When the earth moves, nothing can stand its ground.

IOW,
the only way to avoid an earthquake is to take to the air at
just the right time. Fortunately, however, earthquakes don't occur
very often; those that do occur are generally confined to small
areas; and the vast majority of earthquakes strong enough to be
noticeable are not strong enough to do any harm.

At the other extreme, a violent earthquake can crumble a cliffside,
crack open a flat field, and move countless tons of rock and
earth in the space of a few seconds. Any living thing in the immediate
vicinity of a strong quake is in jeopardy -- not so much from
the tremor itself, but from what the quake does to the countryside.
If a strong earthquake occurs in a heavily populated area, the
shock waves and their after-effects can devastate entire cities.
But since the scope of this book does not include cities, villages,
and other places where large groups of people live, this discussion
of earthquakes is limited to quakes that occur in wilderness
areas that have no stable human or humanoid populations.
As stated above, most earthquakes occur only in specific areas.
Those areas are generally located along fault lines, where
two sections of bedrock come together and exert pressure
against each other. (The DM has information to help
him determine where fault lines should be located; it is up to characters <where would this info be?>
to find out for themselves where those places are.) A
quake occurs when this continually building pressure becomes
so intense that the strength of the rocks themselves cannot contain
or absorb it. Something has to give, so one or both of the rock
masses shifts along the fault line - and the energy released by
this movement travels through the ground in the form of shock
waves. Because they extend over a much larger area, these
shock waves can do much more actual damage than the fault-line
shift that started the quake. One earthquake can cause another
one, if the tremors from the first one happen to set off activity
along a second, nearby fault line. And a quake hardly ever ends
with just one tremor. Aftershocks can occur sporadically for several
days afterward as mountain-sized slabs continue to settle
into what will be their final configuration - final, that is, until the
next earthquake.

Types of Quakes

For game purposes, earthquakes are classified as light, moderate, or severe.

A light earthquake is perceived as a shivering or vibrating force
that travels through the ground and anything in contact with it.
The tremor lasts for a few seconds, often even less than that, and
produces no effects that are directly hazardous to characters.
However, it could happen that a quake strikes at the precise
time when a character is performing some task that requires concentration,
delicate balance, or careful movements. A M-U
who is trying to prepare a spell will have his concentration
disrupted, and the spell will be ruined. A thief engaged in the meticulous
business of defusing or avoiding a trap will be jostled,
and there is a 50% chance that this will cause him to fail even if
an earlier dice roll indicated that he would succeed. Anyone attempting
to walk a tightrope will have his chance of success cut in
half, and will fall to the ground unless he manages to grab the
tightrope by making a successful DEX Check. Any character
involved in some other similar type of activity may likewise be affected;
the DM must exercise his judgment to determine
when a light quake is more than just momentarily upsetting.

A moderate quake normally lasts for 10-15 seconds (d6+9),
occasionally (20%) less than that, and sometimes (30%) more
than that -- up to 25 or 30 seconds. It has all the effects of a light
quake, and in addition has the power to dislodge a character from
a precarious position even if the position is not particularly unstable.
For instance, a character who is straddling a limb or branch
of a tree when a moderate quake hits will be toppled from his
perch unless he makes both a DEX Check (reacting to his
predicament in time) and a STR Check (grabbing the limb
and managing to hold on until the quake subsides). Someone
climbing a severe or moderate slope must make a Climbing
Check at a -50% penalty to his CR to avoid losing
his footing and tumbling down the slope. A character specialized
in the use of a missile weapon takes a -2 penalty “to hit” if he
gets a shot off during a moderate quake, unless the shot is at
point blank range. A non-specialist using a missile weapon does
so with a -3 penalty “to hit” at short range, a -5 penalty at medium
range, and a -7 penalty to long range - in addition to the
usual penalties for medium and long range. Any character attempting
to engage in melee combat takes a -3 penaltyon all attack
rolls, and if he misses he will slip and fall as if he was trying to
attack while standing on a slippery horizontal surface.

Besides the bothersome and potentially harmful effects of the
tremor itself, a moderate quake can endanger characters in the
area who are in a position to be hit or buried by sliding earth and
natural debris generated by the force of the quake. A character at
the base of a cliff || a severe slope may find himself set upon by a
rockfall or a mudslide, depending on the makeup of the terrain.
(Of course, anyone climbing either of those surfaces is also subject
to these perils.) A character standing under a tree has a 1 in 8
chance of being hit if the tree is uprooted and falls. It is rare, but
possible, for a moderate quake to open a fissure in the ground,
and an unfortunate character who is standing in that exact spot
will take falling damage (and possibly abrasion damage as well, if
he bounces off the sides of the crevass before hitting bottom). A
very unfortunate character, if he isn’t killed by the fall, will be suffocated
or crushed if the two sides of the fissure happen to move
toward each other after separating.

A severe earthquake has all the effects of a moderate quake,
but all penalties and all chances of unfortunate occurrences are
multiplied by two:

* The penalty to a character’s CR is -100% instead of -50%;
* the penalty to attack rolls in melee combat is -6 instead of -3;
* the chance of a rockfall occurring adjacent to a cliff face is twice as great;
* and so forth.

A severe quake will last from 40-90 seconds (1d6+3), during which time
the only sensible course of action is to hit the ground (but not from
50 feet in the air), cover your head, and pray to the deity of your
choice.