NEW YORK (Reuters) - U.S. gasoline demand fell last week, compared with a year earlier, as a sharp uptick in southern California gasoline prices raised the average price motorists paid across the nation, a biweekly report from MasterCard showed on Tuesday.

Gasoline demand last week fell 3.9 percent from a year earlier, on top of its 2.4 percent decline, year-over-year, in the previous week, MasterCard's report showed.

On average, demand over the two weeks ending October 12 fell 3.1 percent lower than a year ago.

The average price of a gallon rose 4.00 cents-a-gallon to $3.82, 11.7 percent higher than in the same week last year. This followed a sharp, 31-cent rise in southern California pump prices over the last two weeks, after refinery outages created a severe supply shortage there.

MasterCard's data also showed that the four-week moving average for demand fell 2.1 percent last week. In week-to-week terms, demand first dropped by 1.7 percent in the week to October 5 then rose by 0.7 percent last week, the report showed.

MasterCard Advisors, a unit of MasterCard Inc, estimates retail gasoline demand based on aggregate sales in the MasterCard payments system, coupled with estimates for other payment forms, including cash and checks.

Earlier this year, MasterCard changed the frequency of its report from weekly to every other week.

(Reporting by Selam Gebrekidan; editing by Andrew Hay and M.D. Golan)