LOCATING LONG PERIOD EVENTS AND TREMORBENEATH GALERAS VOLCANO USING SEISMICAMPLITUDES – STOCHASTIC APPROACH
Roberto Armando Torres Corredor ∗MEE09178

Supervisor:Hiroyuki KUMAGAI ∗∗ABSTRACT
A source location method using seismic amplitudes is applied to locate long period (LP) events andtremor at Galeras volcano. Galeras in the Colombian Andes is an active andesitic stratovolcano. Thismethod is appropriate to estimate source locations of seismic signals that show emergent onsets suchas LP events, or stationary signals for a long time that do not exhibit onset times such as tremor. The
method assumes isotropic radiation of S-waves and uses seismic amplitudes corrected for station site
effects. Site amplification was estimated at stations of the Galeras network in five different frequency
bands (1-6, 3-8, 5-10, 7-12, and 9-14 Hz) using the coda normalization method. The estimated site
amplification factors may be consistent with geological features observed at Galeras. For source
location determinations, grid searches with respect to the frequency range, quality factor for medium
attenuation (Q), and space for each origin time were performed in each event to estimate the minimum
normalized residual, which is regard as the source location. The five frequency bands used in theestimations of site amplification factors were used in source location determinations. For LP events,
the best-fit locations were found using a frequency band of 9-14 Hz and Q values of around 70. Also,using these parameters, a reasonable source location was estimated for tremor. It has been interpreted
that the isotropic S-wave radiation assumption becomes valid in high frequency bands because of thepath effect caused by the scattering of seismic waves propagating in structural heterogeneities beneathvolcanoes. This study supports this interpretation and demonstrates the applicability of the source
location method to LP events and tremor at Galeras volcano.
Keywords: Long-period events, tremor, isotropic radiation, site amplification.

1. INTRODUCTION
1.1 BackgroundGaleras is an active andesitic stratovolcano (elevation 4276 m) located in the southwest of Colombia
near the Ecuatorian border at 1°13.73' N, 77°21.55' W. Its active cone is located at 9 kilometers to thewest of Pasto, a city with 450,000 inhabitants and the capital of Nariño province (Figure 1).
Galeras volcano is considered to be one of the most active volcanoes in the country. In thelast 4500 years, most of the eruptive events have been interpreted as vulcanian-type eruptions, withsmall eruptive columns (less than 10 km in height) that produced small pyroclastic flow deposits,
containing a high proportion of non-juvenile materials, lava flow fragments and scoria clasts(Calvache and Williams 1992). During the last 21 years, 20 explosive eruptions have taken place after
the emplacements of lava domes, producing small pyroclastic fall deposits.

∗Colombian Institute of Geology and Mining (INGEOMINAS), Pasto, Colombia.
National Research Institute for Earth Science and Disaster Prevention (NIED), Tsukuba, Japan.

∗∗11.2 Seismic Monitoring Network
The Galeras seismic monitoring network wasestablished in February 1989 by the Colombian
Institute of Geology and Mining, INGEOMINAS.
At present the seismic network consists of ten
permanent radio-telemetered stations with velocitysensors. The seismic stations are located at the range
between 1 km and 7 km from the active crater.
Figure 1 shows the location of the short period
seismometers (blue triangles) and the broad bandseismometers (red triangles) with flat velocity
response between 0.02 and 60 s. Digital data arerecorded in real time at a rate of 100 samples per
second in the Volcanological Observatory (VO)
Figure 1. Location of Galeras volcano and thelocated in the Pasto city.
seismic monitoring network
1.3 Types of seismic events at Galeras volcano

Figure 2. Examples of types of seismic events atGaleras volcano recorded at ANVG short period
station. The velocity values are not corrected byinstrument response. (a) VT earthquake, (b) LPevent, (c) Tornillo type of LP event, (d) tremor, and(e) hybrid event.

According to the classification proposed by
Chouet (1996), the following families ofseismic events are distinguished at Galeras
volcano: volcano- tectonic (VT) earthquakes,long period (LP) events, a particular type of
LP events called “Tornillos” (Spanish wordfor “screws”), tremor and hybrid events(Figure 2).
VT events are associated to tensile orshear fractures in the solid rock due to thepressures induced by magma, while LP eventsand tremor are related to magmatic and/orhydrothermal activity and they are interpreted
as the resonances of fluid-filled cavities orcracks due to the pressures transient originatedby the movement of fluids. Hybrid events areassociated with the fractures in the solidmaterial but in addition involving pressurestransient originated by the movement of fluids(Chouet 1996).
Often VT seismograms show clear
onsets of P- and S-wave arrivals. Sometimes
LP seismograms show clear onset of P-wave
arrival but they usually exhibit emergent
onsets, and the signatures of tremor show
continuous oscillations for a relative long timethat do not exhibit onset times.

1.4 Purpose of Study
Source positions of LP events and tremor are useful in evaluating the volcanic activity and for
improving the knowledge about the structure of volcano. Hypocenter determinations of LP events andtremor based on phase arrival times are usually difficult or impossible. The main problem is the2identifications of phases because their onsets are usually emergent (LP events) or their waveforms are
stationary for a long duration (tremor). As an alternative, relatively a new method has been introduced
to locate these kinds of events. Battaglia and Aki (2003) proposed an approach to locate these eventsusing their amplitudes assuming isotropic radiation of S-wave, and located LP events and volcanic
tremor at the Piton de la Fournaise volcano located in Réunion island (French territory in the Indian
Ocean). Kumagai et al. (2010) used the method to locate LP events, explosions, tremor associated withlahars and pyroclastic flows at Tungurahua and Cotopaxi volcanoes in Ecuador, and their results
provided reasonable source locations in a frequency band around 5-12 Hz and Q factor around 60.
Kumagai et al. (2010) interpreted that the assumption of isotropic radiation may become valid becauseof the path effect caused by the scattering of seismic waves in this frequency band, and based on that,the source location method may be categorized as a stochastic approach based on the nature ofscattering waves.
The purpose of this study is to locate LP events and tremor at Galeras volcano using themethod proposed by Kumagai et al. (2010) to test the validity and applicability of the method.
Furthermore, the implementation of this method running in an automatic way and provision of
permanent hypocenter solutions will contribute to improved monitoring.

2. METHODOLOGY
The source location method of LP events and tremor uses the spatial distribution of their amplitudes
recorded by a seismic network (Battaglia and Aki 2003; Kumagai et al. 2010). The method uses thefar-field approximation for a body wave and assumes isotropic radiation of S-waves. The seismic
amplitude at the i-th station, Ai , excited by a point seismic source can be expressed as (Kumagai et al.
2010):
πf⎛ r ⎞
1 − Qβ ri
Ai ⎜⎜ t + i ⎟⎟ = A0 s0δ (t − t s )
e,ri
⎝ β⎠

(1)

where A0 is the coefficient for radiation pattern of S-wave (constant value for isotropic wave radiation),s0 δ(t-ts) is the rate source time function represented by the delta function, ri is the distance between
the i-th station and the seismic source, f is the frequency, Q is the quality factor for medium
attenuation, β is the S-wave velocity, and ts is the source origin time.
Observed amplitudes are fitted by minimizing the normalized residuals E2⎧ o⎛ri ⎞ri ⎞⎫⎛⎨ Ai ⎜⎜ t s + ⎟⎟ − Ai ⎜⎜ t + ⎟⎟⎬∑
β⎠β ⎠⎭ ,i =1 ⎩⎝⎝E=2N⎧ o⎛ri ⎞⎫⎨ Ai ⎜⎜ t s + ⎟⎟⎬∑β ⎠⎭i =1 ⎩⎝N(2)where N is the number of stations and Aio is the observed amplitude at the i-th station corrected bylocal site conditions. Minimizing E leads to the following solution at t = t s :A0' =r ⎞1 N o⎛Ai ⎜⎜ t s + i ⎟⎟ri e Bri .
∑β⎠
N i =1 ⎝(3)

Since observed amplitudes at individual stations are affected by local site conditions, wecorrect those by using coda normalization method (Aki and Chouet 1975; Phillips and Aki 1986).
According to Aki and Chouet (1975) the time- and frequency-dependent amplitudes of coda waves fora lapse time t greater than about twice the S-wave travel time can be described as:3

Ai ( f , t ) = S ( f )C ( f , t )Gi ( f ) ,(4)where Ai(f,t) is the Fourier amplitude of a coda wave at the i-th station, S(f) is the source term, C(f,t) isthe path term, and Gi(f) is the site term of the i-th station. Relative site amplification between twostations i and j can be determined by using spectral ratio of Ai(f,t) to Aj(f,t) at the same t from the same
event (e.g., Kato et al. 1995):

Ai ( f , t ) Gi ( f )=.
Aj ( f ,t) G j ( f )(5)

3. RESULTS AND DISCUSSION
3.1 Site amplification factors
The relative site amplification factors were estimated at individual stations of the Galeras network byusing CUVZ (Figure 1) as the reference station due to its continuous recording and high signal tonoise ratio. First, a set of 14 well recorded regional tectonic earthquakes between September 2008 andMay 2009 were selected (Figure 3a). The magnitudes of these earthquakes were in a range between
3.2 and 4.8. These earthquakes with the epicentral distances between 50 and 234 km from the active
crater are diversely distributed around the volcano except in the east direction, where a zone of low
seismicity exists. Sometimes, the seismograms were not available at all stations of the Galeras network
because of equipment
troubles or low signal
to noise ratio. So asecond set of 17 local
tectonic and volcanotectonic earthquakes
betweenOctober
2008 and May 2009were also selected(Figure 3b). The
magnitudes of this setof earthquakes wereranging between 0.8
and 2.1 and theFigure 3. Location of regional tectonic earthquakes (a) and local tectonic and distances were bevolcano-tectonic earthquakes (b) used to estimate the site amplification tween 4 and 16 km
theactivefactors. Black circles and numbers indicate the locations and occurrence dates. fromcrater.
Site amplification factors for the stations at the Galeras network in five individual frequency
bands (1-6, 3-8, 5-10, 7-12 and 9-14 Hz) were estimated by using coda waves of regional
earthquakes, and local tectonic and volcano-tectonic earthquakes. In general, smaller errors in the coda
amplitude ratios were obtained by using codas from regional earthquakes. This may be due to theavailability of high signal-to-noise ratio seismograms in most of the stations, as opposed to thevolcano-tectonic earthquakes with smaller magnitudes, of which codas were contaminated by the
background noise of the volcano activity. Consistent results for ANGV, URCR, COB3 and OBVR
were obtained by using codas of regional and local earthquakes, and some discrepancies were found
for the remaining stations (Figures 4a and 4b).

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Figure 4. Site amplification factors for all
stations in individual
frequency bands byusing regional earthquakes (a), and localand volcano- tectonicearthquakes (b).
3.2 Source location of LP events and tremor
LP events and tremor at Galeras volcano were located by using the method proposed by Kumagai et al.
(2010). Grid searches with respect to the frequency range, quality factor, and space for each origin
time t s were performed in each event to estimate the spatial distributions of normalized residuals.
Five different frequency ranges were considered (1-6, 3-8, 5-10, 7-12, and 9-14Hz), fifteen Q values
from 5 to 180 were used (5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 150 and 180), and point
sources were positioned at individual grid nodes spaced 400 m apart covering an area of 10x10 km
centered at the active crater and a vertical extent of 6 km covering the summit. Topography grid datawith a 20-m resolution were used in the spatial grid search. For LP events, a set of 6 events recordedon November 20 and 21, 2008, were selected. The criteria for this selection were: (1) good quality ofseismograms, (2) a good coverage involving most of the stations of the Galeras network, and (3) abroad spectrum with significant frequency contents above 5 Hz. The global minimum normalized
residual, which is regards as the source location, was found by using a frequency band of 9-14 Hz, Q
values between 60 and 80, and the amplitudes corrected by the amplification factors obtained fromregional earthquakes (Figure 5). Using these parameters, the spatial distributions of the normalized
residuals and best-fit location were conducted for each LP-event (Figure 6).

Figure 5. Plots of the minimum normalized
residuals for the LP-event recorded on
November 21, 2008 as a function of Q values in
the five frequency bands, in which theamplitudes were corrected by the coda
amplification factors of regional tectonic
earthquakes.
The estimated source locations ofthese six LP events are beneath the active crater
region at shallow depths. Using theseparameters, a reasonable source location was
estimated for tremor recorded on November 20
and 21, 2008 at Galeras.

Figure 6. Spatial distributions of the normalized
residuals estimated for the LP-event recorded on
November 21, 2008 using amplitudes band-passed
between 9 and 14 Hz, Q of 70, and amplitudes
corrected by the site amplification factors obtained
from regional earthquakes. Star indicates theminimum residual and the best fit source location.

5The estimated source locations of LP-events at Galeras are in agreement with hypocentersestimated for other LP events that occurred in 2008 by using first arrivals with Hypo71. However, wenote that the hypocenter determination with Hypo71 using first arrivals is not always reliable due tothe difficulties arising from the identification of phases when the onsets are usually emergent as wellas due to the oversimplification of the structure (flat layers) used in Hypo71.
A frequency band of 9–14 Hz and Q value around 70 for the best fit locations of LP events atGaleras are in agreement with the results found by Kumagai et al. (2010) at Ecuadorian volcanoes: afrequency band of 7–12 Hz and Q value of 60 for the best-fit location of a VLP/LP event at Cotopaxi,
and a frequency band of 5–10 Hz and Q value of 60 for the best-fit source location of an explosion
event at Tungurahua. Kumagai et al. (2010) interpreted that the isotropic radiation assumption
becomes valid in high frequency bands because of the path effect caused by the scattering of seismic
waves propagating in structural heterogeneities through the volcanoes. This study supports theinterpretation of Kumagai et al. (2010).
4. CONCLUSIONS
A source location method using seismic amplitudes was used to locate LP events and tremor atGaleras. The method assumes isotropic radiation of S-waves and uses seismic amplitudes corrected for
local site effects. Site amplification factors were estimated at stations of the Galeras network in fivedifferent frequency bands (1-6, 3-8, 5-10, 7-12, and 9-14 Hz) by using the coda normalization method,which geographically diverse sets of well-recorded regional and local earthquakes were used. For thesource location determinations, grid searches with respect to the frequency range, quality factor, andspace for each origin time were performed in each event to estimate the minimum normalized residual,
which is regard as the source location. The five frequency bands used in the estimations of site
amplification factors were used in the source location determinations.
For LP events, best-fit locations were found when using a frequency band of 9-14 Hz, Qvalues of around 70, and the amplitudes corrected by site amplification factors obtained from regional
earthquakes. Also, a reasonable source location was estimated for tremor using these parameters.
These results demonstrate the applicability of the method proposed by Kumagai et al. (2010) to LP andtremor at Galeras. This source location method, without requiring an expert intervention for picking
arrival times, offers a possibility of automatic estimations of source locations of LP events and tremor.
A permanent provision of source location determinations will contribute to improve volcanicmonitoring.
The introduction of this method at Galeras is worth in evaluating volcanic activity and in
improving the knowledge about the structure of the volcano.
AKNOWLEDGEMENT
I would like to express my sincere gratitude to my adviser Dr. T. Hara for his continuous support,
valuable suggestions and guidance during my study. I am very grateful to Dr. T. Yokoi for reviewing
the manuscript and giving valuable comments.
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Battaglia, J. and Aki, K., 2003, J. Geophys. Res., 108, 2364, doi:10.1029/2002JB002193.
Calvahe, M., Cortes, G., and Williams, S., 1997, J. Volcanol. Geother. Res., 77, Issues 1-4, 5-19.
Chouet, B., 1996, Nature, 380, 309-316.
Kato, K., Aki, K., and Takemura, M., 1995, Bull. Seism. Soc. Am., 85, 467-477.
Kumagai, H., Nakano, M., Maeda,T., Yepes, H., Palacios, P., Ruiz, M., Arrais, S., Vaca, M., Molina,I., Yamashina, T., 2010, J. Geophys. Res., 115, 1308303, doi:10.1029/2009JB006889.
Phillips, S., and Aki, K., 1986, Bull. Seism. Soc. Am., 76, 627-648.

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