U.S. Department of the Interior

U.S. Geological Survey

Scientifi c Investigations Report 2004–5211

Seismic Monitoring at Cascade Volcanic Centers, 

2004—Status and Recommendations

Mount Baker Mount Baker 

Glacier Peak Glacier Peak 

Mount Rainier Mount Rainier 

Mount

St. Helens St. Helens Mount Adams Mount Adams 

Mount Hood Mount Hood 

Mount Jefferson Mount Jefferson 

Three Sisters Three Sisters 

Newberry Volcano Newberry Volcano 

Crater Lake Crater Lake 

Mount Shasta Mount Shasta 

Medicine Lake Medicine Lake 

Lassen Peak Lassen Peak 
Seismic Monitoring at Cascade Volcanic Centers, 

2004—Status and Recommendations

By Seth C. Moran

Scientific Investigations Report 2004–5211

U.S. Department of the Interior

U.S. Geological Survey 
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Version 1.0, 2005

This publication is available only online at:

http://pubs.usgs.gov/sir/2004/5211/

Manuscript approved for publication, September 30, 2004

Production and design by Stephen L. Scott

Text edited by James W. Hendley II

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 
Contents

1—Introduction --------------------------------------------------------------------------------------------- 1

2—Seismic monitoring at Cascade volcanic centers—the big picture ------------------------------ 1

3—Seismic networks by volcano ------------------------------------------------------------------------- 1

4—Assessment of the ability of networks to detect anomalous seismicity ------------------------- 2

4.1—Earthquake Catalogs – what has been detected? ------------------------------------------ 3

4.1.1—Detection threshold -------------------------------------------------------------------- 3

4.1.2—Magnitude of completeness ----------------------------------------------------------- 3

4.1.3—Theoretical detection threshold ------------------------------------------------------ 7

4.1.4—Theoretical “good” location threshold ----------------------------------------------- 8

4.2—Important seismic-monitoring criteria other than earthquake detection 

 and location ----------------------------------------------------------------------------------- 8

5—Recommendations ------------------------------------------------------------------------------------- 16

5.1—Seismic networks should be upgraded before any unrest occurs ---------------------- 16

5.2—Priority A volcano networks ------------------------------------------------------------------ 16

5.2.1—Mount Baker --------------------------------------------------------------------------- 16

5.2.2—Newberry Volcano --------------------------------------------------------------------- 18

5.2.3—Medicine Lake ------------------------------------------------------------------------- 18

5.2.4—Crater Lake ----------------------------------------------------------------------------- 19

5.2.5—Three Sisters --------------------------------------------------------------------------- 19

5.3—Priority B volcano networks ------------------------------------------------------------------ 19

5.3.1—Glacier Peak ---------------------------------------------------------------------------- 19

5.3.2—Mount Adams -------------------------------------------------------------------------- 20

5.3.3—Mount Jefferson ----------------------------------------------------------------------- 20

5.4—Priority C volcano networks ------------------------------------------------------------------ 20

5.4.1—Mount Rainier -------------------------------------------------------------------------- 21

5.4.2—Mount Hood ---------------------------------------------------------------------------- 21

5.4.3—Mount Shasta -------------------------------------------------------------------------- 21

5.4.4—Lassen Peak ---------------------------------------------------------------------------- 21

5.5—Priority D volcano networks ------------------------------------------------------------------ 21

5.5.1—Mount St. Helens ---------------------------------------------------------------------- 21

6—Conclusions -------------------------------------------------------------------------------------------- 21

Acknowledgments ----------------------------------------------------------------------------------------- 22

References -------------------------------------------------------------------------------------------------- 22

Figures

1. Shaded relief map showing locations of all seismic stations operating in the Pacific 

 Northwest as of November, 2003 ----------------------------------------------------------------- 2

2. Plot showing number of stations within X km of 11 Cascade volcanoes ------------------------ 3

3. Single-channel seismic-reflection profiles relative to ODP Site 1015 ---------------------------- 4 

4. Glacier Peak seismic network ------------------------------------------------------------------------ 4

5. Mount Rainier seismic network ---------------------------------------------------------------------- 4

6. Mount St. Helens seismic network ------------------------------------------------------------------- 6

7. Mount Adams seismic network ----------------------------------------------------------------------- 5

8. Mount Hood seismic network ------------------------------------------------------------------------- 5

III
Figures–continued

 9. Mount Jefferson seismic network ------------------------------------------------------------------- 5

10. Three Sisters seismic network ----------------------------------------------------------------------- 5

11. Newberry Volcano seismic network ----------------------------------------------------------------- 6

12. Crater Lake seismic network -------------------------------------------------------------------------- 6

13. Mount Shasta and Medicine Lake seismic networks ---------------------------------------------- 6

14. Lassen Peak seismic network ------------------------------------------------------------------------ 6

15. Smallest magnitude earthquake within 20 km of volcano between 1990-2003 ------------------ 7

16. Magnitude of completeness for earthquakes within 20 km of volcano between 

 1990-2003 ------------------------------------------------------------------------------------------------ 8

17. Estimates of detection thresholds at the summit of each volcano ------------------------------- 9 

18. Mount Baker and contours of estimated minimum magnitude for earthquake 

 detection ------------------------------------------------------------------------------------------------ 9

19. Glacier Peak and contours of estimated minimum magnitude for earthquake 

 detection ------------------------------------------------------------------------------------------------ 9

20. Mount Rainier and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 10

21. Mount St. Helens and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 10

22. Mount Adams and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 10

23. Mount Hood and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 10

24. Mount Jefferson and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 11

25. Three Sisters and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 11

26. Newberry Volcano and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 11

27. Crater Lake and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 11

28. Mount Shasta and Medicine Lake contours of estimated minimum magnitude 

 for earthquake detection ----------------------------------------------------------------------------- 12

29. Lassen Peak and contours of estimated minimum magnitude for earthquake 

 detection ----------------------------------------------------------------------------------------------- 12

30. Minimum magnitudes required for events located at each summit to record on 

 enough stations to have well-constrained locations --------------------------------------------- 12

31. Contours of magnitude thresholds for “good” locations at Mount Baker ---------------------- 13

32. Contours of magnitude thresholds for “good” locations at Glacier Peak ---------------------- 13

33. Contours of magnitude thresholds for “good” locations at Mount Rainier --------------------- 13

34. Contours of magnitude thresholds for “good” locations at Mount St. Helens ----------------- 13

35. Contours of magnitude thresholds for “good” locations at Mount Adams --------------------- 14

36. Contours of magnitude thresholds for “good” locations at Mount Hood ----------------------- 14

37. Contours of magnitude thresholds for “good” locations at Mount Jefferson ------------------ 14

IV
Figures–continued

38. Contours of magnitude thresholds for “good” locations at Three Sisters ---------------------- 14

39. Contours of magnitude thresholds for “good” locations at Newberry Volcano --------------- 15

40. Contours of magnitude thresholds for “good” locations at Crater Lake ------------------------ 15

41. Contours of magnitude thresholds for “good” locations at Mount Shasta and 

 Medicine Lake ----------------------------------------------------------------------------------------- 15

42. Contours of magnitude thresholds for “good” locations at Lassen Peak ---------------------- 15

43. Multistation spectrogram showing a minutes-long tremor episode at Great Sitkin 

 Volcano, Alaska ---------------------------------------------------------------------------------------- 17

44. Contours of magnitude thresholds for “good” locations at Mount Baker network 

 with three additional stations ------------------------------------------------------------------------ 18 

45. Contours of magnitude thresholds for “good” locations at Three Sisters with four 

 additional stations to be installed in 2004 ---------------------------------------------------------- 19

46. Contours of magnitude thresholds for “good” locations at Three Sisters with eight 

 additional stations, four of which are scheduled to be installed in 2004 ----------------------- 20

47. Seismic monitoring network around Mount Etna, Italy ------------------------------------------- 22

V
1

1—Introduction

The purpose of this report is to assess the current (May, 

2004) status of seismic monitoring networks at the 13 major 

Cascade volcanic centers. Included in this assessment are 

descriptions of each network, analyses of the ability of each 

network to detect and to locate seismic activity, identification 

of specific weaknesses in each network, and a prioritized list 

of those networks that are most in need of additional seismic 

stations. 

At the outset it should be recognized that no Cascade 

volcanic center currently has an adequate seismic network 

relative to modern-day networks at Usu Volcano (Japan) or 

Etna and Stromboli volcanoes (Italy). For a system the size 

of Three Sisters, for example, a modern-day, cutting-edge 

seismic network would ideally consist of a minimum of 10 to 

12 short-period three-component seismometers (for determining particle motions, reliable S-wave picks, moment tensor 

inversions, fault-plane solutions, and other important seismic 

parameters) and 7 to 10 broadband sensors (which, amongst 

other considerations, enable detection and location of very 

long period (VLP) and other low-frequency events, moment 

tensor inversions, and, because of their wide dynamic range, 

on-scale recording of large-amplitude events). Such a dense, 

multi component seismic network would give the ability to, 

for example, detect in near-real-time earthquake migrations 

over a distance of ~0.5km or less, locate tremor sources, determine the nature of a seismic source (that is, pure shear, implosive, explosive), provide on-scale recordings of very small 

and very large-amplitude seismic signals, and detect localized 

changes in seismic stress tensor orientations caused by movement of magma bodies. 

However, given that programmatic resources are currently 

limited, installation of such networks at this time is unrealistic. Instead, this report focuses on identifying what additional 

stations are needed to guarantee that anomalous seismicity 

associated with volcanic unrest will be detected in a timely 

manner and, in the case of magnitude ≥ 1 earthquakes, reliably 

located. 

2—Seismic monitoring at Cascade 

volcanic centers—the big picture

Seismic activity at Cascade volcanic centers is monitored 

by a composite network of seismic stations (fig. 1) operated 

by several groups. These groups include the Pacific Northwest 

Seismograph Network (PNSN), operated by the University of 

Washington in cooperation with several other entities (such as 

the University of Oregon), and the Northern California Seismic Network (NCSN), jointly operated by the U.S. Geological 

Survey (USGS) Earthquake Hazards Team in Menlo Park and 

the University of California Berkeley. Both groups receive 

funding from the USGS Volcano Hazards Program, the USGS 

Earthquake Hazards Program through the National Earthquake 

Hazards Reduction Program (NEHRP) and Advanced National 

Seismic System (ANSS) programs, the National Oceanic and 

Atmospheric Administration (NOAA) through the Cooperative 

Remote sEnsing Science and Technology (CREST) program 

for tsunami mitigation, and other sources. Waveform data is 

shared in real time between these networks and other groups 

(including the Cascades Volcano Observatory (CVO)) via the 

USGS Earthworm data acquisition system (Bittenbinder and 

others, 1994). The PNSN also uses this system to receive data 

from several Canadian stations operated by the Pacific Geoscience Centre in Sydney, British Columbia. 

The PNSN/NCSN combined network has a total of 44 

stations within 20 km of individual stratovolcanoes, and 66 

within 40 km (figs. 1, 2). The vast majority of these are shortperiod, vertical-component stations that use analog telemetry. 

This number also includes four broadband seismometers (one 

each near Rainier, Hood, Newberry, and Shasta; most are 

CREST-funded sites), two short-period three-component seismometers (Three Sisters, Lassen Peak), and five ANSS- and 

CREST-funded strong-motion accelerometers (near Mount 

Baker, Mount Rainier, Mount Hood, Three Sisters, and Newberry Volcano; three of these are co located with broadband 

instruments). CVO currently receives real-time data from 50 

of these stations.

3—Seismic networks by volcano

Figures 3 through 14 show seismic stations within ~40 

km of the 13 individual Cascade volcanic centers. Maps are 

drawn to similar, but not identical, scales, with the exception 

of Shasta/ Medicine Lake. Yellow triangles represent shortperiod vertical-component instruments, cyan represent shortperiod three-component instruments, red represent broadband 

(BB) seismometers, and green represent strong-motion (SMO) 

accelerometers. White circles correspond to locations of shallow (< 10 km) earthquakes occurring between 2000-2003. Signals from short-period instruments are telemetered in real time 

Seismic Monitoring at Cascade Volcanic Centers, 

2004—Status and Recommendations

 By Seth C. Moran
2 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

frequency” events, long-period (LP) events, VLP events, 

tremor, and surface events (such as rockfalls, avalanches, and 

explosions)) is one of the chief roles of a volcano seismic network. Detection occurs either with automated computer-based 

programs or the human eye. Automated detection of earthquakes and surface events works well for VTs, hybrids, impulsive LP events, and large-amplitude surface events, as long as 

a sufficiently dense network is in place. It does not work well 

for non impulsive and (or) low-amplitude events, particularly 

Figure 1. Shaded relief map showing locations of all seismic stations (red triangles) operating in the 

Pacific Northwest as of November, 2003. Locations of the 13 major Cascade volcanic centers are 

indicated by yellow stars.

124o W 120o W

40o N

44o N

48o N

50 km

Mount Baker

Glacier Peak

Mount Rainier

Mount

St. Helens

Mount Adams

Mount Hood

Mount Jefferson

Three Sisters

Newberry Volcano

Crater Lake

Mount Shasta

Medicine Lake

Lassen Peak

Volcano PNSN/NCSN station

to analysis centers for the respective networks using analog 

radios.

4—Assessment of the ability of networks to detect anomalous seismicity

Detection of anomalous seismicity in all its possible 

forms (volcano-tectonic (VT) earthquakes, hybrid or “mixed-
3

tremor. Detection of these signals is more often dependent 

upon human analysts, even with relatively dense networks in 

place.

The easiest way to measure a network’s ability to detect 

seismicity is to look at earthquake detection, as this is (mostly) 

done by computer algorithms and is thus relatively straightforward to quantify. There are two ways to characterize the detection capabilities of a network. One is the detection threshold 

magnitude, which is the magnitude of the smallest earthquake 

that can be automatically detected (but not necessarily located). 

The other is the magnitude of completeness, which is the 

magnitude of the smallest event that will, in theory, always be 

detected and located. Both can be assessed by looking at earthquake catalogs, providing there are sufficient numbers of earthquakes. This is done in sections 4.1.1 to 4.1.2. However, most 

volcanoes in Oregon have insufficient seismicity, so section 

4.1.3 presents theoretical detection thresholds for each volcano. 

4.1— Earthquake Catalogs – what has been 

detected?

4.1.1—Detection threshold

The detection threshold can be estimated by looking at 

the smallest-magnitude earthquake to be located by a seismic 

network. However, this may underestimate the true detection 

threshold slightly, as earthquakes can be detected without 

being located (as would happen if a detected earthquake only 

appeared on two stations). It also is a poor estimate for regions 

with sparse seismicity 

Figure 15 shows the smallest magnitude earthquake 

located within 20 km of each of the major Cascade volcanic 

centers between 1990 and 2003. Note that this is a poor measure for most Oregon volcanic centers, specifically Mount Jefferson (1 located event), Three Sisters (6), Newberry Volcano 

(0), and Crater Lake (4). On the other hand, the M=0.0 value 

for Lassen Peak is a true detection threshold that I determined 

by first estimating coda-duration magnitudes for VT events 

in a February 2004 swarm, then examining NCSN trigger log 

files (courtesy of D. Oppenheimer, 2004) to determine which 

events did and did not trigger the NCSN acquisition system.

4.1.2—Magnitude of completeness

The magnitude of completeness (Mc) is determined 

through a b-value analysis of an earthquake catalog. This analysis uses the Gutenberg-Richter relationship between earthquake magnitude and the number of events with magnitude M:

 log N(M) = a - b * M (1)

N(M) is the number of events with magnitude M, b is the 

slope, and a the x-axis intercept. In general the relationship 

between M and N(M) is log-linear, and the “b-value” is the 

slope of this linear relationship. However, small earthquakes 

are less reliably detected and located than larger events, as 

their signals can be more easily masked by natural noise 

Number of seismic stations within X km of 

major Cascade volcanic centers

0

4

8

12

16

20

24

10 20 30 40 50 60

Distance (km)

Number of stations

Baker

Glacier Peak

Rainier

St. Helens

Adams

Hood

Three Sisters

Crater Lake

Shasta

Medicine Lake

Lassen

AVO Standard

Figure 2. Plot showing number of stations within X km of 11 Cascade volcanoes (Newberry and Jefferson, 

which are equivalent to Adams out to 50 km, were omitted for clarity). “AVO Standard” reflects typical 

network installed by the Alaska Volcano Observatory (AVO).

Assessment of the ability of networks to detect anomalous seismicity
4 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

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Figure 3. Mount Baker seismic network. For this and all other mapview plots ( figs. 4-14, 18-29, 31-42, and 44-45), yellow triangles 

represent short-period vertical component seismometers, cyan 

triangles represent short-period three component seismometers, 

green triangles represent strong-motion accelerometers, red 

triangles represent broadband seismometers, and white circles 

represent earthquakes with circle size scaled to magnitude.

Figure 4. Glacier Peak seismic network. See figure 3 for symbol 

explanation.

Figure 5. Mount Rainier seismic network. Note that one station (LON) 

has both a broadband and a strong-motion sensor. See figure 3 for 

symbol explanation.

Figure 6. Mount St. Helens seismic network. See figure 3 for 

symbol explanation.
5

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Figure 7. Mount Adams seismic network. See figure 3 for symbol 

explanation.

Figure 9. Mount Jefferson seismic network. See figure 3 for 

symbol explanation.

Figure 8. Mount Hood seismic network. Note that one station 

(HOOD) has both a broadband and a strong-motion sensor. See 

figure 3 for symbol explanation.

Figure 10. Three Sisters seismic network. See figure 3 for symbol 

explanation.

Assessment of the ability of networks to detect anomalous seismicity
6 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

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Figure 11. Newberry Volcano seismic network. Note that one 

station (PIN) has both a broadband and a strong-motion sensor. 

See figure 3 for symbol explanation.

Figure 12. Crater Lake seismic network. See figure 3 for symbol 

explanation.

Figure 13. Mount Shasta and Medicine Lake seismic networks. 

Signals are telemetered via satellite (Nanometrics VSAT) to U.S. 

Geological Survey offices in Menlo Park, Calif. See figure 3 for symbol 

explanation.

Figure 14. Lassen Peak seismic network. See figure 3 for symbol 

explanation.
7

sources, such as wind. Additionally, their small amplitudes 

make their detection and location more vulnerable to station outages, as they show up on far fewer stations than large 

events. In all catalogs there is a critical magnitude Mc below 

which the Gutenberg-Richter relationship breaks down. This 

is widely taken to represent the magnitude below which earthquakes are sometimes missed; alternatively, it is the magnitude 

above which earthquakes are always detected.

Figure 16 shows Mc for each volcano network. Note that 

there were insufficient numbers of earthquakes for b-value 

analyses at most volcanic centers except Mount Rainier, 

Mount St. Helens, Mount Hood, Mount Shasta, and Mount 

Lassen. This demonstrates that a theoretical approach is 

needed to assess detection and location capabilities at most 

Cascade volcanic centers.

4.1.3—Theoretical detection threshold

When there are too few earthquakes to empirically 

assess detection capabilities of seismic networks, the next 

best approach is to generate theoretical estimates of detection 

thresholds. Because earthquake detection is usually done automatically, computer programs can be written to mimic these 

automated detection systems. 

Most automated event-detection systems (such as the 

Earthworm system used by the PNSN and NCSN) employ a 

subnet-trigger algorithm. All stations are continuously monitored for significant short-term increases in seismic amplitude. 

If enough stations within a defined subnet experience significant increases at roughly the same time, then the subnet “triggers” and waveform data is saved for subsequent analysis by a 

human (if the event is big enough, an automated location may 

also be produced). Thus event detection is dependent upon the 

number of seismic stations in each subnet, their proximity to a 

volcano, their sensitivity (including site and path effects), and 

the number of stations required for a subnet to trigger.

For simplicity, all stations and sites are assumed to have 

identical sensitivities. The theoretical detection threshold is 

then computed by (1) calculating the distance from each latitude/longitude grid point to the Xth closest station, where X 

is the number of stations required for a given subnet to trigger (so if a subnet has a 4-station trigger requirement, then 

compute the distance to the 4th closest station); (2) Using a 

modified version of the Richter magnitude scale to convert 

distance and minimum amplitude required to trigger a given 

station into a magnitude (ML). The relationship (modified to 

best reflect minimum magnitude earthquake distributions at 

Mounts St. Helens and Rainier) is:

 ML = log(Amplitude) + 1.7*log(distance_to_Xth_station) – 2.38 (2)

Amplitude here isn’t truly a peak-peak amplitude; instead 

it is the minimum amplitude above background required 

to trigger a single station. In most cases this number was a 

priori chosen to be 2. At Three Sisters and Lassen Peak the 

Amplitude factor was empirically determined to be 3 and 3.5, 

respectively, using recent swarms to gauge the actual detection 

threshold (as was described above for Lassen Peak).

The resulting magnitude is an estimate of the smallest 

event at each latitude/longitude grid point that would trigger 

a given volcano subnet. Note that this estimate neglects variations in equation (1) with increasing depth, assumes that all 

Smallest earthquake located within 20 km of volcano

Lassen

Glacier

Peak

Rainier

Adams

Hood

Newberry

Three

Sisters

Crater

Lake

Shasta

Jefferson

St.

Helens

Medicine

Lake

Baker

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

Magnitude

No data (no earthquakes)

Figure 15. Smallest magnitude earthquake within 20 km of volcano between 1990-2003. This is an estimate of 

the magnitude below which earthquakes will never be detected.

Assessment of the ability of networks to detect anomalous seismicity
8 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

sites have identical sensors and site responses, assumes that 

all sites are functioning, and assumes that background noise is 

constant. In other words, these are only estimates of detection 

thresholds based on ideal conditions and a simplified view of 

seismometers and the Earth. Thus the detection threshold estimates have an unknown error, which I estimate to be +/- 0.2-

0.3 magnitude units. 

Figure 17 shows detection thresholds at the latitude and 

longitude of the centroid of each volcanic center, and figures 

18-29 show detection threshold contours for each network.

4.1.4—Theoretical “good” location threshold

In the absence of seismicity, it is even more difficult to 

estimate the magnitude of completeness (see section 4.1.2 and 

fig. 16). One way of addressing this is to use the same methodology described in section 4.1.3 to determine the minimum 

magnitude required for an event to appear on enough stations 

to allow for the determination of a “good”, or well-constrained, location. Two network-geometry-dependent criteria 

are important for a “good” location; maximum azimuthal gap 

(or “maximum gap”) between stations relative to the epicenter, 

and distance to the nearest station. For reasonable accuracy 

in latitude and longitude determinations, the maximum gap 

should be << 180o. Any gap larger than 180o can cause artificial epicentral patterns, such as dike-like linear patterns of 

epicenters trending away from a network. For accurate depth 

determinations, at least one station needs to be within one 

“skin depth” of an earthquake’s epicenter (i.e., the distance to 

the nearest station needs to be ≤ earthquake depth). 

Figures 30 through 42 show the estimated minimum 

magnitudes required for an event to have a well-constrained 

location, defined here as having a maximum azimuthal gap 

between stations of <= 135 o (one of the PNSN criteria for a 

quality “B” solution). As with the estimated detection thresholds in figures 17 through 29, it should be remembered that 

the magnitude contours are only estimates that probably have 

an error of a few tenths of a magnitude unit. It should also be 

noted that these contours were not computed using a closest 

station criterion (10 km or less for a PNSN quality “B” location), as this is solely dependent upon location and thus holds 

for all magnitudes. Volcanoes without a station within 10 km 

of the summit (red star in the map-view figures) are shown in 

pink in figure 30. 

4.2— Important seismic-monitoring criteria 

other than earthquake detection and location

Detection and location of VT and impulsive LP events 

can be achieved by standard vertical-component short-period 

instruments. If there are sufficient numbers of these instruments, first-motion focal mechanisms can also be determined. 

However, other types of sensors are needed to adequately 

record and detect nonstandard seismic sources. Three-component instruments are necessary for (1) accurately picking 

S-wave arrivals, which are particularly important for depth 

Magnitude of completeness (Mc) for eqs w/in 20 km

Baker

Lassen

Medicine

Lake

St.

Helens

Jefferson

Shasta

Crater

Lake

Three

Sisters Newberry

Hood

Adams

Rainier

Glacier

Peak

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

Mc

Not enough data

Figure 16. Magnitude of completeness (Mc) for earthquakes within 20 km of volcano between 1990-2003. 

Earthquakes with magnitudes ≥ Mc will, in theory, always be detected. Centers with too few earthquakes to 

determine b-values are colored pink.
9

Theoretical earthquake detection threshold

Baker Adams

Hood

Crater

Lake

Glacier

Peak

St.

Helens

Medicine

Lake

Lassen

Jefferson

Newberry

Three

Sisters

Shasta

Rainier

-1.5

-1

-0.5

0

0.5

1

1.5

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Figure 17. Estimates of detection thresholds at the summit (red star) of each volcano. This estimate 

represents the magnitude below which an event will never be detected.

Figure 18. Mount Baker and contours of estimated minimum 

magnitude for earthquake detection. 

Figure 19. Glacier Peak and contours of estimated minimum 

magnitude for earthquake detection.

Assessment of the ability of networks to detect anomalous seismicity
10 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

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Figure 20. Mount Rainier and contours of estimated minimum 

magnitude for earthquake detection.

Figure 22. Mount Adams and contours of estimated minimum 

magnitude for earthquake detection.

Figure 21. Mount St. Helens and contours of estimated minimum 

magnitude for earthquake detection.

Figure 23. Mount Hood and contours of estimated minimum 

magnitude for earthquake detection.
11

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Figure 24. Mount Jefferson and contours of estimated minimum 

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Figure 26. Newberry Volcano and contours of estimated 

minimum magnitude for earthquake detection.

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Figure 27. Crater Lake and contours of estimated minimum magnitude 

for earthquake detection. 

Assessment of the ability of networks to detect anomalous seismicity
12 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

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Figure 28. Mount Shasta and Medicine Lake contours of estimated 

minimum magnitude for earthquake detection.

Figure 29. Lassen Peak and contours of estimated minimum 

magnitude for earthquake detection. 

Theoretical "good" location threshold

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Figure 30. Minimum magnitudes required for events located at each summit to record on enough stations 

to have well-constrained locations (maximum gap < 135o). Mount Adams and Crater Lake are colored pink 

because they have no station within 10 km, so events beneath each summit will have poorly constrained 

depths.
13

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Figure 31. Contours of magnitude thresholds for “good” locations 

at Mount Baker. “Wobbles” in contours shown here and in figures 

32 through 42 are artifacts due to tight contour spacing and an 

inexact contouring program.

Figure 32. Contours of magnitude thresholds for “good” locations 

at Glacier Peak.

Figure 34. Contours of magnitude thresholds for “good” locations 

at Mount St. Helens.

Figure 33. Contours of magnitude thresholds for “good” locations at 

Mount Rainier.

Assessment of the ability of networks to detect anomalous seismicity
14 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

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Figure 36. Contours of magnitude thresholds for “good” 

locations at Mount Hood.

Figure 35. Contours of magnitude thresholds for “good” locations at 

Mount Adams.

Figure 37. Contours of magnitude thresholds for “good” 

locations at Mount Jefferson.

Figure 38. Contours of magnitude thresholds for “good” locations at 

Three Sisters.
15

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Figure 41. Contours of magnitude thresholds for “good” locations at 

Mount Shasta and Medicine Lake.

Figure 39. Contours of magnitude thresholds for “good” 

locations at Newberry Volcano.

Figure 40. Contours of magnitude thresholds for “good” locations at 

Crater Lake.

Figure 42. Contours of magnitude thresholds for “good” locations 

at Lassen Peak.

Assessment of the ability of networks to detect anomalous seismicity
16 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

determinations; (2) determining particle motions, which can be 

used to estimate locations of tremor sources (for example, see 

Arciniega-Ceballos and others, 2003) as well as phase identification for non standard events (for example, see White, 1996); 

and (3) potentially determining moment-tensor solutions, 

which are important for assessing whether sources have a 

volumetric (explosive or implosive) component (for example, 

see Julian and others, 1998). Broadband instruments are necessary for detecting VLP events (Hill and others, 2002; Chouet 

and others, 2003) and tremor with dominant frequencies < 1 

Hz, as well as greatly facilitating the computation of moment 

tensors (for example, see Chouet and others, 2003). Digital 

stations (on-site digitizers and digital telemetry) are necessary 

for providing wide dynamic range, enabling stations to be sensitive enough to record small events and to also stay on scale 

during energetic events such as large earthquakes or eruptions. 

Finally, for tremor identification, which is often accomplished 

through visual cross-correlation, having several stations in the 

near-field is vital for distinguishing between low-amplitude 

tremor (see fig. 43) and wind or other noise sources.

One additional consideration is that near-field microphones (or infrasonic recorders) have proven useful in 

determining whether or not explosions are occurring and 

constraining the depth of such sources. Microphones are not 

vital for monitoring networks on dormant volcanoes, but they 

should be considered in any monitoring effort on restless or 

erupting volcanoes.

5—Recommendations

5.1—Seismic networks should be upgraded 

before any unrest occurs

The March 23-25 swarm in 2004 at Three Sisters illustrated the importance of having an adequate seismic network 

installed before any unrest occurs. In March 2004 the Three 

Sisters seismic network consisted of two stations within 20 

km of the Three Sisters edifices, one of which was offline 

and the other intermittent due to telemetry interference from 

an above-average snow pack, and three others within 50 km. 

This network was sufficient for detecting the swarm and for 

locating earthquakes as small as ML = –0.1. However, of the 

more than 300 events that occurred, only 73 were recorded 

on enough stations to allow for a location, and only 11 of the 

locations had gaps of < 135o and thus, by one measure, had 

“well-constrained” epicenters. Furthermore, because of the 

loss of station HUO (see fig. 10) the nearest operating station 

was MOON at ~14 km, and, as a result, depths were poorly 

constrained. Even if station HUO had been in operation, the 

east-west planar alignment of the five closest stations would 

still have resulted in few “good” locations, as most of the 

swarm earthquakes occurred north of this planar alignment. 

In other words, the network in place was sufficient for detection, but not location. This meant that, among other things, 

any lateral or vertical migration of hypocenters with time was 

undetectable. 

Had the swarm continued, installation of new telemetered 

stations near the source would have been difficult and timeconsuming due to winter conditions and the time required to 

establish new communication channels to transmit data from 

new stations in real time. With snow depths of 5 to 10 feet in 

many places, the only quick way to access sites close to the 

swarm (which occurred entirely within the Three Sisters Wilderness) is by helicopter, requiring at least 24 hours to make 

necessary arrangements (if an available nearby helicopter 

can be found) and good weather. Telemetering signals also 

requires site visits to repeaters and receiving nodes, as well as 

having available slots on phone or microwave circuits. Thus 

had the swarm continued, it would have taken at least several 

days to establish new telemetered sites near the swarm and 

several days before well-constrained locations could have 

been determined. This further demonstrates that, particularly 

for sites close to a volcanic system, it is critical to have sufficient numbers of telemetered stations installed well before 

any seismic unrest. Once unrest occurs, it may be difficult or 

impossible to install sufficient numbers of seismometers to 

effectively monitor a restless volcano.

In the next section, I review the status of seismic networks 

at each of the 13 Cascade volcanic centers. The primary goal 

is to identify weaknesses in these networks that require installation of additional stations (before unrest occurs) to ensure a 

baseline capability exists at each center for detecting seismic 

unrest.

5.2—Priority A volcano networks

Of all the Cascade volcanoes, Mount Baker, Newberry 

Volcano, Medicine Lake, and Crater Lake have the poorest 

network coverage. The first three are problematic because, 

unlike all other Cascade volcanoes, they lie at or very near a 

margin of the PNSN/NCSN combined network and thus have 

at least one sector in which earthquakes of any magnitude 

cannot be located reliably without use of distant (>150 km) 

non-PNSN/NCSN stations. They also have few near-field 

stations (< 10 km from the presumptive vent), making our 

ability to detect unrest extremely vulnerable to station outages. Although Crater Lake is mostly surrounded by PNSN 

stations, none are closer than 45 km, giving Crater Lake the 

worst detection threshold of all Cascade volcanoes. Three Sisters is also included as a Priority A volcano because, although 

it has a network with two near-field stations already in place, 

the years-long deformation, the poorly located March 23-25, 

2004, seismic swarm, and the large geographical area in which 

an eruption could occur strongly argue for placing a high priority on improving detection and location capabilities. 

5.2.1—Mount Baker

Mount Baker lies at the northern PNSN margin. It has one 

near-field station and three stations within 40 km, including 
17

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Figure 43. Multistation spectrogram showing a minutes-long tremor episode at Great Sitkin Volcano, 

Alaska.

Recommendations
18 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

one in Canada (VDB). In Canada there are two stations north 

and east of Mount Baker; VDB (34 km to the northwest (fig. 

44)) and PNT (172 km to the east-northeast). Data from these 

stations are shared in real-time with the PNSN, effectively 

expanding the PNSN aperture into Canada. 

Since Mount Baker has a sparse network and is at the 

northern margin of the PNSN, it has significantly limited 

detection and location capabilities. These limitations include:

• For much of the northeast sector of the Baker edifice the 

 gap between the two Canadian stations is > 135o, meaning 

 that well-constrained locations from PNSN stations alone 

 are impossible to achieve for much of the northeast sector 

 (fig. 44) unless events are large enough to be located by 

 the National Earthquake Information Center (M ~3.5- 

 4.0): 

• With only three stations within 40 km, the “good” loca- 

 tion threshold is unacceptably high on all sectors of the 

 edifice (M ~1.7; see fig. 44). 

• Earthquake detection at Baker is highly dependent upon 

 the single near-field station (MBW), which counts twice 

 in the PNSN automated triggering algorithm. If MBW 

 is offline (as was the case for most of the 2003-2004 

 winter), detection thresholds are significantly degraded, 

 as the next nearest station is 28 km from the summit.

• With just one near-field station, tremor detection capabili- 

 ties are also severely compromised.

To address these shortcomings, at least three stations 

should be added to the Baker network (fig. 44); one located 

north of Baker as close as possible to the summit (to provide 

another near-field station), another 15 km northeast of Baker 

(to make it possible to reliably locate M ≥ 1.0 earthquakes 

within 10 km of the edifice), and a third within 10-15 km on 

the southern or southeastern flank (to lower the “good” location threshold to M ~0.5 at the summit itself). The resultant 

network would still be somewhat vulnerable to single-station 

outages, and its sparsity would likely not allow for reliable 

tracking of hypocenter migrations. Future improvements 

should include making arrangements for installation of at least 

one more station to the north-northeast of Baker in Canada.

5.2.2—Newberry Volcano

Newberry Volcano lies at the eastern margin of the PNSN. 

It has one near-field station and one broadband located 30 km 

to the east-northeast. The next nearest eastern station lies in 

Idaho, the next nearest southeastern station lies 255 km away 

in southeastern Oregon, and the nearest station to the south is 

164 km away in Klamath Falls, Oregon. There is thus a risk 

that large events occurring off the southeastern flank of Newberry could not be reliably located. With only one near-field 

station, event detection capabilities are very vulnerable to 

single-station outages.

To address these shortcomings, two stations should 

be added, one to the east-northeast and one to the southsoutheast, both 10 to 20 km from the caldera. A third station 

installed within the caldera would provide redundancy, greatly 

improve detection thresholds and improve our ability to detect 

tremor, and provide better depth constraints for shallow intracaldera seismic events.

5.2.3—Medicine Lake

Medicine Lake lies at the eastern margin of the NCSN. 

There are two near-field stations, with the next nearest eastern 

station lying ~260 km to the east-northeast in southeastern 

Oregon. There are also relatively few stations to the north (the 

nearest lies ~64 km away in Klamath Falls, Oregon). As a 

result, there is a ~180o swath covering most of the eastern half 

of the edifice where earthquakes cannot be located reliably 

below M 1.4 (fig. 41). With only two near-field stations, earthquake detection is also highly vulnerable to station outages. 

Finally, the NCSN uses a five-station trigger for an event to be 

automatically detected, resulting in a high detection threshold 

even with both near-field stations functioning. All of these factors give Medicine Lake the second-worst theoretical detection 

threshold in the Cascades (M 1.0; see fig. 4). 

To address these shortcomings, at least two stations 

should be added 5 to 15 km north and east of Medicine Lake. 

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Figure 44. Contours of magnitude thresholds for “good” locations 

at Mount Baker network with three additional stations (pink 

triangles). Extreme wobbliness in contours north of Baker is 

an artifact of tight contour spacing and an inexact contouring 

program.
19

In addition, detection thresholds would be greatly improved 

by lowering the NCSN trigger threshold from five stations 

to (ideally) three (for comparison, the Three Sisters network 

has a two-station trigger threshold). As with Baker, two new 

stations will greatly improve the ability to detect and roughly 

locate earthquakes in all sectors of the volcano, although location quality will still be marginal for small events.

5.2.4—Crater Lake

With no near-field stations and the nearest station 46 km 

distant, Crater Lake has the worst detection threshold (M 1.1; 

see fig. 4) of all Cascade volcanoes. Earthquake detection and 

location are entirely dependent upon PNSN regional stations. 

The lack of any near-field stations also means that tremor 

would be extremely difficult to detect unless it was very energetic.

Installing just one station will not improve this situation, as automatic detection requires at least two near-field 

stations (the PNSN currently employs a 2-station trigger at 

Three Sisters). In addition, at least two stations are required to 

distinguish between low-level seismic tremor and other noise 

sources (see fig. 43). However, ideally three stations should be 

installed, as a third near-field station would provide a safety 

margin in case of station outages and would yield a much 

better network geometry. A 3-station network would still likely 

not be able to provide “good” locations for events too small 

to show up on regional stations (M ≤ ~1) or track hypocenter 

migrations very reliably, but it would be infinitely better than 

no network at all.

5.2.5—Three Sisters

As discussed above, the March 23-25, 2004, seismic 

swarm demonstrated that location capabilities of the current 

Three Sisters network are not good, even with the nearest station (HUO) in operation. Since the swarm one new station has 

been added to the north near Black Crater, and three more stations will be installed in the summer of 2004 at The Wife, near 

Two Buttes to the northwest, and near Trout Creek Butte to the 

east (magenta diamonds in fig. 45). These four new sites will 

greatly improve the situation for the area around the “bulge” 

and March 23-25 swarm (see fig. 45 for revised location 

thresholds with these four stations in place). After these new 

sites are in place the Sisters network will include two threecomponent short-period and two broadband sites, making it 

the most modern network operated at a Cascade volcano. 

However, this improved network will still have serious 

deficiencies. One can be seen in fig. 45; the “good” location 

threshold along the Three Sisters themselves will be M ~0.2-

0.4, but will degrade markedly to the north of North Sister 

and spectacularly south of South Sister. The location threshold 

ideally should be M ≤ 0 over the entire area where future eruptions could occur, a 20 by 30 kilometer swath that surrounds 

the Three Sisters (W. E. Scott, oral commun., 2004). In addition, at least one near-field station is needed near North Sister 

to ensure well-constrained depths should future seismicity 

occur north of the March 23-25 swarm area. These two deficiencies require installation of at least four more stations; two 

sites in the near-field near North Sister within the wilderness, 

and two others south of South Sister (fig. 46).

5.3—Priority B volcano networks

All other Cascade volcanoes lie within the broader PNSN/

NCSN network, providing a baseline ability to determine 

reasonably well-constrained locations for ~M > 1.5 events 

within ~10 km of the summit (fig. 30). However, Glacier Peak, 

Mount Adams, and Mount Jefferson networks have only one 

near-field station, and thus have only a relatively poor ability 

to locate small earthquakes and detect other types of seismic 

sources, in particular seismic tremor. Their detection and location capabilities are also vulnerable to outages of their sole 

near-field stations. 

5.3.1—Glacier Peak

Despite installation of one very-near-field station (GPW) 

in 2001, detection capabilities at Glacier Peak are still poor. 

Glacier Peak is a noisy volcano, with GPW records dominated 

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Figure 45. Contours of magnitude thresholds for “good” locations at 

Three Sisters with four additional stations (magenta diamonds) to be 

installed in 2004.

Recommendations
20 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

by what are presumably glacier quakes that occur several 

times an hour. With only one near-field station and a nextnearest station 33 km to the southeast, it is impossible to determine whether any of these events are volcanic earthquakes of 

some sort. It would also be next to impossible to distinguish 

low-level tremor from wind noise. Another consideration is 

location; GPW is located high on a flank of Glacier Peak (necessary for data from it to be radioed directly to the University 

of Washington) and so is prone to weather-related outages in 

the winter (for example, it was damaged in the early spring 

of 2004 and remains off-line as of August 2004). This makes 

detection and location capabilities more vulnerable to a single 

point-of-failure than at other single-station volcanoes. 

To address these shortcomings, at least one additional station should be installed within 20 km of Glacier Peak. Given 

the high glacier-related noise level seen on GPW, this site 

should be placed several kilometers away from the summit 

glaciers. Telemetry and wilderness access are problematic 

around Glacier Peak, perhaps more so than any other Cascade 

volcano. Installing another near-field station would likely also 

require installation of a repeater site. Given these limitations, 

it is perhaps most prudent to pursue a two-pronged approach: 

(1) Focus first on installing an intermediate-field (15-25 km) 

station outside of the wilderness that can be telemetered with 

relative ease. One possible site is ~20 west-northwest of 

Glacier Peak on the northwest flank of Pugh Mountain. This 

could be accomplished in a several-months time frame, assuming permits and equipment are readily attainable; (2) Begin 

the process of identifying a new near-field site and all of the 

telemetry path requirements, and, once a site is identified, 

begin the permitting process. This would likely take one to 

several years, based on past experience with the installation of 

GPW (C. A. Gardner, oral commun., 2004).

5.3.2—Mount Adams

Mount Adams has one station (ASR) that lies ~11 km 

to the west, with the next-nearest station 33 km to the southsouthwest near the Washington/Oregon border. The St. Helens 

network to the west and the Mount Hood network to the south 

bolster detection and location capabilities. The lack of a nearfield station means that depth determinations are compromised, as are tremor detection capabilities. These issues can be 

addressed by addition of one near-field station, ideally on the 

eastern side of the volcano.

5.3.3—Mount Jefferson

Mount Jefferson also has only one near-field station 

(BPO), with the next-nearest station located 44 km to the 

north. Detection capabilities are marginally bolstered by the 

Mount Hood network to the north and the presently expanding Three Sisters network to the south. Nevertheless, Mount 

Jefferson has the third-worst theoretical detection threshold 

(M 0.9) of all Cascade volcanoes, illustrating that detection of 

small earthquakes and tremor is compromised by presence of 

only one near-field station. To address these issues, at least one 

additional near-field station should be installed, ideally on the 

west side.

5.4—Priority C volcano networks

Networks at the other Cascade volcanoes (Mount Rainier, 

Mount St. Helens, Mount Hood, Mount Shasta, and Lassen 

Peak) have enough stations near the summits to enable adequate detection, although not necessarily adequate location, of 

seismic unrest, as well as to be less vulnerable to single-station 

outages. However, none are ideal, even Mount St. Helens. For 

example, only three volcanoes have a digital station nearby 

(Rainier, Hood, and Newberry), meaning that most networks 

will “clip” during any energetic event, resulting in loss of 

information vital both for monitoring and research into the 

nature of eruptive processes. In addition, only five volcanoes 

have a three-component station nearby (Rainier, Hood, Three 

Sisters, Newberry, and Lassen). This means that, amongst 

other things, tremor sources cannot be located (which requires 

many (8 to 10) three-component stations). In addition to these 

limitations, other improvements described below should be 

made at some point to each network.

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Figure 46. Contours of magnitude thresholds for “good” locations at 

Three Sisters with eight additional stations (magenta diamonds), four 

of which are scheduled to be installed in 2004.
21

5.4.1—Mount Rainier

Mount Rainier has three near-field stations and six within 

20 km of the summit. In addition, it is one of two volcanoes to 

have a broadband close to the summit (LON). However, two 

of the near-field stations (RCM and RCS) are at high altitudes 

and are particularly prone to winter-weather-related outages. 

Another problem is that the five nearest stations are aligned in 

a NE-SW direction (fig. 5), which can increase location errors 

for events locating to the northwest or southeast. Improvements to be made at Rainier include improving the robustness 

of RCS and RCM, perhaps by putting them in more robust 

enclosures, and installing an additional site near or on the 

northwest flank of Rainier.

5.4.2—Mount Hood

Mount Hood has two stations in the near-field, including 

one broadband (the only Cascade volcano with a near-field 

broadband). However, the “good” location threshold is only 

~0.4 over the entire western flank of Hood and drops off markedly east of the Hood River, a result of there being only four 

stations within 30 km of the summit. To address these shortcomings two additional seismometers should be added, one 

on the northwest flank of Hood (ideally within 10 km of the 

summit), the other east-northeast of the summit (ideally 10-15 

km of the summit).

5.4.3—Mount Shasta

Mount Shasta has two near-field stations and six within 30 

km of the summit. Furthermore, detection and location capabilities benefit greatly from Shasta being located within a relatively dense network of regional stations. However, none are 

closer than 8 km to the summit, meaning that depth constraints 

may not be sufficient to enable detection of hypocentral migration in the uppermost several kilometers (including most of the 

edifice, which lies above the two closest seismometers). Also, 

the sheer size of the Shasta edifice means that more stations 

are needed to lower the good location threshold from the current M ~0.4 down to M 0.0. To address these shortcomings, at 

least one station should be added near the summit. The detection threshold (M ~0.4) could also be lowered if the NCSN 

reduced its trigger requirement for automatic event detection 

from five to three or four stations. 

5.4.4—Lassen Peak

Lassen Peak volcano has three near-field stations and six 

within 30 km of the summit. Because of this network geometry, the detection threshold is quite good at Lassen. However, 

the “good” location threshold is M ~0.9 at the summit (fig. 

42), a result of the east-west alignment of the six nearest stations and the lack of nearby stations east-northeast of Lassen. 

This could be addressed by adding one station ~10 km northeast of Lassen. Placing a second new station ~10 km to the 

southwest would further improve network geometry. Finally, 

reducing the NCSN’s current five-station trigger requirement 

for automated detection to three or four stations is a cheap and 

easy way to lower detection thresholds even further without 

installing new stations.

5.5—Priority D volcano networks

5.5.1—Mount St. Helens

Mount St. Helens has eight near-field stations and thirteen 

within 20 km of the lava dome. This is the densest network 

operating in the Cascades with the best detection and location 

thresholds, and can withstand simultaneous outages of several 

stations with minimal impact on detection and location capabilities. It would be an even better network if it had another 

station 10-20 km to the northeast, as location capabilities in 

this sector are currently vulnerable to the loss of station SOS, 

the only northeast-sector station outside of the crater. Another 

issue is that the network is in need of modernization. All stations are short-period vertical-component sites; at least a few 

should be considered for upgrades to three-component and (or) 

broadband sites. 

6—Conclusions 

All Cascade volcano networks have shortcomings compared to modern-day networks operated at, for example, 

Mount Etna in Sicily, where activity is monitored by ~35 

short-period stations, eight broad-band stations (telemetered 

using satellite uplinks), and three short-period three-component stations (fig. 47). In the face of limited resources, 

however, installation of modern-day networks at one or more 

of the thirteen Cascade volcanic centers is at present highly 

unlikely. 

Instead, this report has focused on relatively inexpensive 

and (mostly) easy-to-achieve improvements that are needed 

to enable or improve baseline detection and interpretation of 

seismic unrest. Volcano networks most in need of additional 

seismometers include Mount Baker, Three Sisters, Newberry 

Volcano, Crater Lake, and Medicine Lake:

• Crater Lake is essentially unmonitored in the sense of 

 being able to detect small (M < 1) magnitude earthquakes 

 and low-amplitude tremor. At least two stations should be 

 installed on or near the rim.

• Mount Baker, Newberry Volcano, and Medicine Lake are 

 all at the edge of the PNSN/NCSN network, and as a 

 result even moderate-sized (M 2.0 or larger) earthquakes 

 can fall outside of the network and be poorly located. 

Conclusions
22 Seismic Monitoring at Cascade Volcanic Centers, 2004—Status and Recommendations

 Each of these networks requires at least one, preferably 

 two, optimally placed stations to solve this.

• The March 23-25, 2004, swarm at Three Sisters demon- 

 strated that the Sisters network is capable of detecting 

 small earthquakes but is not able to locate them very well. 

 The current unrest argues for significantly improving 

 detection and location capabilities now to ensure that 

 future seismicity will be detected and located well enough 

 to allow for relatively unambiguous interpretations. 

In addition to these five networks, three other networks 

(Glacier Peak, Mount Adams, and Mount Jefferson) have 

only a single near-field station. Installation of at least one, 

and preferably two, new stations near each of these volcanoes 

would yield significant improvements in detection thresholds. 

It would also greatly reduce the dependence of our monitoring capabilities at these volcanoes on a single station and our 

consequent vulnerability to weather-related or other types of 

outages. 

Mount Rainier, Mount Hood, Mount Shasta, and Lassen 

Peak volcanoes all have reasonable networks in place that 

should be able to detect most types of volcano-related seismicity. However, all have weaknesses that are readily addressable 

via changes in trigger thresholds (Shasta, Lassen Peak) and 

(or) by additions of one or two stations. Finally, Mount St. 

Helens is the only Cascade volcano with a seismic network 

that, other than a wholesale upgrade to modern equipment, 

requires no new seismic stations. 

Acknowledgments

Bobbie Myers modified and significantly improved the 

PNSN/NCSN network figure shown on the front page. Elliot 

Endo, David Hill, Willie Scott, and Jeff Wynn gave detailed 

comments that significantly improved the clarity and presentation of this report, and final edits by Jim Hendley improved 

readability and ensured consistency throughout. David Oppenheimer provided information on NCSN triggering thresholds, and Steve Malone gave the same for the PNSN. Both 

also gave valuable feedback on the theoretical detection and 

“good” location thresholds presented in this report. All mapview figures were generated using Generic Mapping Tools 

(GMT) software (Wessel and Smith, 1991).

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Figure 47. Seismic monitoring network around Mount 

Etna, Italy (image from the Istituto Nazionale di Geofisica e 

Vulcanologia (Catania) web site (URL: http://www.ct.ingv.

it/Sismologia/images/Rete_Satellitare.jpg)).