Earthquake Information

Earthquake Preparedness

Modified Mercalli Intensity Scale of 1931

(Adapted from Sieberg's Mercalli—Cancani scale, modified and condensed)

I. Not felt -- or, except under especially favorable circumstances.

Under certain conditions, at and outside the boundary of the area in which a great shock is felt:

  • Sometimes birds, animals, reported uneasy and disturbed;
  • Sometimes dizziness or nausea experienced;
  • Sometimes trees, structures, liquids, bodies of water, may sway; doors may swing, very slowly.

II. Felt indoors by few, especially on upper floors, or by sensitive or nervous persons.

Also, as in grade I, but often more noticeably:

  • Sometimes hanging objects may swing, especially when delicately suspended;
  • Sometimes trees, structures, liquids, bodies of water, may sway, doors may swing, very slowly;
  • Sometimes birds, animals, reported uneasy and disturbed;
  • Sometimes dizziness or nausea experienced.

III. Felt indoors by several, motion usually rapid vibration.

  • Sometimes not recognized to be an earthquake at first.
  • Duration estimated in some cases.
  • Vibration like that due to the passing of light or lightly loaded trucks or heavy trucks some distance away.
  • Hanging objects may swing slightly.
  • Movements may be appreciable on upper levels of tall structures.
  • Rocked standing motor cars slightly.

IV. Felt indoors by many, outdoors by few.

  • Awakened few, especially light sleepers.
  • Frightened no one, unless apprehensive from previous experience.
  • Vibration like that due to the passing of heavy or heavily loaded trucks.
  • Sensation like heavy body striking building or falling of heavy objects inside.
  • Rattling of dishes, windows, doors; glassware and crockery clink and clash.
  • Creaking of walls, frame, especially in the upper range of this grade.
  • Hanging objects swung, in numerous instances.
  • Slightly disturbed liquids in open vessels. Rocked standing motor cars noticeably.

V. Felt indoors by practically all, outdoors by many or most: outdoors direction estimated.

  • Awakened many, or most.
  • Frightened few, -- slight excitement, a few ran outdoors.
  • Buildings trembled throughout.
  • Broke dishes, glassware, to some extent.
  • Cracked windows -- in some cases, but not generally.
  • Overturned vases, small or unstable objects, in many instances, with occasional fall.
  • Hanging objects, doors, swing generally or considerably.
  • Knocked pictures against walls, or swung them out of place.
  • Opened, or closed, doors, shutters, abruptly. Pendulum clocks stopped, started, or ran fast, or slow.
  • Moved small objects, furnishings, the latter to slight extent.
  • Spilled liquids in small amounts from well-filled open containers.
  • Trees, bushes, shaken slightly.

VI. Felt by all, indoors and outdoors.

  • Awakened all.
  • Frightened many excitement general, some alarm, many ran outdoors.
  • Persons made to move unsteadily.
  • Trees, bushes, shaken slightly to moderately.
  • Liquid set in strong motion.
  • Small bells rang -- church, chapel, school, etc.
  • Damage slight in poorly built buildings.
  • Fall of plaster in small amount.
  • Cracked plaster somewhat, especially fine cracks; chimneys in some instances.
  • Broke dishes, fall of knick-knacks, books, pictures.
  • Overturned furniture in many instances.
  • Moved furnishings of moderately heavy kind.

VII. Frightened all -- general alarm, all ran outdoors.

  • Some, or many, found it difficult to stand.
  • Noticed by persons driving motor cars.
  • Trees and bushes shaken moderately to strongly.
  • Waves on ponds, lakes, and running water.
  • Water turbid from mud stirred up.
  • Incaving to some extent of sand or gravel stream banks.
  • Rang large church bells, etc.
  • Suspended objects made to quiver.
  • Damage negligible in buildings of good design and construction, slight to moderate in well-built ordinary buildings, considerable in poorly built or badly designed buildings, adobe houses, old walls (especially where laid up without mortar), spires, etc.
  • Cracked chimneys to considerable extent, walls to some extent.
  • Fall of plaster in considerable to large amount, also some stucco.
  • Broke numerous windows, furniture to some extent.
  • Shook down loosened brickwork and tiles.
  • Broke weak chimneys at the roof-line (sometimes damaging roofs).
  • Fall of cornices from towers and high buildings.
  • Dislodged bricks and stones.
  • Overturned heavy furniture, with damage from breaking.
  • Damage considerable to concrete irrigation ditches.

VIII. Fright general — alarm approaches panic.

  • Disturbed persons driving motor cars.
  • Trees shaken strongly — branches, trunks, broken off, especially palm trees.
  • Ejected sand and mud in small amounts.
  • Changes: temporary, permanent; in flow of springs and wells; dry wells renewed flow; in temperature of spring and well waters.
  • Damage slight in structures (brick) built especially to withstand earthquakes.
  • Considerable in ordinary substantial buildings, partial collapse: racked, tumbled down, wooden houses in some cases; threw out panel walls in frame structures, broke off decayed piling.
  • Fall of walls.
  • Cracked, broke, solid stone walls seriously.
  • Wet ground to some extent, also ground on steep slopes.
  • Twisting, fall, of chimneys, columns, monuments, also factory stacks, towers.
  • Moved conspicuously, overturned, very heavy furniture.

IX. Panic general.

  • Cracked ground conspicuously.
  • Damage considerable in (masonry) structures built especially to withstand earthquakes;
  • threw out of plumb some wood-frame houses built especially to withstand earthquakes;
  • great in substantial (masonry) buildings, some collapse in large part; or wholly shifted frame buildings off foundations, racked frames;
  • serious to reservoirs; underground pipes sometimes broken.

X. Cracked ground, especially when loose and wet, up to widths of several inches; fissures up to a yard in width ran parallel to canal and stream banks.

  • Landslides considerable from river banks and steep coasts.
  • Shifted sand and mud horizontally on beaches and flat land.
  • Changed level of water in wells.
  • Threw water on banks of canals, lakes, rivers, etc.
  • Serious damage to dams, dikes, embankments.
  • Severe to well-built wooden structures and bridges, some destroyed.
  • Developed dangerous cracks in excellent brick walls.
  • Destroyed most masonry and frame structures, also their foundations.
  • Bent railroad rails slightly.
  • Tore apart, or crushed endwise, pipe lines buried in earth.
  • Open cracks and broad wavy folds in cement pavements and asphalt road surfaces.

XI. Disturbances in ground many and widespread, varying with ground material.

  • Broad fissures, earth slumps, and land slips in soft, wet ground.
  • Ejected water in large amount charged with sand and mud.
  • Caused sea-waves ("tidal" waves) of significant magnitude.
  • Damage severe to wood-frame structures, especially near shock centers.
  • Great to dams, dikes, embankments, often for long distances.
  • Few, if any (masonry), structures remained standing.
  • Destroyed large well-built bridges by the wrecking of supporting piers, or pillars.
  • Affected yielding wooden bridges less.
  • Bent railroad rails greatly, and thrust them endwise.
  • Put pipe lines buried in earth completely out of service.

XII. Damage total -- practically all works of construction damaged greatly or destroyed.

  • Disturbances in ground great and varied, numerous shearing cracks.
  • Landslides, falls of rock of significant character, slumping of river banks, etc., numerous and extensive.
  • Wrenched loose, tore off, large rock masses.
  • Fault slips in firm rock, with notable horizontal and vertical offset displacements.
  • Water channels, surface and underground, disturbed and modified greatly.
  • Dammed lakes, produced waterfalls, deflected rivers, etc.
  • Waves seen on ground surfaces (actually seen, probably, in some cases).
  • Distorted lines of sight and level.
  • Threw objects upward into the air.