Emile A. Okal
Ph.D., California Institute of Technology, 1978
firstname.lastname@example.org Personal page here.
Seismology, structure of the Earth, marine geophysics, low-frequency acoustics, tsunamis.
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.
1. Regional Structure under a Large Igneous Province
[E.A. Okal, W.P. Richardson;
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).
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
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.
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
3. Deep Seismic Sources and Structures
C.R. Bina; S.H. Kirby (Menlo Park);
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.
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).
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
Earth Planet. Sci. Letts., 186, 347-361, 2001.
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.,
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.