Police and firefighters were at the resort north of Tokyo combing through an area where the volcanic rocks had rained down. They say their operation will resume on Thursday if the situation stabilizes.
The Meteorological Agency is keeping the alert level at 3 on a scale of 5, saying more eruptions may occur.
On Tuesday, one person died after being struck by volcanic debris and 11 others were injured.
Dozens of people stuck near the summit had to be rescued by helicopters and snowmobiles. Meanwhile, the agency is taking heat for not immediately issuing a notice after the eruption.
Officials say their monitoring cameras were not facing the direction of the blast, and it took time to confirm what happened. They say new high-definition cameras were installed on Wednesday.
NHK World's Miki Yamamoto spoke with our correspondent Kazuaki Hirama at the ski resort one day after the eruption.
Hirama said authorities were monitoring the mountain but were keeping a closer eye on another location based on past eruptions. They said the latest eruption occurred at a location that was slightly outside the view of their cameras.
Hirama said people there are wondering why the experts could not predict this event and why they didn't get a warning. They say they have been warned in the past of smaller seismic activities, so they're worried that another eruption could come without warning. The Meteorological Agency says it's taking steps to overhaul its monitoring method.
About 24 hours after the eruption, Hirama was getting more reports from rescuers. One of them described how he got to the top of a ski slope to help the people who were stuck there.
He said the falling rocks left holes on the slope and that nobody could ski because of volcanic ash on the trail. He said they reached the gondola station near the top by snowmobile.
Hirama said he's hearing that the falling rocks were as big as 30 centimeters in diameter.
The area is popular with foreign tourists. Hirama spoke with an Australian family staying at the resort.
The 11-year old daughter was with her brother at the time of the eruption and took a photograph. The brother said he was quite scared the eruption would trigger an avalanche, which it did.
An eruption also may have occurred at another crater
Professor Toshitsugu Fujii of Tokyo University has analyzed footage from the cameras in Kusatsu. He says the color of volcanic smoke in one area a little removed from the main site turned from white to dark when an eruption was observed near Kagami pond. He says this means the smoke was mixed with volcanic ash, suggesting a minor eruption may have occurred at another crater.
Fujii says he examined other footage showing volcanic rocks that appear to be tens of centimeters across, and pyroclastic flows of hot gas and ash surging down the slope.
He says the volcanic activity could spread if there was an eruption at another crater. He advises people to stay away from the area until the Meteorological Agency conducts an in-depth probe.
Another expert calls for close monitoring
Professor Kenji Nogami of the Tokyo Institute of Technology says he's been watching Mount Kusatsu-Shirane based on the assumption that eruptions will occur at a different crater on Mount Shirane. He says an eruption at Mount Motoshirane was the first he observed. He says he never expected it and hadn't studied how to prepare for it.
He says the latest eruption that killed one person and injured many others is serious. He says it reminds him of how difficult they are to predict.
Nogami says he probed an area near the mountaintop ropeway station but bad weather prevented him from making an assessment.
He says he still doesn't know the crater's exact location, but that eruptions have occurred in succession at Mount Kusatsu-Shirane in the past. He says this means they need to keep monitoring volcanic activities in the area closely.
Nogami is planning a detailed analysis of the volcanic ash and rocks collected to determine the mechanism of the latest eruption.
Chief of expert panel calls for stepping up observation
Kyoto University Professor Kazuhiro Ishihara has warned of a possible rise in underground magma. Ishihara heads an expert panel on volcanic eruption prediction.
He says this is likely to occur along with other changes, including an increase in volcanic earthquakes. He says surveillance should be stepped up to avoid overlooking these events.
Ishihara also referred to the Meteorological Agency's decision to more closely monitor a different crater. He says eruptions can occur within a 5 km radius of craters of many volcanoes and stresses the need to monitor these areas. He says authorities have to take steps, including making hazard maps, based on the assumption that eruptions will occur at multiple craters.