Summary
- KDDI’s 7:30 p.m. update on 28 July added full-mode JAPAN Roaming in affected parts of Yatsushiro and Hitoyoshi, Kumamoto Prefecture.
- au, UQ mobile, povo and MVNO services using KDDI’s network were unavailable or difficult to use, including emergency calling.
- KDDI attributed the disruption to earthquake-related base-station power failures and failures on transmission routes to base stations.
- Where a handset displays JPN-ROAM, the service offers voice including emergency calls, SMS and data capped at 300kbps in each direction over another carrier’s 4G LTE network.
- Android users may need to enable data roaming, and some devices may require manual network selection; the notice does not report usage volumes or successful-session rates.
Emergency roaming is designed for a moment when the ordinary network cannot be trusted. In Kumamoto, Japan’s cross-carrier arrangement reached that moment.
The earthquake occurred at about 4:27 p.m. Japan time. The official seismic record gives magnitude 7.1 at a depth of roughly 10 kilometres, while the government reported maximum seismic intensity 7 in Hikawa Town, Uki City and strong shaking across a broad part of Kyushu.
That event preceded this reporting window by about 66 minutes. The qualifying news event is KDDI’s 7:30 p.m. update: the operator had made full JAPAN Roaming available in affected parts of two cities.
A national design produced a local route
Japan’s five mobile carriers launched the framework in March for service availability from 1 April 2026. It allows temporary use of another carrier’s 4G LTE network when a major disaster or outage damages a subscriber’s primary network.
The framework is nationwide in design. This activation was not nationwide in geography. KDDI named parts of Yatsushiro and Hitoyoshi, and users still depended on donor-network radio coverage at their location.
That distinction matters. “Nationwide emergency roaming” describes the shared operating contract; it does not mean every handset across Japan began roaming or that every street in the two cities had an alternative signal.
The full mode preserves a narrow communications floor
Full mode supports ordinary voice, emergency calls, SMS and data at up to 300kbps in both directions. It is meant to preserve essential transactions, not recreate the damaged operator’s normal 4G or 5G experience.
Three hundred kilobits per second can carry text, a basic web page or lightweight coordination. It is not an unrestricted broadband promise. Congestion, radio conditions and application behaviour can reduce what users actually complete.
The arrangement also has an emergency-calls-only mode, although KDDI’s update identified full mode for this activation. Which mode is chosen depends on damage, affected users and infrastructure conditions.
A visible network name is only the first operational step
KDDI instructed users to look for JPN-ROAM. Android data use may require enabling data roaming, and some devices can require manual network selection. Compatibility and software state can therefore stand between a person and the alternative route.
These steps are understandable in a service that must bridge separate carrier systems, but they create a usability test under stress. A customer dealing with shaking, power loss or an urgent call may not know which setting to change.
Future design work should minimise configuration and make the safety message unmistakable. In a disaster, the best fallback is the one a person can enter without a manual.
Availability is not session success
The KDDI notice establishes that roaming was activated and describes its service contract. It does not report how many devices attached, which donor networks they used, how many emergency calls completed, the SMS delivery rate or data-session performance.
It also does not specify the percentage of either city covered at each moment. The presence of an alternative route is important evidence of institutional readiness. It is not yet evidence of restoration for every affected user.
The next useful measures are registration success by device type, time to attach, completed emergency-call rate, SMS delay, data task completion, load on donor cells and the number of users who needed manual intervention.
Resilience depends on failure domains, not brand boundaries
KDDI said earthquake-related loss of power at base stations and failures in transmission routes caused the disruption. Emergency roaming can bypass a failed operator access path only where another carrier retains power, backhaul and radio capacity.
If several carriers share the same physical route, power supply or damaged area, commercial independence may not create technical diversity. The resilience value therefore depends on mapping common failure domains and reserving enough donor capacity for a sudden influx.
The Kumamoto activation is significant because a policy mechanism became an operating route in a real disaster. Its success should now be judged at the completed-call and completed-message level, not by activation alone.