Stephen R. McNutt

Variations in the frequency‐magnitude distribution with depth in two volcanic areas: Mount St. Helens, Washington, and Mt. Spurr, Alaska

The frequency-magnitude distribution of earthquakes, characterized using the b-value, is examined as a function of space beneath Mount St. Helens (1988–1996), and Mt. Spurr (1991–1995). At Mount St. Helens, two volumes of anomalously high b (b > 1.3) can be observed at depths of 2.6–3.6 km below the crater floor and below 6.4 km. These anomalies coincide with (1) the depth of vesiculation of ascending magma, and (2) the suggested location of a magma chamber at Mount St. Helens. Study of Mt. Spurr reveals an area of high b-value (b ≥ 1.3) at a depth of about 2.3–4.5 km below the crater floor of the active vent Crater Peak. We propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b-value at shallow depths. Alternatively, interaction of magma with groundwater may have increased pore pressure and lowered the effective stress. The deeper anomaly at Mount St. Helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber. Our results indicate that detailed mapping of the frequency-magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers.

New constraints on source processes of volcanic tremor at Arenal Volcano, Costa Rica, using broadband seismic data

Broadband seismic data recorded 2.3 km from the active vent of Arenal Volcano, Costa Rica, provide new constraints on tremor source processes. Arenals tremor contains as many as seven harmonics, whose frequencies vary temporally by up to 75 percent, from initial values of 1.9 Hz for the first peak immediately following explosive eruptions to 3.2-3.5 Hz several minutes later. We infer that gas bubble concentration is variable within the conduit and also changes as a function of time, thereby changing the acoustic velocity. We infer that the source is a shallow, 200- 660 m-long, vertically oriented 1-D resonator with matched boundary conditions, radiating seismic energy from a dis- placement antinode. Polarization analyses show that particle motion azimuths abruptly rotate, which may be explained by a decrease of the incidence angle. We suggest that energy is radiated predominantly from a displacement antinode that is changing position with time. Tremor consists mainly of transverse waves that travel at speeds of about 800 m/s. P waves in the magma conduit will couple very efficiently into S waves in the surrounding medium when there is virtually no impedance contrast between the two media for these two types of waves. The tremor at Arenal is similar to tremor at nine other volcanoes.

Uturuncu volcano, Bolivia: Volcanic unrest due to mid-crustal magma intrusion

Uturuncu volcano, SW Bolivia, is a dormant stratovolcano (∼85 km3) dominated by dacitic lava domes and flows. 39Ar/40Ar ages show that the volcano was active between 890 ka and 271 ka, with the lavas becoming younger and less extensive at higher elevations. There are current signs of unrest. Between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm/yr. Deformation indicates volume changes of 400 × 108 m3 over 14 years, an average of ∼1 m3/s (10−2 km3/yr). The deformation is attributed to magma intrusion into the Altiplano-Puna regional crustal magma body. Deformation models indicate a source at depths of 17 to 30 km beneath current local relief. In a reconnaissance survey, persistent seismic activity (mean of 2.6 earthquakes per hour with a maximum of 14 per hour) was recorded at about 4 km depth below the center of the uplift, 4 km SW of the volcanos summit. The seismic events have a normal b value (∼1.04) and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion. The porphyritic dacite lavas (64−68% SiO2) have a plagioclase-orthopyroxene-biotite-magnetite-ilmenite assemblage and commonly contain juvenile silicic andesite inclusions, cognate norite nodules and crustal xenoliths. Temperature estimates are in the range 805 to 872°C for the dacites and about 980°C for the silicic andesites. The dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust. Compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite. Reversely zoned orthopyroxene phenocrysts in the andesitic end-member are explained by changing oxidation states during crystallization. Fe3+/Fe2+ ratios from orthopyroxene crystals and Fe3+ in plagioclase provide evidence for a relatively reduced melt that subsequently ascended, degassed and became more oxidized as a consequence of degassing. The geophysical and petrological observations suggest that dacite magma is being intruded into the Altiplano-Puna regional crustal magma body at 17 km or more depth, consistent with deformation models. In the Late Pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together. The current unrest, together with geophysical anomalies and 270 ka of dormancy, indicate that the magmatic system is in a prolonged period of intrusion. Such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation.

Aseismic inflation of Westdahl volcano, Alaska, revealed by satellite radar interferometry

Westdahl volcano, located at the west end of Unimak Island in the central Aleutian volcanic arc, Alaska, is a broad shield that produced moderate-sized eruptions in 1964, 1978–79, and 1991–92. Satellite radar interferometry detected about 17 cm of volcano-wide inflation from September 1993 to October 1998. Multiple independent interferograms reveal that the deformation rate has not been steady; more inflation occurred from 1993 to 1995 than from 1995 to 1998. Numerical modeling indicates that a source located about 9 km beneath the center of the volcano inflated by about 0.05 km³ from 1993 to 1998. On the basis of the timing and volume of recent eruptions at Westdahl and the fact that it has been inflating for more than 5 years, the next eruption can be expected within the next several years.

Theory of the airborne sound field generated in a resonant magma conduit

Abstract Explosive sources triggered inside a magma conduit may excite the conduit into acoustic resonance. The acoustic field in the conduit can propagate into the atmosphere through an open vent and ensonify the overlying atmosphere. The character of the airborne sound field is determined by a combination of propagation and source effects: the resonance of the magmatic conduit and the diffraction of the sound field at the volcanic vent are acoustic propagation effects, whereas the explosion pressure signature and the firing rate of the explosions define the source characteristics. For wavelengths larger than the conduit radius, only the longitudinal resonances of the magmatic conduit are relevant, and the open vent radiates like a piston surrounded by an infinite baffle. In this case, the fluid particle velocities are directed along the axis of the conduit and the sound field may propagate into the surrounding bedrock through the conduit wall displacement induced by the fluid overpressure. This coupling may produce seismic signals with banded spectra, such as volcanic tremor and long-period events. The airborne pressure field retains the modal structure of the sound field in the magmatic conduit, which contains information on the conduit geometry and geo-acoustic properties of the magma. The seismic wavefield is driven by the acoustic field in the magma and also contains this information, although it may be filtered by propagation effects in the bedrock. The theoretical sound field in the magma conduit is used to interpret seismic tremor signals recorded at Mt. Spurr Volcano, Alaska. Rapid variations in the acoustic impedance of the magma conduit terminations can create systematic changes in the tremor spectra, which can be used to monitor changes in the magmatic system. The results of the modelling illustrate the difference between source effects and conduit resonance, as well as the value of seismoacoustic measurements in volcanic environments.

Eruption of Alaska volcano breaks historic pattern

In the late morning of 12 July 2008, the Alaska Volcano Observatory (AVO) received an unexpected call from the U.S. Coast Guard, reporting an explosive volcanic eruption in the central Aleutians in the vicinity of Okmok volcano, a relatively young (∼2000-year-old) caldera. The Coast Guard had received an emergency call requesting assistance from a family living at a cattle ranch on the flanks of the volcano, who reported loud “thunder,” lightning, and noontime darkness due to ashfall. AVO staff immediately confirmed the report by observing a strong eruption signal recorded on the Okmok seismic network and the presence of a large dark ash cloud above Okmok in satellite imagery. Within 5 minutes of the call, AVO declared the volcano at aviation code red, signifying that a highly explosive, ash-rich eruption was under way.

Seismicity at the volcanoes of Katmai National Park, Alaska; July 1995–December 1997

Abstract Upper-crustal seismicity located within Katmai National Park occurs mainly within four distinct clusters originating near Martin–Mageik (MM subnet) volcanoes, Trident volcano, Katmai caldera and Snowy volcano. Analyses of earthquake frequency–magnitude distributions reveal high b -values beneath MM subnet (∼1.5) normal b -values at Trident volcano (∼1.0) and intermediate b -values at Katmai caldera (∼1.3) for all seismicity occurring between July 1995 and December 1997. Detailed analyses of subsets of b -values and hypocenter locations at MM subnet reveal a temporal increase in b -value associated with an increase in the maximum depth of seismicity. The changes occurred during a shallow earthquake swarm beneath Mageik volcano between October 16 and 25, 1996 and again in November–December 1997. Before the swarm, the weighted least squares b -value was 1.01 at MM subnet, increased to 1.59 during the swarm and remained anomalously high until April 1997. The corresponding maximum depth of seismicity is generally less than 5 km for well located earthquakes occurring after September 18, 1996, but shifted to ∼10 km during both the October 1996 swarm and the November–December 1997 period. The November–December 1997 event is not associated with an increase in the rate of seismicity or the b -value. The October 1996 swarm had a cumulative moment release of 5.0×10 20 dyn-cm, and decayed from a peak rate of 110 events per day with a modified Omori law p -value of 1.06±0.11. Modelling by the flow law with a 10 km depth limit for seismicity suggests that temperature gradients are on the order of 20–40°C km −1 in agreement with the p -value estimate. We infer that a simple pressurizing intrusion was not associated with the October swarm because higher stresses should increase seismicity and lower the b -value, the opposite of what we observed. Alternatively, an actively degassing intrusion would reduce the effective stress and increase the b -value at shallow depths while the increased stress would induce seismicity at depth. Surface temperature measurements taken at the Mageik crater lake in July 1995, and again in July 1997, revealed an increase of about 10°C in the lake water temperature, consistent with a degassing intrusive event.

Spatial Variations in the Frequency-Magnitude Distribution of Earthquakes at Mount Pinatubo Volcano

The frequency-magnitude distribution of earthquakes measured by the b -value is mapped in two and three dimensions at Mount Pinatubo, Philippines, to a depth of 14 km below the summit. We analyzed 1406 well-located earthquakes with magnitudes M D ≥0.73, recorded from late June through August 1991, using the maximum likelihood method. We found that b -values are higher than normal ( b = 1.0) and range between b = 1.0 and b = 1.8. The computed b -values are lower in the areas adjacent to and west-southwest of the vent, whereas two prominent regions of anomalously high b -values ( b ∼ 1.7) are resolved, one located 2 km northeast of the vent between 0 and 4 km depth and a second located 5 km southeast of the vent below 8 km depth. The statistical differences between selected regions of low and high b -values are established at the 99% confidence level. The high b -value anomalies are spatially well correlated with low-velocity anomalies derived from earlier P -wave travel-time tomography studies. Our dataset was not suitable for analyzing changes in b -values as a function of time. We infer that the high b -value anomalies around Mount Pinatubo are regions of increased crack density, and/or high pore pressure, related to the presence of nearby magma bodies. Manuscript received 16 December 2002.

25 - Volcano Seismology and Monitoring for Eruptions

Seismology is considered one of the most useful tools for eruption forecasting and monitoring. Volcanoes are the sources of a great variety of seismic signals that behave differently than those from events on earthquake faults. Every recorded volcanic eruption is preceded by an increase in earthquake activity beneath or near the volcano and accompanied and followed by varying levels of seismicity. This chapter reviews some of the developments in volcano seismology that have led to an improved understanding of volcanoes and the volcanic processes that cause earthquakes and other seismicity. It also describes patterns and relationships in volcano seismology that form the physical basis of contemporary monitoring and forecasting. The key developments in volcano seismology are often driven by different types of seismic events that volcanoes produce. Some of the significant characteristics explored by volcano seismology include: (1) tomography and b value anomalies, which have identified large magmatic or near-magmatic structures, while low frequency events and volcanic tremor are associated with small scale movement of magma or water, (2) normal earthquake activity at volcanoes varies widely with a number of systematic trends, and (3) understanding and modeling of physical processes and study of many case histories helps to know about the next happening.

Application of wave‐theoretical seismoacoustic models to the interpretation of explosion and eruption tremor signals radiated by Pavlof volcano, Alaska

Tremor and explosion signals recorded on September 29 during the Fall 1996 Pavlof eruption are interpreted using video images, field observations, and seismic data. Waveform analysis of tremor and explosions provided estimates of the melts volcano-acoustic parameters and the magma conduit dimensions. Initial mass fractions of 0.25% water and 0.025% carbon dioxide in the melt can explain the resonance characteristics of the tremor and explosion pulses inferred from seismic data. The magma conduit is modeled as a three-section rectangular crack. We infer that the tremor-radiating region consists of the lowermost two sections, both with cross-sectional areas of ∼10 m2. The deeper section is 43 m long, with magma sound speed of 230 m/s, density of 2600 kg/m3, and viscosity of 1.0×106 Pa s. The section above it, defined by the water nucleation depth, is 64 m long, with magma sound speed of 91 m/s, density of 2000 kg/m3, and viscosity of 1.4×l06 Pa s. An average magma flow velocity of 1.2 m/s, with superposed random oscillations, acts as the tremor source. Explosions are postulated to occur in the uppermost part of the magma conduit after water comes out of solution. The explosion source region consists of a 15 m long section, with cross-sectional area of 20 m2, sound speed of 51 m/s, density of 1000 kg/m3, and viscosity of 1.5×103 Pa s. A burst pressure of 220 MPa at 14 m depth would generate an acoustic pulse whose amplitude and character match the observed signal. Waveform analysis of the explosion pulses shows that the explosive event may be preceded by a long-period fluid transient which may trigger the metastable magma-gas mixture. The modeling procedure illustrates the synergy of fluid dynamic, seismic, and acoustic models and data with geological and visual observations.