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The upper crustal microseismicity image from the North Chilean subduction zone
Salazar Reinoso, Pablo

HaupttitelThe upper crustal microseismicity image from the North Chilean subduction zone
Titelzusatzimplications for tectonics and fluid migration
TitelvarianteDie Verteilung der krustalen Mikroseismizität über der Nordchilenischen Subduktionszone
Zusatz zur TitelvarianteFolgerungen für Tektonik und Fluid Migration
AutorSalazar Reinoso, Pablo
Geburtsort: Antofagasta, Chile
GutachterProf. Dr. Serge Shapiro
weitere GutachterProf. Dr. Serge Shapiro
PD Dr. Günter Asch
Freie SchlagwörterMicroseismicity; fluids; Tectonics; Andes; Subduction
DDC550 Geowissenschaften
558 Geologie Südamerikas
ZusammenfassungIn November 2005, a temporary short-period seismic network was installed in the Chilean Precordillera, around 21°S. This set of twelve 3-component stations that form the West Fissure seismological network has been recording continuously since then, however, only the crustal microseismicity data recorded during the years 2005-2009 were processed and analyzed here.

Thus, the microseismicity studies in the forearc of the north Chilean subduction have permitted the retrieval of important tectonic features of the crust. A new image of the crustal structure and the West Fissure Fault System (WFFS) as a westward-dipping compressive structure, geometrically opposed to Nazca plate subduction, has been constrained by microseismicity.

The origin of this particular geometry is not clearly defined yet, but this could be a tectonic response to differences in the rheological behavior of the crust.

The characterization of this structure, in terms of its stress distribution and kinematics, has been obtained by a good fit with some deeper focal mechanisms and also through stress tensor analysis in the zone.

The stress tensor analysis also shows that the state of stress in the convergence margin is related to compression, whereas the extension is only a manifestation of local forces associated with the highest areas in the Andes.

Two seismic clusters have also been identified and possibly associated with a blind branch of WFFS. The study of these two clusters with a focus on their sources and possible connection with fluid migration was carried out.

It is observed that some characteristics of the two clusters found in the zone differ from each other. The central cluster has characteristics of an earthquake swarm with two phases which can clearly be identified, whereas the SW cluster has a clear main shock associated and can be separated into two subclusters (A and A', respectively).

In contrast, the similarities permit to infer that they could have a common origin. The b-values for both are characteristic of tectonic plate boundaries. The spatial spreading of the events -which is approximately confined to one plane- reveals a progressively growing of the main fracture that form the swarm and subcluster A activities. It is also found that earthquakes themselves trigger aftershocks near the border of their rupture areas.

In addition, the spatio-temporal migration of hypocenters, as well as, spatial correlation -with areas that were interpreted as fluid migration zones- lead to believe that there is a close relation between fluids and the sources of the swarm and subcluster A.

These observations above, point to think in a stick-slip behavior of the rupture propagation, which can be explained by stress transfers and induced fluid flows due to earthquakes in a fluid-permeated critically loaded fault zone.
InhaltsverzeichnisList of Figures .................................................................................... iii
List of Tables ..................................................................................... vi

1 Introduction 1
1.1 The problem .... ............................................................... 1
1.2 The scientific questions...................................................... 2
1.3 Hypothesis ....................................................................... 3
1.4 State of investigations ....................................................... 3
1.5 A brief overview of this study ............................................. 4

2 Geotectonic setting 6
2.1 The Andean convergence system .......................................... 6
2.2 Central Andes ..................................................................... 9
2.3 The Altiplano segment ......................................................... 11
2.3.1 Forearc ................................................................... 11
2.3.2 Arc, backarc and foreland ......................................... 11
2.3.3 Deformation and uplift of the Altiplano Plateau ............ 13
2.4 West Fissure Fault System ................................................... 15

3 Theoretical background of the methods used 17
3.1 Precise location technique NonLinLoc ..................................... 17
3.2 Local magnitude determination ............................................. 19
3.3 Focal mechanisms ............................................................... 19
3.3.1 Introduction ............................................................. 19
3.3.2 First-motion of P-waves ............................................ 20
3.3.3 Amplitude ratios ....................................................... 21
3.4 Stress tensor analysis .......................................................... 23
3.5 Moment tensor analysis ........................................................ 24
3.6 Spectral analysis .................................................................. 26
3.6.1 Determination of M0 and MW ...................................... 28
3.7 Frequency-magnitude distribution .......................................... 28
3.8 Interevent-time distribution ................................................... 29
3.9 Principal Components Analysis ............................................... 31
3.10 3D rotation matrix ............................................................... 33
3.11 Spatiotemporal analysis ....................................................... 34
3.12 Spatiotemporal migration ..................................................... 35

4 Result-characterization of the West Fissure Fault System 36
4.1 Experiment set-up and data ................................................... 36
4.2 The observed seismicity ........................................................ 40
4.3 Focal mechanisms-data processing and results ........................ 47
4.4 Stress tensors ...................................................................... 51
4.5 Moment tensor analysis ......................................................... 57
4.6 Discussion of the results and comparison with other studies ...... 69

5 Detecting possible fluid signals from the clusters 78
5.1 Introduction to the cluster analysis .......................................... 78
5.2 Data ..................................................................................... 78
5.3 Chronological evolution of the energy release in the clusters ...... 80
5.4 Statistical characteristics ......................................................... 82
5.4.1 Frequency-magnitude distribution ................................. 82
5.4.2 Interevent-time distribution .......................................... 82
5.5 Spatial analysis ...................................................................... 85
5.6 Spatiotemporal analysis .......................................................... 88
5.7 Spatiotemporal migration ........................................................ 94
5.8 Discussion of the results and the different clusterization models .. 95
5.8.1 Intrinsic differences in the earthquake clusterization ....... 95
5.8.2 Mainshock-aftershock model ......................................... 96
5.8.3 Ambiguities of the mainshock-aftershock model .............. 96
5.8.4 Stick-slip model ........................................................... 97
5.8.5 The proposed model .................................................... 98
5.8.6 Other geophysical anomalies taken in consideration ....... 98

6 Conclusions 103
6.1 Tectonic implications ............................................................. 103
6.2 Fluid migration implications .................................................... 104

Bibliography 107

A Focal mechanisms parameters 125
A.1 Events used in the stress tensor analysis ............................... 125

B Location parameters 132
B.1 Events used in the swarm analysis ....................................... 132
B.2 Events used in the subcluster A analysis ............................... 135
B.3 Events used in the subcluster A' analysis .............................. 139

C Acknowledgments 141

D Curriculum Vitae 143
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SeitenzahlVI, 145 S.
Fachbereich/EinrichtungFB Geowissenschaften
Erscheinungsjahr2011
Dokumententyp/-SammlungenDissertation
Medientyp/FormatText
SpracheEnglisch
Rechte Nutzungsbedingungen
Tag der Disputation05.07.2011
Erstellt am07.07.2011 - 11:29:52
Letzte Änderung11.07.2011 - 13:28:57
 
Statische URLhttp://www.diss.fu-berlin.de/diss/receive/FUDISS_thesis_000000023718
URNurn:nbn:de:kobv:188-fudissthesis000000023718-4
Zugriffsstatistik
E-Mail-Adressepablo@geophysik.fu-berlin.de
E-Mail-Adressepasalaz@ucn.cl