Section Meetings and the Annual Meeting).

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Splitting continued from p. 149 level that is in accordance with the seeing a snapshot of the internal next several years close collaboration seismological observations. If we are dynamics of the inner core. Karato between mineral physicists, geodynam- lous PKIKP traveltime observations willing to accept that the predominant (1993) proposed that the hcp iron min- icists, and seismologists will, one may be explained in terms of cylindri- mineral in the inner core is hcp iron, erals are oriented by Earth’s magnetic hopes, uncover more of the mysteries cal anisotropy of Earth’s inner core. this raises questions about the mecha- field. The feasibility of this mechanism of this fascinating region of Earth’s This robust observation has important nism for the preferential alignment of critically depends on the magnetic interior. implications for the composition, these minerals. Only if most of the susceptibility of iron under core condi- growth, and internal dynamics of the minerals are more-or-less aligned does tions. In this case, the three-dimen- ACKNOWLEDGMENTS inner core. First of all, we need to deter- the aggregate behave as an anisotropic sional distribution of anisotropy would Supported by the David and Lucille mine the mineralogy of Earth’s core. body; a random orientation would reflect the history of the geometry of Packard Foundation and by the Recent theoretical calculations by result in a seismically isotropic inner the magnetic field at or near the inner- National Science Foundation. I thank Stixrude et al. (1994) and Stixrude and core. Jeanloz and Wenk (1988) and core boundary. A third possibility is the Guy Masters for providing Figure 1C, Cohen (1995) confirm earlier calcula- Wenk et al. (1988) suggested that this preferred orientation of iron minerals and Wei-Jia Su for assistance with Fig- tions by Brown and McQueen (1985) preferred orientation is a result of due to rotation and self-gravitation ures 4A and 5. Wei-Jia Su and Adam which suggest that the inner core con- degree-one convection in the solid during the solidification and growth Dziewonski provided their PKIKP trav- sists of hexagonally close-packed (hcp) inner core. Much as in the mantle, this of the inner core. At the moment, it is iron. Under inner-core conditions, this convection would be a manifestation not at all unlikely that the inner core phase of iron exhibits anisotropy on a of thermal effects, and we would be is one giant single crystal. During the Splitting continued on p. 151

150 GSA TODAY, July 1995 CLASSIFIED ADVERTISING Mt. Eden Books Published on the 1st of the month of issue. Ads (or The suerpvisor must also direct students who are vice activities of the Geology/Geophysics and Geog- cancellations) must reach the GSA Advertising office assigned to help operate the system. In addition to raphy Departments in College Station. The Associate & one month prior. Contact Advertising Department the knowledge needed to keep our system running, Dean is expected to encourage interactions between (303) 447-2020, 1-800-472-1988, fax 303-447-1133, we are looking for a person with a good knowledge the research programs and the acadmeic depart- or E-mail:[email protected]. Please include of computer applications in the geosciences. 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Splitting continued from p. 150 Karato, S., 1993, Inner core anisotropy due to Shearer, P. M., Toy, K. M., and Orcutt, J. A., 1988, Tromp, J., 1995, Normal mode splitting due to magnetic field-induced preferred orientation of Axi-symmetric earth models and inner core inner core anisotropy: Geophysical Journal Inter- iron: Science, v. 262, p. 1708–1711. anisotropy: Nature, v. 333, p. 228–232. national , v. 121, p. 963–968. eltime anomalies, and Xiang-Feng Liu and Adam Dziewonski provided their Lay, T., and Wallace, T. C., 1995, Modern global Song, X. D., and Helmberger, D. V., 1993, Tromp. J. and Zanzerkia, E., 1995, Toroidal split- seismology: New York, Academic Press (in press). Anisotropy of the Earth’s inner core: Geophysical ting observations from the great 1994 Bolivia and latest mantle model. Göran Ekström Research Letters, v. 20, p. 2591–2594. Kuril Islands earthquakes: Geophysical Research developed the initial software for edit- Lehmann, I., 1936, P': Bureau Centrale Seismo- Letters (in press). loguique Internationale, Travaux Scientifiques, Song, X. D., and Helmberger, D. V., 1994, Depth ing the data and answered many ques- v. A14, p. 3–31. dependence of inner core anisotropy: Eos (Trans- Vinnik, L., Romanowicz, B., and Dreger, L, 1994, tions about time-series analysis. John actions, American Geophysical Union), v. 75, fall Anisotropy in the inner core from the broadband He assisted in editing the data and Li, X.-D., Giardini, D., and Woodhouse, J. H., meeting suppl., p. 71. records of the GEOSCOPE network: Geophysical 1991, Large-scale three-dimensional even-degree Research Letters, v. 21, p. 1671–1674. making splitting measurements. I structure of the Earth from splitting of long- Stixrude, L., and Cohen, R. E., 1995, High pressure thank Adam Dziewonski, Wei-Jia Su, period normal modes: Journal of Geophysical elasticity of iron and anisotropy of Earth’s inner Wenk, H.-R., Takeshita, T., Jeanloz, R., and John- and Göran Ekström for numerous dis- Research, v. 96, p. 551–557. core: Science (in press). son, C. G., 1988, Development of texture and elastic anisotropy during deformation of hcp met- cussions; IRIS, the USGS, and Geoscope Love, A. E. H., 1927, A treatise on the theory of Stixrude, L., Cohen, R. E., and Singh, D. J., 1994, als: Geophysical Research Letters, v. 15, p. 76–79. for providing the data used in this elasticity (4th edition): Cambridge, United King- Iron at high pressure: Linearized augmented plane dom, Cambridge University Press. wave computations in the generalized gradient Widmer, R., Masters, G., and Gilbert, F., 1992, study; and two anonymous reviewers approximation: Physical Review, v. 50, p. 6442. Observably split multiplets—Data analysis and for their helpful suggestions and Masters, G., and Gilbert, F., 1981, Structure of interpretation in terms of large-scale aspherical the inner core inferred from observations of its Su, W. J., and Dziewonski, A. M., 1994, Degree comments. structure: Geophysical Journal International, spheroidal shear modes: Geophysical Research 12 model of shear velocity heterogeneity in the v. 111, p. 559–576. Letters, v. 8, p. 569–571. mantle: Journal of Geophysical Research, v. 99, REFERENCES CITED p. 6945–6980. Woodhouse, J. H., Giardini, D., and Li, X. D., Morelli, A., Dziewonski, A. M., and Woodhouse, 1986, Evidence for inner core anisotropy from Brown, J. M., and McQueen, R. G., 1986, Phase J. H., 1986, Anisotropy of the inner core inferred Su, W. J., and Dziewonski, A. M., 1995, Inner core splitting in free oscillation data: Geophysical transitions, Grüneisen parameter, and elasticity from PKIKP travel times: Geophysical Research anisotropy in three dimensions: Journal of Geo- Research Letters, v. 13, p. 1549–1552. for shocked iron between 77 GPa and 400 GPa: Letters, v. 13, p. 1545–1548. physical Research (in press). Journal of Geophysical Research, v. 91, Manuscript received January 12, 1995; Poupinet, G., Pillet, R. R., and Souriau, A., 1983, Tromp, J., 1993, Support for anisotropy of the ■ p. 7485–7494. accepted April 1, 1995 Possible heterogeneity of the Earth’s core deduced Earth’s inner core: Nature, v. 366, p. 678–681. Creager, K. C., 1992, Anisotropy of the inner core from PKIKP travel times: Nature, v. 305, from differential travel times of the phases PKP p. 204–206. and PKIKP: Nature, v. 356, p. 309–314. Ritzwoller, M., Masters, G., and Gilbert, F., 1986, Dziewonski, A. M., and Anderson, D. L., 1981, Observations of anomalous splitting and their Preliminary Reference Earth Model: Physics of interpretation in terms of aspherical structure: Earth and Planetary Interiors, v. 25, p. 297–356. Journal of Geophysical Research, v. 91, p. 10,203–10,228. Giardini, D., Li, X.-D., and Woodhouse, J. H., 1987, Three-dimensional structure of the Earth Ritzwoller, M., Masters, G., and Gilbert, F., 1988, from splitting in free oscillation spectra: Nature, Constraining aspherical structure with low-degree GSA Today ADS v. 325, p. 405–411. interaction coefficients: application to uncoupled multiplets: Journal of Geophysical Research, v. 93, Giardini, D., Li, X.-D., and Woodhouse, J. H., p. 6369–6396. 1988, Splitting functions of long-period normal modes of the Earth: Journal of Geophysical Rosovsky, J. S. and Ritzwoller, M. H., 1995, Con- ads are Research, v. 93, p. 13,716–13,742. straining odd-degree Earth structure with coupled free-oscillation overtones: Geophysical Research GET Jeanloz, R., and Wenk, H.-R., 1988, Convection Letters (in press). and anisotropy of the inner core: Geophysical Research Letters, v. 15, p. 72–75. Shearer, P. M., 1994, Constraints on inner core Online anisotropy from ISC PKP(DF) data: Journal of Johnson, S., Masters, G., Shearer, P., Laske, G., Geophysical Research, v. 99, p. 19,647–19,660. and Bolton, H., 1994, A shear velocity model of the mantle: Eos (Transactions, American Geophys- Shearer, P. M., and Toy, K. M., 1991, PKP(BC) To access GSA Today on the World Wide Web RESULTS ical Union), v. 75, fall meeting suppl., p. 475. versus PKP(DF) differential travel times and aspherical structure in the Earth’s inner core: Jour- use the Universal Resource Locator (URL): nal of Geophysical Research, v. 96, p. 2233–2247. http://www.aescon.com/geosociety/pubs/index.html GSA TODAY ADVERTISING and then select GSA Today. 1-800-472-1988 Each month, GSA Today features a short science article on fast-breaking items or Classified ads are located on the last page; current topics of general interest to the 15,000 members of GSA. What do you think P.O. BOX 9140 display ads appear throughout the issues. BOULDER, CO 80301 of these articles? Do you have an idea for an article that you would like to see pub- 303-447-2020, FAX 303-447-1133 lished in GSA Today? If so, please contact Eldridge Moores, Science Editor, GSA Today, (916) 752-0352, fax 916-752-0951.

GSA TODAY, July 1995 151