Anatolian tectonic plate, north anatolian fault, historical earthquakes Greece, Attica, Earthquakes, Tsunami, , Hurricanes, Volcanic Eruptions and other Natural and Man-Made Hazards and Disasters - by Dr. George Pararas Carayannis

 

Tsunami, Earthquakes, Hurricanes, Volcanic Eruptions and other Natural and Man-Made Hazards and Disasters

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Development of a Model for the Real Time Evaluation of the Tsunami Risk

George Pararas-Carayannis

(Excerpts from Paper presented at the The International Association for the Physical Sciences of the Oceans (IAPSO) Conference in Honolulu, Hawaii, August 1995)

Abstract


The real time assessment of the tsunami risk from large Pacific-rim earthquakes is not always possible because the dynamic interactions of major and minor crustal plates and source mechanisms for tsunami generation are not well established or understood. The tectonic interactions of crustal subplates that have caused recent destructive earthquakes and local tsunamis have distinctively different seismic source characteristics than those of major crustal plates which involve extensive subduction and are responsible for destructive Pacific-wide tsunamis. Determination of earthquake parameters of Richter and moment magnitude, as well as epicenter and focal depth is not adequate seismic information to differentiate and evaluate, in real-time, the tsunami risk potential from all large earthquakes.

Using the Japan/Kuril islands region as an example, this paper reviews, compares and evaluates the seismic and hydrodynamic source parameters of recent and historical earthquakes, the tectonics of the region, the complex dynamic source interactions responsible for destructive tsunami generation and the conventional tsunami risk assessment methods. Finally, the present study proposes a model for a more accurate, real-time, evaluation of tsunami generation and tsunami risk for the source area as well as for distant terminal points.

The proposed model would provide a real-time analysis of periods and patterns of unaltered, long seismic waves and trends of first motions of such waves, to deduce earthquake characteristics at the source region through the development of a pattern recognition methodology. The latter can be developed from comparison of long period waves of an actual earthquake with mathematically equivalent theoretical radiation patterns of simulated sources and analytical solutions of all possible hypothetical torque couples embedded in an unruptured elastic medium. These analytical torque couples, simulating numerous theoretical deformation models, rotating in opposite directions, would deform the medium thus radiating elastic waves in patterns identical or similar with those in which an earthquake source radiates seismic waves. By using a number of such pre-solved, theoretical-analytical models, the pattern of the actual seismic radiation pattern from an earthquake source could be compared to arrive at a fairly reasonable source mechanism for the actual event. This could include dimensions of the source area, orientation and length of faulting, degree and percentage of vertical subduction, energy transfer to the overlying elastic medium, energy radiation and other pertinent source data.

Thus deduced source parameters could serve independently to assess qualitatively, in real-time, the relative tsunami risk from the earthquake or they could be used as input to existing hydrodynamic models to provide quantitative estimates of the distribution of tsunami energy flux and expected tsunami run-up at terminal points. An integrated computer model with algorithms can be developed which can provide, in real time, fairly good evaluation of the tsunami potential and possible tsunami risk for the source area as well as for the rest of the Pacific Ocean. In addition to the benefits in the real time assessment of the tsunami risk for warning purposes, the proposed model would help in the understanding of earthquake-tsunami focal mechanisms. Furthermore, the proposed model could become a useful research tool which could be applied to evaluate the plate and subplate tectonic interactions of different regions of the world.

Introduction

The real time assessment of the tsunami risk from large Pacific-rim earthquakes is not always possible because the dynamic interactions of major and minor crustal plates and source mechanisms for tsunami generation are not well established or understood. The tectonic interactions of crustal subplates that have caused recent destructive earthquakes and local tsunamis have distinctively different seismic source characteristics than those of major crustal plates which involve extensive subduction and are responsible for destructive Pacific-wide tsunamis. Determination of earthquake parameters of Richter and moment magnitude, as well as epicenter and focal depth is not adequate seismic information to differentiate and evaluate, in real-time, the tsunami risk potential from all large earthquakes.

In assessing the tsunami risk for the region the present study reviews historical tsunamis and makes comparisons of seismological and hydrodynamic parameters and reviews the dynamics of tectonic interactions that are responsible for tsunami generation. Finally the study provides a quantitative estimate of risk for each subsection of the region.

Historical Earthquakes and Tsunamis in the Japan/Kurile Island Region

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Subduction along the Japanese Trench is responsible for many large historical earthquakes and catastrophic tsunamis in the immediate region of Japan/Kuril islands with minor damaging effects elsewhere in the Pacific. Tectonic movements in the East Sea/Sea of Japan, Sea of Okhtsok and inland seas have been responsible for large earthquakes and tsunamis but their effects have been confined to the region.

The Earthquakes of October 4, 1994 and January 17, 1995

Two major and very destructive earthquakes ocurred in the region in October 4, 1994 and on January 17, 1995. The October 4, 1994 earthquake generated a small tsunami which was damaging in Japan and the Kuril islands but did not pose a Pacific-wide threat. The January 17, 1995 earthquake produced no noticeable tsunami. A Pacific-wide Tsunami Warning was issued by PTWC at great cost to the Pacific community on October 4, 1994.


Comparison of Similarities of the 1963 and 1994 Earthquakes and Tsunamis


The historical record shows that this event was very similar in magnitude and epicenter to two quakes which occurred on October 13, and October 19, 1963, from the same exact region (Northern Hokkaido, Kuril islands). for which two Pacific-wide tsunami warnings were issued then. Neither of these events generated a Pacific-wide damaging tsunami in Hawaii or elsewhere.

A simple reference to the historical Tsunami data Tsunamis in the Hawaiian Islands, would have revealed that there has never been a Pacific-wide tsunami threat, even from the largest earthquakes in the Northern Hokkaido, Kuril Islands region; only local damaging tsunamis have been generated. Table 1 provides a comparison between the earthquakes and tsunamis of October 4, 1994 and October 13, 1963.



Table 1. Comparison of Similarities of the 1963 and 1994 Earthquake and Tsunami Events (Re: Catalog of Tsunamis in the Hawaiian Islands/Pacific Catalog)

Oct 12,1963 :

Magnitude 8.2 , Epicenter 44.8 N, 149.5 E
Depth: 33 km
Time 05:18 UTC

(Area Affected by local damaging tsunamis: South Kuril Islands (Urup, Shikotan,Kunashir, Urup, Iturup, Paramushir, etc )

Japan: Hanasaki, Kushiro, Hachinohe,Chichijima

Highest Recorded at Midway 1.03 meters adjusted to 0.6 meters peak to trough
Highest in Hawaii: Kahului 0.4 meters adjusted
to 0.8 meters peak to trough

PTWC issued Pacific-wide Tsunami Warning

Oct 4, 1994 :

Magnitude 8.2 Epicenter 43.668 N, 147.333 E
Depth: 33 km
Time 0:323 UTC

(Area Affected by local damaging tsunamis: South Kuril Islands (Shikotan,Kunashir,Iturup, etc.)

Japan: Hanasaki, Kushiro,Hachinohe,Chichijima

Highest Recorded at Midway I. 0.54 meters (peak to trough)
Highest in Hawaii: Kahului 0.8 meters (peak to trough)

PTWC issued Pacific-wide Tsunami Warning

There are similarities in magnitude, and epicenter location. There is overlap in tsunami generation area. Tsunami heights and damage in immediate area are very similar. Both tsunamis recorded similarly at the Midway Island. The Kahului tide gauge recordings were the highest for both events in the Hawaiian islands. Although it is only a coincidence, both events occurred in the same month in October and even at approximately the same UTC time.

Table 2 , provides a similar comparison for the major aftershocks of the two events.


Table 2. Comparison of the two major aftershocks of the October 4,1994 and October 19, 1963 earthquakes and tsunamis.


Oct 19,1963:

Magnitude 6,75-7 Epicenter 44.7 N , 150.7 E
Depth: 33 km
Time 00:53 UTC

(Area Affected by small observe or recorded local tsunamis: South Kuril Islands (Urup, Shikotan,Kunashir, Urup, Iturup, Paramushir)
Japan: Recorded or observed at Hanasaki, Kushiro, Hachinohe,Chichijima

Highest Recorded at Midway I. 0.2 meters adjusted to 0.4 meters peak to trough
Highest in Hawaii: Kahului 0.4 meters adjusted to 0.8 meters peak to trough

PTWC issued Pacific-wide Tsunami Warning

Oct 9, 1994:

Magnitude 7.2 Epicenter Same approximate location as major quake
Depth: Shallow
Time 0756 UTC

(Area probably affected by local tsunamis: South Kuril Islands, unknown height but expected to be small

Japan: Hanasaki (9cm), Kushiro (3cm),

Highest Recorded at Midway I.: unknown
Highest in Hawaii: unknown
PTWC did not issue a Watch or a Warning.




The Great Sanriku Earthquake and Tsunami of 1933


Even the great Sanriku earthquake of 1933 did not generate a tsunami that was of any consequence elsewhere in the Pacific. The 1933 tsunami had originated from the Sanriku area, but was considerably south and westward of the October 4, 1994 quake's generating area. The epicenter of the 1933 earthquake had been at 39.1 N, 144.7 E,, in a region of much greater vertical subduction along the Japanese Trench, near the island of Honshu and approximately 300 nautical miles south and 120 nautical miles westward of the October 4, 1994 event. Furthermore, the orientation of the fault zone and the tsunami generating area for the 1933 earthquake were different; greater amount of energy radiated unobstructed towards Hawaii. Crescent City, California, recorded the largest tsunami oscillation (1.1 meters), anywhere in the Pacific from that event even with the more optimum tsunami energy orientation effect from that source.



Plate Tectonics of the Japan/Kuril Island Region



The plate tectonics of the Southern Kuril islands-Northern Hokkaido region are quite different than those near Honshu or the islands near the southern portion of the Japanese Trench. Specifically, the October 4, 1994 earthquake (and the 1963 earthquakes) ocurred at the Pacific side boundary of a smaller tectonic subplate which includes the Sea of Okhotsk and perhaps a portion of the northern part of the East Sea/Sea of Japan. This subplate is characterized with large earthquakes such as the 1963 and 1994 events but with lesser vertical subduction and rotational movement (possibly counterclockwise) as the North Pacific Plate grinds against it. The whole area appears to be highly fractured in an east-west direction and the crustal displacements appear to be ocurring along the boundaries of subplates that may not be longer than 200-250 miles. The fractured smaller plates along the northern part of the Japanese Trench limit the extent of crustal displacements and therefore the size of the resulting tsunami. The historical record supports this as well. This is the reason why very large magnitude earthquakes from that region produce only locally catastrophic tsunamis.

The East Sea/Sea of Japan represents another subplate with some subduction and possible clockwise or counterclockwise rotational movement as it interacts against the Okhotsk plate, along the inland sea boundary. The 1983 earthquake and tsunami in the East Sea/Sea of Japan was along the boundary of these interacting subplates. Thus, the mechanisms and tectonic interactions of subplates that caused both the 1983 earthquake and Sea of Japan tsunami and the October 4, 1994 earthquake and tsunami, are different than those of other typical subductions caused by the Pacific plate movement along the northern part of the Japanese Trench. This appears to hold true for other regions of the Pacific.

Development of a Model for the Real Time Evaluation of the Tsunami Risk

With proper applied type of research and analysis, the source mechanisms of large earthquakes can be analyzed in real time to assess the relative tsunami risk at distant terminal points. Similarly, a methodology can be developed which may provide in real time even quantitative estimates of tsunami run-up for warning purposes. Unecessary tsunami warnings can be eliminated through such real-time assessment

Presently, the only data available within the time constraints of a potential warning decision is some basic seismic data. Such data is simply not sufficient. Tsunami risk evaluation for warning decisions, must concentrate in the real time understanding of the earthquake-tsunami focal mechanisms and evaluation of the seismic parameters. For example we now use seismic moment (Mo) as the more reliable method for measuring the energy and magnitude of the larger earthquakes which saturate the Richter scale. Why not extend these measurements for further assessment of earthquake source parameters and mechanism, in real time?

The same measurements that provide an estimate of the moment magnitude can be used to study other earthquake source parameters. For example, since the seismic moment is related, not only to the size of the earthquake but also to the overall deformation at the source and to the fundamental parameters of the faulting process, why not use these measurements to determine indirectly earthquake source mechanism that can help with the real time tsunami evaluation?

Proposed Methodology and Model Development for the Real Time Assessment of Earthquake Source Parameters.

We know that the amplitude of every long period seismic wave is proportional to the surface area of the fault, to the average displacement on the fault plane, and to the rigidity of the material at the fault. In fact, using such measurements of the long-period seismic waves we arrive at the seismic moment calculation. However we can go a step further and use the long period waves in a different way. For example, we do not smooth out the details of the rupture through a Fourier analysis of the long period waves and we do not treat the entire fault as a point-source. Instead we examine and analyze the periods and patterns of the unaltered seismic long waves and trends of first motions (from one or more stations) to understand earthquake source characteristics. A pattern recognition methodology can be developed for long period seismic waves that can become a useful tool in the real time assessment of earthquake source parameters and subsequent tsunami risk evaluation. Research could begin by looking at such recordings of long period waves of major historical earthquakes

Furthermore, the following can be done. We know that the periods of the seismic waves increase with the size of the fault. Additionally, we assume that the slip from unruptured to ruptured state along an earthquake fault to be instantaneous. In reality it doesn't happen that way, but it is a reasonable approximation to make since the rupture rate is very fast. Seismic moment is calculated by comparing the actual pattern of the seismic radiation emitted by such instantaneous rupture to a mathematically equivalent theoretical radiation pattern emitted by two hypothetical torque couples embedded in an unruptured elastic medium. The torque couples, rotating in opposite directions, deform the medium thus radiating elastic waves in a pattern identical with that in which an earthquake source radiates seismic waves.Since the seismic moment is calculated by comparison to this theoretical deformation model, it seems to me that it would be possible to develop numerous theoretical deformation models, involving different torque couples which simulate a number or possible earthquake sources, mechanisms and solutions Then, by using a number of pre-solved, theoretical-analytical models, we could compare the pattern of the actual seismic radiation from an earthquake source as revealed by the long period waves and recorded at a seismic station, to the pre-solved analytical solutions to arrive at a fairly reasonable source mechanism for the actual earthquake. This could include dimensions of the source area, orientation of faulting, degree of vertical subduction, energy transfer to the overlying elastic medium, energy radiation and other pertinent source data.

Once the earthquake source characteristics and the displacement field are approximated, the Tsunami source mechanism, and the approximate source energy passing into a gravity wave can be deduced from the earthquake source parameters. Then, the gravity wave can be propagated with existing hydrodynamic models to provide estimates of expected tsunami runups at terminal points. Furthermore, historical earthquake and tsunami data could be used for calibration purposes of such models.

Similarly, short period seismic waves and first motions from seismometers and accelerometers near an earthquake source could be used in a similar fashion to provide some assessment for tsunami warning purposes. A lot can be learned from studies of historical records. Alternatively, telemetering data to PTWC could be used in real time in a computer model to arrive at focal plane solution for a given earthquake. The difficulty of this approach would not be the development of the computer model but in the difficulty and expense of placing instruments and telemetering the data.

In conclusion, a series of such integrated computer models and algorithms could be developed through appropriate research that would provide, in real time, a fairly good evaluation of the tsunami potential and possible tsunami height predictions for warning purposes. Only if such work is done tsunami evaluation for warning purposes will be possible - Otherwise, it will continue to be a hit or miss proposition, same as it was in 1963 and, obviously, at the present time.

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