Hi!
As far as I understand, the problem was in very uneven watering. The tree might be stressing without sufficient water when it got plenty all of a sudden. The plant had significant water uptake but because of previous drought the fruit rind lost it's elasticity and could not expand to accommodate juice... Grapefruits are less affected since thir rind is thicker... Try avoiding water stress in the future.
Good luck!

Thanks, Gleb, for the reply and info. How often would you recommend watering the citrus trees in my location (northern California) during the dry months? I'll try to be more consistent next year. marilano

Marilano,
Am not an expert by all means and my own citrus trees are suffering from the same water stress as yours... :( Also, I am in humid se tx and never been to California... However, this is what I read somewhere about water requirements for citrus:
it is recommended to have two light and one deep watering per week,
alternatively, when the soil feels dry two inches below the surface, it is time to water.
Is this info helpful?
I am thinking about finding or building a simple sensor that would signal when the soil dries to that depth... Meanwhile, an old fashion "dig and touch" ...

Sounds good, I don't water mine unless it get very dry, but do know that if after a long dry spell without water if we get heavy downpours during the fruiting period I can expect to lose fruit to spliting.

http://redwoodbarn.com/DE_citrusfreeze.html

An article you will like

Normally, tangerines are exempt from splitting unless heavily drought stressed and then overwatered. Even then they are are not as bad as tight skinned citrus (like round oranges) when it comes to fruit splitting. That plus being somewhat compact size is why we often prefer them as container citrus.

Well.. Some people prefer lemons in containers for the same reasons plus the frequent fragrant blossoms.

Larry

I am also in San Jose experiencing the same problem . Orange trees are OK, just the Tangerine effected. I assumed it was from lack of water. I have the trees on a water system, but since they are 30 years old like yours I don;t water often.

I have had scale infestations that caused the fruit to split as well as excess water.

I'm in southern California and my fruit is splitting also. I have suspected it was because of less watering and then a deep water. The tree if full of tangerines and I'm hoping I can save the rest by careful watering...

Here's some interesting reading about citrus splitting. Thought you all might find this interesting... :)

http://farmerfredrant.blogspot.com/2016/10/the-year-of-splitting-orange.html?m=1