'which occur earlier' is correct and usable in written English.
It is often used when comparing two or more events or objects to indicate the one that happened or came before the other(s). For example: "He was the fifth person to arrive at the party, which occur earlier than everyone else."
Nuclear medicine scintigraphic imaging is an excellent non-invasive method of whole-body scanning, allow to determinate infectious foci in all parts of the body, based on pathophysiological changes which occur earlier in the infection process.
Explore face-to-face contacts which occur earlier in pregnancy (i.e. the first hospital visit).
An essential component to chemopreventive agent development, highlighted by consensus groups in the United States (Crowell, 2005) and Europe (Gerhauser et al, 2006), is a need to identify end points, which occur earlier than cancer, that accurately predict the potential to reduce cancer incidence.
Alcohol is responsible for around 150,000 hospital admissions in England and Wales each year, the majority of which occur between 12.00 and 05.00 [ 5].
Judge Henry Bramwell Brooklyn Federal Court Judge which occurred May 28 , 2010
Different reasons have been offered for the change, which occurred this week.
The case, which occurred several years ago, touched off an emotional debate at the hospital.
In the second incident, which occurred two weeks later, the head of the black child was snipped out.
The cartel is also afraid of overproducing and touching off a free fall in prices, which occurred two years ago.
Diplomats from France, the United Kingdom and the United States condemned protesters' attacks on members of Parliament, which occurred last Friday.
The attack, which occurred March 23, took place as the victim, Guarionex Montas, was playing basketball, the police said.
