Modern space geodetic techniques such as InSAR (synthetic aperture radar interferometry) and GPS have revolutionized the study of volcanic deformation, where one of the primary goals is to estimate the geometry of magma bodies in space and in time. These time-space volumetric relations are needed to predict changes in the state of stress within the volcanic system and to build robust eruption forecast models. However, the geodetic observations are restricted to the ground surface, and the models quickly lose resolving power with depth. This poor spatial resolution at depth can be improved significantly by using independent subsurface information. Magma intrusion events are often accompanied by increased seismicity in the vicinity of the intrusion. The migrating swarms of earthquakes have been used as an indication for the propagation of cracks that fill with magma. However, the time-space constraints provided by earthquake locations have never been used quantitatively to constrain the geometry of the magma body in an inverse problem.
Colleagues Sang-Ho Yun (JPL) and Paul Segall (Stanford) have developed and are testing a joint inversion algorithm that uses seismicity rate and hypocentral location together with geodetic observations, and to apply the method to 9 large volume Afar dike intrusions where both geodetic and seismicity data are available to this study. We will address the following questions:
- What are the time-space relations between magma intrusion and fault slip in the brittle rock surrounding the intrusion?
- Does the seismicity constrain the shape of the intruded magma body in a mechanically consistent way?
- What is the geometry of the magma reservoir feeding the dike system, and can we estimate the volume changes in time?
- What are the implications for the spatiotemporal variability in the stress state in the surrounding rock for the future occurrence of magma intrusions and earthquakes?
Physics and Astronomy graduate student Gabrielle Tepp, Cindy Ebinger, and former graduate student Manahloh Belachew have developed tools to distinguish earthquakes associated with dike propagation and faulting above the dike. Work in preparation for publication documents several unusual features of low-frequency earthquakes accompanying magma intrusion: waves trapped along the 10’s of km long, meters wide dikes; waves scattered from possible magma bodies; low frequency body waves associated with fault rupture to the surface. Comparison of stress drops of the different classes of earthquakes enables re-assessment of fault scaling relations, and remote detection of dike intrusions with and without surface ruptures.