Tsunami - to hit or not to hit..

I must admit that I did not notice the tremors that hit Colombo in the afternoon although most people had. I was at McDonalds having my lunch at the time, and when I stepped out I saw so many people gathered on the Galle road with their cellphones held to their ears. I was wondering about this strange behaviour when my mother called and told me about the earthquake and a possibility of a tsunami and asked me to come to her office at once. At this moment I just had a hunch that a tsunami was not going to hit although I had no information at that time whatsoever to come to such a conclusion.
The main shock had a Richter scale magnitude of 8.6 and caused widespread panic and also brought about the release of tsunami warnings in several countries including Sri Lanka. Having had an  extremely bad experience with the 2004 indian ocean tsunami these actions were appropriate. However even after the estimated arrival times had been issued, no tsunami arrived.
Earthquakes are not rare. Hundreds of earthquakes occur throughout the globe at any given time. Most of them are too small to even notice. The rare ones with the large magnitudes are the ones that cause trouble. The following diagram shows the data from USGS of some recorded earthquakes. In order to minimize the number of earthquakes displayed, only the details of earthquakes that are greater than 5.4 in magnitude in the region depicted in the map are given. (click on image to enlarge)

source - USGS -  http://earthquake.usgs.gov/earthquakes/map/

Note that in total 6985 earthquakes have been recorded in the past 30 days. The one that is highlighted is the 8.6 main shock that raised concerns of a tsunami. The rest are aftershocks. Clearly this earthquake took place very close to the epicenter of the 2004 one and therefore was expected to replicate the same effects, but it did not.

While Tsunamis can be generated by various methods such as earthquakes, landslides and even meteor strikes earthquake generated tsunamis are the most common, although the largest recorded tsunami was caused by a landslide at Lituya bay, Alaska (source : http://geology.com/records/biggest-tsunami.shtml). It must be noted however that not all earthquakes can cause a tsunami. Several conditions need to be fulfilled for the generation of a tsunami wave. They are as follows
1.  The earthquake should have a large magnitude ( greater than 7.0)
2.  The earthquake should be a shallow focus earthquake (focal depth less than 70km)
3.  The epicenter of the earthquake must be at an active plate margin near or in the ocean.
4.  There should be a vertical displacement of the plate (a thrust fault).
5.  The displacement should occur over a considerable area to displace a large volume of water.
Although not a hard and fast rule, these conditions need to be satisfied for a tsunami. The earthquake that hit today(11.04.2012) did not fulfill all these conditions.
Given below are the details of today's earthquake.

source - USGS -  http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/usc000905e.php 

While the earthquake had a large enough magnitude and a shallow focus, the earthquake occurred as a result of strike slip faulting and not thrust faulting (source : http://earthquake.usgs.gov/earthquakes/eventpage/usc000905e#summary) . Therefore there has not been a considerable vertical motion of the plates in order to displace a large volume of water. This was the reason why a devastating tsunami was not generated.
While we were lucky today and the warnings and preparation were seemingly unnecessary, we can now be assured that proper warnings will be given and that we are prepared to take necessary measures in the case of an actual threat.