Active tectonics of the Gulf, , the fastest growing rift on planet Earth

(Dr. Athanassios Ganas)

Abstract: Since the beginning of the instrumental seismology in Greece, the most intense seismic activity has been recorded within the Corinth rift and its vicinity such as the central . This rift is the most active continental seismic region of the Mediterranean and one of the fastest opening continental rifts in the world. Its seismic history since the nineteenth century has exhibited more than 10 shallow events with M>6.0. A dense GPS velocity field is used to model the present-day deformation by means of a continuous strain and rotation rate field. The geodetic results quantify and refine previous findings as well as yield new insights for the present-day deformation pattern in the rift. The consistency of the contemporary deformation field with earthquake activity is also evaluated by the calculation of geodetic moment rates and comparison with moment rates determined from earthquake catalogs. In this café, Dr. Athanassios Ganas presented briefly why Corinth Gulf is known as the fastest growing rift on planet earth by the support of his recent research results.

Host: Good evening ladies and gentlemen. This is English GeoScience Café session 18. It’s a great pleasure for me to introduce our speaker tonight, who is going to talk to us about “Active tectonics of the Corinth Gulf, Greece, the fastest growing rift on planet Earth”. We are honored and delighted to have Dr. Athanassios Ganas with us today. Dr. Athanassios Ganas completed his PhD in Geological Remote Sensing from the University of Reading, UK. His expertise is in Active Tectonics and Geophysical Earth Observation including Fault Interaction, Tectonic Geomorphology and GPS networks. He has published 66 papers in major international journals and has 1019 citations on the Science Citation Index. He serves as member of the Greek National Committee for seismic hazard Assessment and was elected member of the Executive Board of the Geological Society of Greece (Vice-president since 2016). He is a regular member at the Board of Directors of the EPPO (Earthquake Planning and Protection Organization, Greece). This whole session is divided into two parts. The speaker will give his valuable talk in the first part followed by a question and answer session. Please be calm and keep your mobile phone in silent mode. Ladies and gentlemen, please join me to welcome our honorable speaker, Dr. Athanassios Ganas. Dr. Athanassios Ganas: Good afternoon everybody. Thank you for coming to my talk today. If anyone has any question, please feel free to ask me. Right, is it possible to turn off the light, please? Only those lights above the projector so that you can see better slide color. I remember when I did my Master’s degree in Canada 30 years ago, there was no Power Point, no computers; people were used to slides. So, we had talks by these great geologists, going to the Great Slave lake in the north-west territory upon the North Pole or North America or Europe, wherever they went, so we had really good slide projectors and that’s why we didn`t need any light in the room, as that prevented natural color to show on the slides.

Fig. 1 Dr. Athanassios Ganas in EGSC Session 18 (photo by Suo Yingbo)

Today I am going to talk about the Corinth gulf in Greece, which is the fastest growing rift on this planet. I will present some evidence for that, basically coming from three different geo- sciences (Geology, Seismology, and Geodesy). The primary objective of my research is to know the way earth deformation generates faults. Then, I need to map faults if I want to prove this is true. Different kinds of faults, the rate of faulting, big structures, small structures, massive structures and so on. 1. What is a Fault?

Α fault is a discontinuity inside the upper part of the lithosphere. When the faults grow they generate earthquakes. Actually, a fault locality is the basic parameter to estimate in a deterministic way the effects of large earthquakes on the built environment. So, if you want to study about an earthquake in a certain place, you need to have a very clear concept about the faults in this area, (especially how many, where, what size and how often). So, that's why we need fault models. This is the prime reason why I do my research.

2. Why fault models are necessary?

First, you need spatial information. This is necessary for all sort of studies. You need to have a location, dimensions, geometry and seismic potential of each fault. Second, you need to estimate the slip-rate of faults. How fast the fault slips? Actually, we geologists, estimate it in mm per year. Third, you need to estimate the ground motion using a Shake-Map. Fourth, you need to know where the faults are. If you are able to know where a fault is, then you can predict the damage it will cause. There are some relationships to calculate ground motion. This methodology is true for every part of the world. All you need is good field data and precise scientific data to support the model. In the end, this fault can be imported to a digital database for open access by citizens. Fault maps are very important for risk assessment in a hazardous area. Today you can create fault maps in GIS. For that you need to export the file in .kmz, .kml or .shp file. This file can be exported into either commercial or open access database like SQL or any like that. Look at figure 2. This represents Greece, situated on the south-east of Europe. To the south of Greece, the African-Mediterranean plate is situated and this plate is moving towards north. Using GPS measurements, it is estimated that the motion of African plate is 5-7 mm per year. What do we see there? The Hellenic Arc which is the most seismically active part of Europe, but little is known about its mechanics. We modeled deformation along the arc using a finite element model. The model was intended to capture the large-scale 3-D structure of Nubian block subducting beneath the Aegean block and its deformational consequences. We found that the Nubian block is moving 4.9 mm/year in the north-west direction and the Aegean block is moving 33 mm per year at south-west direction. Fig. 2 The two blocks of the finite element model showing the 3-D shape of the subducting Nubian slab and the applied displacement vectors. Deformation of the upper plate is shown exaggerated 100 times. Color shading is differential stress in the upper plate.

3. Neotectonic configuration of

What do we get out of these pictures? That the African-Mediterranean plate is moving beneath the Eurasian plate and the Eurasian plate is moving over the African plate. This deformation generates the faults, both in central Greece and in the Gulf of Corinth (Figure 3). The red lines are the big faults here and these faults bound four blocks over the whole crust. So, what actually happens in-terms of earthquake size? Along the boundary of each block, the big faults generate big earthquakes (Up to Magnitude 7). There is also some inter-block deformation. Inter-block deformation is the cause of middle size faults and these faults generate earthquakes of magnitude 6 (see figures 6 and 7). So, sometimes we can predict the size of an earthquake and not the time of it, because we know how big the faults are! Therefore, this is the neotectonic configuration of central Greece. Fig. 3 Neotectonic configuration of central Greece, orientation of section is north-south

4. Shape of African-Mediterranean Plate

Again, we have already figured out what`s happening in the down going African- Mediterranean plate with the help of seismic reflection data. On figure 4, all these red points are seismic stations aligned to the NE-SW direction. We also have one cross-section here (see lower left of Fig. 4) representing the top of African-East Mediterranean slab (thick grey line). According to these results, the depth of the slab varies between 40 and 100 km. These results come from passive sources (distant large earthquakes). Not from active sources. We have also used big earthquakes in this region to map the shape of this subducting plate. Then, with this information, figure 4 shows how the seismicity of this subducting plate is organized. So, what is the outcome of this study? When the African plate moves beneath Eurasia, it is not any more remaining as one body but it is rather broken like slices or panels. So, generally speaking, there is no chance to generate big earthquakes like those in Japan or Chili. This is another tool we have in geology, that is to use seismology to estimate deformation of subducting slabs. Fig. 4 The African Mediterranean Plate`s panel shape

5. So, what`s happening on the crust?

Fig. 5 The important faults of the fast-growing rift of the Gulf of Corinth There is a 130 km rift on the Gulf of Corinth (Fig. 5). In this area, lots of sediments have been stored during 5 million years. Since the last 1 million years, the storage rate became faster than before. So, this 130 km rift is very young and growing very fast. Some of you may want to know why it`s growing so fast? What`s the mechanism? To understand the reason, we collected all these data from geology, seismology, and geodesy. At the eastern part of the rift there are some young faults and all of these faults are normal in nature. As is shown in Fig6, my assistant is showing the direction of slip vector of the fault. The bedrock is limestone. In Fig.7, the left image shows the earthquake occurred in 3rd, March, 1981, at the magnitude of 6.3. This is the evidence of deformation on the earth surface. In the right image, I also put a sketch here just to show how the things happen.

Fig. 6 Assistant showing the direction of the slip vector of the fault. The bedrock is limestone.

Fig. 7 One Example of Deformation on earth surface. 6. What`s happening on the west side of Corinth rift?

Now, if we go to the west side of the Corinth rift, we will see this normal fault (Fig. 8; the area) which is a normal fault separating sediments on both blocks (hangingwall and footwall). So, this fault is working now but what we have found here is that, it gradually extended its footwall space towards both ends. This is the mature stage of fault growth or footwall expansion.

Fig. 8 Triangular facets, west side of Corinth Rift

7. How can we study the fault?

Now, there is a question. How can we study this fault? Geomorphology is a very useful tool to study this fault. Facets are the feature of erosion, so we map the facets (Fig. 8) because we found that there is fast erosion along the slope of the mountain (Fig. 9) just above the sea-side of the fault. This area is a very nice area to study active tectonics. This whole area was under water just 1 million years ago. Fig. 9 A diagram of Slip rate Vs average facet height for 10 active normal faults, read the paper by Tsimi and Ganas (2015)

So, if you put this relationship in x, y diagram, slip rate (mm/yr) versus average facet slope for 10 active normal faults, you will find that the slip rate is about 0.8 mm/year. If you have data you can try this relation in China and find out if it works.

8. Can seismology predict an earthquake?

We already talked about what seismology can tell us about deformation and extension. So, what we do when we have this figure (10)? We have the data of all the major earthquakes from 550 BC to 1900 AD. In the western part of the Corinth rift, near Aigion, during the study period, we identified 28 confirmed historical earthquakes. Most of the earthquakes are of magnitude 6 and above within a radius of 50 km. Fig. 10 Data of major earthquake locations from 550 BC to 1900 AD, in the western part of the rift, Aigion.

If we consider all of these earthquakes as a cluster of earthquakes and we just consider the mean value, we can easily say that one earthquake happens every 87.5 years in this area. So, statistically, we can predict an earthquake. So, if anyone wants to study active tectonics this is a nice area to go and work. There is a lot of evidence for a major earthquake in the past and such major earthquakes will tell you some things about the future seismicity.

9. The first-ever map of an earthquake on the earth.

Now, I want to show you something. A historical evidence. A map, made by Schmidt, a German Director of the National Observatory at Athens. In 1861, a big earthquake happened near the western part of the Corinth gulf and Schmidt mapped the ground ruptures (Fig. 11). 15 years later, Schmidt went to Germany to work and created the first-ever map of an earthquake on the earth. There is some controversy about who made the first ever earthquake map! Some say an American geologist created the first ever earthquake map but I think this is Schmidt who made the first such map. Fig. 11 First earthquake map made by SCHMIDT

10. The CMT (Global Centroid-Moment-Tensor) project

Moving to seismology, we have moment tensors to analyse large earthquakes. Moment tensor inversion is a method to find out fault parameters from seismological data.

Fig. 12 The CMT (Global Centroid-Moment-Tensor) project solution for the 1995 earthquake near Aegion and its ERS interferogram (upper right)

We use the CMT (Global Centroid-Moment-Tensor) project to collect moment tensor history. CMET has been continuously funded by the National Science Foundation of USA. Almost 23 years ago in 1995, we have seen the last major earthquake in this part of the Corinth rift with magnitude 6.5 Mw (Fig. 12). Fig. 13 Instrumental Earthquakes in Aegion

We can see there are 16541 instrumental earthquakes within a radius of 30 km only in Aegion from 1964 to 2014 from Fig.13. The other figure indicates that 9795 earthquakes took place in this area from 01-01-2012 to 01-06-2016 and we also see that most of the seismicity occurrence is offshore.

Fig. 14 Results of relationship between Depth ~ Distance Now look at the results in Fig.14, what I found is amazing! Fantastic picture of seismicity. I can see where the faults are! So these are the offshore faults rooting at a very low angle fault. The low-angle fault shows on almost all north-south cross-sections. We see profile-3 and profile- 4 are almost similar. So, what do we see? We have thousands of small earthquakes formed along the low angle normal faults. In geology, we cannot see it because it is happening several kilometres below the earth’s surface, but in seismology, we can see what`s the primary mechanism. The mechanism of the fast extension on this rift is the motion along the low angle normal fault.

11. Method to find out the Inland deformation

We have a lot of deformation inland. To find out the deformation of the inland, we cannot use In-SAR because there is a lot of noise. So, we used GPS.

Fig. 15 ASPIDA-3 GPS campaign Aigion region

According to figure 15, red lines are the active faults. The city of Aigion is situated above the normal fault. So, what did I? we drilled the rock and put some GPS points and observed for 8 hours. Then we collected GPS observation data and measured the antenna position at each time. During this job, you have to be very careful. You cannot play with errors. The purpose is to minimize the errors. Less errors bring good data and good data bring precise results. People may not like you for some reason but they cannot disagree with the data.

12. Understanding Horizontal and Vertical Extension

Now we can see the overall picture, and we can map the information with GPS over larger scale e.g. 500 kilometers or 1000 kilometers. In this data set (Fig. 16) what I present is the position of vertices of triangles which are GPS stations. These GPS stations have data for at least 3 years, so I took the velocities, the horizontal components, east, north and vertical. Then, this is the deformation of horizontal components here. Most of the extensions are horizontal. Red arrows indicate extensions and blue arrows are for compression. Here I have the scale, so according to this, you can see that in this triangle crosses the rift there is extension of about three hundred nanostrains per year. This is one of the biggest on the earth. I give you another example: there is change of 2 mm over two antennas but the distance of two antennas is 100 kilometers. This is a very small deformation. But in this data set of Corinth it is 10 times faster. The deformation increases from east to west. This deformation is mostly accommodated by earthquakes. We have some other data sets but this rift is expanding very fast indeed, that can be proved by geodesy.

Fig. 16 Principal axes of strain rates in central Greece from Cardozo and Allmendinger using Delaunay triangulation. Strain rate ranges from 10 ns/yr up to 270 ns/yr. Red arrows indicate extension and dark blue are for compression.

13. Does extension affect central Greece near the Gulf of Corinth?

Now we have crustal extension. We know what extensions are in mechanics. They create space, and we have basin information here. There is a small basin here and also in the north. We know that because we have marine geophysical data. We see results about extensions on the south coast, so the question is how was this deformation generated? We used the process of axes of strain rates in central Greece; moreover, what we did later is collecting the geodetic data for 10 years (Fig. 17). So, we see the extension really affects this area in central Greece near the Gulf of Corinth. The extension of this rift is affecting 800 kilometers square, at least. On the other hand, what we see in the Ionian Sea is compression but all this area of central Greece is extended. In south it is one of the few places where sigma one changes its orientation from north-south to east-west. I emphasize on the study of geodesy to understand the deformation.

Fig. 17 Tectonic strain (2003-2013)

From this data we got a 10 by 10 km grid where we have calculated how much is the extension. There are some areas really on the edge of network. The shear strain map shows a lot of horizontal movements for example in the north Aegean Sea.

14. Deformation and its Consequences

The deformation is the combination of translation and rotation. Because deformation as we know causes changes in shape of bodies, it follows that if it doesn’t change shape it is not deformation. This change is mostly due to the result of two processes, translation and rotation. According to figure-18, you see rotation of two areas in central Greece and these are very big areas. Then, area A and area B are under clockwise rotation with a rate of 4 degrees per million years. So, in every million year the crust rotates 4 to 5 degrees about a vertical axis. Today we can study all these processes, that is to find the shape changes according to time very easily with the help of geodesy, using geodetic data. So, the whole thing rotates within the primary deformation, that is extension and translation. In the picture to the right (Fig. 18) you see the red dots that are permanent GPS stations and you can see the spacing among stations which is not homogenous. It has clusters in spacing, that is why there were several gaps in data. Because of fewer stations in some of the areas, we don’t have enough data that means we need more stations to get detailed data. But overall the results are very reliable.

Fig. 18 The deformation mechanism of area A and area B

15. Movement in Northern and southern faults Fig. 19 Left-Lateral Shear between right-stepping rifts.

Between two grabens or rifts on this map there is a lake. So, the question is how these two rifts are connected together? We can solve this, if you consider the extension across the lake: this part moves to the north and the other part moves to the south. In between you can merge in the horizontal-slip fault. Because of differential rotation this area is just opening. We combined various data sets to get a model.

16. Can we estimate hazard (Earthquake) from fault model?

According to my research, if we can create a fault model then we are able to explain hazards. According to figure-20, all these yellow lines in the maps represent active faults and ruptures in the last few centuries. These faults are either in north or south of the Corinth rift. So, what we have to do is to create a fault model, link these faults and calculate the cumulative probability of a large earthquake. It turns that the probability of a M6 earthquake, is 70% during the next 30 years. Fig. 20 Evaluating probability and aggregation by a fault model for the western Corinth rift

Here in figure 21, the list represents some of the key findings of my research.

Fig. 21 Key findings of the research 17. Q&A

Question 1: When you collected the data through GPS, does the ionospheric problem affects the result? Answer: Yes, this is a common problem scientists face when they use GPS data. There are two ways to process the GPS data. One way is to process it on double difference and the second way is to use the precise point positioning method. So, for both methods there are algorithms where a global ionospheric correction model is introduced, that mitigates for such errors. But, the longer the data set span the less will be the errors in position or station velocity. For example, we have 10 years of positioning data where the fit to the time series is linear. So, we are very confident about these data and the errors we accept are 10% of the signal or less. So, to sum up the answer is we cannot get rid of all ionospheric error but we can compensate it by 10 years of data. So, when we come to make the trends of time series this error is really minimized.

Question 2: On slide 8 is it an anticline area? Answer: No, this is not an anticlinal area. This area consists of relatively young sediments and the apparent shape of those formations is like a monocline, but it is not in reality. These are sediments on the footwall of the active fault that were uplifted and warped during the uplift process, so it gives you this impression when you view it from a perspective view, but this is not an anticline. Moreover, the normal fault is not exactly straight, in other words east-west in strike, but it changes from WSW (west side) towards ESE (east side).