DARWIN Digitale Dissertationen German Version Strich

FU Berlin
Digitale Dissertation

Arturo Belmonte-Pool :
Crustal Seismicity, Structure and Rheology of the upper Plate between the Pre-Cordillere and the volcanic Arc in northern Chile (22°S-24°S)
Krustale Seismizität, Struktur und Rheologie der Oberplatte zwischen der Präkordillere und dem magmatischen Bogen in Nordchile (22°S-24°S)

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Abstract

(Type a title for your page here) At the beginning of 1994, the PISCO '94 experiment (Proyecto de Investigación Sismológica de la Cordillera Occidental), which is part of the so-called Collaborative Research Center 267 (SFB 267) "Deformation Processes in the Andes" (FU-Berlin, TU-Berlin, GFZ Potsdam, U. Católica del Norte and U. de Chile), was carried out in northern Chile. PISCO '94 focused on active and passive seismological investigations of the upper plate and the Benioff zone. During a period of 100 days, a network of 32 continuous and digital recording stations with three components registered the local seismicity between the Pre-Cordillere and the volcanic arc between 21,8° and 24,3°S. Stations were arranged over a surface of 230 x 180 km² with an average distance to each other of 40 to 50 km. The geometry of this temporal seismological network was laid out primarily for registering the Benioff-zone seismicity in the subducted Nazca Plate. During this period, about 5300 events were recorded (PISCO-Catalog). Apart from the large number of earthquakes in the Benioff zone, some events in the upper plate (described in this work as crustal events) were also registered. Their low magnitude and small number of about 2 events/day as compared to the Benioff zone events (~100 events/day) made the recognition of these crustal earthquakes more difficult.

It was the aim of this work to identify those earthquakes which occur in the upper plate and which were not registered in the PISCO-Catalog. In order to distinguish those events lying in the upper plate of the Benioff zone, different criteria were investigated:

(1) S and P travel time differences.

(2) P travel time differences as well as signal amplitudes.

(3) the spectral frequency.

The former proved to be the most successful method for recognizing crustal earthquakes.

The search for crustal events produced about 300 events, of which 215 earthquakes were selected to be further investigated. The remainder were comprised of events with a very small magnitude (mb<0,5) or occurred outside the network (gap>200°). The P and S onsets-determination was undertaken using the program PITSA, the localization using the routine HYPO71 with a 1-D-velocity model. Relocalizations were carried out using the routine VELEST. For earthquakes occurring above the depth of 15 km, the vertical error can amount to 10 km, for deeper events, on the contrary, only to 3-5 km.

The seismicity is distributed unevenly in the crust. A concentration of earthquakes can be recognized in the pre-Andean Depression south of 22,7°-23°S. The number of hypocenters decreases at the northern part of the investigation area in the Western and Pre-Cordillere. Most of the events lie between 5 and 30 km in depth. At the north-west part of Salar de Atacama (Cordillera de Domeyko-Cordillera de la Sal) a cluster of earthquakes occur at a depth of 40-50 km. At a depth of 60-70 km, a further group of events can be recognized under the Pre-Cordillere around 22,8°S. These events lie about 10 km above the Benioff zone. The seismicity in the Western Cordillere appears to be related to the volcanic activity. Earthquakes are situated between the surface and a depth of 20 km. In the Pre-Cordillere earthquake, clusters also lie at about 20 km in depth. In the transition zone between the Western Cordillere and the pre-Andean Depression as well as between the pre-Andean Depression and the Pre-Cordillere, two zones without seismicity can be identified.

Zones without seismicity under the Pre-Cordillere and under the pre-Andean Depression (Atacama Block) are explained by taking into consideration in these areas an elastic deformation in a rigid rock. Contrarily the cluster at about 40-50 km depth at the north-west part of Salar de Atacama and the seismicity at 60-70 km depth under the Pre-Cordillere show that the deformation here takes place in a brittle zone.

Magnitudes of the most events vary between 0,2 and 2,5. As focal mechanisms of these events show, local stress field presents strong changes in its direction. A variable direction of main stresses determines the type of mechanism. Both reverse and normal mechanisms characterize the observed pattern above a depth of 30 km. Those earthquakes being situated at a depth of about 40-50 km show a predominant reverse mechanism, while those situated at 60-70 km depth a normal mechanism.

The derived b-value varies between 0,5 and 1,2. Such a variation does not allow to make clear conclusions about the predominant stress field.

Finally, reflections about the rheology and tectonic are proposed. The regional W-E-effect of isotherms in the upper plate as well as the changing regional stress appear to be due to the heterogeneous structure of the upper plate both in the upper crust and above a depth of 60 km - a condition for the observed seismological data. The volcanic arc shift to the east and the supposed existence of an anomalous rigid body (Atacama block) correspond with the data processed in this study.

Table of Contents

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Titelblatt
Inhaltsverzeichnis
1. Einleitung 1
2. Strukturen und Prozesse in den Zentralen Anden 5
2.1. Einleitung 5
2.2. Geologische Entwicklung und morphotektonische Gliederung der Zentralen Anden 6
2.2.1. Der Bereich der präandinen Depression 8
2.3. Seismizität in den Zentralen Anden 9
2.3.1. Stil der Subduktion, Inter-Platten-Erdbeben und Strukturen im Tiefseegraben 9
2.3.2. Beben in der Benioff Zone 12
2.3.3. Beben in der Oberplatte 12
2.4. Geophysikalische Anomalien im Untersuchungsgebiet 14
2.4.1. Überblick über die prominentesten Anomalien und ihre Interpretation 14
2.4.2. Laufzeit Tomographie und Dämpfung 14
2.4.3. Refraktionsseismik 15
2.4.3.1. Diskontinuitäten im Bereich 0-40 km Tiefe 15
2.4.3.2. Hydratisierter Mantelkeil und Verlauf der Kruste-Mantel-Grenze (Moho) 16
2.4.4. Gravimetrie 16
2.4.5. Magnetotellurik 17
2.4.6. Geothermie 17
2.5. Rolle der Fluide im Untersuchungsgebiet 17
3. Datengewinnung 19
3.1. Das seismologische Experiment PISCO`94 19
3.2. Auswertung der PISCO-Daten 20
4. Datenbearbeitung: Die Suche nach Krustenbeben 21
4.1. Definition von seismischen Bereiche und Selektionskriterien 21
4.2. Seismische Registrierungen 23
4.3. Selektionskriterien 25
4.3.1. Laufzeit und Signalamplituden 25
4.3.1.1. Hauptkriterium: S- und P-Laufzeitdifferenzen unter Verwendung einer Station 25
4.3.1.2. Zusatzkriterium: S- und P-Laufzeitdifferenzen unter Verwendung zweier Stationen 27
4.3.1.3. Zusatzkriterium: P-Laufzeitdifferenzen und Signalamplituden 28
4.3.2. Das Frequenzspektrum als mögliches Selektionskriterium 29
4.3.2.1. Das Erdbebenspektrum 29
4.3.2.2. Darstellung eines Sonogramms 31
4.4. Beispiele 34
4.5. Schlussfolgerungen 40
4.6. Zusammenfassung 40
5. Ergebnisse der Suche von Krustenbeben 43
5.1. Lokalisierung und eliminierte Beben 43
5.2. Relokalisierung 47
5.2.1. Lokalisierungsgenauigkeit und VELEST als Lokalisierungsroutine 47
5.2.2. Relokalisierung unter Verwendung von Subnetzen 50
5.3. Magnitudenbestimmung 55
5.3.1. Bestimmung und Verteilung der Magnituden 55
5.3.2. Bestimmung des b-Wertes 56
6. Bestimmung von Herdflächenlösungen 59
6.1. Bebenmechanismen 59
6.2. Grundlage zur Darstellung der Quelle eines Erdbebens 59
6.3. Bestimmung von Herdflächenlösungen 60
6.4. Darstellung des Spannungsfeldes 62
6.5. Darstellung der Herdkugel und der Herdflächenlösungen 63
6.6. Darstellung der Ergebnisse 64
7. Interpretation 71
7.1. Rheologie 72
7.1.1. Spröde und duktile Bereiche 72
7.1.2. Seismizität und Wärmefluss 74
7.1.3. Seismizität und Rheologie der Oberplatte 76
7.2. Vergleich mit geophysikalischen Anomalien 84
7.2.1. Rigidität 84
7.2.2. Der Atacama Block und die verdickte Kruste im Zusammenhang mit der Seismizität 85
7.3. Tektonik und geologische Strukturen im Bereich der präandinen Depression 87
7.3.1. Das Neogene Spannungsfeld 87
8. Schlussfolgerungen 93
Zusammenfassung 95
Summary 97
Resumen 99
Literaturverzeichnis 101
Appendix A 107
Appendix B 113
Danksagungen 119

More Information:

Online available: http://www.diss.fu-berlin.de/2002/202/indexe.html
Language of PhDThesis: german
Keywords: Crustal Seismicity, stress field, Rheology
DNB-Sachgruppe: 31 Geowissenschaften
Date of disputation: 19-Jul-2002
PhDThesis from: Fachbereich Geowissenschaften, Freie Universität Berlin
First Referee: Priv. Doz. Dr. Günter Asch
Second Referee: Prof. Dr. Peter Giese
Third Referee: Prof. Dr. Götze, Prof. Dr. Kind, Prof. Dr. Scheuber
Contact (Author): belmonte@geophysik.fu-berlin.de
Contact (Advisor): giese@geophysik.fu-berlin.de
Date created:25-Sep-2002
Date available:27-Sep-2002

 

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