Volcanic Seismicity

Chapter 59

Volcanic Seismicity Stephen R. McNutt School of Geosciences, University of South Florida, Tampa, FL, USA

Diana C. Roman Department of Terrestrial Magnetism, Carnegie Institution of Washington, DC, USA

Chapter Outline 1. Introduction 1.1. Some Famous Early Eruptions 1.2. Brief Review of Earthquake Seismology 2. Types of Volcanic Earthquakes and Terminology 2.1. HF Earthquakes 2.2. LF Events 2.3. Hybrid Events 2.4. Volcanic Tremor 2.5. Very-Long-Period Events 2.6. Explosion Earthquakes 2.7. Surficial Events 3. General Features of Earthquakes at Volcanoes 3.1. Seismicity Rates and Background 3.2. Seismicity Associated with Large versus Small Eruptions

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3.3. Locations of Volcanic Earthquakes 3.4. Large Regional Earthquakes Near Volcanoes 3.5. Caldera Earthquakes 3.6. Seismicity at Volcanoes with Long Repose Times 3.7. Synchronous Volcanic and Tectonic Activity 3.8. Seismic Features of Magma Chambers 3.9. Source versus Path and Site Effects 4. Case Histories 4.1. Mount St Helens, 2004e2008 4.2. Soufrie`re Hills, 1995eContinuing 4.3. Okmok, 2008 4.4. Augustine, 2006 5. Conclusions See also the Following Articles Further Reading

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GLOSSARY explosion earthquake Events that are recorded as seismic waves traveling through the ground (including P waves and S waves) often followed by air waves. The air waves travel through the air and are coupled back into the ground near the recording site. high-frequency (HF) earthquake Discrete events with clear P waves and S waves. The dominant energy is above 5 Hz (synonyms are A-type earthquake or volcano-tectonic (VT) earthquake). low-frequency (LF) earthquake Discrete events with weak P and no discernable or distinct S waves. Recordings show monotonic, low frequency (1e5 Hz) waveforms that resonate for many cycles (synonyms are B-type event and long-period (LP) event). magnitude A measure of earthquake size, usually determined by measuring the highest-amplitude waves and correcting for distance and instrument type. The scale is logarithmic, so each increase of one unit corresponds to an amplitude increase of a factor of 10.

The Encyclopedia of Volcanoes. http://dx.doi.org/10.1016/B978-0-12-385938-9.00059-6 Copyright Ó 2015 Elsevier Inc. All rights reserved.

P wave Primary wave, a compressional elastic wave with primarily longitudinal particle motion parallel to the propagation direction. P waves travel faster than other waves and are thus the first to appear on seismograms. S wave Secondary wave, a shear elastic wave with particle motion perpendicular to the direction of propagation. S waves are usually the most prominent phases on a seismogram; they travel more slowly than P waves and thus arrive later on seismograms; they cannot pass through liquids. swarm A group of many earthquakes of similar magnitude occurring closely clustered in space and time with no dominant mainshock. volcanic tremor Continuous seismic signal with regular or irregular sine wave appearance and characterized by low frequencies (0.5e5 Hz). Harmonic tremor has very uniform appearance, whereas spasmodic tremor is pulsating and consists of higher frequencies with a more irregular appearance.

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Volcano seismology is the study of earthquakes of volcanic origin as well as of velocity structure, attenuation, and other physical properties of the Earth materials that affect the passage of seismic waves at volcanoes. Volcanic earthquakes may be defined as earthquakes that occur at or near volcanoes, generally within 15 km, or that are related to volcanic processes. Volcanoes are places where heat and mobile fluids are concentrated, so the number of earthquakes per unit time is high when compared with normal crust. Most volcanic earthquakes take place at shallower depths (1e9 km) than tectonic earthquakes on typical faults (generally to depths of about 15 km in the crust; as deep as 700 km in subduction zones). Volcanic events also differ in their patterns of occurrence: they often occur in swarms, which are groups of many small events with similar magnitudes and locations. This is in contrast to the typical mainshockeaftershock sequences characteristic of tectonic earthquakes. Volcanoes produce different types of earthquakes that are thought to represent different physical processes. Among these earthquakes are high-frequency (HF) (also known as volcano-tectonic (VT) earthquakes), lowfrequency (LF) (also known as long-period (LP) events), explosions, and volcanic tremor. These typically occur in increasing numbers prior to or during eruptions. Seismicity also increases during intrusionsdepisodes of magma ascent that do not culminate in an eruptiondand even though the eruptive outcomes differ in these cases, the processes that cause various types of volcanic earthquakes are similar. The source mechanisms of some volcanic earthquakes are not yet well understood.

1. INTRODUCTION Volcanoes are the sources of a great variety of seismic signals that differ from those produced by tectonic earthquakes. Nearly every recorded volcanic eruption has been preceded by an increase in earthquake activity beneath or near the volcano and accompanied and followed by varying levels of seismicity. For this reason, seismology has become one of the most useful tools for eruption forecasting and monitoring. At present, approximately 200 of the world’s volcanoes are seismically monitored, although the number and quality of stations at each volcano varies considerably. This represents about one-third of the 541 volcanoes that have erupted in historic times. Over the last several decades, 55e70 individual volcanoes have erupted each year. Because erupting volcanoes draw attention both publicly and scientifically, over half of these are seismically monitored (see Chapter 63). This chapter reviews some of the developments in volcano seismology over the last 10 years that have led to an improved understanding of volcanoes and the volcanic processes that cause earthquakes and other types of seismicity. The chapter also describes several patterns and

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relationships in volcano seismology that form the physical basis of contemporary monitoring and forecasting, although these topics are covered in greater detail elsewhere.

1.1. Some Famous Early Eruptions Throughout history, the relationship between earthquakes and volcanoes has been close, although there has been confusion about causal relations between the two. This is because both volcanoes and earthquakes form parallel belts at subduction zones, where 95% of the Earth’s subaerial volcanoes are located. It is common in the historic and recent record to find an increase in reports of regional earthquakes around the times of eruptions. For example, the reports of the eruption of Vesuvius in AD 79 tell of numerous local earthquakes preceding the eruptive activity. People living near Mount Nuovo, Italy, in 1538 and Mount Usu, Japan, in 1663 fled from the areas before the eruptions because of many felt earthquakes. Early observatories at Vesuvius starting in 1856, Usu in 1910, and Hawaii in 1912 recorded different types of earthquakes and began the systematic study of eruption precursors. Three weeks of felt earthquakes heralded the formation of the new volcano Paricutin in Mexico in 1943. These and many other examples illustrate that volcanoes are seismically active features.

1.2. Brief Review of Earthquake Seismology In order to appreciate the unique features of earthquakes at volcanoes, it is first necessary to review some features of typical earthquakes. Most earthquakes, which volcanologists often refer to as “tectonic earthquakes” to distinguish them from volcanic earthquakes, are caused by shear failure of rock. That is, rock masses on either side of a planar fracture slip past each other in a shearing motion, generating heat, deformation, and elastic waves. It may surprise most readers to learn that only about 1% of the energy of an earthquake is released in the form of elastic waves, which is the part that is felt or recorded by instruments as ground shaking. Most of the energy is liberated as heat and deformation. While rocks are fractured and faulted at all scales, it is only where they are organized into large planar features that we can identify them as faults. Faults can be the sites of large earthquakes, those that involve a large surface area of the fault, as well as many smaller ones that rupture only a small surface area. In general, there are many more small earthquakes than large ones; in fact, the distribution of earthquake numbers versus magnitude follows a power law, known to seismologists as the frequencye magnitude distribution. The relation is log10 N ¼ a  bM where M is magnitude, N is the cumulative number of earthquakes greater than or equal to M, and a and b are

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constants. The slope, b, is known as the b-value, and usually takes on values of 0.9e1.0 for tectonic earthquake sequences. This means that for every M3 earthquake, there are ten M2 earthquakes, one hundred M1 earthquakes, and so on. There are several different measures of earthquake size. The most widely known is magnitude, which is the amplitude of the elastic waves recorded on a particular instrument and corrected for distance. Magnitude was originally developed by C. Richter using the Woode Anderson torsion seismometer. A more physical measure of earthquake size is seismic moment, which is the product of fault area, slip, and the rigidity or strength of the rock. The ranges of these parameters and how they relate to each other are called scaling relations, and they permit the estimation of one earthquake parameter from another. Table 59.1 shows typical dimensions and parameters for earthquakes of different magnitudes. Once an earthquake occurs, the elastic waves travel away from the source (the fault) along different paths and eventually reach the seismometers. Characteristics of the seismogram (the way the earthquake appears on a paper record or other media) may be generated at the source, along the path, and at the recording site; thus, a major task for seismologists is to determine, for each seismogram, the relative contributions of source, path, and site effects. Once these are known, a basis exists to demonstrate that a particular earthquake is LF, harmonic, etc., and physical models are then constructed to evaluate the likely causes of peculiar (and normal) waveforms. Earthquakes are located using a velocity model of the Earth’s structure. Most velocity models are 1-D models consisting of horizontal layers of varying thicknesses and velocities that generally increase with depth. Velocities in the shallow crust (0e10 km) typically have values of w5 km/s, while velocities near the Earth’s surface (upper

TABLE 59.1 Typical Earthquake Parameters (Circular Fault, 30-bar Stress Drop) Magnitude

Radius

Slip

Moment1 (Nm)

2

0.85 m

0.06 mm

4  106

0.00053

0.6 mm

4  10

0.0053

6 mm

4  10

0.053

6 cm

4  10

0.53

60 cm

4  10

5.3

6m

4  10

53

0 2 4 6 8 1 2

8.5 m 85 m 0.85 km 8.5 km 85 km

Rigidity ¼ 3  10 N/m . Assumed b ¼ 3.18 km/s. 10

2

9 12 15 18 21

Duration2 (s)

1 km) have lower values of w3 km/s near volcanoes. To locate an earthquake, a trial location and depth are assumed and rays are traced from the earthquake to the various stations using Snell’s law according to the velocity model. The theoretical travel times are then compared with observed travel times. If the waves arrive at the station too early (or too late), then the earthquake is moved away from (or toward) that station by a small amount and the rays are traced again. Each step is known as an iteration, and computer programs are designed to iterate with predefined small increments until the travel time errors (difference between observed and theoretical time) reach a minimum. At this point, further iterations do not result in reduced errors, and the event is considered to be located. Note that an earthquake location is a model-dependent interpretation and not a fact. If the velocity model is changed or arrival time picks are modified, then the location changes as well. Seismology is also concerned with the distribution of velocities within the Earth. Seismic waves have much higher velocities than what we typically deal with on a dayto-day basis and are measured in kilometers per second. For example, a car traveling on a highway at 60 mph is only going about 0.027 km/s, whereas air waves (sound waves) travel at 0.331 km/s (740 mph) and typical P waves at volcanoes travel at about 3e5 km/s (6710e11,180 mph) or faster. Seismic wave velocities, like all wave velocities, depend on the characteristics of the material they are passing through; for example, seismic waves travel more slowly through molten rock than they do through solid rock. Therefore, seismic tomography, similar to medical tomography, uses variations in the speed of elastic waves to recover information about where magma is located. With this brief overview as a reference frame, we now turn our attention to some of the features of volcanic earthquakes.

2. TYPES OF VOLCANIC EARTHQUAKES AND TERMINOLOGY Active volcanoes are the sources of a great variety of seismic signals. Traditionally, volcanic earthquakes have been classified based on seismogram appearance into four different types: (1) HF, VT, or A-type; (2) LF, LP, or B-type; (3) explosion quakes; and (4) volcanic tremor. This general classification scheme works well at a large number of volcanoes. Other local signals, such as those caused by glaciers or landslides, are also recorded at volcanoes. Although the original classification included restricted depth ranges for various events, these have often been relaxed because of improvements in location accuracy and better understanding of source and propagation effects. This section also provides additional discussion on terminology. Examples of volcanic events from several volcanoes are shown in Figure 59.1.

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HSR EHZ - 11/20/2004 15:06:49

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Volcanic Hazards

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FIGURE 59.1 Example waveforms (top) and spectrograms (bottom) of volcanic seismicity: (A) Volcano-tectonic earthquake recorded at Kilauea volcano, Hawaii. (Data courtesy HVO.) (B) Hybrid event recorded at Mount St Helens, Washington. (Data courtesy CVO.) (C) Long-period event recorded at Shishaldin volcano, Alaska. (Data courtesy AVO.) (D) Volcanic tremor recorded at Kilauea volcano, Hawai’i. (Data courtesy HVO.) (E) Deep long-period earthquake recorded at Akutan volcano, Alaska. (Data courtesy AVO.) (F) Very long-period earthquake recorded at Fuego volcano, Guatemala. (Data courtesy G. Waite.) (G) Explosion earthquake recorded at Soufrie`re Hills volcano, Montserrat. (Data courtesy MVO.) Note that this event begins with a dome collapse e the explosion begins at approximately 130 s. (H) Rockfall event recorded at Soufrie`re Hills volcano, Montserrat. (Data courtesy MVO.) All signals have been bandpass filtered between 0.1 and 10 Hz, with the exception of (F), which was bandpass filtered between 60 and 12 s periods. Note different timescales (given in seconds at the bottom of each plot). Station name, channel code, and event date and time are given on each waveform. HVO, Hawaiian Volcano Observatory; CVO, Cascades Volcano Observatory; AVO, Alaska Volcano Observatory; MVO, Montserrat Volcano Observatory.

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Volcanic Seismicity

2.1. HF Earthquakes Most HF earthquakes are thought to be caused by shear failure or slip on faults and differ from their tectonic counterparts only in their maximum magnitudes and patterns of occurrence. At volcanoes, earthquakes typically occur in swarms rather than mainshockeaftershock sequences. A swarm may be defined as a group of many earthquakes clustered in space with no dominant shock. In practical terms, the difference in magnitude between the largest and second largest event of a swarm is 0.5 magnitude unit or less, as opposed to 1.0 magnitude unit or more for most mainshockeaftershock sequences. HF earthquakes have clear P- and S-wave onsets, and dominant frequencies are 5e15 Hz. Higher frequencies are generated at the source but are not recorded because of instrumental limitations and high local attenuation. Their magnitudes are typically lower than M ¼ 3.0. A typical HF (VT) earthquake is shown in Figure 59.1(A).

2.2. LF Events Most LF events are thought to be caused by fluid pressurization processes, such as bubble formation and collapse, and also by shear failure or tensile failure of the rock, or nonlinear flow processes that occur at very shallow depths for which attenuation and path effects also play an important role. These events often have emergent P waves, lack distinct S waves, and have dominant frequencies between 1 and 5 Hz, with 2e3 Hz being the most common. Examples of LF events, including LP and deep LP events are shown in Figures 59.1(C) and (E). Prior to eruption, magma must move from depth to the Earth’s surface through conduits, dikes, sills, reservoirs, or various combinations of these. Thus, models of LF volcanic earthquakes often select a suitable geometry, such as a rectangular crack or a cylindrical pipe, and then attempt to reproduce seismograms by appropriate choice of length, width, and velocity of the material within it, which can be either magma or water (seismicity similar to that at volcanoes is found at geysers and geothermal areas). A source of mechanical energy is needed; this can be a small earthquake adjacent to a conduit, a flow transient or pressure fluctuation within a conduit, gas bubbles expanding or contracting, a shock wave from choked flow, or other causes. Some researchers consider the source to be the mechanical energy alone, while others treat the source as the ensemble of the mechanical energy and the resonant response of the magma or water in conduits or dikes.

2.3. Hybrid Events Some earthquakes share attributes of both HF and LF events. These are called hybrid events. For example, the

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event shown in Figure 59.1(B) displays the HF onset of an HF event, but the later part of the signal (the coda) is similar to LF events. It is thought that such events may represent a mixture of processes such as an earthquake occurring adjacent to a fluid-filled cavity and setting it into oscillation. Others suggest that hybrids are shallower than LF events and thus preserve most of the HF energy that is attenuated for deeper events.

2.4. Volcanic Tremor Volcanic tremor is a continuous signal with duration of minutes to days or longer. The dominant frequencies of tremor are 1e5 Hz (2e3 Hz is the most common), similar to LF events, and many investigators have concluded that tremor is a series of LF events occurring at intervals of a few seconds. Harmonic tremor and spasmodic tremor are two special cases of more general volcanic tremor. Harmonic tremor is an LF, often single-frequency sine wave with smoothly varying amplitude, or sometimes it consists of a fundamental frequency with a set of harmonics. Spasmodic tremor is an HF, pulsating, irregular signal. An example of volcanic tremor is shown in Figure 59.1(D). It has previously been stated that earthquakes obey a power law, known in seismology as the frequencye magnitude relation. There are more small earthquakes than large ones, and the relative numbers of events form a power law distribution. Volcanic tremor, however, obeys a different type of relation, which is an exponential law distribution (Figure 59.2). For tremor, which is a continuous signal, we cannot count the number of events, so we compute the duration at different elevated amplitudes. Synthetic experiments have shown that this is equivalent to counting events because large and small earthquakes have different durations (large ones shake longer). The exponential law has implications for the source of volcanic tremor. For earthquakes, both the fault area and the slip increase as magnitude increases, but for tremor, either the source size (such as conduit length) or the magnitude of pressure fluctuations (analogous to slip) must remain constant to produce the observed exponential scaling. The available evidence suggests that the source (conduit) size remains constant.

2.5. Very-Long-Period Events Over the past two decades, new types of seismometers known as broadband seismometers have been deployed at volcanoes. These have the ability to detect ground motions over a wider frequency band, particularly at the LF end (down to f ¼ 0.016 Hz, or periods of 60 s and lower). Not surprisingly, a new class of events called very-long-period (VLP) events has been observed at some volcanoes using broadband seismometers. Observations to date show events with periods of

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FIGURE 59.2 Volcanic tremor data from Mount Spurr, Alaska, comparing an (A) exponential and a (B) power law model. Note that the vertical axis is logarithmic in both cases, but the horizontal axis is linear for the exponential model and logarithmic for the power law model. The circles show the duration of tremor observed at various amplitudes; strong tremor occurs less often than weaker tremor. The lines are weighted least squares mathematical fits of the data to the two models shown. The data fit better using the exponential model as shown by the higher value of the correlation coefficient R2. After Benoit et al. (2003).

3e50 s originating from shallow depths of 1.5 km or less under several active volcanoes, including Kilauea, Stromboli, Aso, Sakurajima, Fuego, Augustine, and Satsuma-Iwojima. The VLP events at these volcanoes have been associated with either eruptions or vigorous fumarolic activity, and the observed events have fairly small amplitudes. An example of a VLP event is shown in Figure 59.1(F). Normally, it takes a large structure, such as a long fault, to produce significant waves with these long periods. However, many of the VLP events at volcanoes appear to come from small source zones despite the long wavelengths. Some current models suggest that these events may be produced as magma moves through a flapper valve (such as a restriction in a dike or sill) in distinct pulses. Additionally, theoretical calculations show that it is possible for a crack or tube filled with fluid to produce long-wavelength (and LP) waves if the acoustic velocity in the fluid is slow compared with the rigid walls. The interaction between the fluid and walls produces low phase velocities along the interface. Such waves are called crack waves or tube waves. During its climactic eruption on June 15, 1991, Mt Pinatubo, Philippines, produced strong seismic waves with periods of 228e270 s. These are thought to be caused by a type of oscillation in the atmosphere in which thermal energy from the vent travels up and then reflects off the top of the ash column (in this case, in the stratosphere). These waves also coupled into the ground and appeared on seismic stations thousands of kilometers away, lasting for over 2 h. During the strong initial phase of the 2008 eruption of Okmok volcano, Alaska, a nearby broadband seismometer recorded several hours of similar waves with periods of 540 s.

Broadband sensors have been deployed at a number of volcanoes that produced no observed VLP signals, even though several of these were erupting. They include Mt Spurr, Montserrat, Arenal, and Akutan volcanoes. VLP events have been observed as precursors to only a few large eruptions, one example of which was the August 2008 eruption of Kasatochi, Alaska. There VLP signals were associated with some of the large precursory earthquakes (M ¼ 4e5.8) even though the nearest broadband seismometer was 70 km away. The VLP signals made visible by the new broadband instruments are an exciting part of volcano seismology, and it is anticipated that many new advances will be made in this area over the next few years.

2.6. Explosion Earthquakes Explosion quakes accompany explosive eruptions, and many are characterized by the presence of an air-shock phase on the seismogram. There is a partitioning of energy at the source: part of the energy travels through the ground as seismic waves and part travels through the air as acoustic or air waves. The air wave then couples back into the ground and is detected by the seismometer. An example of an explosion earthquake is shown in Figure 59.1(G). The air waves also show up clearly on microphones or infrasound sensors. Infrasound data have been used increasingly to study volcanoes and are a very useful complement to seismic data. The instruments are typically deployed in small arrays (a few hundred meters across) and instruments may be colocated with seismic stations. The data recording and telemetry are similar, using sample rates of 20e100 samples per second and recording pressure

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Volcanic Seismicity

instead of ground velocity. Infrasound signals have the advantage that they travel through a simple medium (the air) and, at least for local stations, do not suffer from many of the complications present in seismic data such as reflections, refractions, and other path and site effects (see also Chapter 63).

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2.7. Surficial Events Seismometers located on volcanoes may record a variety of local signals caused by shallow processes. These include nonvolcanic processes, such as glacial events, shore ice movement, and landslides, as well as volcanic processes, such as outburst floods, lahars, pyroclastic flows, and rockfalls, from crumbling lava domes. Some of the seismograms are similar, so they must be recognized and properly treated by analysts. l

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Glacial events. Volcanoes at high latitudes and tall volcanoes are frequently covered by glaciers, which produce seismic events along the iceerock boundary and within the ice when cracks and crevasses form. Because the seismic wave velocity in ice is lower than that in rock, the seismic waves may become partially trapped in the ice and set up oscillations of the ice itself. Seismograms from such events resemble those for volcanic LF events. Glacial events are more common in the summer months, when the glaciers are moving faster. Shore ice. Ice forms from seawater when the air temperature falls below 10  F (12  C). Large blocks of such ice gather on the shores of Augustine Island, Cook Inlet, Alaska, and move around as the tides ebb and flood. When the blocks collide or break, they produce seismic events because the blocks are coupled to the ground. The events are shallow (surface), so the seismic waves are mostly LF surface waves traveling in thin sediment layers. The individual ice events resemble volcanic LF events, and the events occur in swarms of a few hours duration because of the tides. Both of these features are similar to seismicity preceding eruptions, so thermometers are needed to measure air temperature in such situations! Landslides. Avalanches and landslides of various sizes occur on volcanoes. Those associated with partial melting of ice and snow mainly occur in the spring and summer. The avalanche or landslide generates a seismic signal of several minutes’ duration, depending on the size and run-out. The amplitude varies with the amount of material, with larger avalanches generating stronger seismic signals. Some landslides on volcanoes (and elsewhere) are preceded by small discrete seismic events, whose rate of occurrence increases up to the time of the main event. This is similar to a common pattern of earthquakes before some eruptions, so again,

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additional information is needed to distinguish between volcanic and nonvolcanic causes. Rockfalls. Rockfalls are a special type of landslide in which one or a few pieces of bedrock become detached and free-fall. They are especially common at the edges of volcanic domes and at volcanoes with very steep slopes or cliffs. In the summer of 1995, workers at Mt Spurr, Alaska, observed rocks more than 1 m in diameter falling several hundred meters and impacting on the crater floor. The corresponding seismograms showed strong local signals lasting more than 1 min. An example of a seismic signal from a rockfall is shown in Figure 59.1(H). Pyroclastic flows. Pyroclastic flows are products of explosive eruptions and consist of particles of hot rock and gases that move rapidly over the ground surface. Large-scale collapses of volcanic domes can also produce pyroclastic flows. These latter events are characterized by complex seismograms that have been modeled as a sequence of forces. First, rock is removed from the dome (the dome moves up slightly in response); second, the falling rock collides with the slope below (an impact); and third, the rock breaks apart and the fragments continue to move down the slope as an irregular flow. These seismograms last several minutes, they may contain high frequencies, and the larger flows are associated with larger-amplitude signals. Pyroclastic flows have also been associated with another seismic feature: At Pinatubo volcano, Philippines, pyroclastic flows traveled down a prominent valley between two seismic stations, one of which transmitted its signal to the other. As the ash clouds rose into the air, the telemetry path was interrupted for several tens of minutes after which the signal returned. Thus, the temporary absence of the telemetry could be used to infer that pyroclastic flows were occurring. Outburst floods and lahars. The surface melting of ice and snow, or heavy rains, may cause floods and lahars (volcanic mudflows). Similarly, a volcanic eruption under a glacier will melt the ice, and the flooding that subsequently occurs is called a jokulhlaup. All of these floods or lahars cause long-duration seismic signals that resemble volcanic tremor, except that the signal is stronger near the flow channel as opposed to tremor, which is stronger nearest the volcanic vent. The floodwater travels downstream, so the source is moving. Also, the recorded frequencies may be high if the seismometer is near the flow channel, even though this may be far from the vent.

Different groups of investigators and various observatories have used several different local terminologies for volcanoseismic event types, and no consensus has yet emerged about an appropriate global terminology. The lack of a global terminology makes it difficult to evaluate and

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TABLE 59.2 Selected Volcano Seismology Terminology This Article

Minakami (1960)

Latter (1979)

AVO1

Other names

Example (Figure 59.1)

High-frequency (HF)

A-type

Tectonic, volcano-tectonic

Volcano-tectonic (VT)

Short-period earthquake

(A)

Low-frequency (LF)

B-type

Volcanic

Long-period (LP)

Long-coda event, tornillo2

(C), (E)

Hybrid

e

Medium-frequency

Hybrid

Mixed-frequency

(B)

Explosion quake

Explosion quake

Volcanic explosion

Explosion

e

(G)

Volcanic tremor

Volcanic tremor

Volcanic tremor

Volcanic tremor

Harmonic tremor, Spasmodic tremor

(D)

1

Alaska Volcano Observatory. Tornillo is the Spanish word for “screw.” The codas of these events resemble a wood screw in profile.

2

compare observations from different volcanoes and creates confusion in the literature. Table 59.2 compares several of the more common local terminologies. Different mechanisms acting at the seismic source are responsible for many of the features of the different events, and path and site effects, which are extreme at many volcanoes, may greatly modify the signals. Four principal seismic sources are shear failure, also called “double-couple” mechanisms; tensile failure, or other “non-double-couple” mechanisms; single forces resulting from explosive injection of material into the atmosphere; and passive and active fluid involvement in producing LF events and volcanic tremor. Nonlinear flow has also been modeled as a source for volcanic tremor. Some authors have investigated differences between volcanic and tectonic earthquakes by comparing factors such as magnitude using different waves, and seismic moment versus fault length. Such comparisons reveal that many volcanic earthquakes are enriched in low frequencies and occur on smaller structures (faults) than tectonic earthquakes of the same magnitude. Intermittent or “banded” tremor occurs in regular, periodic bursts separated by periods of quiescence of uniform duration. The resulting pattern looks like stripes or bands on seismograms recorded on rotating drum recorders. Many other patterns are observed and various terms are used as adjectives to describe them, but this does not necessarily mean that different source processes are required.

3. GENERAL FEATURES OF EARTHQUAKES AT VOLCANOES The goals of volcano seismology include monitoring the present state of a volcano, forecasting eruptions and other changes in a volcano’s activity (the end of an eruption, for example), estimating the size of eruptions in progress, locating magma chambers, and understanding the physical

processes that are occurring within the magmatic system. The following topics clarify many features that are common to earthquakes at volcanoes.

3.1. Seismicity Rates and Background Volcanoes nearly always have a nonzero background level of seismicity caused by heat; movement of groundwater, volatiles, or magma; glaciers (if present), landslides, and rockfalls; and reactions to stresses from regional tectonics, tides, or other forcing functions. Thus, a baseline of measurements of at least a few years duration is necessary to characterize and understand the background seismicity. The background may consist of several different types of events that are related to different processes. Typical background rates of seismicity are a few to a few tens of events per day, depending on the locations and sensitivity of the seismic stations. In contrast, the rates of seismicity before and during eruptions are typically several tens to several hundreds or more events per day and include larger-magnitude events. Examples of rates and magnitudes of seismicity prior to selected eruptions are shown in Table 59.3. Many successful eruption forecasts have been made because of an increase in seismicity above previously recorded background levels. This is based on the idea that the level of seismicity reflects the level of volcanic activity and suggests that there is a constant long-term probability of eruption, usually assumed to be a Poisson process. This means that eruptions behave as a random variable and that volcanic systems have no memory (mathematically) of previous eruptions. The average rate of eruptions is also assumed to be constant, and the probability for a new eruption increases when earthquake swarms occur. Commonly, the outcome of an increase in seismic activity is magma intrusion without an eruption. Therefore, there will always be the possibility of a false alarm because some intrusions remain at depth, whereas others reach the surface and erupt.

Location

Detected1

M>1

M>2

M>3

M>4

LP2/tremor

Eruption

M ¼ 2.5

No

Yes

M ¼ 1.7

Yes

Yes

M¼4

No

No

M¼4

No

No

640/day

M ¼ 4.6

Yes

Yes

360/day

M ¼ 3.5

Yes

Yes

M ¼ 2.8

No

No

33/day

M ¼ 5.2

?

No

12/day

M ¼ 5.8

Yes

Yes

M ¼ 3.3

Yes

Yes

M ¼ 6(4)

No

No

M ¼ 4.1

No

No

M ¼ 2.8

No

No

M ¼ 5.3(2)

No

No

M¼3

No

No

M¼3

No

No

M ¼ 5.0

No

No

Augustine Feb 1986

>5000/day

Jan 2006

80/hour

17/hour

Campi Flegrei Oct 1983

>300/day

Mar 1984

z500/day

5/day

El Hierro Aug 2011

Volcanic Seismicity

Mmax

Chapter | 59

TABLE 59.3 Seismicity Rates for Selected Earthquake Swarms

Eyjafjallajo¨kull Apr 2010 Fuego Jan 1977

2000/day

70/day

z5/day

Galapagos Jun 1968

90/day

Kasatochi Aug 2008

530/day

Kilauea Jan 1983

>1100/day

92/day

18/day

1/day

Long Valley z15/day

May 1980 May 1982

7/3 days

Nov 1982

z100/day

Jan 1983

z800/day

Jun 1989

z40/day

Mar 1990

>300/day

z9/day 25/hour z25/day

2/day

z10/day

Matsushiro Nov 1965

z2000/day

>200/day

1019

(Continued)

1020

TABLE 59.3 Seismicity Rates for Selected Earthquake Swarmsdcont’d Location

Detected1

M>1

M>2

M>3

M>4

Mmax

LP2/tremor

Eruption

M ¼ 4.2

No

No

M ¼ 2.8

No

No

M¼4

Yes

Yes

M ¼ 5.5

Yes

Yes

Medicine Lake Sep 1988

80/hour

2/hour

Mt Hood Jul 1980

20/hour

14/hour

z600/day

>70/day

Mt St Helens Mar 1980

z50/day

1e4/day

Off-Ito Jul 1989

z400/day

40/day

Okmok 32/hour

24/hour

M ¼ 2.4

Yes

Yes

800/day

400/day

M ¼ 2.1

Yes

Yes

z1700/day

M ¼ 4.8

No

No

Dec 1989

z150/hour

M<2

Yes

Yes

Mar 2009

120/hour

M ¼ 3.4

Yes

Yes

M ¼ 1.2

Yes

No

M ¼ 3.8

Yes

Yes

Jul 2008 Pavlof Apr 1986 Rabaul Apr 1984 Redoubt

May 2003

1505/day

247/day

Usu Aug 1977 2

Minimum magnitude not specified but generally M < 1. LP: Long-period earthquake.

5/hour

Volcanic Hazards

1

200/hour

PART | VII

Shishaldin

Chapter | 59

Volcanic Seismicity

3.2. Seismicity Associated with Large versus Small Eruptions In general, large eruptions are relatively easier to forecast than smaller ones. The amount of magma involved is large, and precursory earthquakes tend to be numerous as well as distributed over a large volume. Other precursors such as deformation and steaming are usually observed in conjunction with earthquake swarms. Recent examples include Pinatubo, Philippines, 1991; Spurr, Alaska, 1992; Augustine, Alaska, 2006; Kasatochi, Alaska, 2008; Redoubt, Alaska, 2009; Eyjafjallajo¨kull, Iceland, 2010; and Merapi, Indonesia, 2010. Small eruptions and many phreatic (water-driven) eruptions, in contrast, involve much smaller amounts of magma and have subtle precursors or sometimes no observable precursors. They are, therefore, generally harder to forecast. Recent examples include Galeras, Colombia, JanuaryeJune 1993; Arenal, Costa Rica, August 1993; White Island, New Zealand, February 1992; and Ontake, Japan, September 2014. Occasionally, “unexpected” eruptions occur; often these take place during later phases in a sequence of eruptions. Even though these may be large, they may have subtle or undetectable seismic precursors. This is a consequence of the vent remaining “open” or mechanically weak following an initial stage of activity, preventing buildup of stresses that would cause earthquakes. A recent example is the second eruption of Crater Peak (Mt Spurr), Alaska, in August 1992, which occurred without immediate detectable precursors.

3.3. Locations of Volcanic Earthquakes Generally, volcanic earthquakes occur beneath the point of eruption (i.e., in proximal clusters). This helps reduce the monitoring problem to that of estimating the time of eruption, since the place is considered known. For example, seismicity preceding the 2004 eruption of Mount St Helens, Washington, occurred in a small shallow volume approximately 1 km below the eruptive vent. However, several cases have occurred in which the locations of initial precursory earthquake swarms did not coincide with the eruptive vent (i.e., distal clusters). For instance, the initial earthquake swarms prior to the dacitic eruption of Mt Pinatubo, Philippines, in 1991, occurred about 5 km northwest of the volcano. Twelve days before the climactic eruption, activity of these swarms declined and vigorous shallower swarms began beneath the eventual eruption site. Similarly, prior to the onset of eruption at Soufrie`re Hills, Montserrat, the first recorded earthquakes were located w4 km west-northwest of the volcano. After 2 days, the distal earthquake swarm ceased and a swarm began directly under the Soufrie`re Hills vent. These and other cases demonstrate that earthquakes occur where stresses are concentrated, which is not necessarily where the magma is

1021

located. It is expected that earthquake locations would migrate upward toward the Earth’s surface as magma rises from depths prior to eruption. There are, however, surprisingly few well-documented cases of such a trend associated with eruptions. This overall pattern was first shown for Kilauea volcano, Hawaii, in 1960; a more recent example is a seismic swarm that migrated upward over a period of 35 h, preceding the 1998 eruption of Piton de la Fournaise volcano, la Reunion. One spectacular case of lateral migration of hypocenters was observed at Krafla volcano, Iceland, in September 1977; when the earthquakes passed a geothermal borehole, fresh pumice actually erupted from the hole! Most well-documented examples of hypocenter migration involve noneruptive swarms, e.g., at Mammoth Lakes, California; Upptyppingar, Iceland; and Paricutin, Mexico, suggesting that migrating swarms are caused by a process other than preeruptive magma ascent such as hydrothermal fluid circulation.

3.4. Large Regional Earthquakes Near Volcanoes Large regional earthquakes are believed to occasionally trigger eruptions. Proposed examples include the eruption of Puyehue, Chile, 48 h after the M9.5 earthquake of 1960 and the 1999 eruption of Cerro Negro, Nicaragua, 3 days after a series of nearby M6.5 earthquakes. However, such examples of “static” triggering are difficult to prove conclusively, and the exact mechanism of eruption triggering is also not well understood. Proposed mechanisms include local stress changes near the volcano induced by the nearby large earthquake (analogous to squeezing a tube of toothpaste) and several mechanisms involving the displacement or growth of bubbles by passing seismic waves. Most well-documented examples of “dynamic” triggering involve triggered episodes of noneruptive volcanic unrest. For example, the only documented example of volcanic unrest triggered by the 2004 M9.1 Sumatrae Andaman earthquakes was at Wrangell volcano, Alaska (11,000 km away), which experienced a series of 14 volcanic earthquakes during the passage of surface waves through the volcano. Immediately after the 2011 M9.0 Tohoku earthquake in Japan, a swarm of over 1600 earthquakes was recorded at the quiescent Hakone volcano, 450 km away, that included multiple felt earthquakes. Proposed examples of static triggering of noneruptive unrest also abound: for example, it has been demonstrated convincingly that the June 1992 M7.3 earthquake in Landers, California, remotely triggered earthquake swarms at 17 volcanic areas in the western United States at distances of 165e1250 km. Although no eruptions occurred, these swarms demonstrated that small transient stress changes can trigger significant changes in seismicity in volcanic areas.

1022

PART | VII

Vulcan

Y-profile

Tovakundum 0

Caldera

Raluana Pt

Kokopo

Volcanic Hazards

Tokua Airport

Depth (km)

2 4 6 8 10

24/09/433

0

20

10 2.0

Tovakundum 0

P-wave velocity

30 6.5 km/s

Raluana Pt Vulcan Caldera

Y-profile

40 km

Kokopo

Tokua Airport

Depth (km)

2 4 6 8 10 0

20

10

24/09/433

30

40 km

–1.0 1.0 km/s P-wave velocity contrast FIGURE 59.3 Tomographic images of P-wave velocity along an NEeSW profile at Rabaul caldera, Papua New Guinea. (Top) P-wave velocity and (bottom) residual velocity difference after subtracting the regional one-dimensional velocity from the top. Note the significant low-velocity region under the center of the caldera, interpreted to be a region of high-temperature magma accumulation. After Finlayson et al. (2003).

3.5. Caldera Earthquakes 0

-5

Depth (km)

-10 3 2.5

-15 2 1.5 -20 b-value

-25 Lo

14.9 ng 15 itu 15.1 de

37.65

37.7 Latitude

37.75

37.8

Large earthquakes often occur at large structures such as calderas. For example, M6 earthquakes have occurred at or near calderas in Long Valley, California; Yellowstone, Wyoming, Montana and Idaho; and Aso, Japan. M5.1 earthquakes have occurred at Rabaul caldera, Papua New Guinea, in 1982 and 1984. While small eruptions have occurred frequently at the central vent of Aso, no eruptions have occurred at Long Valley or Yellowstone. Rabaul finally erupted on September 19, 1994, following 13 years of caldera-wide seismicity and 27 h of intense local seismicity. It is normal for large calderas to show frequent signs of unrest, the vast majority of which are not precursors to eruptions. This adds to the uncertainty in dealing with earthquake swarms at calderas: false alarms are more likely, but large eruptions are also possible.

=

FIGURE 59.4 Three-dimensional map of b-values (here expressed as mean earthquake magnitude) under Mt Etna. Red volumes mark locations characterized by a lack of larger earthquakes. These volumes are interpreted as places of magma storage. b-values are estimated every 1 km by samples of N ¼ 50 events, which occur in radii between 2.5 and 6 km. The period covered by the data is 1990 through 1997e1999, and using M > 2.5 earthquakes. The numbers 1 and 2 mark a stronger and weaker anomaly, respectively. After Murru et al. (1999).

Chapter | 59

Volcanic Seismicity

3.6. Seismicity at Volcanoes with Long Repose Times Large eruptions often follow long repose periods, and the precursory earthquake swarms may include relatively large events. Unfortunately, because large eruptions are relatively rare and volcanoes with long repose times are usually unmonitored, very few observations exist to date. Recent examples include Chaite´n volcano, Chile, 2008 (volcanic explosivity index (VEI) 4, repose 9400 years; M ¼ 5.2) and Kasatochi volcano, Alaska, 2008 (VEI 4, repose 248 years; M ¼ 5.8). At Mount St Helens, 1980 (VEI 5, repose 123 years), an M ¼ 5.0 earthquake that occurred seconds before observation of the cataclysmic eruption may have been a short-term precursor or a manifestation of summit failure. Large eruptions commonly result in caldera formation, which may also result in large-magnitude syn- or posteruptive earthquakes. For example, at Mt Pinatubo, Philippines, in 1991 (VEI 6, repose 600 years), two M ¼ 5þ earthquakes occurred shortly after the start of the cataclysmic eruption. In 1912, approximately 11 h into a large-volume eruption at Katmai volcano, Alaska, a series of fourteen M ¼ 6e7 earthquakes accompanied caldera formation. The presence of large structures such as caldera-bounding faults may result in large earthquakes not directly related to an eruption. For example, a sequence of four M ¼ 6 earthquakes at Long Valley caldera, California, in May 1980, and some of the later seismicity at Long Valley may have been related to intrusions of magma at depth.

3.7. Synchronous Volcanic and Tectonic Activity A 40-km-wide belt of volcanoes and frequent shallow crustal earthquakes is located above the subduction zone in Central America. While the general structural features are similar to most other subduction zones, the Central American tectonic seismicity is more abundant and appears to be more closely linked to volcanism. Both eruptions and damaging earthquakes have occurred in isolation, but occasionally the two have been coincident. For example, at Boqueron volcano, El Salvador, an M6.2 earthquake occurred alone in May 1965, while in June 1917 and September 1659, eruptions and destructive (M6) earthquakes occurred simultaneously. These may be cases of one phenomenon modifying or triggering the other, or an external agent may be the cause of both.

3.8. Seismic Features of Magma Chambers Magma is thought to be stored in some type of chamber or reservoir underground, and seismology is useful for determining the size, shape, and location of such

1023

chambers. Magma chambers at stratovolcanoes appear to be generally equant in shape and on the order of 1e20 km3 in volume. Those at calderas are larger. The majority of depths to the tops are mapped at about 5e20 km or deeper based on tomography, S-wave screening, and posteruption seismicity. Seismic tomography, similar to medical tomography, requires seismic waves to travel through the target region. Thus, a good distribution of earthquakes and seismic stations is needed; both local and distant earthquakes have been used as sources for such studies. The technique exploits the fact that P waves speed up in competent rock and slow down in unconsolidated materials or magma. An example of a tomographic image of a magma chamber under Rabaul caldera, Papua New Guinea, is shown in Figure 59.3. S-wave screening exploits the fact that S waves cannot pass through liquids. Thus, the target region will be illuminated by the presence of P waves but lack of S waves for those earthquake rays that pass through magma. Posteruption seismicity is often concentrated at depths of 5e20 km. Here, the upward removal of magma during eruption causes stress changes at depth that induce earthquakes. The idea is that the magma chamber walls collapse inward, and earthquakes are concentrated where these processes are most pronounced. A technique that requires well-distributed seismicity (as opposed to earthquakes concentrated at one spot) looks for systematic spatial variation in the b-value, discussed earlier. It has been shown in the laboratory that heat, high pore pressure, low applied stresses, or heterogeneous materials all produce high b-values. These are also the conditions that would be expected in the vicinity of magma bodies. Although this technique cannot uniquely identify which process is dominant, strong anomalies have been found at depths of 3e4 km and 7e9 km beneath more than 10 volcanoes studied. Examples of b-value anomalies thought to represent magma chambers beneath Mt Etna volcano, Italy, are shown in Figure 59.4. The parameter plotted is mean magnitude: If the b-value is high, there will be many small events and the mean magnitude will be low, whereas if the b-value is low, the sample will have more number of larger events and hence a high mean magnitude.

3.9. Source versus Path and Site Effects A long-standing issue in volcano seismology is the relative contribution of source and propagation (path and site) effects on the resulting seismogram. Two examples illustrate why this issue is important. The first shows threecomponent seismograms from three small earthquakes near Mammoth Mountain, California (Figure 59.5). Both the P and S waves are distinct for the events that occur at a depth of approximately 4 km and epicentral distance of 3 km (Figure 59.5(A)). The station MMB is located on an

1024

PART | VII

Volcanic Hazards

(A)

(B)

(C)

FIGURE 59.5 (A) Map of temporary seismic stations (dots) deployed near Mammoth Mountain, California, in 1989. (After Julian et al., 1998.) The edge of Long Valley caldera is the heavy curved line. The approximate location of the earthquakes is shown as a cross. (B) Vertical, northesouth, and eastewest seismograms from station MMB. Data for 10 seconds are shown. Clear P and S waves can be seen for three small earthquakes beginning roughly 1.5, 3, and 7 s into the record. (C) The same events for vertical, northesouth, and eastewest seismograms from station MMF. Note that the seismograms look like a single low-frequency event. The third event can be identified with close scrutiny, but the small second one cannot be seen. This is a clear example of site effects.

old dome and is essentially a bedrock site. Figure 59.5(C) shows the same events at an adjacent site that is approximately at epicentral distance of 4 km. The seismograms look completely different. Whereas the earthquakes appear to be ordinary HF events at station MMB (Figure 59.5(B)), on station MMF (Figure 59.5(C)) the events look like a single LF event with extended coda. The third event can be discerned barely, but the small second one cannot even be identified. Station MMF is a soft sediment site. The network geometry was designed to record optimally for focal mechanism studies. This example clearly demonstrates the high impact of path and site effects on the resulting waveforms. The second example shows that when seismometers are right on top of the source (0e2 km), LP events can appear

as simple pulses, whereas at distances of 3e10 km, the seismograms show typical long codas (Figure 59.6(A)). Because of steep topography, most stations at volcanoes are in the range of 3e10 km. The simple pulse can be caused by slow rupture failure in unconsolidated sediments (such as tephra), and the long codas are caused by propagation effects such as resonance in layers of the volcanic pile. Thus, fluids need not be involved. Simple pulses have been observed at close distances at several volcanoes and at Mt Etna during several eruptions (Figure 59.6(B)). These observations demonstrate that alternative explanations need to be considered. No sweeping generalizations can be made regarding source versus site and path effects, and it is recommended that each case be evaluated independently.

Chapter | 59

Volcanic Seismicity

1025

(A) 8 6 2.5 2

4

1

2 Offset (km)

Offset (km)

1.5

0

0.5 0

−0.5 −1

−2 −1.5 −2

−4

−2.5

0

1

2

3

4

5 6 Time (s)

7

8

9

10

−6 −8 −10 2

4

6 8 10 12 14 16 18 20 22 Time (s)

(B) Turrialba

2.5

0.2

0

Turrialba

-2.5 Ubinas

1

0

0

0.1 Ubinas Etna04

50 0 -50

Etna08.1

10

Amplitude (a.u.)

Amplitude µm/s

-1 0 5

Etna04

0 1

0 -10

Etna08.1

25

Etna08.2

0

0 1 Etna08.2

-25 0 0

5

10 Time (s)

15

0.1

0.5 1 Frequency (Hz)

5

FIGURE 59.6 (A) Spatial distribution of waveforms for a long-period (LP) event family at Mt Etna in 2008. Each seismic trace is a stack of about 60 events in the family. Events are located beneath the summit at depths of <800 m. Normalized vertical component traces are plotted as a function of the station’s distance from the volcano summit. (B) Left panel: Shallow pulse-like LP events (vertical component) detected on near-summit stations at Turrialba, Costa Rica (2009); Ubinas, Peru (2009); and two different time periods on Mt Etna (2004 and 2008). Right panel: amplitude spectra for the data shown in the left panel. a.u. stand for arbitrary units. After Bean et al. (2013).

1026

PART | VII

Volcanic Hazards

TABLE 59.4 Parameters of Volcanic Activity: Seismic Case Histories

Volcano Name

Date

VEI1

Swarm Duration (days)

Times since previous eruption (years)

Event type2

Maximum Magnitude

Mount St Helens

Sep 2004 to Jan 2005

2

130

13

H,L,T

2.9

Soufrie`re Hills

Mid-1995 to present

3

>6700

365

H,L,T,R

3.3

Okmok

Jul 12, 2008 to Aug 19, 2008

4

0.21

>800 (11)

H,L,T

2.4

Augustine

Apr 30, 2005 to May 2006

3

256

20

H,L,T,R

1.7

3

1

Volcanic Explosivity Index. H, high-frequency earthquakes; L, low-frequency earthquakes; T, volcanic tremor; R, rockfalls. Okmok volcano erupted in 1997 from a different vent (Cone A), located 4 km W of the 2008 event.

2 3

4. CASE HISTORIES

chapter gives at least one recent reference for each of the case histories.

We consider the earthquake activity at four volcanoes to illustrate the variety of seismic activity. The four examples include small and large eruptions and systems dominated by basalt, andesite, and dacite magmas. The volcanoes chosen all had high-quality seismic data. Parameters of the four case studies are summarized in Table 59.4. The Further Reading section at the end of this

Frequency, in hertz

a b

4.1. Mount St Helens, 2004e2008 Mount St Helens, Washington, USA, experienced a 4-year-long, near-continuous eruption involving gradual extrusion of dacitic magma beginning in October 2004. e

d

c

14 10 6 2

Event spacing, in seconds

800 600 400 200

/0 26 11 /

4/ /0 10

5

05

05 /1 08

07

/0

2/

8/

05

05 05

/1

6/

05 0/ /3 03

/1 1/ 02

6/ /2 12

05

04

4 /0 09 11 /

09

/2

3/

04

0

Time, in pacific daylight time FIGURE 59.7 Plot showing Earthquake Spectral Amplitude Measurement (top) and interevent spacing (bottom) for detected events at Mount St Helens on station HSR between September 23, 2004 and December 31, 2005. Roughly 370,000 events were detected during this time period, including occasional noise glitches and other false triggers. Note the brief initial swarm of volcano-tectonic earthquakes, followed by several months of extremely regular and repetitive “drumbeat” events (Figure 59.1(B) shows an example of a “drumbeat” event from Mount St Helens), and the gradual slowing of the rate of seismicity beginning in April 2005. See text for additional details. After Moran et al. (2008), courtesy of S. Moran/CVO.

Chapter | 59

Volcanic Seismicity

a

VTs per day

150

1027

c

b

e

d

g

i

h

100

50

0 1995

1997

1999

2001

2003

2005

2007

2009

2011

2013

1995

1997

1999

2001

2003

2005

2007

2009

2011

2013

2007

2009

2011

2013

800 700

Hybrids per day

600 500 400 300 200 100 0 700 600

LPs per day

500

f

400 300 200 100 0 1995

1997

1999

2001

2003

2005

FIGURE 59.8 Histograms of the number of volcano-tectonic (VT) (top), hybrid (middle), and long-period (LP) (bottom) events per day at the Soufrie`re Hills volcano, Montserrat, from 1996 to 2013 (period of operation of the MVO digital seismic network). The precursory phase and the first year of the eruption, which preceded the installation of the digital seismic network, are indicated by the gray stippled areas. Solid gray areas indicate periods during which Soufrie`re Hills was in eruption, and white areas indicate intereruptive periods. Note that hybrid and LP activity occurs primarily during eruptive (gray) periods, while VT activity declines during eruptive periods and is strongest during inter-eruptive periods and immediately before the onset of each eruptive period. See text for additional details. MVO, Montserrat Volcano Observatory. Data courtesy P. Smith/MVO.

Seismicity preceding the eruption began on September 23, 2004, with a shallow swarm of VT earthquakes beneath the 1980 dome (Figure 59.7: phase a). VT activity increased on September 24, the first LP event was recorded on September 25, and the first hybrid event was recorded on September 26. Seismicity

continued to increase in intensity until the first phreatic explosion on October 1. From October 1e5, recorded seismicity included one M2þ earthquake every minute, with tremor bursts beginning on October 2. Magma reached the surface and began to erupt from the dome on October 11 in a series of spines, accompanied by

1028

PART | VII

Volcanic Hazards

FIGURE 59.9 Seismogram of 24 h of data from station OKWE, 10 km NW of the vent of the 2008 eruption of Okmok volcano. Each line shows 30 min of data, and time increases from top to bottom. A regional earthquake occurs at 00:02 UTC (upper left). The first visible precursors at this station are small earthquakes at 15:08 and 16:05 UTC. Precursors intensify at 18:41 UTC, and the eruption begins with continuous tremor at 19:43 UTC, which increases in strength at 19:47 UTC. The strong initial phase of the eruption lasted for 10 h. Red color indicates strong signal saturating the plot. Gaps represent telemetry dropouts.

strikingly regular “drumbeat” earthquakes beginning on October 16 (Figure 59.7: phase b). Drumbeats had HF onsets and predominantly LF codas, with many having nearly identical waveforms. An example waveform from a drumbeat earthquake is shown in Figure 59.1(B). Drumbeat seismicity continued throughout the domebuilding phase of the eruption, at a remarkably steady rate until April 2005 (Figure 59.7: phase c), then at a declining rate through the remainder of the eruption (Figure 59.7: phase d). A swarm of VT earthquakes

occurred in JulyeAugust 2005 (Figure 59.5: phase e), coincident with the disintegration of the largest dacitic spine extruded during the 2004e2008 eruption. On January 16, 2008, steam was observed seeping from the lava dome, coincident with an M2.9 earthquake, followed by a small episode of tremor that lasted nearly 90 minutes, and an M2.7 earthquake. By the end of January 2008, the lava dome growth stopped along with elevated seismic activity. Overall more than one million earthquakes were recorded during this eruption.

Chapter | 59

Volcanic Seismicity

1029

(A)

h

(B)

(C)

FIGURE 59.10 Summary monitoring data from Okmok volcano, JulyeAugust 2008 eruption. (A) Deformation (up (N) corresponds to deflation) from GPS station OKSO, 12 km SW of the vent. (B) Plume height in km. (C) Real-time Seismic Amplitude Measurement (RSAM) from station OKRE. After Larsen et al. (2009).

4.2. Soufrie`re Hills, 1995eContinuing In the century preceding its current eruption, the Soufrie`re Hills volcano, Montserrat, experienced three major seismic crises that occurred approximately every 30 years (1897e1898, 1933e1937, and 1966e1967). The current eruption began in mid- to late 1995 following approximately 3 years of escalating seismic activity, with a series of VT earthquake swarms beginning in January 1992, which intensified in 1994. As of mid-2013, the eruption has been ongoing for over 17 years and has consisted of five discrete phases of extrusive activity to date (Figure 59.8). A phreatic explosion on July 18, 1995 marked the onset of the phreatic phase of the eruption, which continued until the first emergence of lava at the volcano’s summit in September 1995. Soufrie`re Hills then remained in continuous eruption until March 1998, accompanied by high rates of VT, LP, and hybrid earthquakes (Figure 59.8: phase a). The exact appearance of the VT, LP, and hybrid events varied by station. An intereruptive pause then lasted approximately 20 months and was accompanied by the highest levels of VT earthquakes recorded to date and almost no LP or hybrid seismicity (Figure 59.8: phase b). The second phase of the eruption began in November 1999 and lasted until July 2003 and was characterized by dome growth punctuated by episodes of dome collapse and explosions, reflected by

moderate rates of LP seismicity and declining rates of VT and hybrid seismicity (Figure 59.8: phase c). After a second inter-eruption pause characterized by minimal LP and hybrid activity and increasing VT seismicity (Figure 59.8: phase d), eruptive activity resumed approximately 2 years later with the onset of the third phase of lava extrusion in August 2005 (Figure 59.8: phase e), which lasted until early 2007. Dome growth during this phase was initially sluggish, with extrusion rates increasing significantly in FebruaryeMarch 2006. Significant explosions occurred in May 2006 and January 2007, and a major dome collapse occurred on May 20, 2006, accompanied by a strong swarm of LP events (Figure 59.8: phase f). Fourth and fifth phases were relatively short (August 2008eDecember 2009 and October 2009eFebruary 2010, respectively, Figure 59.8: phases g and h). As of mid-2014, the volcano is showing low levels of surface activity but is still seismically active, with a moderately high rate of VT seismicity (Figure 59.8: phase i) and undergoing inflation. In general, during each phase of eruptive activity, seismicity is dominated by LP/hybrid events, and pauses between eruption phases are dominated by VT seismicity. Overall, there has been a gradual decline in VT seismicity throughout the eruption, with subsequent extrusive phases preceded by weaker and shorter VT swarms.

1030

PART | VII

Volcanic Hazards

FIGURE 59.11 (A) Map of Augustine volcano showing the permanent AVO monitoring network stations, including broadband seismic station AUL and the pressure sensor colocated with a short-period seismic station at AUE (small squares). The active vent is located at the summit. The four summit stations and AUL were destroyed over the course of the eruption. Nearly all the earthquakes were located at shallow depths under the summit in a zone 2-km across. (B) Number of earthquakes per month from 2000 to 2006. From a variable background, the long swarm started 8.5 months before the eruption, and the short swarm started 10 h before the eruption onset. After Jacobs and McNutt, (2010).

4.3. Okmok, 2008 Okmok volcano, Alaska, erupted on July 13, 2008 following a 5-h-long earthquake swarm that intensified approximately 1 h before the eruption onset (Figure 59.9). A new vent was opened at a location where no previous eruptions had occurred for over 800 years. The onset was

accompanied by volcanic tremor that became stronger about 5 min later (Figure 59.9). The eruption quickly formed a 16-km-high ash column (Figure 59.10(B)) and was sensed by several satellites. The initial main phase lasted about 10 h as determined from seismic and infrasound data (Figure 59.10(C)). Continuous tremor was observed on stations up to 160 km away. During the

Chapter | 59

Volcanic Seismicity

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FIGURE 59.12 Seismogram of 24 h of data from station AUE, 3 km E of the vent of the 2006 eruption of Augustine volcano. Each line shows 30 min of data, and time increases from top to bottom. Over 700 earthquakes occurred in the 10 h preceding the eruption. Two eruptions are shown in the central part of the figure at 13:44 and 14:12. Because they are 30 min apart, the traces overlap. Red color indicates signal saturation.

strongest portion, gravity waves with a period of 9 min (540 s) were also observed on a broadband seismometer 14 km east of the vent. Later analyses of InSAR and GPS geodetic data showed deflation at the time of eruption (Figure 59.10(A)) followed by inflation as the magma chamber began to refill. The eruption lasted 5 weeks with variable intensity and seismicity. The plume height varied and there was a rough correlation with the strength of the seismic signal as shown by RSAM (real-time seismic amplitude measurement) data (Figure 59.10(B) and (C)). Given the eruption size and the fact that a new vent was formed, the seismic precursors were remarkably small and short lived.

4.4. Augustine, 2006 Augustine volcano, Alaska (Figure 59.11), began to erupt on January 14, 2006, following an 8.5-month-long earthquake swarm, inflation of the edifice, and increased steaming. A distal cluster of earthquakes was also observed 25 km NE of Augustine at the same time as the later stages of the long-lasting precursory swarm. The local earthquake swarm intensified in the 10 h immediately preceding the eruption with 722 events recorded (Figure 59.12). Of the 722 events, 221 or 31% had VLP

energy as revealed by filtering. This suggests magma injection in a series of pulses during the final ascent of magma before the eruption. A series of strong explosive eruptions from January 14 to 28 produced ash columns from 4 to 14 km (Figure 59.13(A)). Each was accompanied by strong seismic and infrasound signals (Figure 59.13(B)) as well as lightning in the ascending ash plumes. Ash clouds were tracked for many hours in satellite images. Following the explosive phase, dome growth occurred for several months and persistent small ash clouds were produced. Vigorous dome growth was accompanied by repetitive small earthquakes, which were similar to the “drumbeats” observed at Mount St Helens. Numerous rockfalls produced secondary deposits and accompanying clouds; these also produced seismic signals and were recorded on low-light cameras. Two previous eruptions of Augustine, in 1976 and 1986, had similar precursory seismic sequences (8.5 and 9 months, respectively) as well as similar eruptions including both explosive (4e18 days) and dome growth (several months) stages. The close similarities suggest that Augustine has a characteristic eruptive activity. Thus, a useful background investigation for other volcanoes would be to determine the parameters of typical eruptions at candidate volcanoes. If there are no

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Volcanic Hazards

FIGURE 59.13 (A) Histogram of infrasound data from Augustine volcano, showing zero-to-peak pressures of the 13 acoustic signals associated with the explosive eruption phase (January 11e28, 2006). Preliminary plume heights determined by National Weather Service are indicated by dots. Horizontal axis is event number, with dates shown at the top. (B) Examples of traces and cumulative energies of acoustic signals associated with the Augustine January 2006 explosive eruptions recorded at AUE. The signals are high-pass filtered with a corner frequency of 0.1 Hz (a, c, and d) are “impulsive events”; (b) example for an “emergent event.” Note that impulsiveness of individual degassing events is clearly visible as the vertical part in the cumulative energy plots. After Petersen et al. (2006).

previous eruptions at a candidate volcano, then a systematic search for activity at similar volcanoes may be helpful.

5. CONCLUSIONS This chapter has discussed many general features of earthquake activity at volcanoes, and it has been shown that activity varies widely. This is true because the volcanoes and their eruptions vary, and also because the

stresses differ and volcanoes have different geometries as well as combinations of rock, magma, water, and gases within them. Although seismic data are plentiful, the complexities of the volcanic environment often make interpretations ambiguous. Different processes can generate similar seismograms, and similar processes can generate different seismograms, although detailed diagnostics can often distinguish between them. This keeps volcano seismology challenging, but also suggests that seismology alone may be a somewhat limited tool.

Chapter | 59

Volcanic Seismicity

There is a close link between earthquakes and volcanoes. This is true at a large scale in the parallel belts of volcanoes and earthquakes at subduction zones. It is also true at the smaller scale of individual volcanoes, where rates of local seismicity often mimic eruptive activity. Although seismology is often viewed as a separate subject, seismology is an integral component of volcanology. Similarly, volcanology may be viewed simply as a component of the larger study of tectonics. Volcano seismology has helped to answer some fundamental questions: (1) Where is the magma? Tomography and b-value anomalies have identified large (kilometer scale) magmatic or near-magmatic structures, while LF events and volcanic tremor are associated with small-scale movement of magma or water. (2) What is “normal” earthquake activity at volcanoes? The case studies and general features sections showed that activity varies widely, but with a number of systematic trends. Successful separation of source, site, and path effects of seismograms helps to elucidate the causes of specific activity. (3) What will happen next? Understanding of physical processes and study of case histories are the two main tools used to evaluate this question, which is treated more fully elsewhere. It is also possible, and indeed likely, that new high-quality data will lead to fresh insights about the new as well as older examples of earthquake activity at volcanoes.

SEE ALSO THE FOLLOWING ARTICLES Calderas l Hawaiian and Strombolian Eruptions l Lahars l Magma Chambers l Migration of Melt l Plate Tectonics and Volcanism l Seismic and Infrasonic Monitoring l Synthesis of Volcano Monitoring.

FURTHER READING Battaglia, J., Ferrazzini, V., Staudacher, T., Aki, K., Chemine´e, J.-L., 2005. Pre-eruptive migration of earthquakes at the Piton de la Fournaise volcano (Re´union island). Geophys. J. Int. 161, 549e558. Bean, C.J., De Barros, L., Lokmer, I., Me´taxian, J.-P., O’ Brien, G., Murphy, S., 2013. Long-period seismicity in the shallow volcanic edifice formed from slow-rupture earthquakes. Nat. Geosci. http:// dx.doi.org/10.1038/NGEO2027. Benoit, J.P., McNutt, S.R., Barboza, V., 2003. The duration-amplitude distribution of volcanic tremor. J. Geophys. Res. 108 (B3), 2146. http://dx.doi.org/10.1029/2001JB001520. Chouet, B.A., Matoza, R.S., 2013. A multi-decadal view of seismic methods for detecting precursors of magma movement and eruption. J. Volcanol. Geotherm. Res. 252, 108e175. De Angelis, S., McNutt, S.R., Webley, P.W., 2011. Evidence of atmospheric gravity waves during the 2008 eruption of Okmok volcano from seismic and remote sensing observations. Geophys. Res. Lett. 38. http://dx.doi.org/10.1029/2011GL047144.

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Diez, M., La Femina, P.C., Connor, C.B., Strauch, W., Tenorio, V., 2005. Evidence for static stress changes triggering the 1999 eruption of Cerro Negro Volcano, Nicaragua and regional aftershock sequences. Geophys. Res. Lett. 32, L04309. http://dx.doi.org/10.1029/ 2004GL021788. Finlayson, D.M., Gudmundsson, O., Itikarai, I., Nishimura, Y., Shimamura, H., 2003. Rabaul volcano, Papua New Guinea: seismic tomographic imaging of an active caldera. J. Volcanol. Geotherm. Res. 124, 153e171. Gardine, M., West, M., Cox, T., 2011. Dike emplacement near Paricutin volcano, Mexico in 2006. In: Roman, D.C., Moran, S.C., Newhall, C.G., (Eds), Failed eruptions: Late-stage cessation of magma ascent. Bull. Volcanol. 73(2), 123e132. Jacobs, K.M., McNutt, S.R., 2010. Using Seismic b-values to Interpret Seismicity Rates and Physical Processes During the Augustine 2005e2006 Pre-eruptive Earthquake Swarm. U.S.G.S. Prof. Paper 1769. Julian, B.R., Pitt, A.M., Foulger, G.R., 1998. Seismic image of a CO2 reservoir beneath a seismically active volcano. Geophys. J. Intl. 133, F7eF10. Larsen, J., Neal, C., Webley, P., Freymueller, J., Haney, M., McNutt, S., Schneider, D., Prejean, S., Schaefer, J., Wessels, R., 2009. Eruption of Alaska Volcano breaks historic pattern. EOS Trans. Amer. Geophys. Union 90, 173e174. Lees, J., 2007. Seismic tomography of magmatic systems. J. Volcanol. Geotherm. Res. 167, 37e56. Manga, M., Brodsky, E., 2006. Seismic triggering of eruptions in the far field: volcanoes and geysers. Annu. Rev. Earth Planet. Sci 34, 263e291. Moran, S.C., Malone, S.D., Qamar, A.I., Thelen, W., Wright, A.K., Caplan-Auerbach, J., 2008. Seismicity associated with renewed dome-building at Mount St. Helens, 2004e2005. In: Sherrod, D.R., Scott, W.E., Stauffer, P.H. (Eds.), A Volcano Rekindled: The Renewed Eruption of Mount St. Helens, 2004e2006. U.S.G.S. Prof. Paper 1750, pp. 27e60. Murru, M., Montuori, C., Wyss, M., Privitera, E., 1999. The locations of magma chambers at Mt. Etna, Italy, mapped by b-values. Geophys. Res. Lett. 26, 2553e2556. Neuberg, J.W., Tuffen, H., Collier, L., Green, D., Powell, T., Dingwell, D., 2006. The trigger mechanism of low-frequency earthquakes on Montserrat. J. Volcanol. Geotherm. Res. 153, 37e50. Power, J.A., Lalla, D.J., 2010. Seismic observations of Augustine Volcano, 1970e2007. In: Power, J.A., Coombs, M.L., Freymueller, J.T. (Eds.), The 2006 Eruption of Augustine Volcano, Alaska. U.S. Geological Survey Professional Paper 1769, pp. 3e35. Power, J.A., Stihler, S.D., White, R.A., Moran, S.C., 2004. Observations of deep long-period (DLP) seismic events beneath Aleutian arc volcanoes; 1989e2002. J. Volcanol. Geotherm. Res. 138, 243e266. Petersen, T., De Angelis, S., Tytgat, G., McNutt, S.R., 2006. Local infrasound observations of large ash explosions at Augustine Volcano, Alaska, During January 11e28, 2006. Geophys. Res. Lett. 33, L12303. http://dx.doi.org/10.1029/ 2006GL026491. Roman, D.C., Cashman, K.V., 2006. The origin of volcanotectonic earthquake swarms. Geology 34, 457e460. Roman, D.C., De Angelis, S., Latchman, J.L., White, R., 2008. Patterns of volcanotectonic seismicity and stress during the ongoing eruption of the Soufrie`re Hills Volcano, Montserrat (1995e2007). J. Volcanol. Geotherm. Res. 173 (3e4), 230e244.

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Ruppert, N.A., Prejean, S., Hansen, R.A., 2011. Seismic swarm associated with the 2008 eruption of Kasatochi Volcano, Alaska: earthquake locations and source parameters. J. Geophys. Res. 116, B00B07. http://dx.doi.org/10.1029/2010JB007435. West, M., Sanchez, J.J., McNutt, S.R., 2005. Periodically-triggered seismicity at Mt. Wrangell Volcano following the Sumatra-Andaman Islands earthquake. Science 308, 1144e1146.

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White, R.A., Power, J.A., 2001. Distal volcano-tectonic earthquakes (DVT’s): diagnosis and use in eruption forecasting. EOS Trans. AGU 82 (47). Yukutake, Y., Ito, H., Honda, R., Harada, M., Tanada, T., Yoshida, A., 2011. Fluid-induced swarm earthquake sequence revealed by precisely determined hypocenters and focal mechanisms in the 2009 activity at Hakone Volcano, Japan. J. Geophys. Res. 116, B04308. http://dx.doi.org/10.1029/2010JB008036.