Seismotectonic implications for the Alborz mountains (Iran) from regional seismic moment tensorsStefanie Donner ¹, Frank Krüger ¹, Dirk Rößler ², Abdolreza Ghods ³,

Angela Landgraf ¹, Paolo Ballato ¹

Vernant et al., 2004

3b. The NE Alborz region

centroid are very deep (down to 28 km) thrust mechanisms seem to correlate with Khazar thrust fault and/or a wedge-shaped imbrication zone north of it orientation of thrust mechanisms and Khazar fault contradict each other in the eastern part of the area (along profile B)

3a. General resultsThese results are based on an extended data processing and inversion approach, especially developed for the study area (talk S2-1.001). We determined 40 moment tensors (3.1 ≥ Mw ≥ 6.2) to show the potential of this method for the investigation of seismotectonics in the study region. They revealed the following implications:

sources separate in strike-slip and pure and oblique thrust mechanisms

shallow centroid depths (< 20 km), except close to the coast of the South Caspian Basin (down to 28 km)

mechanisms form a complex pattern of slip-partitioning with possible depth- and magnitude-dependence, probably with local influences

d

b

c

4. OutlookA comprehensive retrieval of seismic moment tensors (Mw ≥ 3.0) will allow a much more detailed investigation on the seismotectonics of the active faults within the Alborz mountains. A broad catalogue of MTs in connection with other data (e.g. EQ locations, GPS data) and techniques (e.g. stress modelling, statistics) can contribute to the general understanding of the tectonics and deformation of transpressional systems. Additionally, it provides a good data base for future studies on seismic hazard.

contact: stefanie.donner@uni-potsdam.de

2. Method

regional seismic moment tensors: complete description of equivalent body forces of a general seismic source

inversion of amplitude spectra from broadband and short-period regional waveforms using an iterative least-squares algorithm (mtinvers)

full waveform theory far-field Green's functions calculated using reflectivity method

explosion: M11 + M22 + M33

strike-slip: M23 +M32 thrust: M11 - M22 M11 M12 M13

M ij = M21 M22 M23

M31 M32 M33

∣u∣=√(Gkℜ⋅mk)

2+(Gk

ℑ⋅mk )

2 with m1=1/2(M 22−M 11);m2=M 12;m3=M 13 ;m4=M 23

m5=1 /3 [0.5(M 22+M 11)−M 33 ];m6=1/3(M 11+M 22+M 33)

Example: 15.08.2009 00:28 UTC M0 7.85e14 Nm Mw 3.9 depth 10 km DC 79% CLVD 21%

spectra ofdata and synthetics

observed and from solution calculated synthetic waveforms

Affiliations: 1 - Institute of Earth and Environmental Sciences, University of Potsdam, Germany; 2 - GFZ German Research Centre for Geosciences, Potsdam, Germany; 3 - Institute of Advanced Studies in Basic Sciences, Zanjan, Iran

Acknowlegements: This study was funded by the German Research Council (DFG Project KR 1935/12-1) and funds from the DFG Leibniz Award to M. Strecker (DFG project STR 373/20-1). We greatly appreciate the data supply from the Iranian Institute of Earthquake Engineering and Seismology (IIEES) and the Institute of Geophysics, University of Tehran (IGUT).

indications for a change in the tectonic regime from left-lateral (SE) to right-lateral (NW)

3c. The Zanjan region

1. Motivation

The northward movement and collision of the Arabian with the Eurasian plate generates compres-sive stresses in Iran. The Alborz mountains are a double vergent mountain belt between two aseismic blocks. Within these mountains, a complex and poorly understood sys-tem of strike-slip and thrust faults accom-modates a fundamental part of the NNE-SSW oriented shortening. So far, this region is analysed from structural and geo-morphologic field work butlacks comprehensive studies on earthquakes and earthquake source processes. Several questions concerning the seismotectonics, such as fault kinematics and possible slip-partitioning, remain open. A comprehensive retrieval of moment tensors so far was hampered by a missing appropriate velocity model and heterogeneously distributed seismic stations.

3d. The Mw 6.2 Baladeh earthquake sequence of 2004

Light-coloured mechanisms from Tatar et al. (2007).

aftershocks occurred in two distinct clusters: east (blue) and west (orange) with intermediate events (black)

western aftershocks are not related with the fault plane of the main shock (green box)

depth-dependent slip-partitioning occurred in the western aftershock cluster