For a person in Israel, geographically, Tarshish is in the opposite direction from Nineveh. Ellicot correctly observes:
Tarshish.—This can hardly be any other than Tartessus, an ancient
Phœnician colony on the river Guadalquivir, in the south-west of
Spain. (See Genesis 10:4; 1Chronicles 1:7.)
However, I do not believe that Jonah's "fleeing from the LORD" should be understood geographically. Jonah's own actions clearly show that we are all in the presence of the LORD because he would have also known:
Ps 139:7-12 - Where can I go to escape Your Spirit? Where can I flee
from Your presence? If I ascend to the heavens, You are there; if I
make my bed in Sheol, You are there. If I rise on the wings of the
dawn, if I settle by the farthest sea, even there Your hand will guide
me; Your right hand will hold me fast. If I say, “Surely the darkness
will hide me, and the light become night around me”— even the
darkness is not dark to You, but the night shines like the day, for
darkness is as light to You.
In modern theological techno-speak we say God is "omnipresent". Jonah's own actions also betray this when he prayed to the LORD from beneath the waves in the belly of the fish! (as pointed out in the OP) Jonah knew he could NOT escape from the presence of the LORD.
So why did Jonah sail for Tarshish? There are several possibilities:
- Jonah thought that a simple "no" to God was not enough and had to move himself geographically to reinforce the point
- He was effectively renouncing his prophetic office and leaving the land of Israel to go to a pagan country
- He wanted to move himself away from the Temple dedicated to God and the visible presence in the Holy of Holies in the hope that God would find someone else in Israel
- He knew God was gracious and that when the Ninevites repented his prophetic reputation would be tarnished (Jonah 4:2)
... and so forth. It may have been a combination of all of these. Note that the literal Hebrew in Jonah 1:3 says that Jonah "fled from the face of the LORD". That is, Jonah did not want to stand in before (= in the face of) the LORD.
Several commentators reach the same conclusion. The Pulpit commentary has this:
From the presence of the Lord; literally, from the face of Jehovah.
This may mean, from God s special presence in Jerusalem or the Holy
Land, as banishment from Cannaan is called "casting out of his sight"
(2 Kings 17:20, 23; 2 Kings 23:27); or, from serving the Lord as his
minister (Deuteronomy 10:8), Jonah preferring to renounce his office
as prophet rather than execute his mission. The former seems the most
natural explanation of the phrase.
From the presence of the Lord.—Rather, from before the face of
Jehovah. The words may imply (1) the belief in a possibility of hiding
from the sight of God (as in Genesis 3:8), a belief which, as we
gather from the insistence on its opposite in Psalms 139, lingered
late in the popular conception; (2) a renunciation of the prophetic
office. (Comp. Deuteronomy 10:8; 1Kings 17:1); (3) Flight from the
Holy Land, where the Divine presence was understood to be especially
manifested. Commentators have generally rejected the first of these as
implying ignorance unworthy of a prophet; but, on embarking, Jonah
went below, as if still more securely to hide, and used the same
expression to the mariners, who would certainly take it in its literal
and popular sense.
The Cambridge commentary observes:
from the presence of the Lord This may mean from standing before the Lord or being in His presence, as His servant or minister
(Deuteronomy 10:8, 1 Kings 17:1, Matthew 18:10, Luke 1:19. See Dr
Pusey, Commentary on Jonah, p. 247, note d.); i. e. he renounced his
office of prophet rather than obey so unwelcome a command. It may,
however, only refer to that special presence of God in the Holy Land,
which all Jews recognised. Either view is compatible with a belief on
the part of Jonah in the omnipresence of God (Psalms 139). It is said
of Cain (Genesis 4:16) that he “went out from the presence of the
Lord” (and the Heb. phrase is the same as here), when he forfeited the
favourable regard, together possibly with some local manifestation of
the presence of the Almighty.
The reason of Jonah’s disobedience is given by himself, ch. Jonah 4:2.
Knowing well the lovingkindness of God, he anticipated that He would
spare the Ninevites on their repentance, and he could not bring
himself to be the messenger of mercy to heathen, much less to heathen
who (as the Assyrian inscriptions state) had already made war against
his own people, and who as he may have known were destined to be their
conquerors. See the statements of his probable contemporary, Hosea
9:3; Hosea 11:5.