When God lets earthquakes hit, is it like Him saying straighten up?
That question cannot be answered for certain in the absence of a word of knowledge or prophetical word about any specific earthquake. It is true in the Old Testament, He occasionally used physical destruction to take care of problems and correct His people.
An example was the destruction of the pre-Adamite world by flood due to Lucifer's rebellion against God, which perhaps was the flooding of the Mediterranean Sea from the Atlantic Ocean when the strait of Gibralter opened. The results of this can be seen in Genesis 1:2 and Jer. 4:23-28.
Another example of destruction by flood is Noah's flood (Gen. 6:1-7, 17) due to Satan's angels trying to pollute the blood line from Adam to Christ, which may well have been the filling of the Black Sea basin from the Mediterranean Sea when the Bosphorus and Dardanelles straits were opened.
An example of the earth opening to consume people in judgment is found in Numbers 16:20-35 when Korah and his company were destroyed. There are many prophecies of earthquakes to come in the tribulation period as well (Rev. 6:12, 8:5, 11:13, 19, 16:18) as God tries to get those who have rejected Christ to turn from their evil ways and punishes mankind for their rebellious acts.
Earthquakes can also be used by God to help people out. When Paul and Silas were in prison, an earthquake from God broke them free of their bonds and made it so they could leave. They kept the jailer from killing himself over this and eventually led the jailer and his house to God. (Acts 16:16-34).
In Amos 4:6-11, God speaks of using drought and rain to move people to turn to Him, so even basic forces of nature can be put under His control if He chooses.
So it can safely be said that God does use natural disasters and general weather for His purposes. In most cases when He is involved He has people on the ground preaching repentance already. The people know they are in trouble with God due to their actions and this is a last resort approach.
On the other hand, the world is in a fallen state and plate tectonics are a fact of life. As continental plates gradually move above and below one another, and slide along each other, they stick and build up pressure and then release that pressure. Whether the result is catastrophic for man depends on where the rupture happens and the energy released by the rupture. Many earthquakes happen all around and cause no difficulty whatsoever. Others happen in crowded areas and are devastating or happen offshore and cause tsunamis that are sometimes even more destructive. To say that all of these are the result of God would be equally wrong.
The times that God is involved seem to be known in advance according to the scripture record by at least a few - or don't do significant damage. The ones that happen due to the general forces of nature don't appear to have any warning and probably should not be laid at God's doorstep in the guilt department. The thing I can be sure of is that if He does cause nature to be destructive, He has reasons that we have no business second guessing as we rarely know any of the facts.
The last tsunami that hit Asia was caused by a deep sea earthquake the day after Christmas. There was a second one the day after Easter the following year. Coincidence or God's judgment of the area and their religion? You be the judge. The earthquake and tsunami that damaged Japan... God's judgment of the area and their religion or coincidence? You be the judge. We will never know until we get to heaven and ask God.
I do know for sure that the religions of the area are not Christian. That is an indisputable fact, although many Christians undoubtedly suffered along with the rest. Is God still using natural forces? Are we in a great lull between the earthquake when Christ died and the tribulation? There's no way to know for sure. It would always be better to turn to Him before we stand in the way of an angry God.