Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1918 Source analysis of multiplet earthquakes (two case studies in Iran) SAMAR AMINI ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-513-0909-5 UPPSALA urn:nbn:se:uu:diva-407247 2020 Dissertation presented at Uppsala University to be publicly examined in Hambergsallen, Geocentrum, Villavägen 16, Uppsala, Friday, 12 June 2020 at 10:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Kuvvet Atakan (University of Bergen, Norway). Abstract Amini, S. 2020. Source analysis of multiplet earthquakes (two case studies in Iran). Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1918. 45 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0909-5. Multiplet earthquakes are large earthquakes of similar magnitude which occur close in time in the same limited geographical area. They are not common but they considerably increase the potential hazard in the area in which they occur. This thesis studies source properties and triggering mechanisms of two sets of multiplet events in Iran, which both occurred in unexpected areas, but close to some major active fault systems. The first multiplet is an earthquake doublet (Mw 6.5 and Mw 6.4) which occurred in northwestern Iran and caused more than 300 fatalities and significant injuries. In paper I, a teleseismic body-waveform inversion was used to deduce the slip distribution pattern on the fault plane of the first mainshock. The estimated slip pattern was utilized to calculate the Coulomb stress changes on the second fault plane and on the following aftershocks. Based on this analysis, the ambiguity between the primary and auxiliary fault plane of the second mainshock could be resolved. The second set of events is a triplet (Mw 6.1 - 6.0) that occurred in eastern Iran, close to the Kerman province. In paper II, the rupture propagation patterns of the three mainshocks were analyzed using Empirical Green’s Function (EGF) deconvolution. Two different approaches were used: One, the analysis of the azimuthal variation of the apparent rupture duration based on the width of the observed relative source time functions, and the second, the analysis of along-strike rupture directivity by assessing azimuthal variations of the relative amplitude spectra. The second approach was also used to investigate the rupture directivity of the largest aftershocks in the sequence (Mw 5 - 5.5). A clear tendency for rupture propagation towards the northwest was observed for the sequence, which suggests that the regional stress field has a central role in controlling the rupture propagation direction. In paper III, the slip distribution patterns of the triplet earthquakes were analyzed using teleseismic body-waveform inversion, and the similarities and differences in the rupture processes of the three mainshocks were investigated. Using the Coulomb stress analyses, the stress interactions between the mainshocks were examined, leading to identification of the primary and auxiliary planes. Finally, we suggest that the hazard estimates in complex continental regions such as Iran need to consider the probability of multiplets, which might allow a reduction of the seismic risk associated to the occurrence of further large earthquakes subsequent to a devastating earthquake. Keywords: Multiplet earthquakes, slip inversion, Coulomb stress, rupture directivity Samar Amini, Department of Earth Sciences, Geophysics, Villav. 16, Uppsala University, SE-75236 Uppsala, Sweden. © Samar Amini 2020 ISSN 1651-6214 ISBN 978-91-513-0909-5 urn:nbn:se:uu:diva-407247 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-407247) Dedicated to those, who made this happen Supervisor Roland Roberts Proffessor at Department of Earth Sciences, Geophysics Uppsala University, Uppsala, Sweden Assistant Supervisor Björn Lund Associate Professor at Department of Earth Sciences, Geophysics Uppsala University, Uppsala, Sweden Assistant Supervisor Hossein Shomali Researcher at Department of Earth Sciences, Geophysics Uppsala University, Uppsala, Sweden Opponent Kuvvet Atakan Proffessor at Department of Earth Sciences University of Bergen, Bergen, Norway List of papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Amini, S., Roberts, R., Raeesi, M., Shomali, Z.H., Lund, B., Zarifi, Z. (2018) Fault slip and identification of the second fault plane in the Varzeghan earthquake doublet. Journal of Seismology, 22, 815-831 II Amini, S., Roberts, R., Lund, B.(2020) Directivity analysis of the 2017 December Kerman earthquakes in Eastern Iran. Journal of Seismology, DOI: 10.1007/s10950-020-09913-8 III Amini, S., Raeesi, M., Roberts, R. (2020) Fault slip and rupture properties of the December 2017 Hojedk triplet in Eastern Iran. submitted to Geophysical Journal International Reprints were made with permission from the publishers. An additional journal article, published during my Ph.D. studies, that is not included in the thesis is: Raeesi, M., Zarifi, Z., Nilfouroushan, F., Amini, S., Tiampo, K. (2017) Quantitative Analysis of Seismicity in Iran. Pure Appl. Geophys. 174, 793- 833 Contents 1 Introduction .................................................................................................. 9 2 Study area ................................................................................................... 12 2.1 Seismotectonic setting of northwestern Iran and the doublet on August 11, 2012 ............................................................................. 13 2.2 Seismotectonic setting of eastern Iran and the triplet on December 2017 .............................................................................. 13 3 Methodology .............................................................................................. 16 3.1 Waveform Modeling ...................................................................... 16 3.2 Coulomb stress ............................................................................... 19 3.3 Directivity analysis and Empirical Green’s Function .................. 20 4 Summary of papers .................................................................................... 23 4.1 Paper I: Fault slip and identification of the second fault plane in the Varzeghan earthquake doublet ................................................ 23 4.1.1 Motivation ........................................................................ 23 4.1.2 Results .............................................................................. 23 4.2 Paper II: Directivity analyses of the 2017 December Kerman earthquakes in Eastern Iran ........................................................... 27 4.2.1 Motivation ........................................................................ 27 4.2.2 Results .............................................................................. 27 4.3 Paper III: Investigations of rupture properties of the December 2017 Hojedk triplet in Eastern Iran .............................................. 31 4.3.1 Motivation ........................................................................ 31 4.3.2 Results .............................................................................. 31 5 Concluding remarks .................................................................................. 36 6 Sammanfattning på svenska ...................................................................... 39 7 Acknowledgements ................................................................................... 41 References ........................................................................................................ 43 Abbreviations 3D Three-dimensional EGF Empirical Green’s Function E-W East-West g-CMT Global Centroid Moment Tensor Hz Hertz km kilometer km/s kilometer per second Ml local Magnitude Mw moment Magnitude NE Northeast NW Northwest N-S North-South s second SE Southeast SW Southwest IRIS Incorporated Research Institutions for Seismology IRSC Iranian Seismological Center RSTF Relative-source-time-function STF Source-time-function 1. Introduction Throughout human history some areas have been repeatedly devastated by earthquakes, the origins of which remained a mystery. As civilization and human constructions developed, understanding and mitigating the effects of these destructive events became ever more important. A major step towards the development of our understanding of earthquakes occurred in 1668 when Hooke introduced the theory of elasticity, explaining the deformation of solid objects due to external forces. Hooke’s law was extensively used to explain various aspects of the mechanical behavior of material and physical phenom- ena including earthquakes. Two and a half centuries later, the faulting the- ory of earthquakes was presented by Reid (1910) whose analysis was based on investigations of the San Andreas fault and the 1906 San Francisco earth- quakes. Reid’s concept, known as the elastic rebound theory, explains earth- quakes as the sudden release of strain energy which has been accumulated slowly on two sides of a fault plane over an extended period of time. The theory was a major conceptual development in understanding the mechanisms of earthquakes. Further important developments towards understanding the large scale geological processes causing earthquakes came in the mid-1960’s after the installation of the World Wide Standardized