Les manifestations actuelles de l'activité volcanique sont représentées par une activité hydrothermale et fumerolienne au sein de la caldeira. Les séries ignimbritiques de Nemrut et Kantaşı, principales manifestations de l'activité de la caldeira, sont constituées d'unités pliniennes et de coulées ignimbritiques.
INTRODUCTION
T ECTONICS AND V OLCANISM IN A NATOLIA
The convergence between African and Eurasian plates, which began in the Late Cretaceous (Şengör and Yılmaz, 1981), resulted in the gradual closure of these ocean basins and the coalescence of the surrounding continental fragments (Bozkurt, 2001). Reilinger et al., 2006) relative to the Eurasian plate (Fig. 1.1) is located in the Caucasian belt that forms the thrust zone.
N EMRUT C ALDERA
Close to the Nemrut caldera, İncekaya tuff cone and Mazik and Girekol domes (Fig. 1.7) can be considered as in the system of Nemrut volcanism. Study area, main roads, populated places and important hydrogeological, topographical and volcanic features in the study area.
P REVIOUS STUDIES
S TRUCTURAL A PPROACH AND N ATURAL R ISK OF THE N EMRUT C ALDERA
- Methodology
A wide variety of methodological approaches have been used to introduce the structural evolution of a volcano and the associated natural risk. Former geological maps of the volcano were presented by Özpeker (1973a), Güner (1984) and Atasoy et al.
P RE - VOLCANIC BASEMENT
- Bitlis Metamorphics
- Çatak Ophiolites
- Tertiary Sediments (Ahlat formation)
Small granitic plutons and associated granitic dykes intrude the pre-Devonian basement of the Bitlis massif (Ustaömer et al., in press). North to the Nemrut volcano they are probably overlain by volcanic products (Atasoy et al., 1988).
- Süphan Volcano
- Bilican volcano and Kolango dome
- İncekaya Tuff Cone
Tertiary sedimentary rocks outcrop west and northwest of Nemrut volcano and east of the city of Ahlat. To the north of the building are five adjacent cones forming a linear cluster, heading N12ºE (Adıyaman et al., 1998).
G EOLOGICAL EVOLUTION OF N EMRUT VOLCANO
- Pre-caldera activity
- Caldera forming eruptions (Sub-stage V)
- Post-caldera activity
Map of spatial color change of the upper level of Nemrut ignimbrite around the volcano. Products of the Great Lake (Fig. 2.28) are widespread in the caldera, pasted on the western shore of Lake Nemrut.
S YNTHESIS
For scale, please note the presence of our team member in the red circle. The evolution of the volcano has been investigated in two main phases (pre-caldera and post-caldera) separated by the catastrophic collapse of the caldera. Recently, the most prominent activity of the volcano is hydrothermal activity and fumaroles.
Fumarole activity is best seen around Lake Ilığ; bubbling in hot springs is also evident. Quantification of hydrothermal activity in the caldera and its relationship with caldera structure has been investigated using geophysical surveys and remote sensing methods. The second paper presented (chapter 3.2) deals with the geophysical and remote sensing approach of the hydrothermal dynamics of the Nemrut caldera.
We used the self-potential (SP) method to describe the hydrothermal fluid circulation and reveal the relationship between the structural features of the caldera and hydrothermal system.
I MAGE BASED RETRIEVAL AND CORRECTION OF ALTITUDE AND ASPECT EFFECTS
Correcting nighttime TIR images for the elevation and slope of the region can reveal more reliable data. The Nemrut volcano has been corrected using STcorr, and the quality of the final image is further addressed. Image thermal gradient correction is performed against this calculated thermal gradient image.
After selecting the three input images (ST image, DEM and Aspect image) a masking option is given to the user. Depending on the unit of the input ST image, the units of the output images are °C or . We have used basic statistical analysis to show the quality of the produced images compared to the original image.
Based on the value of the standard deviation, an increase in the quality of the produced images can be seen (table 1: analysis - 2).
S TRUCTURE OF THE N EMRUT CALDERA (E ASTERN A NATOLIA , T URKEY ) AND
Two large collar collapses were observed on the western and SSW flanks of the caldera wall (Fig. 4c). A NW-oriented line of the volcanic centers in the caldera is evident in the aspect map. Several anomalies from the caldera are visible on the image (Fig. 7c: black and white arrows).
Lake Nemrut with a volume of 1264 km3 is located in the western half of the caldera. Two larger slips were observed on the southwestern part of the caldera wall (Fig. 5.1a: slip faults). In the structural context of the volcano, we should not neglect the S-oriented rifting in the northern area (Fig. 5.5).
Structural evidence of this localized extension occurs in the western and northwestern parts of the caldera.
SEISMIC MONITORING OF NEMRUT VOLCANO
S EISMIC N ETWORK
- Limitations, problems faced and efficiency of the network
The stations are established in the headquarters of the Gendarmerie in the towns of Tatvan, Güroymak and Ahlat. The stations' power system is reinforced with a regulator, a UPS (Uninterruptible Power Supply) and a surge protector. It is therefore important to discuss these issues and the effectiveness of the surveillance network.
Although stations are equipped with UPS units, prolonged electrical interruptions or voltage drops go beyond the capabilities of the stations. At the time of the installation of the seismic stations (summer 2003), the teleprocessing capacity of the telecommunications infrastructure in Bitlis city was limited. The network efficiency between October 2003 and October 2005 is 77%, 62% and 67% for the Tatvan, Ahlat and Güroymak stations respectively.
To overcome the data loss caused by the telecommunications system and increase the efficiency of the network, precautionary measures were necessary.
D ATA P ROCESSING
At the end of 2007, the local telecom added the ADSL (Asymmetric Digital Subscriber Line) data transfer protocol in the cities of Tatvan, Güroymak and Ahlat. Teleprocessing with the frame-relay system will be completed and transferred to the ADSL system before the end of 2008. Lines with different colors are assigned to two horizontal and one vertical component channels of the seismometers.
Depending on the magnitude or relative location of the triggered event, the event is not always triggered by all three seismometers. During the STA/LTA triggering process, when a possible event is only triggered by one or two seismometers, we use Nemtract to trim the related data part using "Nemtrig" for the missing (non-triggered) station(s). Our third code, "Nemspec" (Nemrut spectrogram), visualizes the spectrogram of the event to determine the type of seismic event (Fig. 4.5).
For a given part of the seismic signal, Nemspec plots the spectrum of the signal against time, frequency and amplitude.
S EISMICITY OF THE VOLCANO
- Seismicity between October 2003 and October 2005
After hypocentral solutions of seismic events, event types are determined by frequency spectrum and depth analyses. The local magnitude equations of the recorded events were calculated using the event magnitudes reported by the National Earthquake Monitoring Center (Kandilli Observatory). Where "Md" is the duration-dependent magnitude of the earthquake, "T" is the total duration of the signal, "D" is the epicentral distance in km, and "a", "b" and "c" are the regression coefficients.
Shallow sites of the volcanic events that occurred beneath the Nemrut volcano (Fig. 4.10) are indicative evidence of a shallow magma chamber at 4-5 km depth. However, a slight increase starting in spring and ending in autumn (Fig. 4.11a, dotted line) should not be ignored in future monitoring of the volcano. The “b-value” is the slope of the straight line that usually fits the magnitude and logarithm of the frequency distribution of the observed data.
The frequency-magnitude distribution of the earthquakes associated with the epicenters was analyzed to obtain a spatial distribution of the b-value (Fig. 4.13).
S YNTHESIS
S TRUCTURE OF N EMRUT C ALDERA
- Tectonic evolution and initiation of Nemrut volcanism
- On the formation of Lake Van
Eruption of Nemrut and Kantaşı ignimbrite series led to the collapse of the building. Intra-caldera hydrothermal activity appears to be mainly controlled by the internal structure of the caldera (Ulusoy et al., 2008). In addition, WNW-ESE reverse faulting on the western caldera rim (Fig. 5.1b; Ulusoy et al., 2008) and arcuate faulting (and its en-échelon pairs) on the northwestern flank of the volcano have drawn attention to the western lace.
In their experimental study, Holohan et al. 2005), stated that aspect ratio of the calderas in stress-free conditions is equal to 1. Arcuate faulting on the northwestern flank of Nemrut caldera is most likely a trace of a horsetail crack formed due to the oblique character of the Otluk fault. In addition, except for the dykes on the western wall (Yellow pointed circle, Fig. 5.8), radial placement of the dykes is visible on the caldera wall (Fig. 5.8).
However, this dissection is of great importance for the origin of Nemrut volcanism; it is best observed north of the two basins.
N ATURAL R ISK POTENTIAL OF N EMRUT VOLCANO
- Small scale cold lahars
Flank landslide is one of the most important and common hazards that can occur in volcanic terrains (Merle et al., 2008). In addition to the flanking landslide, the large volume of Lake Nemrut doubles the danger. The first event was observed on the northeast and southeast flanks of the volcano and occurred just after a heavy rain.
The flow completely destroyed the road between the village of Yumurtepe (Fig. 2.28) and the edge of the caldera, channeled into the drainage canals, filled them and reached Lake Van (Fig. 5.13c). 19.07.2007 The lahar is flowing. a,b) Lahar flows originate from the northeastern flank of the caldera (the destroyed road was dug by villagers) and, b) Flood fans of cold lahar flows in Lake Van. The second event occurred on the northwest flank of the volcano and this event could be observed on satellite imagery (Figure 5.14).
Press services reported that the event was caused by melting snow due to rising air temperature.
S UGGESTIONS
Two ASTER false color composite images demonstrating the difference before and after lahar flow event (images are from ASTER image database of volcanoes - Geological Survey of Japan). a) ASTER VNIR image obtained on traces of older lahar flows is clear in the image. Other current codes such as "Nemtract" and "Nemspec" can still be used in the analysis workflow without any problems. The modeling should mainly include the modeling of the impact areas of lahar risk, as well as pyroclastic activity risk should also be considered.
High resolution images will provide a huge amount of data and information for the hazard modelling. A new Self-potential survey covering the area outside the caldera will quantify the hydrothermal system precisely. Multiple, parallel resistivity imaging profiles, which can be set in the NE-SW direction, will precisely demonstrate the intra-caldera structural system.
Together with the Self-potential data, resistivity imaging will provide valuable information on the surface structure of the calderas.
O NGOING WORKS
- On the calculation method of Ce Map
- Lightweight multi-electrode resistivity cabling system
The difference between the system and other intelligent switching systems is the number of wires in the main cable. Lithospheric structure of the Arabian Eurasian collision zone in eastern Turkey from S-wave receiver functions, Geophys. Evolution and relationship between volcanism and tectonics in the central-eastern part of the Oligocene Borovitsa caldera (Eastern Rhodopes, Bulgaria).
Evolution of volcanism in the interaction zone between the Arabian, Anatolian and Iranian plates (Lake Van, Eastern Turkey), J. Volcanostratigraphy and petrogenesis of the Nemrut stratovolcano (High Anatolian Plateau): Recent post-collisional volcanism in Turkey . Source mechanism of the 15 November 2000 Lake Van earthquake (Mw = 5.6) in eastern Turkey and its seismotectonic implications.
The mythological description given at the beginning of the text is still known in Anatolia as a folklore narrative (with minor variations).
APPENDIX – 1: EVOLUTION OF NEMRUT VOLCANO
APPENDIX – 2: STRATIGRAPHICAL SECTIONS
APPENDIX – 3: “STCORR” AND SHORT USER GUIDE
APPENDIX – 4: “NEMTRIG” AND SHORT USER GUIDE
