Which type of seismic sensors usually used in Earthquake Early Warning? As I'm a newbie in seismology and related area, I would like to know which type of seismic sensors usually used in earthquake early warning systems?
Accelerometers, preferably triaxial (i.e., those that record horizontal and vertical motion). Beyond that your question is indeed a bit too open-ended. Some use many cheap MEMS (http://onlinelibrary.wiley.com/doi/10.1029/2008GL036572/epdf; https://www.theverge.com/circuitbreaker/2017/7/13/15963996/raspberry-pi-seismograph-early-warning-earthquakes; these are similar to those found in phones), some use more expensive but reliable equipment (https://kinemetrics.com/post_products/episensor-es-t/)...