Why seemingly small earthquakes can cause a lot of damage

CIRO DE LUCA/REUTERS

A damaged house and car after a 4 magnitude earthquake hit in Italy earlier this week.

Often, small earthquakes seem to do a lot of damage - take the recent 4 magnitude quakes in Italy and Greenland for example.
The quake in Italy killed two, injured dozens and destroyed buildings. A former Christchurch couple were lucky to escape harm.
Greenland's quake resulted in a tsunami which caused widespread damage, washed away homes and killed at least four people.

In New Zealand, small magnitude aftershocks in Christchurch in 2011 and 2012 caused significant damage.

 

CIRO DE LUCA/REUTERS

Rescue workers check a collapsed house after an earthquake hit the island of Ischia, off the coast of Naples, Italy on August 22.

Why is it that these small to moderate quakes resulted in so much destruction?

SIMPLY, IT HAS TO DO WITH WHERE EARTHQUAKES HIT
GNS Science seismologist Dr John Ristau said "for a small magnitude earthquake to cause significant damage it would have to be very shallow and very close to human-built structures – depth will be very important as the amount of ground shaking drops off quickly with increasing depth."

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The damage resulting from the Greenland tsunami caused by an earthquake included this house that was swept away.

The deeper the quake, the more energy the seismic waves it causes lost as it travelled.

Ristau said in theory, an earthquake of any magnitude could cause damage depending on where it hit relative to human structures and how well built they were.
The rock and soil conditions also played a part. Areas of bedrock experienced less shaking than those with soft sediments.

JOINT ARCTIC COMMAND

Large chunks of ice appear to have been swept into a Greenland village by the tsunami in June.

Topography could also contribute - for example, the shape of a valley could shake intensity.
WHAT HAPPENED IN CHRISTCHURCH, ITALY AND GREENLAND?
"In 2011 Christchurch experienced extensive damage due to the amount of liquefaction caused by the soil conditions around Christchurch. Structures around Wellington Harbour, built on re-claimed land, are more susceptible to damage than bedrock sites in the capital."

DAVID HALLETT/THE PRESS

Damage in Christchurch following the 2011 earthquake including a collapsed building on the intersection of Tuam And Manchester St.

Ristau said small aftershocks could also cause serious damage to structures affected by the initial quake.
"The 2010 Boxing Day 4.7M aftershock in Christchurch after the M 7.1 Darfield earthquake, plus the smaller aftershocks that day, caused extensive damage... even though they weren't too large."
Ristau guess that in the case of the Italian quake, it was very shallow, and the buildings that were damaged were un-reinforced brick and masonry structures which performed poorly in earthquakes.
As for Greenland, experts believed the magnitude 4 earthquake likely triggered a landslide into the ocean and caused the tsunami.
In June, Niwa marine geologist Joshu Mountjoy said similar events had led to tsunamis in New Zealand in the past and could continue to do so, especially in areas where mountains rose up from small bodies of water.
DID YOU KNOW
The opposite to small earthquakes causing lots of damage can also be true.
Large quakes that were deeper and further away from populations centres weren't as harmful, the ABC reported earlier in 2017.
Quakes also cause two types of seismic waves, p-waves and s-waves.
P-waves travel quickly and force the ground to move backwards and forwards as it is compressed and expanded.
S-waves follow more slowly, shaking the ground up and down at right angles to the direction of the P-wave.

https://www.stuff.co.nz/science/96149239/why-seemingly-small-earthquakes-can-cause-a-lot-of-damage