can you give an idea of a good quality and reliable vibration sensor that detects the vibration of the ground for landslide monitoring?
A Google search suggests that geologists use 'extensometers' rather than vibration sensors.
Talk to a geologist or a structural engineer.
If you are doing it on the cheap then a laser tape measure may work. You will need to compensate for drift on a daily basis due to the small (1mm) movements involved.
thanks 
At the risk of diverting readers from the wonderful world of Arduino, if you want to have fun with earthquakes/ landslides, consider....
http://qcn.stanford.edu/index.php
If you are "deserving", they'll loan you what you need. The rest of us: For $50, they'll sell you a USB device to plug into your computer, and then you become part of a Stanford University/ BOINC project to collect earthquake data. The software runs in the background on your PC (Linux/ Windows/ Mac), as in the "good old days" of the SETI project, for instance.
No Arduino... but a well developed "answer".