The PICTURES sources are also being recorded on 50 seismometers onshore. Addition of these seismometers to the recording array extends the imaging capability of PICTURES to the east and to greater depth on the plate boundary. The land seismometers were installed by faculty and graduate students from the University of Liverpool and the Universidad of Chile, who learned of this project 3 days before we left port. Many of the instruments were shipped from England, where they are packed up and ready to go whenever and wherever an earthquake or other exceptional seismological opportunity occurs. Getting these instruments through customs and into the ground in a remote part of Chile in 2 weeks was a monumental achievement that required the dedicated efforts of many people! These seismometers will stay in place recording local earthquakes until late December, when they will be moved south to record the Langseth source in south-central Chile as part of Langeth’s next expedition – CEVICHE (Crustal Experiment from Valdivia to Illapel to Characterize Huge Earthquakes).
Figure 1. A Liverpool rapid-deployment seismic station. Not shown, but also indispensable, is a shovel. The entire station will be buried. Careful notes are needed to recover the station and data!
Figure 2. The intrepid deployment team on the coast of Chile near Iquique. This team did a remarkable job of installing a large number of stations in a very short time, traveling on any road possible to distribute the stations across the study region. From left to right: Sergio Leon-Rios, Efrain Rivera, Javier Ojeda and Daniela Calle.
The development of GPS technology in the past 2 decades has produced a revolution in the scientific community’s ability to monitor deformation of Earth’s surface, including providing direct measurements of the deformation leading up to large plate boundary earthquakes. The radio waves carrying GPS data, however, do not penetrate into the ocean, and many of the most dangerous plate boundaries are located beneath the sea. Development of seafloor geodesy is therefore a high priority for geoscientists, and several different approaches are currently in development.
In December 2015, 23 acoustic geodetic seafloor transponders were successfully installed on the seafloor during GEOMAR’s R/V Sonne cruise SO244. These seafloor transponders are located in 3 arrays to form GeoSEA (Geodetic Earthquake Observatory on the SEAfloor). GeoSEA’s target is the segment of the Nazca-South American plate boundary near 21°S that is in the seismic gap left after partial rupture of much larger seismic gap in 2014 (see PICTURES scientific objectives). Array 1 on the middle continental slope consists of 8 transponders located in pairs on four topographic ridges, which are surface expressions of faults at depth. Array 2 is located on the outer rise seaward of the trench, where 5 stations monitor extension across plate-bending related normal faults. Array 3 is located on the lower continental slope where an array of 10 stations measures diffuse strain build-up.
The seafloor acoustic ranging methods provide relative positioning by using precision acoustic transponders (Autonomous Monitoring Transponder, AMT) that include: pressure sensors to monitor possible vertical movements as well as provide data to correct for tides; tiltmeters in order to measure changes in inclination; and sound velocity (SV) sensors to correct for sound speed variations in the water column. Data are stored internally and can be uploaded to either a High Performance Transducer (HPT) lowered from the side of a ship or an autonomous vehicle developed by Liquid Robotics and controlled via satellite that uses wave action for forward propulsion (the GeoSURF Wave Glider). PICTURES provided the first opportunity to upload data since installation of the array. Array 1 and Array 2 are located within the PICTURES footprint, and data were uploaded to the HPT. The waveglider was deployed to upload data from Area 2. However, although the Wave Glider was able to travel to Array 2 and obeys commands from its operator, communications turned out to be too slow for practical data upload. A future upgrade to the communications system is expected to solve this problem, but in the meantime, we have decided to transit 5 hours to Array 2 to upload data from that site using the HPT if we have unused contingency time near the end of the cruise.
Figure 1: The High Performance Transducer lowered from the side of the R/V Marcus G. Langseth. Photo by Jan Steffen.
Figure 2: The Liquid Robotics GeoSURF Wave Glider. The Wave Glider has two main parts: a float, which contains all sensors and communication units, and a subsurface wing rack, which is connected to the float by a 6-m long flexible umbilical tether. Directional control is accomplished with a rudder on the Glider sub unit. The float is equipped with satellite communication systems (Iridium Satellite LLC) for remote transmission of data, a GPS unit, and a weather station. It also contains batteries that are recharged by a solar panel to provide power at night. The Wave Glider periodically transmits is position via satellite to the “watchkeeper.” During PICTURES, the “watchkeepers” have had to be vigilant to command the Wave Glider (nicknamed “SUZI”) to stay out of the path of other ships. We will be recovering SUZI before returning to port.
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The science party for PICTURES includes five biologists (called PSO, for Protected Species Observers) who are charged with scanning the ocean around the ship to detect the presence of protected species. They take photos, identify the different species and keep detailed logs of their observations. The National Oceanographic and Atmospheric Administration (NOAA) has developed a well-defined and strict protocol to protect whales, dolphins, sea lions, seals, sea turtles to protect them from the sound generated to image the subsurface. This protocol includes the requirement that the sound level be ramped up gradually and that it be cut down when protected species are detected within a specified radius that depends on the water depth, source strength and type of animal. This leaves gaps in the data – a small price to pay to share the ocean with its majestic inhabitants.
Short-beaked common dolphins (Photo courtesy PICTURES PSO)
Green sea turtle (Photo courtesy PICTURES PSO)
Because our seismic source is on 24/7, the marine mammal specialists work shifts around the clock (like the rest of the science party). From dawn to sunset, two observers are outside on the observation tower, which provides a spectacular view of the ocean, the ship, and occasionally the Chilean coast, where desert mountains reach the sea. One observer is always stationed in the main lab, with their passive acoustic monitoring system (PAM), which enables 24/7 monitoring. The PAM records signals from a hydrophone (an underwater microphone) that is towed by the ship and records the sounds of the sea e.g. airgun shots, higher frequency chirps for mapping the seafloor (referred to as “the bird” because of the bird-like sound it makes) and, of course, the conversations of whales and dolphins. These sounds are displayed as spectrograms on two computer monitors as well as listened to with earphones.
PAM (Passive Acoustic Monitoring system) display
So far during PICTURES, we have seen many marine mammals and sea turtles. The PSOs are always willing to answer questions from the geophysicists about marine life, or to simply chat and tell jokes. 🙂
El límite oeste de Sudamérica corresponde a una zona de subducción activa, donde la placa oceánica de Nazca se desliza por debajo de la placa Sudamericana a una velocidad relativa de aproximadamente 7 cm/año. A lo largo de este límite ocurren frecuentemente terremotos, los que han sido reconocidos y catalogados desde hace varios siglos. Este registro ha permitido la identificación de brechas sísmicas (seismic gaps), segmentos de este límite donde no han ocurrido terremotos por largos períodos de tiempo, pero donde históricamente han ocurrido grandes eventos sísmicos. Es de notar que en una de estas brechas sísmicas se originó el terremoto del Maule Mw 8.8 en el año 2010, y que en el 1 de Abril de 2014 ocurrió un evento de magnitud Mw 8.2 (terremoto de Pisagua) en la conocida brecha sísmica del norte de Chile – Perú, donde no se había producido un terremoto de gran magnitud desde el par de eventos de los años 1868 y 1877. La brecha sísmica del norte de Chile ha sido desde algún tiempo de interés científico internacional, y es así como ha sido monitoreada por el programa IPOC (Integrated Plate Boundary Observatory of Chile) desde el año 2007 por medio de estaciones sismológicas y geodésicas instaladas en tierra. El terremoto de Pisagua, sin embargo, cubrió solo parcialmente la brecha sísmica, dejando una amplio segmento de ella sin activarse (ver Figura 1). Este terremoto es considerado como un evento con gran potencial para la contribución a un mejor entendimiento de cómo evoluciona el deslizamiento interplaca en la generación grandes terremotos. La Figura 1A muestra la historia sísmica del norte de Chile y sur del Perú desde el año 1868.
Dos características del terremoto de Pisagua son la base científica del proyecto PICTURES: una extensa y bien caracterizada secuencia de eventos sísmicos precursores en los meses y semanas previas al evento principal, y una notable correlación entre la secuencia de estos eventos y anomalías del campo gravitatorio terrestre en la zona de ocurrencia de estos, indicativas de características particulares de la composición y estructura geológica de la corteza terrestre de la zona. El proyecto PICTURES está diseñado para capturar imágenes de la estructura de esta región mediante técnicas geofísicas indirectas no invasivas, análogas a las utilizadas en medicina para obtener imágenes del interior del cuerpo humano (ecografías, CAT scanner), con el objetivo de entender el origen geológico de las anomalías gravimétricas observadas y su relación con la generación y propagación de terremotos.
(français)
La frontière oust de l’Amérique de sud est une zone de subduction active, où la plaque de Nazca plonge sous la plaque Sud-Américaine à une vitesse d’environ 7 cm/an.
Des tremblements de terre se produisent souvent le long de cette frontière, ce qui a permis d’identifier plusieurs «Lacune sismiques», segments d’une zone de la plaque où un grand tremblement de terre est retardé. L’une de ces lacunes était rompue en 2010 avec le tremblement de terre Maule d’une magnitude 8.8 sur l’échelle de Richter. Depuis 2007, une autre lacune, dans le nord du Chili et au Pérou, a été surveillée par le programme de l’Observatoire des frontières de la plaque intégrée – Chili (IPOC). En conséquence, la région terrestre a été bien équipée pour surveiller l’activité sismique et l’accumulation de contraintes lors du tremblement de terre du 1er avril 2014 d’une magnitude 8,1 sur l’échelle de Richter. Cependant, ce tremblement de terre a comblé partiellement la lacune en laissant la limite de la plaque non-rompue au sud.
Deux caractéristiques de ce tremblement de terre ont attiré notre attention:
une séquence étendue et bien caractérisée d’événements sismiques dans les mois et les semaines précédant le choc principal.
une corrélation frappante entre cette séquence et les anomalies dans le champ de gravité terrestre.
Des petites perturbations de la gravité indiquent des différences géologiques dans la composition (la densité) de la croûte terrestre. PICTURES est conçue pour imager cette région à l’aide de techniques analogues à celles utilisées pour l’imagerie non invasive du corps humain (combinaison d’un scanner et d’une échographie) afin de comprendre l’origine géologique des anomalies de gravité et leur impact sur la façon dont les séismes se multiplient et se propagent.
Nature 512, 299 (2014). doi:10.1038/nature13681
Figura 1A. Las líneas verticales muestran la extensión latitudinal estimada para la zona de ruptura de diferentes terremotos con los mayores eventos destacados en rojo. La Figura 1B muestra la topografìa del suelo oceánico (batimetría), el límite entre las placas de Nazca y Sudamericana, la velocidad relativa entre ellas, el modelo de deslizamiento de Schurr et al. (2014) con contornos de intervalo de 1 m, y las áreas de ruptura de otros terremotos recientes ocurridos en la zona. La línea blanca punteada muestra el segmento de ruptura interplaca durante el terremoto de Pisagua de 2014, y la línea sólida el segmento de la brecha sísmica aún sin romper. Si la brecha sísmica se hubiera roto completamente, podría haber generado un gran terremoto de magnitud M ~ 9.
(français) Cette carte montre sur la gauche l’histoire du tremblement de terre du nord du Chili et du sud de Pérou depuis 1868. Les lignes verticales montrent l’estimation étendue nord-sud de la zone de rupture dans différents tremblements de terre, avec les événements les plus importants représentés en rouge. La carte de droite montre la topographie du fond marin, le bleu foncé atteignant une profondeur de 7000 m, le rose et le gris montrant le fond marin peu profond de la crête d’Iquique. La ligne noire avec des triangles montre la limite de plaque où la plaque de Nazca plonge sous l’Amérique du Sud et la flèche indique la direction du mouvement. Les ovales colorés juste au large de la crête Iquique et de Pisagua montrent un modèle du glissement pendant le tremblement de terre du 1er avril 2014, avec des contours d’un mètre d’intervalles. Les zones ombragées de vert clair sont les zones de rupture d’autres séismes récents. La ligne blanche en pointillés montre le segment de la limite de la plaque qui a éclaté pendant le tremblement de terre de 2014, et la ligne blanche pleine indique les lacunes restantes. Si tout les lacunes s’étaient rompu à la fois, il aurait pu engendrer un séisme aussi grand ~ magnitude 9.
-Spanish translation contributed by Felipe Gonzales Rojas and Emilio Vera
(Science Objectives translation prepared by the German members of the PICTURES cruise)
Ziele der Fahrt
Der westliche Teil des südamerikanischen Kontinentes ist eine aktive Subduktionszone, an der sich die Nazca Platte mit einer Geschwindigkeit von etwa 7 cm im Jahr unter die südamerikanische Platte schiebt. Dieser Vorgang führt zu häufigen Erdbeben. Aufzeichnungen dieser Beben über lange Zeiträume ergab unter anderem, dass die Beben entlang der Subduktionszone nicht überall gleichmässig verteilt sind, sondern das sogenannte seismische Lücken („seimic gaps“) vorhanden sind. In diesen Lücken sind starke Erdbeben lange überfällig. Eine dieser Lücke im Grenzgebiet zwischen Chile und Peru wird mit dem „Integrated Plate boundary Observatory – Chile“ (kurz: IPOC) seit 2007 kontinuierlich überwacht. Mithilfe des Observatoriums wurden viele Daten über Erdbeben und den Spannungszustand der tektonischen Platten aufgezeichent. Damit konnten auch wichtige Erkenntinsse aus dem Stark-Beben in der Nähe von Iquique vom 1. April 2014 (Magnitude von 8,1) gewonnen werden: Trotz der starken Erschütterung wurde nämlich dabei nur der nördliche Teil der seismischen Lücke geschlossen, was eine im Süden weiterhin unter starker Spannung stehende Plattengrenze hinterlässt. Zwei Besonderheiten richteten die Aufmerksamkeit der Wissenschaftler auf dieses Beben-Gebiet: (1) Eine ausgedehnte Zeitspanne mit Beben in den Monaten und Wochen vor dem Hauptbeben und (2) eine starke Korrelation zwischen diesen Vorbeben mit Veränderungen des lokalen Schwere- oder Gravitationsfeldes. Schwankungen im Schwerefeld deuten auf geologische Veränderungen in der Zusammensetzung der Erdkruste hin. Die Ziele des Projektes PICTURES sind deswegen darauf ausgerichtet, diese Veränderungen entlang der Region der seismischen Lücke zu finden und zu charakterisieren. Dazu werden geophysikalische Messtechniken eingesetzt, die der nicht-invasiven Abbildung des menschlichen Körpers ähneln, wie z.B. Ultra-sound oder tomographische Verfahren wie CAT-scans. Eine Kernfragestellung für die Wissrenschaflter ist dabei wie die ungewöhnlichen Änderungen im Schwerefeld hervorgerufen werden, und ob sie sich auf die Entstehung und Ausbreitung von Erdbeben auswirken.
Nature 512, 299 (2014). doi:10.1038/nature13681
Diese Abbildung zeigt auf der linken Seite die historische Abfolge von Erdbeben in Nord-Chile und Süd-Peru seit 1868. Die vertikalen Linien zeigen die vermutete Ausdehnung der Zone entlang der Plattengrenze, mit den stärksten Beben in rot eingefärbt. Auf der rechten Seite ist eine topographische Karte der Region um Iquique zu sehen. Die schwarze Linie mit Dreiecken zeigt die Plattengrenze und Bewegungsrichtung an (nach Osten). Die gefärbten Konturlinien vor der Küste von Iquique und Pisagua beschreiben die Laterlverschiebungen innerhalb des Erbebengebietes in Intervallen von je 1m. Die weiß-gestrichelte Linie zeigt das Segment, in dem die seismische Lücke geschlossen wurde und die weiße Linie im Anschluss die Bereiche, wo sie weiterhin besteht. Wäre die gesamte seismische Lücke während eines großen Erdbebens geschlossen worden, so wäre wahrscheinlich ein Beben mit einer Stärke über Magnitude 9 möglich gewesen.
-contributed by Michael Riedel, Jan Handel and Florian Petersen
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The objective of PICTURES is to generate images of faults and other structures beneath the seafloor. Although we have to wait until the end of the cruise to recover the Ocean Bottom Seismometers and get our hands (and computers) on the data, we get the multichannel seismic (MCS) reflection data in real time and have been having a wonderful time processing the data to generate pictures. These data are recorded on a long (up to 12.5 km!) tube that is towed behind the ship and contains thousands of hydrophones. The hydrophones record minute pressure changes generated as sound waves from our seismic source bounce off the seafloor and structures beneath it. The cartoon below shows ray paths of reflected waves recorded on the hydrophone streamer and refracted waves recorded at distances beyond the streamer by OBSs or sonobuoys (expendible floating buoys that contain a hydrophone and send data by radio back to the ship). For this cruise we are using OBSs rather than sonobuoys because they stay at a fixed place on the seafloor and can record data to much greater distance.
The seismic reflection data are rearranged in the computer to generate images like the one shown below, which shows the oceanic crust of the Nazca plate being thrust beneath the continental margin of Chile. Although it looks like the underthrust (i.e. subducted) crust is flat beneath the accretionary prism, this is an illusion. The vertical axis in this image is the time it takes for the acoustic energy to travel from the energy source to the reflector and back to the streamer. To convert this axis to depth, information on the speed of sound is needed, and sound travels more quickly in the accretionary prism than in the ocean. The speed of sound in water is approximately 1500 m/s. A rough guess of the speed of sound in the accretionary prism is 3500 m/s. We therefore estimate that the Nazca plate crust is at about 7 km depth at the trench and at about 9.5 km depth at the eastern edge of this image. The OBS data will provide more detailed information on the speed of sound beneath the seafloor that will be used to improve the imaging.
And for a contrasting view, here is an image we just got this morning showing a large seamount or ridge entering the subduction zone.
We are scrambling to process the data as fast as it comes in to get a first look at the data. Check back again for more pictures from PICTURES.
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Eating a good meal is very important, but luckily here on the R/V Langseth we have a team of top cooks. They always have a lot of ideas to share with us, with cuisine from America to Asia, and everywhere in between. There are always several choices between different dishes just in case you do not like one (so spicy !!). And never forget the delicious dessert!
The meals are always ready on time, at the same time, every day. It’s the superb moments around these meal times that get everyone out of work to talk and laugh. Sometimes these are the only moments of the day that we see the people who work in different parts of the boat than us. Truly, these moments of sharing and socializing are really important for everyone!! Even if you miss a meal or find yourself hungry for snack, there is always something to eat so you’ll never die of hunger 😀 – the salad bar, sandwiches, leftovers from meals and sweets (yummy cakes and cookies!)
I knew as I left France that my biggest problem was figuring out how I will spend 2 months at sea without candy or chips. It was a real concern for me, so I made sure to buy some candy for myself before leaving (but only one box) :D. Luckily, soon after the boat departed, I was surprised with more and more candy every day – all thanks to Tom, who truly thought about everyone when it came to sweets. During our shifts we eagerly wait for Tom to know what surprise he will have for us that day :D. Even when we work on deck he never forgets and always comes with candy or chocolate for everyone. This is not something he does for himself – he is always giving away more candy than he eats himself. Tom, we really appreciate you <3
But we also cannot forget Todd! It is impossible for 4am to pass without seeing Todd sit before his computer with the best pistachios in front of him, sharing them with everyone. Todd has more pistachios in his suitcase than clothes 😀 He prefers the peppered ones, but I prefer the salt. This makes sharing no problem for us!
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An OBS records earthquakes and other seismic and acoustic signals in the deep ocean. The instrument package includes two sensors – a seismometer to record vertical and two components of horizontal ground motion and a hydrophone to measure sound. In addition, batteries, a very accurate clock and a data logger are included in a tube that can withstand very high pressures. The instrument is dropped overboard and falls freely to the seafloor. When it is time to recover it, an acoustic signal is sent to the release system to decouple the anchor from the rest of the instrument. Without the anchor, the OBS is buoyant and floats to the sea surface, where it is fished out of the ocean and the data are downloaded to the computer. The steel anchor rusts on the seafloor and dissolves after several years. Some OBSs have a back-up timed release in case the acoustic release malfunctions.
To make it easier to find the OBS when it surfaces, it is normally equipped with a radio sensor and a blinking light source. These devices have a pressure sensor inside which ensures that they switch off below a specified water depth to preserve battery power while on the ocean bottom.
Here are pictures of the two types of OBSs used during PICTURES:
SIO OBS prepared for deploymentGEOMAR OBS prepared for deployment
The main difference between the two OBSs shown are the way the components are assembled and the type of flotation. The SIO-OBSIP OBS uses pressure-resistant glass balls in a yellow plastic shell, whereas the GEOMAR OBS uses syntactic foam, which allows it to go as deep as 8000 m. The deepest OBS during PICTURES is at 7070 m, and the shallowest is at 674 m water depth. Both types of instruments can be left on the seafloor for up to 1 year to record earthquakes.
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I really should start keeping track of how many times people not only ask me “What do you do out there?!” but how common it is to have it followed up by some quippy comment like “Do you have scurvy yet?” or “How many people have had to walk the plank?” I can’t lie, we are definitely rationing the fresh fruit, but the salad bar is holding on strong. I have never been on a boat for more than 10 days, and this is going to be 49. Seven weeks of constant motion, of constant science, of constant ocean. I really didn’t know what to expect about how my life would drastically change for that time period…
So what is it really like? Well, it’s kind of like a happy, low security prison (we get one 15-minute phone call a week) or like living in a really small town where you greet the same faces every morning. Some days are very busy, filled with logging data on watch in the main lab, assembling and releasing OBS, deploying the streamer and source for active seismic, and other prep on deck. The science going on here is ever changing, with a constant influx of a truly monumental amount of data. Everyone wants to be involved when something new is taking place, and it is easy to fill the day when there are physical tasks going on. However, as we settle into the active source seismic process, there is often much more downtime to fill. Learning the routine of a boat that is operating 24 hours a day certainly involves learning how to balance that work with something else.
For mental relaxation and fun there is plenty available for entertainment. There is a small theater with ample movies, as well as a server with films or TV shows to watch in your cabin. On the first night before leaving port we watched The Life Aquatic, a classic Wes Anderson film perfect for priming us with expectations for this voyage. Now that we are underway, a lot of people have a TV show they are binge watching with the hours to kill they wouldn’t have back at home to power through season after season of Shameless or Grey’s Anatomy (two currently battling examples). Later at night you can often hear boisterous card games and conversations going on in the mess hall. There are few times when many peoples’ shifts overlap, and a large game of cards seems to be like a consistent means to celebrating those times.
Getting out of the windowless seismic lab and outside is also necessary when there are no jobs to do that force you out on the deck. To that end, we have each started to find our favorite places on the boat to sit. Getting a little sun, enjoying the sea breeze, listening to the airguns, looking for whales, and reading a book that isn’t some data processing manual are definitely peaceful ways to get outside. However, after the first week, finishing the only book I brought, I quickly realized I should have brought more. Thankfully, people are willing to share and there is a small library with an assortment of books.
Many of us also try to make time for a visit to the ship’s gym every other day or so, which is in a container on deck. This is one of the best ship gyms I have seen, with an elliptical, rowing machine, treadmill, stationary bike, and an assortment of weights. The motion of the boat, especially when we are caught in a period of increased side to side rolling, makes using the gym quite the sweaty adventure. Never before have I had a treadmill come with such a good view as well as a warning about using it. I personally try to make going to the gym a daily staple, pretending that I’m getting in my daily bike commute and after work run that I would be used to at home. It helps to balance all the good food that I have little willpower to avoid, and the challenge of running completely hands free on the treadmill provides a goal yet to be achieved.
The ship’s container gym – packed but incredibly useful!
Finally, holidays and birthdays do not go unnoticed on the ship either. Halloween was full of as much candy as any other year, though only one person dressed up (the head PSO Cassi was Princess Leia). Everyone did get involved decorating the lab with paper pumpkins and skulls, and we are phasing into a new round of decorations for Thanksgiving later this month. The crew and science party are making photo collages of their loved ones to post on the wall in the lab, a chance for people to feel especially thankful of their family, their animals, and their significant others. We have also had a few birthdays to celebrate, which provide opportunities to embrace working with what you got, using found decorations like the Spongebob Squarepants birthday banner and having the kitchen make a cake with fake candles made of toothpicks. Having people from so many different countries, birthday signs in English, Arabic, German, Russian, and Spanish were hung to surprise Kathy at her midnight watch, along with a seismic reflection birthday card. Welcoming Jan to his very early shift with “Happy Birthday” on the monitors as well as a bowlful of his favorite chocolate ice cream. We might be a little nerdy about how we do it, but I’d say we really know how to enjoy our time at sea and weeks are passing by more smoothly than I ever expected.
Clockwise from upper left: Our Thankful photo wall, Halloween decorations, and celebrating Jan and Kathy’s birthdays!
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The western boundary of South America is an active subduction zone, where the Nazca plate is being thrust beneath the South American plate at a rate of approximately 7 cm/yr. Earthquakes occur along this boundary frequently, resulting in a long record of activity that led to the identification of several “seismic gaps,” sections of the plate boundary where a large earthquake is overdue. One of these gaps ruptured in 2010 with the M8.8 Maule earthquake. Another gap, in northern Chile and Peru, has been monitored by the Integrated Plate boundary Observatory – Chile (IPOC) program since 2007. Consequently the onshore region was well instrumented for monitoring earthquake activity and strain accumulation when a M8.1 earthquake struck on April 1, 2014. This earthquake, however, only partially filled the gap, leaving a long stretch of un-ruptured plate boundary to the south. Two features of this earthquake caught our attention: an extended and well-characterized sequence of seismic events in the months and weeks prior to the mainshock, and a striking correlation between this sequence and anomalies in the earth’s gravity field. Tiny perturbations in gravity are indicative of geologic differences in the composition (and therefore, the density) of the earth’s crust. PICTURES is designed to image this region using techniques that are analogous to those used for non-invasive imaging of the human body (a combination of a CAT scan and an ultrasound) in order to understand the geologic origin of the gravity anomalies and their impact on how earthquakes nucleate and propagate.
This map shows the earthquake history of northern Chile and southern Peru since 1868 on the left. The vertical lines show the estimated north-south extent of the rupture zone in different earthquakes, with the largest events highlighted in red. The map on the right shows the topography of the seafloor, with dark blue being as deep as 7000 m and the pink and grey showing shallow seafloor on the Iquique Ridge. The black line with triangles shows the plate boundary where the Nazca plate plunges beneath South America and the arrow shows the direction of motion. The colored ovals just offshore Iquique and Pisagua show a model for the slip during the April 1, 2014 earthquake, with contours at 1 m intervals. Light green shaded areas are the rupture areas of other recent earthquakes. The dashed white line shows the segment of the plate boundary that ruptured during the 2014 earthquake, and the solid white line shows the remaining gap. Had the entire gap ruptured at once, it could have generated an earthquake as great as ~M9. (adapted from Schurr et al., Nature, 2014)
