Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 144

Table 6.1 Modified Mercalli scale

mm s − 2 ∗ Scale Intensity Description of effect On R. scale

I Instrumental Detected only on seismographs <10

II Feeble Some people feel it <25

III Slight Felt by people resting:

like a large truck rumbling by

<50 <4.2

IV Moderate Felt by people walking;

loose objects rattle on shelves

<100

V Slightly strong Sleepers awake;

church bells ring

<250 <4.8

VI Strong Trees sway;

suspended objects swing;
objects fall off shelves

<500 <5.4

VII Very strong Mild alarm;

walls crack;
plaster falls

<1000 <6.1

VIII Destructive Moving cars uncontrollable;

chimneys fall and masonry fractures;
poorly constructed buildings fall

<2500

IX Ruinous Some houses collapse;

ground cracks;
pipes break open

<5000 <6.9

X Disastrous Ground cracks profoundly;

many buildings destroyed;
liquefaction and landslides widespread

<7500 <7.3

XI Very disastrous Most buildings and bridges collapse;

roads, railways, pipes and cables
destroyed;
general triggering of other hazards

<9800 <8.1

XII Catastrophic Total destruction;

trees driven from ground;
ground rises and falls in waves

<9800 <8.1

∗ maximum acceleration measured in mm s − 2

(from Bryant, 2005 ).

The modified Mercalli scale measures earthquake intensity. It is principally a
qualitative scale that describes the type of damage occurring close to the earth-
quake epicentre. These damage descriptions are presented in Table 6.1. The scale
categories increase with increasing acceleration of the shock wave through the
crust. The interval between each category is approximately proportional to log 2
until the maximum acceleration exceeds 9800 mm s − 2 . This is the acceleration
due to gravity and when this speed is exceeded by the acceleration of the shock