Volcano Seismology

An interesting signal recorded at station REF at Redoubt volcano, on April 1, 1990. This is the seismogram of a long-period earthquake. Because of its extended coda it resembles a screw and has also been called a "tornillo" (spanish word for screw, [Torres et al., 1996], or N-type earthquake [Sawada, 1994]).

Volcano seismology is the study of earthquakes as well as of velocity structure, attenuation, and the properties of earth materials that affect the passage of seismic waves at volcanoes. Volcanic earthquakes may be defined as earthquakes which occur at or near volcanoes, or are related to volcanic processes [McNutt, 1996].

Types of volcanic earthquakes

Volcanic earhquakes have been classsified based on seismogram appearance into four different types [Minakami, 1974]:
  1. High-frequency or A-type
  2. Low-frequency or B-type
  3. Explosion quakes
  4. Volcanic tremor
References

1. High-frequency or A-type earthquakes.

Most high-frequency events are thought to be caused by shear fracture or slip on faults, and are different from tectonic events only in their patterns of occurrence, which, at volcanoes, are typically in swarms, rather than mainshock-aftershock sequences. High-frequency events have clear P- and S-waves, and dominant frequencies are 5-15 Hz.















2. Low-frequency or B-type earthquakes.

Most low-frequency evnts are thought to be caused by fluid-pressurization processes such as bubble formation and collapse, also by shear failure, tensile failure or nonlinear flow processes which occur at very shallow depths for which attenuation and path effects play an important role. B-type events often have emergent P-waves, lack S-waves and have dominant frequencies of 1-5 Hz.














3. Explosion quakes.

Explosion quakes accompany explosive eruptions, and many are characterized by the presence of an air-shock phase on the seismograms. The example shown here was recorded by station SSLW at Shishaldin volcano (Unimak island, Alaska), on January 16, 2000. Note the distinctive "air" (impulsive arrival near the center of the graph).
















4. Volcanic tremor.

Volcanic tremor is a quasi-continuous to continuous signal of duration from minutes to days or longer. The dominant frequencies of tremor are 1-5 Hz, similar to low-frequency events. Harmonic tremor and spasmodic tremor are two special cases of more general volcanic tremor, The former is low-frequency, often monotonic sinusoid with smoothly varying amplitude, and the latter is a higher-frequency, pulsating irregular signal [Finch, 1949].











Aknowledgements: Jackie Caplan-Auerbach kindly provided the codes to plot the time series and compute these spectrograms.

References.

Aoyama, H., and Takeo, M. 2001. Wave properties and focal mechanisms of N-type earthquakes at Asama volcano. Journal of Volcanology and Geothermal Research.105,pp 163-182.

Finch, R.H. 1949. volcanic tremor (Part I). Bulletin of the Seismological Society of America. 39, pp 73-78.

McNutt, S.R. 1996. Seismic monitoring and eruption forecasting of volcanoes: a review of the state-of-the-art and case histories. In: Monitoring and mitigation of volcano hazards. Scarpa/tilling (Eds.). Springer-Verlag Berlin Heidelberg. pp 100-146.

Minakami, T. 1974. Seismology of volcanoes in Japan. In: Civetta et al. (Eds.) Physical Volcanology Developments in Solid Earth Geophysics vol 6. Elsevier Amsterdam pp 1-27.

Sawada, M. 1997. The source mechanism of B-type and explosion earthquakes and origin of N-type earthquakes observed at Asama volcano, central Japan. Ph.D. thesis Univ. of Tokyo, 85 pp.

Torres, R.A., Gómez, D.M., and Narváez, L. 1996. Unusual seismic signals associated with the activity at Galeras volcano, Colombia, from july 1992 to September 1994. ANNALI DI GEOFISICA, 39,2; pp 299-310.