Anomalous Earthquakes Generated by Collapse of Magma Chambers

Anomalous Earthquakes Generated by Collapse of Magma Chambers Focal mechanisms for newly detected earthquakes at Nyiragongo. Schematic diagram on right shows the physical mechanism for these events. Earthquakes are generated by slip on inward-dipping ring faults due to deflation of shallow magma chambers. This can be caused either by diking events during volcanic eruptions (top), or by the transport of magma from deeper to more shallow magma chambers (bottom).
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Recent advances in earthquake detection using intermediate period surface waves have resulted in the discovery of hundreds of earthquakes (Mw>4.5) that previously went unrecorded in global seismicity catalogs [Ekström, 2006]. Many of these events are located in areas with good station coverage, and may have escaped detection due to unusual source properties. A series of five such earthquakes were detected near Nyiragongo Volcano in the Democratic Republic of the Congo between 2002 and 2005. The first three events occurred several days after the massive fissure eruption of Nyiragongo in January 2002. The final two events occurred in 2003 and 2005, and are not linked to a major eruption in the region, but did occur as the level of Nyiragongo’s summit lava lake was rising. This set of earthquakes is anomalous in two regards. First, these earthquakes are depleted in high-frequency energy over approximately 0.1 Hz, and can be considered slow earthquakes. Second, centroid-moment-tensor solutions indicate that these earthquakes are highly non-double-couple, each having a large compensated-linear-vector-dipole component of the moment tensor. This indicates that the double couple model for shear failure on a planar fault cannot explain the radiation pattern of these earthquakes. Drawing on models based on similar observations from other active volcanoes [Nettles and Ekström, 1998; Ekström and Nettles, 2002], we propose that the earthquakes are caused by slip on pre-existing, non-planar faults located beneath the edifice of the volcano. We suggest a mechanism in which these newly detected earthquakes are generated by the collapse of the roof of a shallow magma chamber along an inward-dipping cone-shaped ring fault [Shuler and Ekström, 2009]. As one might expect, these events can occur in association with an ongoing volcanic eruption. In the case of the first three earthquakes, diking events during the 2002 eruption reduced the pressure in a shallow magma chamber, leading it to collapse along the pre-existing ring fault. However, a similar mechanism can also explain the occurrence of these earthquakes due to the transport of magma from deeper to more shallow magma chambers. The earthquakes in 2003 and 2005 occurred during a period of lava lake level rise, and so are associated with magma ascent processes. The detection of this type of earthquake at other active volcanoes may be useful for inferring magma transport and determining the likelihood of future eruptions.
</p><p>References
</p><p>Ekström, G. (2006), Global Detection and Location of Seismic Sources by Using Surface Waves, Bull. Seismol. Soc. Amer., 96(4A), 1201-1212.
</p><p>Shuler, A., and G. Ekström (2009), Anomalous earthquakes associated with Nyiragongo Volcano: Observations and potential mechanisms, J.
Volcanol. Geoth. Res., 181(3-4), 219-230.
</p><p>Nettles, M., and G. Ekström (1998), Faulting mechanism of anomalous earthquakes near Bárdarbunga Volcano, Iceland, J. Geophys. Res., 103(B8), 17,973-17,983.
</p><p>Ekström, G. and M. Nettles (2002), Detection and location of slow seismic sources using surface waves, Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract S72E-06.
</p><p>Acknowledgements: The seismic wave- forms used in this study were obtained from the GSN, GEOSCOPE, GEOFON, and MEDNET. Additional data from the Ethiopia and Kenya Broadband Experiments were also utilized (PI Andy Nyblade). The facilities of the IRIS Data Management System, and specifically the IRIS Data Management Center, were used for access to waveforms and metadata required in this study. The schematic diagram in Figure 1 was rendered by Liz Starin. This work was funded by National Science Foundation Award EAR-0639963. Ashley Shuler was also supported by an NSF Graduate Research Fellowship.</p>

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