This month’s blog from our Community Geologist, Dr Ian Kille, is all about earthquakes and Mystery Rock 17 from last month’s newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.
Swimming pools are not really the best place to appreciate an earthquake. I was enjoying the benefits of Iceland’s geothermal heat in an open-air swimming pool on the only occasion I have been somewhere when an earthquake took place in that location, and I only discovered it had happened when I was told about it afterwards. Iceland, situated on a constructive plate margin, is tectonically active and earthquakes are not uncommon. They do, however, tend to be rather lower down on the Richter scale, in a way that might make it possible to appreciate an earthquake rather than being terrified by it. Constructive plate margins are in tension and are located where the crust is already thin (particularly in oceanic crust) or where the crust is in the process of being thinned. In consequence it is possible for the tension to be released more incrementally (unlike compression where long periods of oh-so-quiet suddenly become oh-
so-loud and very destructive). Not to say that incremental tension release doesn’t cause problems. The Icelandic attempts at harnessing geothermal energy from the very high heat flows encountered on the island have been severely hampered by the water circulation pipes being bent and broken by earth movement. Expensive and costly but not deadly. The earthquakes can however be a signal of something more deadly about to happen. An increase in earthquake activity is a sign that magma is on the move and therefore a helpful predictor of imminent volcanic eruption.
In contrast to Iceland’s constructive margin earthquake experience, that of a destructive plate margin is altogether more devastating. Destructive plate margins are so named because a slab of oceanic crust and lithospheric mantle (solid) are diving back down into the aesthenospheric mantle (fluid and deeper), dragging its trailing plate behind it. It could equally well be called a destructive plate margin for the naked violence of its volcanic eruptions and its earthquakes. Mount Tambora and Krakatoa in Indonesia, Mount Pele in Martinique, Nevada del Ruiz in Columbia and Santorini in Greece top the list for human casualties, and all are found on destructive plate margins. The same is true for the most devastating earthquakes. Tangshan and Sichaun in China, Haiti, Peru, Kashmir and the Indian Ocean earthquake centered just of the coast of Sumatra top the location-list of recent deadly earthquakes. With fatalities in the hundreds-of-thousands, earthquakes are more deadly than volcanoes. The cost is devastating in so many ways. Not 
only in terms of in human lives and injury, but also the misery caused by destruction of homes and businesses and infrastructure resulting in loss of income, famine and illness. There are several physical factors that dictate the scale of damage. The amount of energy released by the earthquake (its magnitude) is the principal measure of how much direct damage an earthquake may cause through shaking and ground rupture. Landslides, lahars (mud flows), flooding and liquefaction of soils and unconsolidated sediments are also direct consequences of earthquakes. Additionally, earthquakes may cause tsunamis. I vividly recollect the news and images from Indonesia and Japan on Boxing Day of 2004 showing the astonishing devastation caused by this earthquake-induced tsunami and the consequent tragedies that followed.
It may seem from this that Earthquakes are literally the great levellers, indiscriminate in who and what gets destroyed. As individuals and societies, we do however have knowledge and experience both of where earthquakes are likely to happen and what can be done to mitigate the consequences when an earthquake hits. There are two places I have visited which illustrate this well. The first is San Francisco. I’m not a great lover of any city, but San Francisco is one of my favourites, with its vibrant mix of peoples and its beautiful 
setting, wrapped around by San Francisco Bay and the Pacific Ocean and its many hills providing beautiful vistas. It is however a city in peril, located as it is on the San Andreas fault, a 1200km-long active transform-fault. The southwestern part of San Francisco is moving north, and the northeastern part is moving south: intermittently and violently. After the devastating 1906 earthquake in San Francisco, much work was done to make buildings and infrastructure more resilient to earthquakes, with good effect. These words from the bible come to mind:
“Therefore whoever hears these sayings of Mine, and does them, I will liken him to a wise man who built his house on the rock: and the rain descended, the floods came, and the winds blew and beat on that house; and it did not fall, for it was founded on the rock.
“But everyone who hears these sayings of Mine, and does not do them, will be like a foolish man who built his house on the sand: and the rain descended, the floods came, and the winds blew and beat on that house; and it fell. And great was its fall.” (Matthew 7:24-27)
However, wise government and judicious engineering is not enough. The central, hilly part of San Francisco is indeed founded on rock, a good place to build not only for rain, but also for earthquakes which have less effect on rock. In contrast the estuarine deposits which fringe the bay area and recent alluvial deposits (sand!) are susceptible to liquification during an earthquake, indeed causing much greater damage. The choices to build and live in these areas are not so much about wisdom versus foolishness as economic necessity. The suburbs built on these soft deposits tend to be the poorest areas in the city. If your house is more likely to be destroyed by an earthquake in an area where earthquakes are inevitable, insurance becomes more expensive and the value of your real estate drops.
In contrast to San Francisco (wealthy albeit with a significant underclass within one of the wealthiest nations) Kathmandu is poor. Nepal is the second poorest nation in Asia. This fact can be attributed to political instability and corruption. I stayed in the Kathmandu valley when I visited Nepal in 2011. We stayed in a lodging house delightfully located in the centre of Bhaktapur. This wonderfully preserved ancient city with its temples and courtyards made of wood, stone, brick and metal is designated as a World Heritage Site. 4 years later, in April 2015, a massive earthquake struck, with an epicentre near to Gorkha to the NE of Kathmandu. The effect of this earthquake was amplified in Kathmandu, Patan and Bhaktapur as they are built on a basin of lake sediments. As distressing as the images were of these ancient temples reduced to rubble it only remains symbolic of what the Nepali people went through, with nearly 9000 killed and nearly 22,000 injured. Nabraj, who had been my guide when in Pokhara, had his family home near to Gorkha. Mercifully his family all survived, but sadly his village was all but wiped out. With no 
insurance and no government help, rebuilding was a major challenge, with his income from tourism also reduced to rubble.
With so many confounding factors it is apparent that earthquakes scoring highest on the Richter Scale don’t necessarily bring the biggest death toll. Population density, poverty and politics have a strong influence too. These are large and seemingly intractable problems, however as a geologist I would suggest that one of the ways to help with this is to understand how earthquakes work so that engineering and socio-political solutions may be put in place.
This takes us, via a few thousand miles and through 340 million years of history to mystery rock number 17 for the Hadrian’s Wall Community Archaeology Project. It is an example of fault gouge, the material which is produced when rocks fracture and move past each other. This fault is part of a fault complex to be found on the Northumberland coast at Howick and is likely to have formed as this Carboniferous sedimentary basin developed. Faults are responsible for earthquakes. On the Howick fault, there has been a total of about 40m of movement, compared to 3m of movement on the fault plane which created the Nepal earthquake. The fault plane at Howick is likely to be much smaller than that in Nepal, which means that each metre of movement on the Howick fault would have generated less energy than each metre on the Nepal fault. Additionally, this total of 40m would have been made through many much smaller movements spread over many millions of years. Given that this fault developed under crustal tension it would have created relatively low energy earthquakes.
The early Carboniferous period (in which these rocks formed) would have been home to many creatures including some of the earliest amphibians. I like to think that, like me, these creatures may have failed to appreciate these earthquakes as I did in my geothermal swimming pool in Iceland.
Attributions
Hekla eruption, February 2000. Photo by: Iceland monitor/Rax
Village on the coast of Sumatra: U.S. Navy photo by Photographer’s Mate 2nd Class Philip A. McDaniel, Public domain, via Wikimedia Commons