"He was sentenced to five years in prison for inciting subversion of state power and deprived of his political rights for three years," said one of his lawyers, Pu Zhiqiang.
"There were no charges related to the quake. All of the proceedings were linked to 4 June (1989)," Mr Pu told AFP news agency by telephone.
Tan's supporters and the London-based rights group Amnesty International has said that his independent investigation was probably the real reason for his detention.
Roseann Rife, of Amnesty International, said it was unusual that the quake was not mentioned in verdict.
Roseann Rife: "He was detained for exercising freedom of expression"
She said Tan had strong public backing for his work after the quake and the authorities risked a backlash if they convicted him over that.
Tan's trial in Chengdu, the capital of Sichuan province, was adjourned without a verdict in August last year.
Tan's brother said that while he had not agreed with Tan's actions at the time, he knew he had "always worked for society and for the country".
"I think that as a citizen, you can't change the reality of society. But today, I have to stand up for him and support my younger brother. Now I understand him," said Tan Liren.
The verdict and sentence, thought to be the maximum possible, was then read out in less than 10 minutes on Tuesday.
Mr Pu said his client planned to appeal.
"I think this is a very important case for China... It shows the Chinese legal system has taken a big step backwards. Tan's 'crime' was entirely one of speech, of conscience," said Chinese artist Ai Weiwei, who has also campaigned for earthquake victims.
Mr Ai, who compiled a list of the children who died in the quake, was roughed up when he travelled to Chengdu to attend the August trial.
A Hong Kong television crew was also prevented from attending that trial and had their hotel room searched under the pretext that they were hiding drugs.
Several thousand school buildings collapsed during the earthquake, fuelling angry allegations by parents that corruption had led to poor construction standards.
In many of the affected towns, schools collapsed but other nearby buildings withstood the quake.
Government officials promised an investigation while, at the same time, pressurising parents to keep their grief - and anger - to themselves.
Tan asked internet users and people who had lost their children in the quake to help compile a detailed database of the victims.
He also asked volunteers to detail any evidence of poor construction at the schools.
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