Floor joist splices appear inadequately reinforced
Holding everything together is just part of the problem. Keeping both of the 2x6’s on the beam is also extremely important. 

During an earthquake, the house is moving in one direction and the foundation, footings, beams etc. are moving in another direction. (Those toe-nails into the beam is just about worthless.) You need to add metal connectors from the joists to the beam so your house “stays together”. (I like Simpson A35 n clips at each joist.)

Likewise, that beam should be secured to a post and the post to the footing. (If it’s a square independent footing, I’d find a way of securing it to the main perimeter footings too.)

Oh, and to answer your question, the double 1x4 splices are fine. They’re not doing much. The plywood subfloor is nailed into each joist, so they’re anchored together pretty good.