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Low-­‐Frequency  Volcanic  Seismicity
Source  processes  and  implica0ons  of  
tremor,  LP  and  VLP  earthquakes  at  open-­‐
vent  volcanoes

TREMOR,  LP,  AND  VLP  CHARACTERISTICS



Primarily  defined  on  the  basis  of  the  frequency  content



These  all  can  be  considered  “low-­‐frequency”  seismicity

TREMOR,  LP,  AND  VLP  CHARACTERISTICS

Short-period seismometers
Broad-band seismometers

TREMOR,  LP,  AND  VLP  CHARACTERISTICS
Low-­‐Frequency
(LF)  Seismicity



Primarily  defined  on  the  basis  of  the  frequency  content



These  all  can  be  considered  “low-­‐frequency”  seismicity

GENERAL  VOLCANIC  EARTHQUAKE  CLASSIFICATION
• HF  (high  frequency)  or  VT  (volcano  tectonic)
– tectonic  earthquakes  (slip  on  a  fault)
– Clear  P  and  S  wave  arrivals

• LF  (low  frequency)
– Typically  no  clear  S  wave
– Low  frequencies  due  to  source  and/or  path
– Includes  long-­‐period  earthquakes,  which  occur  at  all  depths  within  the  crust

– Tremor  -­‐  Semi-­‐con0nuous  signal  with  harmonic  or  irregular  signals
• Hybrid
– Characteris0cs  of  both  HF  and  LF
– Usually  have  high-­‐frequency  onset,  low-­‐frequency  coda
– Could  represent  a  small  VT  that  triggers  an  LP
– Some0mes  dis0nguished  from  LP  on  the  basis  of  mixed  first-­‐mo-on  polari-es
– suggests  the  event  involves  slip  on  a  fault

GENERAL  CHARACTERISTICS  OF  LF  SEISMICITY


May  have  harmonic/narrow  band  signal



Typically  aYributed  to  fluid  interac0ng  with  solid  volcanic  
conduit  walls




gas,  liquid  or  more  likely  mul0phase

Nonlinear  processes  that  vary  with  0me


physical  proper0es  of  the  system  evolve



Transient  or  long-­‐lived



Path  and  site  affects  can  cause  events  to  look  like  LF  events

LP  (LONG-­‐PERIOD)  EARTHQUAKES


Known  by  many  names






Broadband  onset


frequencies  from  .2  to  15  Hz



trigger

Decaying,  harmonic  coda





LP,  B-­‐type,  tornillo,  ...  

frequencies  .5  -­‐  2  Hz

From Kumagai and Chouet, GJI, 1999

resonance

Typically  shallow,  but  can  be  very  deep  (upper  mantle)

PATH-­‐EFFECTED  LPS?


Path  can  filter  some  frequencies  and  enhance  others



Shallow  low-­‐velocity  layers  can  trap  waves




prolongs  the  dura0on  of  the  signal  and  may  mimic  LP  coda  
characteris0cs

Topography  focusses  and  defocusses  waves


Waves  can  be  trapped  beneath  steep  
topographic  features  (hills  and  volcanic  
edifices)



If  underlain  by  strong  reflec0ve  layer,  the  
signal  can  ring  for  10s  of  seconds

Anderson et al.,submitted to G3, 2011

SYNTHETIC  MODELING  OF  STRUCTURE


Shallow  low-­‐velocity  layers  can  trap  waves




prolongs  the  dura0on  of  the  signal  and  may  mimic  LP  coda  
characteris0cs

Topography  focusses  and  defocusses  waves


Waves  can  be  trapped  beneath  steep  topographic  features  
(hills  and  volcanic  edifices)



If  underlain  by  strong  reflec0ve  layer,  the  signal  can  ring  for  
10s  of  seconds

Anderson et al.,submitted to G3, 2011


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