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Does a tsunami warning system automatically issue an alert when it detects a volcanic collapse? Not necessarily. Indonesia’s InaTNT can detect sea-level anomalies and trigger alerts for operators, but the available system descriptions do not show that this automatically issues a public tsunami warning. Detection, an internal alert, an official warning decision, and public dissemination are separate steps.
Why volcanic tsunamis need a different detection path
Indonesia’s InaTEWS was designed around tsunamis caused by tectonic earthquakes. BMKG says that design could not detect the 2018 Sunda Strait tsunami in the usual way because it came from a non-tectonic source: the collapse of part of Anak Krakatau. BMKG characterizes the collapsed mountain-body area as 64 hectares; that is its description of the event, not a universally settled measurement of collapse volume.
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BMKG’s InaTNT background page puts the limitation plainly: “Practically, the BMKG InaTEWS (Indonesia Tsunami Early Warning System), which is designed to detect tsunamis due to tectonic earthquakes, is not able to detect tsunamis with such characteristics.” InaTNT is intended to add a sea-level-based monitoring path for tsunamis that may not begin with a detectable earthquake.
What InaTNT does when it detects an anomaly
1. Collects sea-level observations
BMKG describes InaTNT as integrating observations from tide gauges, buoys, HF radar, IDSL, and water-level devices at automatic weather stations. BRIN reported on September 7, 2026, that IDSL/PUMMA at Rakata was being used with BMKG as a link between PVMBG and BMKG for volcano-tsunami monitoring and early warning.
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2. Identifies unusual signals
InaTNT’s described detection method compares a short-term average with a long-term average (STA/LTA) to identify a departure from background sea-level conditions. If a threshold is exceeded, BMKG says the system treats that as an indication of a sea-level anomaly. An AIC picker then refines estimated arrival timing.
The public system description does not give the numeric thresholds, false-alarm performance, or the operational status and coverage of every data source. Those details matter: a detected anomaly is a signal for assessment, not by itself proof that a destructive tsunami is underway.
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3. Alerts operators
BMKG says a detection trigger can activate a sound alert in the InaTEWS operational room and a notification on its web display. That is an automated internal alert path. It does not establish that the same trigger automatically sends a public warning.
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4. Authorities decide whether and how to warn
The European Commission’s Joint Research Centre describes the Krakatau-area chain as real-time sea-level observations producing alerts to BMKG for further warning decisions and dissemination. In other words, the sensor and detection system can inform authorities; a public warning is a further decision-and-communication step.
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The descriptions available do not specify the complete rules for that step, including decision authority, timing criteria, or public-message procedures. They therefore support neither the claim that every detected anomaly becomes a public warning nor the claim that no algorithm can trigger any alert.
What is known—and not established—about the warning chain
| Stage | What the sources describe | What they do not establish |
|---|---|---|
| Sea-level observation | InaTNT is designed to integrate tide gauges, buoys, HF radar, IDSL, and automatic weather-station water-level devices. BRIN reported IDSL/PUMMA use at Rakata. | Station-by-station uptime and coverage. |
| Signal detection | InaTNT describes STA/LTA anomaly detection and AIC arrival-time picking. | Numeric thresholds and false-alarm performance. |
| Internal alert | BMKG says a trigger can sound an operations-room alert and display a web notification. | The current alert protocol and operator response. |
| Official warning and public dissemination | The Joint Research Centre says abnormal readings alert BMKG for further warning decisions and dissemination. | Exact decision authority, timing criteria, and public-message procedures. |
What recent Anak Krakatau reports do—and do not—say
In a report dated September 5, 2026, Indonesia’s Geological Agency placed Anak Krakatau at Level III (Siaga). It said the rebuilt volcano was relatively small and was not considered prone to a tsunami-generating collapse at that time, while noting that activity and morphology remained under intensive observation.
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BRIN reported on September 17, 2026, that monitoring during increased activity in early September had found no sea-level anomalies indicative of potential tsunami waves. That is a report about that monitoring period, not a forecast or a guarantee of safety. For conditions now, consult current updates from the relevant Indonesian authorities.
How emerging prediction research fits in
A 2025 Ocean Modelling study examined machine-learning prediction using the 2018 Anak Krakatau case. Its abstract reports that nine minutes was the optimal input duration for full-waveform prediction in that study. This is a research result, not an operational response time or evidence that a machine-learning algorithm is deployed to issue warnings.
The broader hazard is not limited to one volcano: a USGS seminar description dated March 18, 2026, gives a global estimate of 400–800 oceanic and coastal volcanic edifices with potential for future collapse and associated tsunamis. This is an estimate of potential hazards, not a count of imminent events.
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