3-D Seismic Wave Propagation on a Global and Regional Scale:
Earthquakes, Fault Zones, Volcanoes

Project Leader: Prof. Dr. Heiner Igel, Institute of Geophysics, Ludwig-Maximilians-University, Munich
Involved scientists: Gunnar Jahnke, Michael Ewald, Johannes Ripperger, Tarje Nissen-Meyer, Max Strasser,
Melanie Reichardt, Markus Treml, Guoquan Wang, Gilbert Brietzke, Miko Fohrmann, Falko Bethmann



The accurate simulation of seismic wave propagation through realistic 3-D Earth models plays a fundamental role in several areas of geophysics: (1) in global seismology knowledge of the structure of the Earth’s deep interior is crucial to understand the dynamic behaviour of our planet such as mantle convection, slab subduction or hot spot activity. Accurate synthetic 3-D seismograms which can be compared with globally recorded data require a numerical approach. The structural resolution of today’s tomographic models can only be improved by exploiting the 3-D wave effects of the geodynamically important regions inside the Earth. (2) As deterministic earthquake fore-casting seems out of sight, the accurate prediction of likely ground motion following earthquakes in seismically active regions is a major goal which will allow measures (e.g. applying strict building codes) to be taken before major events. 3-D modelling will allow local (e.g. amplifying) effects such as low-velocity zone or topography to be studied. These so-called site effects will be investigated for several areas at risk (e.g. San Francisco Bay Area). (3) Active volcanic areas show very characteristic complex ground motion which is usually recorded on local networks monitoring the activity and risk of eruption. The origin of the seismically recorded signals are poorly understood. One of the reasons is the structural complexity of volcanic areas with strong 3-D heterogeneities, topography and sources in the summit region. Realistic 3-D structures and topography will be studied for two particular regions (Merapi, Indonesia and Mount Vesuvius, Italy). Synthetic seismograms will be calculated and compared with observed data from ongoing field projects. The ultimate goal is to detect reliable precursory signals to volcanic eruptions which will allow early warning of  endangered regions. For information on the various ongoing projects please consult the Homepages of the Institute of Geophysics at the Ludwig-Maximilians-University of Munich. Several results of simulations and a brief description of the scientifc rationale are given below. 


Wave Propagation in Volcanoes

Snapshot of wave propagation at the surface of a 3D model of Merapi volcano, Indonesia. The topography is based on a digitalSnapshot of wave propagation at the surface of a 3D model of Merapi volcano, Indonesia. The topography is based on a digital elevation map. The  source immitates an explosive type source in a magma conduit under the summit of the volcano. The goal of this project is to understand the contribution of the topography to the observed wavefield to better understand and separate the effects of the source. Ultimately we hope to be able to understand and quantify the state of the volcanic system based on remote sensing using seismic measurements. (Ripperger, Diplom Thesis, 2001)



Simulations of ground motion for likely earthquake scenarios

This simulation shows the wavefield at the surface for an earthquake happening  in the Cologne basin. The black contour lines denote the topography of the sediment basin with a maximum depth of 1500m. The oscillatory motion is cause by the low-velocity structure close to the surface. Such structure may considerably amplify the ground motion during earthquakes and it is therefore desirable to quantify such effects in seismically active regions.  This finite-difference simulation was carried out on a 800x900x200 grid simulating a crustal block of 140x160x30km. The simulation took 12 hours on 8 nodes of the Hitachi SR8000. Many such simulations for scenario earthquakes are needed to properly quantify the seismic risk in a particular region. A movie of the wavefield can be seen here but careful it is large (16MB). (Michael Ewald, Diplom Thesis, 2001).



Wave Propagation in Fault Zones

Fault zones (FZ) are though to consist of a highly localized damage zone with low seismic velocity and high attenuation. The structure of FZs at depth has important implications for the size of (future) earthquakes and the dynamic behaviour of the rupture. Only recently it was observed that right above FZs a particular wave type (guided waves) can be observed which may allow imaging FZs at depth. Numerical simulations play an important role in developing imaging schemes and assess their reliability. One of the main questions we addressed was, whether disrupted faults will allow guided waves to propagate. (Gunnar, Jahnke, Miko Fohrmann, Yehuda Ben-Zion)


Wave Propagation in Subduction Zones

Subduction zones contain the largest earthquakes on Earth. Knowledge of there structural details not only is important for hazard assessment but also to understand the dynamics of subduction and mantle convection. In this project a 3D algorithm in spherical coordinates was implemented and earthquakes in subduction zones simulated. We were able to simulate particular wave effects observed in nature which – in the future – can be used to further constrain the structure of subduction zones. (Tarje Nissen-Meyer, Diplom Thesis, 2001)