I want to create a project using Arduino that monitor the Earthquake. I used an ADXL345 Accelerometer to detect if there is an earthquake. I used the Adafruit ADXL345 example to determine a acceleration of x-axis, y-axis and z-axis in terms of m/s^2. I want to used the Japan Meteorological Agency seismic intensity scale to determine the intensity level of the earthquake because it gives the Peak Ground Acceleration. If there is a way to determine the earthquake intensity level using ADXL345?
That sensor is not suitable. It has 13 bit resolution at +-16g full range. That gives you a sensitivity of 0.002g for the lsb. In order to detect an earthquake directly below you, you need approx. a few milli-g (e.g. see here), even less for earthquakes with an epicentre elsewhere.
You want a sensor which is much more sensitive by a factor of 100 and probably with more resolution (better ADC), too.
(And you should have been able to do this quick google-search analysis yourself ;) )