Earthquake, Traditional Javanese House, and Newton Law

Even though Java Island is located in near Sunda Trench, the meeting continental boundaries of Australian and Eurasian plates, earthquakes were very rarely before the year of 2006. Several quakes may happen in the ancient time without a significant number of casualties. It can be understood that the population density was lower and most of buildings were constructed from wood. This assumption is based on the fact that big fatalities in 2006 Java earthquake was caused by collapsed houses which are constructed by weak masonry.

Earthquake history in Java Island:

1867 : 372 houses collapsed, 5 fatalities
1943 : 2.800 houses collapsed, 213 fatalities, 2.096 injured
1981 : no casualties 2006 May 27 Central Java: 5.9 (BMG) 6.2 (USGS) 6.234 fatalities, > 50.000 injured, >70.000 houses collapsed
2006 Jul 17 West-Central Java: 7.7 (USGS) 500 fatalities, 9299 injured, 68 houses damaged
2009 Sept 2 West Java: 7.4 (BMG) 7.0 (USGS) 57 fatalities and 300 injured , >10.000 houses damaged
2009 Nov 13 Central Java: 5.4 (USGS) No casualties

When we look back to ancient traditional Javanese houses, wooden based structures are mostly used for houses where stone and brick construction were for temples. Lateral forces from earthquake were easily dissipated by light wooden building since wooden post and beam has flexible adaptability to deal with the forces while heavy (masses) building will be affected more. The simplest form of the equation which expresses the Second Law of Motion is:

F = MA

Beside masses, acceleration has this important influence on damage, because, as an object in movement, the building obeys Newton' Second Law of Dynamics.

This states the Force acting on the building is equal to the Mass of the building times the Acceleration. So, as the acceleration of the ground, and in turn, of the building, increase, so does the force which affects the building, since the mass of the building doesn't change.

The greater the force affecting a building, the more damage it will suffer; decreasing F is an important goal of earthquake resistant design. When designing a new building, for example, it is desirable to make it as light as possible, which means, of course, that M, and in turn, F will be lessened. Various techniques are now also available for reducing A.