* HKMA buys total HK$3.26 bln in 1st intervention since 2015
* Triggers weak-side CU since trading band set in 2005 (Adds latest intervention in U.S. trading hours)
HONG KONG, April 13 (Reuters) - Hong Kong's de facto central bank stepped into the currency market again in U.S. trading hours, buying HK$2.442 billion ($311 million) in Hong Kong dollars from the foreign exchange market as the local currency hit the weaker end of its trading range.
On Thursday, the Hong Kong Monetary Authority (HKMA) said it bought HK$816 million from the currency market.
This was the first time since the trading band was introduced in 2005 that the weak-side convertibility undertaking (CU) at 7.85 to keep the Hong Kong dollar closely pegged to the U.S. currency had been triggered.
The HKMA said the undertaking was triggered in London trading hours.
"I reiterate that the HKMA will buy Hong Kong dollars (HKD) and sell US dollars at 7.85 level to ensure that the HKD exchange rate will not weaken beyond 7.8500," Norman Chan, chief executive of HKMA, said in a statement.
"Such operations are normal and in accordance with the design of the Linked Exchange Rate System."
The Hong Kong dollar touched the lower end of the central bank's trading band target as the interest rate gap between the greenback and the local currency widened.
As the former British colony pegs its currency to the dollar, its money market rates should mirror those of its U.S. counterpart, but the gap has now widened to more than 117 basis points since the U.S. Federal Reserve started raising interest rates from the ultra-low levels adopted in the 2008 financial crisis.
Hong Kong's markets have remained flush with excess cash, keeping a lid on Hong Kong dollar interest rates.
Most market participants do not see the current bout of weakness as a threat to the currency peg even though high liquidity stemming from Chinese and overseas investment into Hong Kong's domestic markets is anchoring short-term interest rates and putting downward pressure on the currency.
The latest intervention, bringing the total amount of local currency bought to HK$3.258 billion, will reduce the aggregate balance - the sum of balances on clearing accounts maintained by banks with the HKMA - to HK$176.52 billion on April 16, according to Reuters data.
The Hong Kong dollar is pegged at 7.8 to the U.S. dollar, but can trade between the high and low limits of 7.75 and 7.85. Under the currency peg, the HKMA is obliged to intervene when the Hong Kong dollar hits 7.75 or 7.85 to keep the band intact.
The currency traded at 7.8499 against the U.S. dollar at 0137 GMT.
"The HKMA is fully capable of maintaining the stability of the HKD and managing large scale capital flows. There is no need to be concerned," Chan said.
The monetary authority last intervened in the market in 2015, selling Hong Kong dollars as the local currency repeatedly hit the strong end of its trading band.
($1 = 7.8496 Hong Kong dollars) (Reporting by Donny Kwok and Twinnie Siu; Additional reporting by Meg Shen; Editing by Anne Marie Roantree and Eric Meijer)