THE RUPTURE PROCESS OF RECENT  TSUNAMIGENIC EARTHQUAKES BY GEOPHYSICAL DATA INVERSION

Subduction zones are the favorite places to generate tsunamigenic earthquakes, where 
friction between oceanic and continental plates causes the occurrence of a strong 
seismicity. The topics and the methodologies discussed in this thesis are focussed to the 
understanding of the rupture process of the seismic sources of great earthquakes that 
generate tsunamis.  
The tsunamigenesis is controlled by several kinematical characteristic of the parent 
earthquake, as the focal mechanism, the depth of the rupture, the slip distribution along 
the fault area and by the mechanical properties of the source zone. Each of these factors 
plays a fundamental role in the tsunami generation. Therefore, inferring the source 
parameters of tsunamigenic earthquakes is crucial to understand the generation of the 
consequent tsunami and so to mitigate the risk along the coasts. 
The typical way to proceed when we want to gather information regarding the source 
process is to have recourse to the inversion of geophysical data that are available. 
Tsunami data, moreover, are useful to constrain the portion of the fault area that extends 
offshore, generally close to the trench that, on the contrary, other kinds of data are not 
able to constrain. 
In this thesis I have discussed the rupture process of some recent tsunamigenic events, as 
inferred by means of an inverse method.  
I have presented the 2003 Tokachi-Oki (Japan) earthquake (Mw 8.1). In this study the 
slip distribution on the fault has been inferred by inverting tsunami waveform, GPS, and 
bottom-pressure data. The joint inversion of tsunami and geodetic data has revealed a 
much better constrain for the slip distribution on the fault rather than the separate 
inversions of single datasets.  
Then we have studied the earthquake occurred on 2007 in southern Sumatra (Mw 8.4). 
By inverting several tsunami waveforms, both in the near and in the far field, we have determined the slip distribution and the mean rupture velocity along the causative fault. 
Since the largest patch of slip was concentrated on the deepest part of the fault, this is the 
likely reason for the small tsunami waves that followed the earthquake, pointing out how 
much the depth of the rupture plays a crucial role in controlling the tsunamigenesis.  
Finally, we have presented a new rupture model for the great 2004 Sumatra earthquake 
(Mw 9.2). We have performed the joint inversion of tsunami waveform, GPS and satellite 
altimetry data, to infer the slip distribution, the slip direction, and the rupture velocity on 
the fault. Furthermore, in this work we have presented a novel method to estimate, in a 
self-consistent way, the average rigidity of the source zone. The estimation of the source 
zone rigidity is important since it may play a significant role in the tsunami generation 
and, particularly for slow earthquakes, a low rigidity value is sometimes necessary to 
explain how a relatively low seismic moment earthquake may generate significant 
tsunamis; this latter point may be relevant for explaining the mechanics of the tsunami 
earthquakes, one of the open issues in present day seismology. 
The investigation of these tsunamigenic earthquakes has underlined the importance to use 
a joint inversion of different geophysical data to determine the rupture characteristics.  
The results shown here have important implications for the implementation of new 
tsunami warning systems – particularly in the near-field – the improvement of the current 
ones, and furthermore for the planning of the inundation maps for tsunami-hazard 
assessment along the coastal area