Android - Increase vibrate strength

AFAIK, the vibrator motor that are used in the phones are called Q-Coin motor as they are in a shape of a coin. They are of permanent magnet type accepting two leads for positive and negative DC voltages. The circuitry that operate this motor just can switch on the motor for a specific period of time and can change its rotation direction. All other parameters of vibration are set by motor's design.

Hence, for your need the amplitude of the vibrator motor has to be increased to make you aware. And unless the motor itself is replaced, this can't be done in my opinion.

This is a specification of such a motor and you can see it's details.

UPDATE: After searching Google, I stumbled on more info about these motors. With that information, it seems that there are two types of such motors. One being fitted with an eccentric rotating mass and another being a linear actuator (working more like a speaker). The article about linear actuator says that this is the present trend and beautifully explains how the motor works.

If the Nexus device that is in question is vibrating at its maximum amplitude, then there is no way any software component to increase its amplitude. Other way, if device is NOT vibrating at its maximum, then by increasing the applied voltage there is a possibility to increase the amplitude.

Thanks to @Konrad Rudolph who made me to know more about this motor.

Just install - Contact Vibrate

From the description:

Set a different vibration for each of your contacts and know who is it.

Set a different a vibration for each of your contacts

Know who is calling or texting you, just feeling it !
When you receive a phone call or a text message (SMS or MMS) Contact Vibrate will respond vibrating differently for every contact you have defined. If this particular number or contact has no vibration set a master vibration will be called by default.

No easy way - you'll need to install a custom kernel like GLaDOS or Trinity kernel and the relevant apps for each kernel from the Play Store to adjust settings - one of which will be the vibration intensity.

Check out the relevant threads at XDA.