Emile A. Okal
 Professor
Ph.D., California Institute of Technology, 1978
[847] 491-3238
emile@earth.northwestern.edu
 Personal page here.

Research Interests
 Seismology, structure of the Earth, marine geophysics, low-frequency acoustics, tsunamis.


THE 2004 SUMATRA EARTHQUAKE:

The Earth's normal modes reveal a slow component which makes the event

MUCH BIGGER THAN REPORTED


FIELD WORK: Surveying the Sumatra tsunami in
Réunion, Rodrigues
Madagascar
Oman



Detection of PKJKP

In 1998, and in collaboration with Y. Cansi (Commissariat à l'Energie Atomique, Paris), we have succeeded in detecting the seismic phase PKJKP which crosses the Earth's inner core as a shear wave, on records of the deep 1996 Flores Sea, Indonesia earthquake. This observation brings direct, irrefutable evidence for the solidity of the innermost part of the planet Earth.

Research Projects

1. Regional Structure under a Large Igneous Province [E.A. Okal, W.P. Richardson; B.M. Gomer]
[Figure 1] In 1994-96, with former Graduate Student Philip Richardson, we ran a deployment of 4 portable seismic stations on the islands of Chuuk, Pohnpei, Kosrae and Nauru. The plateau is believed to be a large plume head erupted in less than 3 m.y. in the Cretaceous, but its precise structure had never been investigated. Figure 1 summarizes the layout of the experiment. Results from this campaign confirmed a 25-35 km thick crust with significant lateral variations. However, by far the most interesting and totally unexpected result was the identification of a large root of slow material resolvable to depths reaching 300 km and featuring 5% deficiency in shear wave velocity (Figures 2 and 3).



[Figure 3]
Former student Brandon Gomer has used multiple-ScS waves reflected under the plateau (Figure 4) to confirm the presence of a root of slow material, but has revealed very low anelastic attenuation (QScS = 366), thus excluding a thermal origin to the root. This then requires a model in which the Plateau would have dragged along a chemically differentiated keel, as it moved over the mantle with the rest of the Pacific plate, during the past 120 million years.





2. Long-Period Seismology and Tsunami Risk [E.A. Okal, C. Synolakis (USC); D. Reymond (Tahiti); S. Kirby, G. Plafker (Menlo Park)]

We continue our collaboration with scientists worldwide on various aspects of tsunami risk.

Following the catastrophic tsunami in Papua New Guinea (PNG) on 17 July 1998, I participated in the post-tsunami field survey, during which we mapped the inundation of the tsunami wave along the coast of PNG (Figure 5).

[Figure 5]

The combination of an earthquake with no slowness anomaly, and extreme wave heights (up to 15 m) on a very concentrated stretch of coast line (25-30 km at most) suggest that the tsunami was generated by an underwater slump triggered by the earthquake, a conclusion later upheld by a marine survey, and by records of T waves propagated in the water column of the ocean, which identify an anomalous event 13 minutes after the mainshock (Figure 6).
Hydrodynamic modeling can explain the main characteristics of the local inundation, only when a slump source is used (Figure 7).

In the context of the generation of tsunamis by landslides, I led a survey to the island of Fatu-Hiva (Marquesas), which in 1999 was the site on a mini-tsunami, that destroyed a number of structures on the water front and severely damaged the school, while miraculously taking no lives. The tsunami was due to the collapse of a volcanic cliff, 5 km from the village.

In July 2001, I led the International Tsunami Survey Team to Southern Peru, in the wake of the destructive tsunami which followed the earthquake of 23 June 2001.

In collaboration with scientists in the US, Polynesia and France, we have reopened the case of the 1946 Aleutian tsunami, one of the strongest transpacific tsunamis of the 20th century. It cannot be satisfactorily modeled by a standard dislocation and thus may have involved a significant underwater slump. In the Summer of 2000, I led a field survey in the Marquesas, where interviews of survivors of the tsunami allowed the precise mapping of its run-up and inundation, which will be inverted into an appropriate source model. Additional surveys were taken in November 2000 on the Chilean Islands of Easter (Rapa Nui) and Robinson Crusoe (Juan Fernández group) and in December 2001 in the Polynesian Austral Islands (Rurutu and Tubuai). In August 2001, we surveyed watermarks in the epicentral area on Unimak and Sanak Islands, in the vicinity of the famous site at Scotch Cap, where the lighthouse was destroyed at an altitude of 10 m.

These various events underscore the hazard posed by aerial and underwater landslides in the generation of tsunamis. I have started theoretical work aimed at understanding the characteristics of landslides (as opposed to dislocation sources) for the generation of tsunamis.

On the theoretical side, I have started a project aimed at identifying robust discriminants between dislocations and landslides as sources of tsunamis. In the near field, we use the aspect ratio of the distribution of run-up along a beach, which is shown to remain bounded (at approximately 10-4 ) by the maximum permissible strain release in a seismic dislocation.


In the field of tsunami warning, and in collaboration with our colleagues in Tahiti, we developed several years ago the TREMORS automated detection system, which uses the mantle magnitude Mm. This system allows the automatic estimation of the moment of a distant earthquake, and thus of teleseismic tsunami risk. It has now been implemented in various locations (Hawaii, Indonesia, Chile).


Recently, we have focused on the concept of seismic energy, which can be measured in real time from body waves.
[Figure 8] The combination of seismic moment and energy can then be used to identify in real time anomalous events, such as the so-called "tsunami earthquakes", whose slow source makes them particularly dangerous in terms of tsunami generation, and which have long been a challenge in tsunami warning. Our results indicate that slow earthquakes, such as the 1992 Nicaragua, 1994 Java, and 1996 Peru "tsunami earthquakes" (in red on Figure 8) exhibit an energy deficiency of 1 to 2 orders of magnitude, with respect to what their seismic moment would predict.
The energy algorithm is being implemented for use in real-time in Tahiti, and at the Pacific Tsunami Warning Center in Honolulu.
Present research (in collaboration with Dr. Kirby) is exploring the case of earthquakes (such as the 2001 El Salvador, Seattle, and Hiroshima earthquakes), occurring inside a slab subducting young lithosphere, and which exhibit faster, "snappier" than normal strain release, as evidenced by a high energy-to-moment ratio. We also aim at extending the use of the algorithm to historical events.

Finally, in collaboration with Dr. Reymond, we have developed a method for the inversion of surface wave amplitude data (as provided by TREMORS) into a source moment tensor, based on datasets of very limited size, as can be transmitted in real-time following a major earthquake recorded world-wide. We are presently adapting this method to the case of historical earthquakes.

3. Deep Seismic Sources and Structures [E.A. Okal, P.-F. Chen, C.R. Bina; S.H. Kirby (Menlo Park); G. Ekström (Harvard)]

Following similar work on deep historical earthquakes, we have conducted a systematic recompilation of the moments of intermediate-depth earthquakes, motivated by the short time span of the Centroid Moment Tensor catalogue, and the poor reliability of traditional magnitudes for deep events. In collaboration with our Harvard colleagues, Graduate Student Po-Fei Chen has inverted 76 new solutions for events in 1962-76, which will complement the Harvard CMT catalogue for that period.
In collaboration with Dr. Bina, Po-Fei is also examining the relationship between volcanism and intermediate-depth seismicity in South America, in the framework of the P-T paths of dehydration reactions.

Following our observation of intense T waves throughout the Pacific, generated by the great 1994 deep Bolivian earthquake, we have systematically studied T waves from deep earthquakes, notably in the case of the so-called "detached" events, occurring a few hundred km in fornt of regular Benioff zones. In most cases, we conclude that a mechanically and thermally (cold) continuous path exists from the detached event to the ocean column at the surface, allowing a successful S-to-T conversion (Figure 9). However, with Dr. Kirby, we have documented the existence of a severed piece of slab under the Fiji Plateau, lying recumbent on the bottom of the transition zone.

With Dr. Bina, we continue to investigate aspects of the cessation of seismicity at the bottom of the transition zone (about 670 km), and have speculated on the possible level of seismicity at greater depths, which would still be compatible with the observation of less than one event since the development of adequate instrumentation in the early 1960s.



4. Marine Low-frequency Acoustics [E.A. Okal; J. Talandier, L. Géli (France); O. Hyvernaud, D. Reymond (Tahiti)]
We study the structure and characteristics of high-frequency seismic and acoustic waves in the marine environment. In particular, we have investigated acoustic waves of exceptionally monochromatic nature (Figure 10) radiated into the Pacific ocean by volcanic sources; we speculate that they express the resonance of bubbly columns of water generated by ex-solution of volatiles during certain forms of underwater eruptions.

[Figure 10]

Following our study of a remarkable episode of such activity in 1991-1992, a 1996 cruise of the French Research Vessel L'Atalante has identified a unique, young and largely unsuspected volcanic stucture, the Hollister Ridge, more than 400 km in length and topping only 135 m b.s.l., in the area of the Eltanin Fracture Zone (Far Southern Pacific; 54°S, 140°W).
These results were discussed briefly in Scientific American's August 1997 issue.

More recently, we have detected yet a new kind of largely monochromatic signals recorded as T phases in Polynesia, which emanate from gigantic icebergs calved off the Ross Sea Ice shelf in 2000. The exact nature of the resonator responsible for these signals, and of their triggering mechanism remains unresolved. These results were described in Science News dated 04 May 2002.

In the context of the monitoring of the Comprehensive Test-Ban Treaty, we study the mechanisms of conversion of acoustic energy in the ocean to and from seismic energy either generated by earthquakes, or recorded by seismic stations in the vicinity of shorelines. In collaboration with Dr. Talandier, we have also developed a discriminant allowing the identification of the source of T waves recorded by seismic stations located on oceanic islands (Figure 11).

Bibliography, 2001 to present

Okal, E.A., T-phase stations for the International Monitoring System of the Comprehensive Nuclear-Test Ban Treaty: A global perspective, Seismol. Res. Letts., 72, 186-196, 2001.

Okal, E.A., and C.R. Bina, The deep earthquakes of 1997 in Western Brazil, Bull. Seismol. Soc. Amer., 91, 161-164, 2001.

Okal, E.A., and A.R. Langenhorst, Reply [to comment by P. Wessel and L.W. Kroenke], Phys. Earth Planet. Inter., 123, 81-83, 2001.

Klosko, E.R., R.M. Russo, E.A. Okal, and W.P. Richardson, Evidence for a rheologically strong chemical mantle root beneath the Ontong-Java Plateau, Earth Planet. Sci. Letts., 186, 347-361, 2001.

Okal, E.A., Converted T phases recorded on Hawaii from Polynesian nuclear tests: A preliminary report, Pure Appl. Geophys., 158, 457-474, 2001.

Talandier, J., and E.A. Okal, Identification criteria for sources of T waves recorded in French Polynesia, Pure Appl. Geophys., 158, 567-603, 2001.

Okal, E.A., and C.E. Synolakis, Comment on "Origin of the 17 July 1998 Papua New Guinea tsunami: Earthquake or landslide?" by E.L. Geist, Seismol. Res. Letts., 72, 363-366, 2001.

Chen, P.-F., M. Nettles, E.A. Okal, and G. Ekström, Centroid Moment Tensor solutions for intermediate-depth earthquakes of the WWSSN-HGLP era (1962-1975), Phys. Earth Planet. Inter., 124, 1-7, 2001.

Okal, E.A., and A.V. Newman, Tsunami earthquakes: The quest for a regional signal, Phys. Earth Planet. Inter., 124, 45-70, 2001.

Okal, E.A., "Detached" deep earthquakes: Are they really?, Phys. Earth Planet. Inter., 127, 109-143, 2001.

Chen, P.-F., C.R. Bina, and E.A. Okal, Variations in slab dip along the subducting Nazca plate, as related to stress patterns and moment release of intermediate-depth seismicity, and to surface volcanism, Geochem., Geophys., Geosyst., 2, Paper Number 2001GC000153, 18 pp., 2001 [Electronic journal].

Synolakis, C.E., J.-P. Bardet, J.C. Borrero, H.L. Davies, E.A. Okal, E.A. Silver, S. Sweet, and D.R. Tappin, The slump origin of the 1998 Papua New Guinea tsunami, Proc. Roy. Soc. (London), Ser. A., 458, 763-789, 2002.

Okal, E.A., J.C. Borrero, and C.E Synolakis, Solving the puzzle of the 1998 Papua New Guinea tsunami: The case for a slump, in: Solutions to Coastal Disasters, Ed. by L. Wallendorf and L. Ewing, Amer. Soc. Civil Eng., pp. 863-877, 2002.

Hébert, H., A. Piatanesi, P. Heinrich, F. Schindelé, and E.A. Okal, Numerical modeling of the September 13, 1999 landslide and tsunami on Fatu Hiva island (French Polynesia), Geophys. Res. Letts., 29, (10), 122_1-122_4, 2002.

Okal, E.A., C.E. Synolakis, G.J. Fryer, P. Heinrich, J.C. Borrero, C. Ruscher, D. Arcas, G. Guille, and D. Rousseau, A field survey of the 1946 Aleutian tsunami in the far field, Seismol. Res. Letts., 73, 490-503, 2002.

Okal, E.A., G.J. Fryer, J.C. Borrero, and C. Ruscher, The landslide and local tsunami of 13 September 1999 on Fatu-Hiva (Marquesas Islands; French Polynesia), Bull. Soc. Géol. France, 173, 359-367, 2002.

Okal, E.A., L. Dengler, S. Araya, J.C. Borrero, B. Gomer, S. Koshimura, G. Laos, D. Olcese, M. Ortiz, M. Swensson, V.V. Titov, and F. Vegas, A field survey of the Camaná, Peru tsunami of June 23, 2001, Seismol. Res. Letts., 73, 904-917, 2002.

Talandier, J., O. Hyvernaud, E.A. Okal, and P.-F. Piserchia, Long-range detection of hydroacoustic signals from large icebergs in the Ross Sea, Antarctica, Earth Planet. Sci. Letts., 203, 519-534, 2002.

Okal, E.A., and S.H. Kirby, Energy-to-moment ratios for damaging intraslab earthquakes: Preliminary results on a few case studies, USGS Open File Rept., 02-328, 127-131, 2002.

Langenhorst, A.R., and E.A. Okal, Correlation of beta-value with spreading rate for strike-slip earthquakes of the Mid-Oceanic Ridge system, Amer. Geophys. Un. Geodyn. Monog., 30, 191-202, 2002.

Stein, S., G. Sella, and E.A. Okal, The January 26, 2001 Bhuj earthquake and the diffuse boundary of the Indian plate, Amer. Geophys. Un. Geodyn. Monog., 30, 243-254, 2002.

Okal, E.A., P.-J. Alasset, O. Hyvernaud, and F. Schindelé, The deficient T waves of tsunami earthquakes, Geophys. J. Intl., 152, 416-432, 2003.

Reymond, D., O. Hyvernaud, J. Talandier, and E.A. Okal, T-wave detection of two underwater explosions off Hawaii on April 13, 2000, Bull. Seismol. Soc. Amer., 93, 804-816, 2003.

Dengler, L., J. Borrero, G. Gelfenbaum, B. Jaffe, E. Okal, M. Ortiz, and V. Titov, Tsunami, in: Southern Peru earthquake of 23 June 2001, Reconnaissance Report, Ed. by A. Rodriguez-Marek and C. Edwards, Earthquake Spectra, 19, Supp. A., 115-144, 2003.

Okal, E.A., G. Plafker, C.E. Synolakis, and J.C. Borrero, Near-field survey of the 1946 Aleutian tsunami on Unimak and Sanak Islands, Bull. Seismol. Soc. Amer., 93, 1226-1234, 2003.

Borrero, J.C., J. Bu, C. Saiang, B. Uslu, J. Freckman, B. Gomer, E.A. Okal, and C.E. Synolakis, Field survey and preliminary modeling of the Wewak, Papua New Guinea earthquake and tsunami of September 9, 2002, Seismol. Res. Letts., 74, 393-405, 2003.

Bardet, J.-P., C.E. Synolakis, H.L. Davies, F. Imamura, and E.A. Okal, Landslide tsunamis: Recent findings and research directions, Pure Appl. Geophys., 160, 1793-1809, 2003.

Okal, E.A., T waves from the 1998 Papua New Guinea earthquake and its aftershocks: Timing the tsunamigenic slump, Pure Appl. Geophys., 160, 1843-1863, 2003.

Okal, E.A., and C.E. Synolakis, Theoretical comparison of tsunamis from dislocations and landslides, Pure Appl. Geophys., 160, 2177-2188, 2003.

Okal, E.A., Normal modes energetics for far-field tsunamis generated by dislocations and landslides, Pure Appl. Geophys., 160, 2189-2221, 2203.

Gomer, B.M., and E.A. Okal, Multiple-ScS probing of the Ontong-Java Plateau, Phys. Earth Planet. Inter., 138, 317-331, 2003.

Okal, E.A., and D. Reymond, The mechanism of great Banda Sea earthquake of 01 February 1938: Applying the method of Preliminary Determination of Focal Mechanism to a historical event, Earth Planet. Sci. Letts., 216, 1-15, 2003.

Beutel, E.K., and E.A. Okal, Strength asperities along oceanic transform faults: A model for the origin of extensional earthquakes on the Eltanin Transform system, Earth Planet. Sci. Letts., 216, 27-41, 2003.

Talandier, J., and E.A. Okal, Hydroacoustic signals from presumed CHASE explosions off Vancouver Island in 1969-70: A modern perspective, Seismol. Res. Letts., 75, 188-198, 2004.

Okal, E.A., J.C. Borrero, and C.E. Synolakis, The earthquake and tsunami of 17 November 1865: evidence for far-field tsunami hazard from Tonga, Geophys. J. Intl., 157, 164-174, 2004.

Chen, P.-F., G. Ekström, and E.A. Okal, Centroid moment tensor solutions for Taiwan earthquakes of the WWSSN era (1963-1975), TAO: Terrestrial, Atmos. Ocean. Sci., 15, 61-73, 2004.

Okal, E.A., Comment on "Source of the great tsunami of 1 April 1946: a landslide in the upper Aleutian forearc", by G.J. Fryer et al., Marine Geology, 209, 363-369, 2004.

Okal, E.A., and C.E. Synolakis, Source discriminants for near-field tsunamis, Geophys. J. Intl., 158, 899-912, 2004.

Chen, P.-F., C.R. Bina, and E.A. Okal, A global survey of stress orientations in subducting slabs as revealed by intermediate-depth earthquakes, Geophys. J. Intl., 159, 721-733, 2004.

Zahibo, N., E. Pelinovsky, E.A. Okal, A. Yalçiner, C. Kharif, T. Talipova, and A. Kozelkov, The earthquake and tsunami of November 21, 2004 at Les Saintes, Guadeloupe, Lesser Antilles, Sci. Tsunami Haz., 23, 25-39, 2005.

Stein, S., and E.A. Okal, Size and speed of the Sumatra earthquake, Nature, 434, 581-582, 2005.

Okal, E.A., A re-evaluation of the great Aleutian and Chilean earthquakes of 1906 August 17, Geophys. J. Intl., 161, 268-282, 2005.

Chen, P.-F., C.R. Bina, and E.A. Okal, Erratum [to "A global survey of stress orientations in subducting slabs as revealed by intermediate-depth earthquakes"], Geophys. J. Intl., 161, 419, 2005.

Park, J., T.-R.A. Song, J. Tromp, E. Okal, S. Stein, G. Roult, E. Clévédé, G. Laske, H. Kanamori, P. Davis, J. Berger, C. Braitenberg, M. Van Camp, X. Lei, H. Sun, H. Xu, and S. Rosat, Long-period behavior of the 26 December 2004 Sumatra-Andaman earthquake from its excitation of the Earth's free oscillations, Science, 308, 1139-1144, 2005.

Weinstein, S.A., and E.A. Okal, The mantle wave magnitude Mm and the slowness parameter THETA: Five years of real-time use in the context of tsunami warning, Bull. Seismol. Soc. Amer., 95, 779-799, 2005.

Synolakis, C.E., and E.A. Okal, 1992-2002: Perspective on a decade of post-tsunami surveys, in: Tsunamis: Case studies and recent developments, ed. by K. Satake, Adv. Natur. Technol. Hazards, 23, pp. 1-30, 2005.

Synolakis, C.E., E.A. Okal, and E.N. Bernard, The megatsunami of December 26, 2004, The Bridge, 35, (2), 26-35, 2005.

Talandier, J., O. Hyvernaud, D. Reymond, and E.A. Okal, Hydroacoustic signals generated by parked and drifting icebergs in the Southern Indian and Pacific Oceans, Geophys. J. Intl., 165, 817-834, 2006.

López, A.M., and E.A. Okal, A seismological reassessment of the source of the 1946 Aleutian "tsunami" earthquake, Geophys. J. Intl., 165, 835-849, 2006.

Blewitt, G., C. Kreemer, W. Hammond, H.-P. Plag, S. Stein, and E.A. Okal, Rapid determination of earthquake magnitude using GPS for tsunami warning systems, Geophys. Res. Letts., 33, (11), L11309, 4 pp., 2006.

Okal, E.A., H.M. Fritz, R. Raveloson, G. Joelson, P. Pancoskova, and G. Rambolamanana, Madagascar field survey after the December 2004 Indian Ocean tsunami, Earthquake Spectra, 22, S263-S283, 2006.

Okal, E.A., H.M. Fritz, P.E. Raad, C.E. Synolakis, Y. Al-Shijbi, and M. Al-Saifi, Oman field survey after the December 2004 Indian Ocean tsunami, Earthquake Spectra, 22, S203-S218, 2006.

Okal, E.A., A. Sladen, and E.A.-S. Okal, Rodrigues, Mauritius and Réunion Islands, field survey after the December 2004 Indian Ocean tsunami, Earthquake Spectra, 22, S241-S261, 2006.

MacAyeal, D.R., E.A. Okal, R.C. Aster, J.N. Bassis, K.M. Brunt, L.M. Cathles, R. Drucker, H.A. Fricker, Y.-J. Kim, S. Martin, M.H. Okal, O.V. Sergienko, M.P. Sponsler, and J.E. Thom, Transoceanic wave propagation links iceberg-calving margins of Antarctica with storms in tropics and Northern hemisphere, Geophys. Res. Letts., 33, (17), L17502, 4 pp., 2006.

Okal, E.A., J.C. Borrero, and C.E. Synolakis, Evaluation of tsunami risk from regional earthquakes at Pisco, Peru, Bull. Seismol. Soc. Amer., 96, 1634-1648, 2006.


Full publication list, including papers in press and submitted.

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This page last updated: 09 OCT (282) 2006; 1423Z