Taking the Earth’s Pulse – Dr Taka’aki Taira Listening to Seismic Noise TAKING THE EARTH’S PULSE – LISTENING TO SEISMIC NOISE Research Seismologist Dr Taka’aki Taira at the University of California at Berkeley and his colleagues investigate changes in Earth’s structures related to earthquake rupture and volcanic eruption by exploring ways to listen to and interpret ambient seismic noise. The Earth Not Only Shouts at Us – as the building of bridges and high-rise It Also Whispers buildings require calculations of the elastic properties of the soil to determine whether Earthquakes are scary things. Just the the structures will be susceptible to shifting shaking and rolling of the normally stable due to earthquakes and other seismic ground in a mild temblor is enough to events. In other words, seismic noise may frighten people and animals. And the indicate whether the ground is firm enough damage large earthquakes cause – including or not. This is where Dr Taka’aki Taira and his the human death toll – seems to make colleagues concentrate much of their recent the news on a regular basis. For example, professional attention – studying seismic recently a violent earthquake struck Gorkha, noise in relation to actual earthquakes for Nepal, killing 9,000 people and injuring determining whether noise is not noise at 22,000. A century ago it was the historic 1906 all, but a possible indicator of stress in the San Francisco earthquake – one of the largest tectonic plates near earthquake prone areas. measure the total strain accumulation by this natural disasters in the history of the United tectonic plate model. But this is not enough. We need to know when the Earth can’t States resulting in the destruction of 80% The Science of Seismic Fault Dynamics is endure the accumulated strain anymore, of the city of San Francisco and the loss of Itself a Dynamic Science over 3,000 lives – that resulted in a veritable i.e., the strength of the crust. Moreover, eruption of scientific interest in earthquakes, Since the 1950s and 1960s, scientific scientists recognise the strength might vary seismic motions and methods to observe understanding has been that the Earth’s rigid over time. When the crust is weakened, and forecast these kinds of disasters. For outer layer – the lithosphere – is broken up earthquakes would occur more frequently the last century, scientists the world over into seven or eight major tectonic plates, even though the accumulated stain is have studied the creaking and cracking along with a number of smaller plates, that smaller. This would lead to a more complex of the Earth’s crust in an effort to warn us essentially float on the fluid inner layers of pattern of earthquake cycles. Without of impending catastrophes. But the big the Earth. These tectonic plates are always knowing the crustal strength, it is extremely sounds that accompany such phenomena as slowly moving, separating in some places difficult to forecast earthquakes. Therefore, earthquakes and volcanic eruptions are not and colliding in others. But the important observing the temporal variations of the fault all that you can detect with a seismograph. thing is when they collide their edges strength has been a long-sought goal of the You can also hear background whispering grind together causing massive friction. earthquake science community over the last of the Earth’s crust – the so-called ‘seismic Where plates collide – called a fault – the few decades. noise.’ friction of that collision can cause energy to accumulate over time, which can suddenly Together with Paul Silver of Carnegie Seismic noise is the rather nonspecific release when the amount of built up energy Institution for Science, Dr Taira and his team term for the fairly persistent, low frequency overcomes the force of friction between developed a new means to monitor fault vibration of the Earth’s crust from any the two edges. According to this model, strength by analysing seismic waveforms and number of causes. Also known as ambient an earthquake results from a sudden slip microearthquake activities, and published vibrations, it is referred to as ‘noise’ on a fault. This slip causes the edges move their findings in Nature. The team found because it is ordinarily an unwanted part against each other, releasing energy in the first field evidence showing the fault of the signals recorded by seismometers. waves that travel through the Earth’s crust strength was temporally weakened, and this Other disciplines besides seismology also and causing vibrations and movement temporal weakening was responsible for term it ‘noise’ because it is a nuisance for that can lead to damage and destruction. clusters of earthquakes in Central California. things that are sensitive to vibrations, like Simply speaking, the tension builds up over More importantly, the reduction of the fault precision telescopes or the commercial time until the edges of the fault can’t hold strength they found in California was induced growing of crystals. On the other hand, it anymore and an earthquake results. Thus by dynamic stress changes from a distant measuring ambient vibrations can be scientists have been focusing on observing 2004 magnitude 9.1 Sumatra earthquake that helpful in engineering, where projects such the deformation at the Earth’s surface to had occurred on the other side of the world. WWW.SCIENTIA.GLOBAL The implication of their finding is that distant Dr Taira and his colleague Florent Brenguier, down with enough force to cause changes large earthquakes may increase the risk of of the Université Grenoble Alpes, analysed in the seismic velocities in the subsurface subsequent earthquakes around the globe. ambient vibrations at six stations in the hydrothermal fluid. Dr Taira feels that this More recently, Dr Taira has concentrated on Northern California Seismic Network is most consistent with a hydrological recording ambient vibrations – generally around Lassen Peak, a mountain in the load model, where surface loading presses considered nonspecific background noise – Lassen volcanic area. The data from these the hydrothermal fluid out, leading to an and correlating them with specific volcanic stations was electronically processed to increase in the opening of cracks in the and earthquake activity. show changes in the speed of seismic waves crust. The weight of snow forces fluid already traveling through this area. Variations in below the surface to cause cracks in the ‘Noise’ Can Be Considered a Heartbeat of seismic wave speed are indicative of changes various subsurface rocks and sediment. At Seismic Activity in tectonic stress in the area. Essentially, any rate, this effect of the hydrothermal fluid they established a quasi-real-time velocity movement is correlated with reductions of In the American Pacific Northwest, Pacific monitoring system through the use seismic seismic velocity. This allowed Dr Taira and his Oceanic tectonic plates have slid below the interferometry with ambient vibrations. Their team to deduce that heated hydrothermal North American Plate for millions of years. monitoring system showed the variability fluid is responsible for the long-term changes Heat from this tectonic subduction has given of seismic velocity over time in response to in seismic velocity, and those changes rise to numerous volcanoes from California stress changes from earthquakes and from in velocity can be used to understand – all the way up to British Columbia over the seasonal environmental changes. basically in real time – what is going on with recent geologic past – say, the last 30 million the subsurface fluids. This is exciting news, years. It is also responsible for seismic activity Interestingly, dynamic stress changes from giving hope that monitoring of the ambient in the Lassen volcanic area, located at the an actual magnitude 5.7 local earthquake vibrations can tell us what the fluids and southern edge of the Cascade Mountain produced a measurable velocity reduction rocks kilometres deep are doing. This in turn Range. Here, at the Lassen Volcanic Center, is 1 km below the surface. Calculations from might give an early warning of volcanic or where Dr Taira and his colleagues have their the changes surrounding this earthquake earthquake activity. seismology listening post. The Center sits indicated that the Lassen hydrothermal above a hydrothermal system that is feared system contained highly-pressurised Is This an Isolated Finding? might be the site of hydrothermal explosions hydrothermal fluid deep beneath the surface. at some time in the future. Dr Taira monitors Dr Taira’s measurements also show that the Dr Taira’s results from the Lassen monitoring this area, aiming to develop a new way of long-term seismic velocity changes closely are very exciting, but they aren’t the first forecasting volcanic phenomena by using follow snow-induced vertical pressure time he’s used ambient vibration monitoring seismic noise correlated with actual seismic almost immediately. That is, winter snow to listen to earthquakes. Recently, in the and geologic activity. accumulation on the surface actually pushes journal Geophysical Research Letters, Dr WWW.SCIENTIA.GLOBAL Taira and his group published their studies of ambient vibration-based ability to understand the underlying mechanisms of earthquakes and monitoring they used to look at the temporal variations of crustal volcanic eruptions. However, their monitoring system is not complete. seismic velocities before, during, and after the 2014 magnitude 6.0 Their present system puts out a daily velocity change of the ambient earthquake in the South Napa area. This South Napa earthquake is vibration pattern. If they had more computing power, however, they the largest earthquake in the San Francisco Bay Area, since the 1989 could perform massive cross-correlation computations and perhaps magnitude 6.9 Loma Prieta earthquake. They saw a velocity drop give an hourly update, even streaming it in real-time online. This would immediately after the South Napa earthquake.