TOKYO, Nov 9 (Reuters) - SoftBank Group Corp (9984.T) shares jumped 10% on Tuesday, the first trading session after the Japanese conglomerate said it would spend up to 1 trillion yen ($8.8 billion) buying back almost 15% of its shares. The company announced the buyback, long speculated by the market, after it revealed its quarterly earnings crashed to a loss amid a decline in the share price of its portfolio companies and a regulatory crackdown in China. The buyback is Softbank"s second largest after a record 2.5 trillion yen buyback launched in the depths of the COVID-19 pandemic last year. Shares of the tech group quadrupled during that buyback, but have since fallen 40% from a peak in May. "Our analysis of buyback history indicates that SBG stock performs (and outperforms indices or BABA) during buybacks," wrote Jefferies analysts Atul Goyal in a note, referring to Alibaba (9988.HK), the group"s largest asset. The slide in the Chinese e-commerce giant"s shares and the broader regulatory backlash contributed to a $57 billion fall in SoftBank"s net assets to $187 billion, a metric that Chief Executive Masayoshi Son has said is the primary measure of SoftBank"s success. The repurchase period for the latest buyback runs to Nov. 8 next year, with the group signalling the programme could take longer than the fast paced programme last year. The buyback "is nice support, but it isn"t rocket fuel," wrote LightStream Research analyst Mio Kato on the Smartkarma platform, adding "there are material downside risks if broader tech, especially unprofitable tech, falters." Speculation SoftBank could launch a buyback has been raging for months as the discount - the gap between the value of its assets and its share price - has lingered to the frustration of executives and as investors push for repurchases. Ongoing uncertainties include the prospect of gaining regulatory approval for the $40 billion sale of chip designer Arm to Nvidia (NVDA.O). Delays to the sale "may have given Softbank the flexibility to announce a buyback now with expectations of ramping up share purchases later," Redex Research analyst Kirk Boodry wrote in a note. ($1 = 113.3500 yen) Reporting by Sam Nussey; Editing by Sam Holmes, Stephen Coates and Jane Wardell