2) Why did the explosion occur? Could it have been prevented? How? 

Understandably, this was a burning question in the months (and years)

after the tragedy.  Initially, it was thought that the Germans might

have had something to do with it; witnesses claimed to have seen

Zeppelins above the city.  This was later discredited, though some

old-timers persisted in the notion that it was somehow "the Germans'

fault".

The captain and bridge crew of the Imo were killed in the blast, but

the crew of the Mont Blanc all survived, except for one crewman who

died from loss of blood.  The Mont Blanc crew were initially charged

with manslaughter and jailed; though they were later set free.  In

April 1918 a Nova Scotia court declared the Mont Blanc solely at

fault, but the following year the Supreme Court of Canada ruled on

appeal that the ships were equally at fault.  This was confirmed by a

ruling of the Privy Council in London, at that time the highest court

ruling on Canadian affairs.  It is hard to escape the impression that

the Mont Blanc crew were seen as a handy scapegoat in the spring of

1918.

So how did it happen?  Well, Captain Haakon From of the Imo was never

able to tell his side of the tale, but much was made later of the fact

that the Mont Blanc was not flying the red pennant which was the

internationally recognized symbol of explosives on board.  Captain Le

Medec of the Mont Blanc, for his part, contended that the red pennant

was customary, not mandatory, and that in any case it would be suicide

for a vessel in wartime...especially one as slow as the Mont Blanc,

with her top speed of 7 1/2 knots.  Official inquiries later pointed

to an inadequate supply of harbour pilots, language problems, and many

areas in which wartime urgency had overridden the harbour's safety

guidelines.  All of these things may have contributed to the disaster,

but there is no way to know for certain.

What is certain is that Captain From was travelling faster than was

customary in the Narrows, and was out of his correct lane.  The rest

seems to have been a tragically large-scale version of the dance that

strangers do when meeting in a doorway.

Could it have been prevented?  As long as there have been humans,

there has been human error; ultimately that is the underlying story of

the explosion.

3) What were the economic and social costs? 

These were, of course, immense; and in some respects incalculable.  In

a city of 50-60,000 people roughly 2000 lost their lives; 9000 more

were injured (some severely); 6000 were left homeless, and

approximately 20,000 were underhoused due to structural damage across

most of the city's residential neighbourhoods.

The names of the 1,951 "known dead" from the blast are preserved in

the Halifax Explosion Memorial Book, available online by courtesy of

the Provincial Archive.  The link I've provided takes you to the first

page of the book.  If you look down the left-hand column of the page,

you will find the names of Amy and Elsie Allen on Allen St in the

relatively intact west side of the city; and Mrs. E. Allen and John R

Allen of Kaye St in the devastated North end.  They were my relatives.

 You will also note that the address "Protestant Orphanage" is given

for several victims; nearly half of those killed in the explosion were

children.  The orphanage was rebuilt and is a community centre today,

located two blocks from my apartment (it was briefly visible in the

film "K-19", which was filmed here).

4) What was done to cope with the immediate and long-term effects of 

the disaster?

Response to the tragedy was all but immediate.  Massachusetts, and

Boston in particular, took a lead role in the international relief

effort.  The first train filled with relief supplies and medical

personnel left Boston on the evening of December 6th, the very day of

the disaster.  Ultimately, over $30 million (1917 dollars) in relief

was received from as far away as Australia and New Zealand, with the

federal government accounting for roughly half of the total.

Of all the assistance received, however, it was the warm, immediate,

and practical response from Boston that would be remembered most

vividly.  To this day, the civic Christmas tree which is raised every

year in Boston's Prudential Plaza is provided as a memorial by the

grateful people of Nova Scotia.

In the days immediately following the disaster, impromptu committees

were raised to oversee relief efforts.  These were quickly centralized

into the official Halifax Relief Commission; created by the federal

government in January 1918.  The commission oversaw the administration

of all benefit claims and their resultant pensions, and looked to the

rehabilitation and rehousing of victims.  The Commission was disbanded

only in 1976, when its remaining pensioners were transferred to the

care of the federal Department of Veterans Affairs.

Much good grew out of the disaster.  When the city was rebuilt, the

formerly lackadaisical attitude of its administration to matters of

public health and medical care was no longer in evidence.  Healthcare

facilities and budgets were greatly improved, and Halifax remains

today a centre of medical excellence.  Regulations governing the use

of the harbour were also revamped and much improved.  The city's

ramshackle trolley system was replaced with new, state-of-the-art

trams, and for the first time (from sheer necessity) many women found

jobs within the transit system.

Perhaps the longest-lasting benefit to the city was the rebuilding

process itself.  Halifax's glory days had begun to pass with the end

of the age of sail; the years-long rebuilding process brought a degree

of bustle and activity to the city which minimized the postwar

economic slump.  Further, Halifax served as something of a laboratory

for one the world's leading figures in urban planning.

Scottish-born Thomas Adams was given the opportunity to rebuild the

North End in accordance with the latest theories.  Together with

protege Horace Llewellyn Seymour, Adams (arguably the most influential

urban planner of his day) laid out a new North End of broad streets,

grassy boulevards, and (a hard-won lesson) fireproof housing. 

Constructed on non-uniform patterns from cinder blocks (known as

"hydrostones" at the time) the Hydrostone District remains the

centrepiece of today's North End.