Tsunami Occurrence Probabilities 



23. Historical data on tsunami generation must be the basis for an 

 analysis that considers the probability of tsunami generation along the 

 Aleutian Trench. A satisfactory correlation between earthquake magnitude and 

 tsunami intensity has never been demonstrated. Not all large earthquakes oc- 

 curring in the ocean even generate noticeable tsunamis. Furthermore, earth- 

 quake parameters of importance to tsunami generation, such as focal depth and 

 vertical ground motion, have been measured only for earthquakes occurring in 

 recent years. Therefore, data on earthquake occurrence cannot be used to 

 determine occurrence probabilities of tsunamis. Historical data of tsunami 

 occurrence generation regions must be used to determine these probabilities. 



24. The concept of tsunami intensity was put forth by Soloviev (1970). 

 He defined intensity as 



i . iog 2 (vTH avg ) m 



where H avg is the average maximum runup (in metres) observed along the 

 coastline adjacent to the source region. 



25. The standard assumption in both earthquake and tsunami analysis is 

 that the logarithm of the probability of occurrence of an event is linearly 

 related to its intensity. In the Aleutian Trench area, only large tsunamis 

 occurring since 1788 have been reliably documented. Assuming an exponential 

 coefficient of -0.71 for this trench area (Soloviev (1970) found this coef- 

 ficient to be the mean value for areas of the Pacific with the most data on 

 tsunamis) and using only the reliable data, Houston (1978) established the 

 following relation for the Aleutian Trench area: 



n(i) = 0.113 e" - 711 (5) 



where n(i) is the probability that, in any given year, a tsunami having in- 

 tensity i will occur somewhere along the Aleutian Trench. Equation 5 gives 

 the ordinates of a histogram where intensities have been grouped in increments 

 of one-half the unit intensity. 



26. Equation 5 was derived by considering tsunamis which occurred any- 

 where along the Aleutian Trench. As discussed in Part II, only tsunamis 



14 



