This map issued Saturday shows the entire coast of Japan under tsunami alert, with red lines indicating waves more than 3 meters (about 10 feet).

As Japan staggers in the aftermath of the devastating quake that pummeled the northern half of the archipelago Friday, killing at least 200 and causing a nuclear emergency, the tech powerhouse's quake and tsunami warning systems predicted that the devastation would continue.

The stunning tsunami warning map above, issued by the Japan Meteorological Agency Saturday, shows the country's entire coastline under threat, with many areas on the Pacific side facing possible waves more than 3 meters (10 feet) tall.

The tsunami warning system worked Friday, with the agency alerting people to imminent tsunamis within three minutes of the quake, and the first waves struck 10 to 15 minutes later. The alert may have saved hundreds of lives, as some residents were able to flee to higher ground.

Japanese broadcasters issued automatic earthquake alerts by the agency predicting more aftershocks for Tokyo, and new, unrelated temblors for the Niigata and Nagano on the other side of the country by the Sea of Japan.

Japan straddles several tectonic plates and is one of the world's most quake-prone countries, with hundreds of tremors every year. The agency has had an Earthquake Early Warning service in place since 2007, issuing alerts to media outlets.

The system is triggered when the agency's myriad seismometers detect primary waves generated by earthquakes. If the tremors are expected to be strong enough, a public alert will be issued and relayed on broadcasters like NHK TV as well as by cell phone carriers.

But since seismic waves travel so quickly, viewers will have seconds at most to prepare. Meanwhile, public address systems in many Japanese communities can relay urgent info, or urge residents to conserve electricity, as they were doing in Tokyo on Saturday.

The quake warning system has had mixed results, failing to predict the massive tremor on Friday. It also failed to issue warnings for relatively strong quakes in 2007 and 2008 (in Kanagawa and Ishikawa prefectures, respectively) by underestimating the intensity.

In 2009, the system warned of a strong quake that threatened Tokyo, causing the subway to shut down. However, only a minor shaking was felt.

There's still little information on how the system dealt with the Friday earthquake, but the tsunami warning may have saved many lives. So far the death toll is relatively low for a quake of such intensity, but officials fear it may climb far higher.