SOME POLITICAL GROUND is shifting - and the California earthquake caused by Proposition 13 is just one sign. There have been tremors in Senate primaries, too. In North Carolina, Democratic moderate Luther Hodges Jr. spent lavishly - and lost a runoff to Insurance Commissioner John Ingram, a Henry Howell-type crusader with a $60,000 campaign. In Arkansas and Mississippi, Democratic governors have run into trouble in Senate nomination bids. And the conservative, anti-tax and generally upstart impulses came together in New Jersey, where Jeffrey K. Bell, a former Reagan aide in his first try for office, upset four-term liberal Republican Sen. Clifford P. Case.

New Jersey also dramatized a generational change. Both Mr. Bell and his Democratic opponent, Bill Bradley, are 34. In South Dakota and Montana, the oldest major-party Senate nominees are 36.

It's rash to read too much from these early returns. New Jersey doesn't prove that GOP conservatives are ascendant everywhere. Sen. Case was hurt by his age (74) and a lackadaiscal campaign; a right-wing primary challenge to Rep. John B. Anderson (R-Ill.) fell flat this spring. And California's voters, while rebelling against soaring property taxes, did renominate all House incumbents on the ballot and chose an old-shoe GOP moderate, Attorney General Evelle J. Younger, to take on Gov. Jerry Brown.

Still, something's stirring. Public disgruntlement over inflation, high taxes and bloated government was plain before last week. So, in a larger framework, were the breaking-down of old political hierarchies and coalitions and the surge of young candidates and special-interest and single-issue action groups.

It's add up to fragmentation and frustation for voters and officials alike. Thus in two spurts of insurgency, people in California elected Jerry Brown - and then slapped him and government generally with Proposition 13. Thus President Carter and all those assertive House Democrats, elected largely on issues of ethics and trust, are now trying to cope with economic strains and make more open, decentralized institutions work. And thus Republicans and conservatives are finding receptive audiences for their limited-government and anti-incumbent themes.

We're not sure what all this portends for 1980 or even for November 1978. Right now everybody is rushing to react to the tax revolt, as the House did Thursday with its somewhat panicky 290-to-87 vote to cut the health and welfare budget by $1 billion. But blunt slicing and slashing-about , in the manner of Proposition 13, is not a durable answer to the tough questions now in the air. Those involve how the public sector should be trimmed, how costs and services should be reallocated - and those decisions ought to be made. The only thing that's obvious is that the season of upsets and unsettledness has really just begun.