I wonder if an earthquake of that magnitude completely saturates the accelerometers in the phones and that leads to either them being mistakenly dismissed as an error or the system fails to solve the location and magnitude and doesn't send out an alert without having that solved (e.g. without location and magnitude they can't decide on the alert radius). Obviously there needs to be a generic large radius warning "Impending large earthquake possible in your area" when a certain threshold of devices all peak out.