Waking up with an alert just before a big earthquake

Early this morning, at around 5:30, I woke up with unfamiliar sound of alert. A few seconds later, everything started to shake with big noise. It was a M6.0 earthquake hit Awaji Island near Kobe. I got a phone call then from my working place for a request for an immediate checking of a machine there. I changed my clothes, washed my face in a few minutes and drove to the working place. After spending a busy quater day there checking and restarting for the normal operation, I found myself wondering about one thing: What was that sound of alert? TV was off and we have no radio in our house and all computers were shut off. Finally I found it was my iPhone5 which received a signal from J-alert system and made that sound of alert. This was very impressive incident.