Can a bad karma happen just only with a thought without any physical or verbal action?

I'm not sure -- there might be something about this in the Abhidhamma, which I don't know.

I think that when a thought arises, that's a result of past karma.

But an "action" can be mental, as well as (or instead of) physical or verbal. And for example, when a thought arises (as a result of past karma), then an action such as deciding whether that thought is skilful or unskilful -- that action is karmic.

Excellent question Tahlia.

Karma follows Dharma, of which intention is a part. You will find Karma is most present surrounding those who are of tranquil mind and heart.

To understand the nature of Karma in a nutshell:

Imagine a bubble that surrounds every individual. When an intention is created the shape of the bubble changes in order to support the intention we send out. If the intentions of the individual continue the bubble will become shaped by the intentions that define the individual.

The if the bubble is perfect it would be round, and a representation of the purest epitome the heart, body and mind of the individual could be, and 'slippery' (for lack of more accurate language) as far as Karmic bubbles go.

The chances of being afflicted by impure intentions of another are dramatically dropped when one is tranquil. When the impure intention slips off, the energy to muster that intention results in what is commonly referred to as 'bad' karma.

So it could be said, karma has little to do with the verbal or the physical. Intention is the wavelength of karma, verbal and physical responses are an easy way to observe karma in action. Lastly, if one does not let the intentions of another reach their inner world, they do not take part in the suffering.

Cultivate in harmony

The text below is taken from the collected teaching of Ajahn Chah - ' about being careful '

As It reads bad karma doesn't happen by sudden arising of thought.

So, some people feel that these sudden arisings of thought are wrong or evil. You may have an impulse to kill someone. But you are aware of it in the next instant, you realize that killing is wrong, so you stop and refrain. Is there harm in this? What do you think? Or if you have a thought about stealing something and that is followed by a stronger recollection that to do so is wrong, and so you refrain from acting on it – is that bad kamma It’s not that every time you have an impulse you instantly accumulate bad kamma. Otherwise, how could there be any way to liberation? Impulses are merely impulses. Thoughts are merely thoughts. In the first instance, you haven’t created anything yet. In the second instance, if you act on it with body, speech or mind, then you are creating something. Avijjā has taken control. If you have the impulse to steal and then you are aware of yourself and aware that this would be wrong, this is wisdom, and there is vijjā instead. The mental impulse is not consummated.

This is timely awareness, wisdom arising and informing our experience. If there is the first mind-moment of wanting to steal something and then we act on it, that is the dhamma of delusion; the actions of body, speech and mind that follow the impulse will bring negative results.

This is how it is. Merely having the thoughts is not negative kamma. If we don’t have any thoughts, how will wisdom develop? Some people simply want to sit with a blank mind. That’s wrong understanding.

Just only with an instant unwholesome thought (without any physical action or verbal action) is generally not bad kamma; particularly if the true nature of that unwholesome thought is fully understood; but habitual unwholesome thoughts (without any physical action or verbal action) is certainly bad kamma because habitual bad thoughts are signs of inherent ignorance that block the progress of the Path.

Yes, from multiple suttas we see that karma can be done by way of the mind and intentional thoughts only. In fact, according to the first verses of the Dhammapada, the mind precedes all mental states. It's more important than words and actions.

From Nibbedhika Sutta:

"Intention, I tell you, is kamma. Intending, one does kamma by way of body, speech, & intellect.

From Mahakammavibhanga Sutta:

After doing an intentional deed to be experienced as pleasant by way of body, speech, or mind, one feels pleasure. After doing an intentional deed to be experienced as painful by way of body, speech, or mind, one feels pain. After doing an intentional deed to be experienced as neutral by way of body, speech, or mind, one feels neutral.”

From Bhumija Sutta:

"When there is a body, pleasure & pain arise internally with bodily intention as the cause; or when there is speech, pleasure & pain arise internally with verbal intention as the cause; or when there is intellect, pleasure & pain arise internally with intellectual intention as the cause.

From Kamma Sutta:

And what is new kamma? Whatever kamma one does now with the body, with speech, or with the intellect. This is called new kamma."