EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Different mechanisms of the pre- and co-eruptive tremor during the 2018 eruption at Sierra Negra volcano, Galapagos

Ka Lok Li1, Meysam Rezaeifar1, Christopher J. Bean1, James Grannell1, Andrew Bell2, Mario Ruiz3, Stephen Hernandez3, and Martin Möllhoff1
Ka Lok Li et al.
  • 1Geophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, Dublin, Ireland (
  • 2School of Geosciences, University of Edinburgh, Edinburgh, UK
  • 3Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador

Volcanic tremor are persistent seismic signals observed near active volcanoes. They are often associated with eruptions, although the exact relationships are not well constrained. To gain a better insight into the generation mechanisms of volcanic tremor, we study tremor that occurred during the 2018 eruption at Sierra Negra volcano, Galapagos. Located 1000 km west of continental Ecuador, Sierra Negra is a shield volcano with a large summit caldera and is one of the most active volcanoes in the Galapagos archipelago. The 2018 eruption started at about 19:55 UTC on 26th June and lasted about two months. Two tremor phases with very different frequency characteristics are identified before and after the eruption onset. The pre-eruptive phase is characterized by a narrow frequency band (2.5 – 4 Hz) and the co-eruptive phase has a broad frequency band (1 – 15 Hz). Location of the two phases by a seismic amplitude ratio method suggests that they are likely to be generated by different physical processes. The pre-eruptive phase is likely generated by dike opening while the co-eruptive phase is associated with lava flow. This interpretation is consistent with a time-lapse P-wave velocity structure of the volcano imaged by local-earthquake travel-time tomography.

How to cite: Li, K. L., Rezaeifar, M., Bean, C. J., Grannell, J., Bell, A., Ruiz, M., Hernandez, S., and Möllhoff, M.: Different mechanisms of the pre- and co-eruptive tremor during the 2018 eruption at Sierra Negra volcano, Galapagos, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18975,, 2020

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Presentation version 1 – uploaded on 06 May 2020
  • CC1: Comment on EGU2020-18975, Eva Eibl, 06 May 2020

    Hi Ka Lok,

    there are some earthquakes at -1.5 km. I therefore presume that 0 is not the surface but sea level? At what elevation is the summit?



    • AC1: Reply to CC1, Ka Lok Li, 07 May 2020

      Hi Eva,

      That is correct. 0 is the sea level. The elevation of the summit is about 1 km. So, some of the earthquakes are actually in the air. That is because for the first run, we didn't account for the topography in the location process.

      Ka Lok

  • AC2: Comment on EGU2020-18975, James Grannell, 07 May 2020

    Hi Eva

    The finalised earthquake catalog was produced with NonLinLoc using a digital eleveation model to constrain hypocenters below the topography (the maximum elevation is -1.1km at the caldera). In addition, station elevation is accounted for by NonLinLoc. The events shown here must have been from an earlier iteration of the catalog, where Hypo71 was used (which does not account for station elevation and where topography is not included).

    James Grannell


    • CC2: Reply to AC2, Eva Eibl, 07 May 2020

      Hi James,

      I see. Thanks for clarifying.




  • AC3: Comment on EGU2020-18975, Andrew Bell, 07 May 2020

    Hi Ka Lok,

    I was wondering if you're able to separate a power spectral density for the pre-euptive tremor? How does it differ from the discrete earthquakes in the pre-eruptive swarm, or the co-eruptive tremor? Does the frequency content change with time through the pre-eruptive phase?

    Best wishes,


    • AC4: Reply to AC3, Ka Lok Li, 07 May 2020

      Hi Andy,

      Separating the power spectral density for the pre-eruptive tremor is quite challenging in this case because of the overlapping of the pre-eruptive tremor with the earthquakes. We are still finding a good way to achieve this. Any idea would be greatly appreciated!

      However, by just looking at the spectrograms, we would expect that the power spectral density for the pre-eruptive tremor is different from those for earthquakes and co-eruptive tremor. The density for the pre-eruptive tremor consists of a few narrow bands while that for the co-eruptive tremor is a single wider band and for the earthquakes are very broad bands extending up to at least 18 Hz.

      The frequency content of the pre-eruptive phase looks quite stable in time, although at VCH1, we see some gliding towards the end of this phase.

      Ka Lok

      • CC3: Reply to AC4, Eva Eibl, 08 May 2020

        Hi Ka Lok,

        maybe look at the presentation of Zahra Zali, she's a PhD student here in Potsdam currently working on separating tremor from earthquakes during the Holuhraun eruption. It looks quite promising.



        • AC6: Reply to CC3, Ka Lok Li, 10 May 2020

          Hi Eva,

          It looks very promising. Thank you very much!

          Best regards,
          Ka Lok

  • AC5: Comment on EGU2020-18975, Andrew Bell, 08 May 2020

    Thanks Ka Lok, you're right, it is tricky. Stephen and I have commented on apparent gliding in frequencies during the pre-eruptive phase, but I'm not sure if that energy is just in the discrete earthquakes, or the tremor too.