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Mount St. Helens in Washington State erupted violently (Volcano Explosivity Index = 5) on 18 May 1980. During the previous two months, intense seismic activity at the volcano was recorded by a combination of continuous analog-tape recordings, paper drum recordings, and a recently installed triggered digital event computer system. Because of the technological constraints of the time, the digital data available cover only a little more than 1% of the two-month period. The paper drum records only exist for a few of the seismic stations and are also quite incomplete. However, the analog-tape data from some stations is near complete for almost the whole two months. During the period 2005–2014, these old analog tapes were recovered from storage and digitized to generate standard digital data for archiving at the Incorporated Research Institutions for Seismology Data Management Center. This recovery process was long and complicated but, for the most part, was fairly successful. Although the quality of these recovered data is nowhere near as good as modern digital seismograms, this dataset does provide a near-continuous record of the significant seismic sequence that led up to the major volcanic eruption. It includes the large variety of seismic signals from different types of volcanic earthquakes and harmonic tremor and should be a valuable resource for those studying volcanic seismicity.

Surficial mass movements, such as debris avalanches, rock falls, lahars, pyroclastic flows, and outburst floods, are a dominant hazard at many volcanoes worldwide. Understanding these processes, cataloging their spatio-temporal occurrence, and detecting, tracking, and characterizing these events would advance the science of volcano monitoring and help mitigate hazards. Seismic and acoustic methods show promise for achieving these objectives: many surficial mass movements generate observable seismic and acoustic signals, and many volcanoes are already monitored. Significant progress has been made toward understanding, modeling, and extracting quantitative information from seismic and infrasonic signals generated by surficial mass movements. However, much work remains. In this paper, we review the state of the art of the topic, covering a range of scales and event types from individual rock falls to sector collapses. We consider a full variety of volcanic settings, from submarine to subaerial, shield volcano to stratovolcano. Finally, we discuss future directions toward operational seismo-acoustic monitoring of surficial mass movements at volcanoes.

We evaluate the performance of earthquake early warning algorithm ElarmS‐2 (earthquake alarm system v. 2) in the Pacific Northwest. Real‐time and prerecorded seismic data from Oregon, California, and Washington in the United States and British Columbia in Canada are used. The earthquakes tested range up to moment magnitude 7.2, the limit for which the ElarmS‐2 magnitude method is accurate. ElarmS‐2 reliably detects catalog magnitude 3 and larger earthquakes within the network, but the cut‐off magnitude for accurate event recognition is higher for events offshore and on the edges of the network. We have made several adjustments that make the ElarmS‐2 algorithm less likely to falsely report multiple alerts for earthquakes. Replaying of past earthquakes shows that the new settings improve the algorithm’s behavior for edge cases while not degrading previously well‐constrained solutions within the dense part of the network. We expect few false alerts, none that would predict significant shaking anywhere in our region. Even though ElarmS‐2 assumes a fixed earthquake depth, the epicenter and magnitude estimates are accurate enough to provide good predictions of shaking intensities. Online Material: Tables listing parameters of 31 calibration earthquakes, ElarmS‐2 real‐time detections, and comparison of ElarmS‐2 performance for calibration events before and after adjusting configuration.

We have detected over 150,000 small (M 3000 m) on the glacier-covered edifice and occur primarily in weeklong to monthlong swarms composed of simultaneous distinct families of events. Each family contains up to thousands of earthquakes repeating at regular intervals as often as every few minutes. Mixed polarity first motions, a linear relationship between recurrence interval and event size, and strong correlation between swarm activity and snowfall suggest the source is stick-slip basal sliding of glaciers. The sudden added weight of snow during winter storms triggers a temporary change from smooth aseismic sliding to seismic stick-slip sliding in locations where basal conditions are favorable to frictional instability. Coda wave interferometry shows that source locations migrate over time at glacial speeds, starting out fast and slowing down over time, indicating a sudden increase in sliding velocity triggers the transition to stick-slip sliding. We propose a hypothesis that this increase is caused by the redistribution of basal fluids rather than direct loading because of a 1–2 day lag between snow loading and earthquake activity. This behavior is specific to winter months because it requires the inefficient drainage of a distributed subglacial drainage system. Identification of the source of these frequent signals offers a view of basal glacier processes, discriminates against alarming volcanic noises, documents short-term effects of weather on the cryosphere, and has implications for repeating earthquakes, in general.

Any good seismologist–carryball (i.e., American football) fan remembers the “beast quake” of 2011. Seahawks fans’ exuberant reactions to a Marshawn Lynch (in beast mode) run caused the stadium to shake enough to be well recorded on a strong‐motion seismograph a block away (Vidale, 2011). With the Seahawks at home for the National Football Conference (NFC) Championship playoffs, both last year and this year, the Pacific Northwest Seismic Network (PNSN) received permission to install some seismometers in the stadium. Motivations for the experiment included testing the field and telemetry equipment, examining analysis and display techniques, and practicing quick reactions to mobilize field experiments. With PNSN students and staff who are also Seahawks fans, it was not hard to get volunteers to do the installations and monitor the on‐field actions and the resulting seismograms. Last year, additional beast quakes were recorded, although none were as high on the beast scale as the 2011 original event. Rapid data acquisition and web‐based, real‐time seismogram displays allowed a viewer to anticipate a successful play before it would be seen on live television (which was delayed about 15 s from real time). This was all fun stuff for many, with the PNSN webservers getting a good test in providing high‐traffic service. Public interest in the real‐time seismograms was so great that the …

We observed several swarms of repeating low-frequency (1–5 Hz) seismic events during a 3 week period in May–June 2010, near the summit of Mount Rainier, Washington, USA, that likely were a result of stick–slip motion at the base of alpine glaciers. The dominant set of repeating events (‘multiplets’) featured >4000 individual events and did not exhibit daytime variations in recurrence interval or amplitude. Volcanoes and glaciers around the world are known to produce seismic signals with great variability in both frequency content and size. The low-frequency character and periodic recurrence of the Mount Rainier multiplets mimic long-period seismicity often seen at volcanoes, particularly during periods of unrest. However, their near-surface location, lack of common spectral peaks across the recording network, rapid attenuation of amplitudes with distance, and temporal correlation with weather systems all indicate that ice-related source mechanisms are the most likely explanation. We interpret the low-frequency character of these multiplets to be the result of trapping of seismic energy under glacial ice as it propagates through the highly heterogeneous and attenuating volcanic material. The Mount Rainier multiplet sequences underscore the difficulties in differentiating low-frequency signals due to glacial processes from those caused by volcanic processes on glacier-clad volcanoes.

Mount Baker volcano displayed a short interval of seismically-quiescent thermal unrest in 1975, with high emissions of magmatic gas that slowly waned during the following three decades. The area of snow-free ground in the active crater has not returned to pre-unrest levels, and fumarole gas geochemistry shows a decreasing magmatic signature over that same interval. A relative microgravity survey revealed a substantial gravity increase in the ~30 years since the unrest, while deformation measurements suggest slight deflation of the edifice between 1981–83 and 2006–07. The volcano remains seismically quiet with regard to impulsive volcano-tectonic events, but experiences shallow ( 10 km) long-period earthquakes. Reviewing the observations from the 1975 unrest in combination with geophysical and geochemical data collected in the decades that followed, we infer that elevated gas and thermal emissions at Mount Baker in 1975 resulted from magmatic activity beneath the volcano: either the emplacement of magma at mid-crustal levels, or opening of a conduit to a deep existing source of magmatic volatiles. Decadal-timescale, multi-parameter observations were essential to this assessment of magmatic activity.

The 2001 Nisqually earthquake occurred within the subducting Juan de Fuca plate. Previous seismic and geodetic studies could not confidently identify its actual fault plane from the two nodal planes. In this study, we apply the recently developed source-scanning algorithm to local seismic waveforms and show unambiguously that the steeply east-dipping plane is the rupture plane. The rupture began near the bottom of the subducting crust and propagated downward into the subducting uppermost mantle. If intraslab earthquakes are assumed to be due to dehydration embrittlement, the source dimension is unlikely to grow any larger because the warm thermal state of the subducting Juan de Fuca plate limits dehydration to a shallow depth below the slab surface. Numerical modeling of the thermal structure indicates that dehydration embrittlement can only take place in the top 10 km of the subducting mantle, implying that the maximum size of an intraslab earthquake in northern Cascadia would be M w∼7 or less.

The reawakening of Mount St. Helens after 17 years and 11 months of slumber was heralded by a swarm of shallow (depth 2 earthquakes were occurring at a rate of ~1 per minute. A gradual transition from volcano-tectonic to hybrid and low-frequency events occurred along with this intensification, a characteristic of many precursory swarms at Mount St. Helens before dome-building eruptions in the 1980s. The first explosion occurred October 1, 2004, 8.5 days after the first earthquakes, and was followed by three other explosions over the next four days. Seismicity declined after each explosion and after two energetic noneruptive tremor episodes on October 2 and 3. Following the last explosion of this series, on October 5, seismicity declined significantly. Over the next ten days seismicity was dominated by several event families; by October 16, spacing between events had become so regular that we dubbed the earthquakes "drumbeats." Through the end of 2005 seismicity was dominated by these drumbeats, although occasional larger earthquakes (M d 2.0-3.4) dominated seismic energy release. Over time there were significant variations in drumbeat size, spacing, and spectra that correlated with changes in the style of extrusion at the surface. Changes in drumbeat character did not correspond to variations in magma flux at the conduit, indicating that drumbeat size and spacing may be more a function of the mechanics of extrusion than of the extrusion rate.

The rapid onset of energetic seismicity on September 23, 2004, at Mount St. Helens caused seismologists at the Pacific Northwest Seismic Network and the Cascades Volcano Observatory to quickly improve and develop techniques that summarized and displayed seismic parameters for use by scientists and the general public. Such techniques included webicorders (Web-based helicorder-like displays), graphs showing RSAM (real-time seismic amplitude measurements), RMS (root-mean-square) plots, spectrograms, location maps, automated seismic-event detectors, focal mechanism solutions, automated approximations of earthquake magnitudes, RSAM-based alarms, and time-depth plots for seismic events. Many of these visual-information products were made available publicly as Web pages generated and updated routinely. The graphs and maps included short written text that explained the concepts behind them, which increased their value to the nonseismologic community that was tracking the eruption. Laypeople could read online summaries of the scientific interpretations and, if they chose, review some of the basic data, thereby providing a better understanding of the data used by scientists to make interpretations about ongoing eruptive activity, as well as a better understanding of how scientists worked to monitor the volcano.

The May 18, 1980 Mount St. Helens eruption perturbed the atmosphere and generated atmosphere-to-ground coupled airwaves, which were recorded on at least 35 seismometers operated by the Pacific Northwest Seismograph Network (PNSN). From 102 distinct travel time picks we identify coherent airwaves crossing Washington State primarily to the north and east of the volcano. The travel time curves provide evidence for both stratospheric refractions (at 200 to 300 km from the volcano) as well as probable thermospheric refractions (at 100 to 350 km). The very few first-hand reports of audible volcano sounds within about 80 km of the volcano coincide with a general absence of ground-coupled acoustic arrivals registered within about 100 km and are attributed to upward refraction of sound waves. From the coherent refracted airwave arrivals, we identify at least four distinct sources which we infer to originate 10 s, 114 s, ∼ 180 s and 319 s after the onset of an 8:32:11 PDT landslide. The first of these sources is attributed to resultant depressurization and explosion of the cryptodome. Most of the subsequent arrivals also appear to be coincident with a source located at or near the presumed volcanic conduit, but at least one of the later arrivals suggests an epicenter displaced about 9 km to the northwest of the vent. This dislocation is compatible with the direction of the sector collapse and lateral blast. We speculate that this concussion corresponds to a northern explosion event associated with hot cryptodome entering the Toutle River Valley.

We propose a new way of looking at the sequence of events leading to different styles of silicic, volcanic eruptions. Small-to-medium sized eruptions, either explosive or effusive, are explained by the ascent of isolated magma batches from mid-crustal magma chambers. We separate magma ascent into four different zones: the Supply System, the Intermediate Storage System, the Transport System and the Eruptive System. Of primary importance is the concept that ascent from the Intermediate Storage System through the Transport System to the Eruptive System first requires the development of a fracture network. Initially, this fracture network allows the ascent of individual magma batches by opening and then closing after their passage. An increase in the complexity of the fracture network with time increases the connectivity of the fractures and hence the ease of upward magma movement. In this model, the dynamics of the ensuing eruptions are controlled entirely by the time spent in the Transport System. Large explosive eruptions require a full interconnectivity of the Transport System from the Intermediate Storage System to the Eruptive System. Moreover, we suggest that a fully connected conduit is rare, develops only under particular conditions, and typically generates catastrophic eruptions during formation. Here we examine two case histories that illustrate the interplay of these processes: Mt St. Helens, USA, between 1980 and 2004, and Mt. Pinatubo, Philippines, in 1991.

The 2004-05 eruption of Mount St Helens exhibited sustained, near-equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes. Here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick-slip motion along the margins of the plug as it was forced incrementally upwards by ascending, solidifying, gas-poor magma. We formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma-plug system and that of a variably damped oscillator. Modelled stick-slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption, and they imply that magma pressure never deviated much from the steady equilibrium pressure. We infer that the volcano was probably poised in a near-eruptive equilibrium state long before the onset of the 2004-05 eruption.

In 1997, the Federal Emergency Management Agency (FEMA), National Oceanic and Atmospheric Administration (NOAA), U.S. Geological Survey (USGS), and the five western States of Alaska, California, Hawaii, Oregon, and Washington joined in a partnership called the National Tsunami Hazard Mitigation Program (NTHMP) to enhance the quality and quantity of seismic data provided to the NOAA tsunami warning centers in Alaska and Hawaii. The NTHMP funded a seismic project that now provides the warning centers with real-time seismic data over dedicated communication links and the Internet from regional seismic networks monitoring earthquakes in the five western states, the U.S. National Seismic Network in Colorado, and from domestic and global seismic stations operated by other agencies. The goal of the project is to reduce the time needed to issue a tsunami warning by providing the warning centers with high-dynamic range, broadband waveforms in near real time. An additional goal is to reduce the likelihood of issuing false tsunami warnings by rapidly providing to the warning centers parametric information on earthquakes that could indicate their tsunamigenic potential, such as hypocenters, magnitudes, moment tensors, and shake distribution maps. New or upgraded field instrumentation was installed over a 5-year period at 53 seismic stations in the five western states. Data from these instruments has been integrated into the seismic network utilizing Earthworm software. This network has significantly reduced the time needed to respond to teleseismic and regional earthquakes. Notably, the West Coast/Alaska Tsunami Warning Center responded to the 28 February 2001 Mw 6.8 Nisqually earthquake beneath Olympia, Washington within 2 minutes compared to an average response time of over 10 minutes for the previous 18 years.

Mount Rainier is one of the most seismically active volcanoes in the Cascade Range, with an average of one to two high-frequency volcano-tectonic (or VT) earthquakes occurring directly beneath the summit in a given month. Despite this level of seismicity, little is known about its cause. The VT earthquakes occur at a steady rate in several clusters below the inferred base of the Quaternary volcanic edifice. More than half of 18 focal mechanisms determined for these events are normal, and most stress axes deviate significantly from the regional stress field. We argue that these characteristics are most consistent with earthquakes in response to processes associated with circulation of fluids and magmatic gases within and below the base of the edifice.Circulation of these fluids and gases has weakened rock and reduced effective stress to the point that gravity-induced brittle fracture, due to the weight of the overlying edifice, can occur. Results from seismic tomography and rock, water, and gas geochemistry studies support this interpretation. We combine constraints from these studies into a model for the magmatic system that includes a large volume of hot rock (temperatures greater than the brittle–ductile transition) with small pockets of melt and/or hot fluids at depths of 8–18 km below the summit. We infer that fluids and heat from this volume reach the edifice via a narrow conduit, resulting in fumarolic activity at the summit, hydrothermal alteration of the edifice, and seismicity.

We present results from a local earthquake tomographic imaging experiment in the greater Mount Rainier area. We inverted P wave arrival times from local earthquakes recorded at permanent and temporary Pacific Northwest Seismograph Network seismographs between 1980 and 1996. We used a method similar to that described by Lees and Crosson [1989], modified to incorporate the parameter separation method for decoupling the hypocenter and velocity problems. In the upper 7 km of the resulting model there is good correlation between velocity anomalies and surface geology. Many focal mechanisms within the St. Helens seismic zone have nodal planes parallel to the epicentral trend as well as to a north-south trending low-velocity trough, leading us to speculate that the trough represents a zone of structural weakness in which a moderate (M 6.5–7.0) earthquake could occur. In contrast, the western Rainier seismic zone does not correlate in any simple way with anomaly patterns or focal mechanism fault planes, leading us to infer that it is less likely to experience a moderate earthquake. A ∼10 km-wide low-velocity anomaly occurs 5 to 18 km beneath the summit of Mount Rainier, which we interpret to be a signal of a region composed of hot, fractured rock with possible small amounts of melt or fluid. No systematic velocity pattern is observed in association with the southern Washington Cascades conductor. A midcrustal anomaly parallels the Olympic-Wallowa lineament as well as several other geophysical trends, indicating that it may play an important role in regional tectonics.

[1] Tectonic tremor has been recorded at many subduction zones, including the Nankai, Cascadia, Mexican, and Alaskan subduction zones. This study, the first to use small aperture seismic arrays to track tremor, deployed three small aperture seismic arrays along the Cascadia subduction zone during a tremor and slow slip episode in July 2004. The tremor was active during virtually all (up to 99%) minutes of the analyzed tremor episode using 5 min sample windows. Individual wave phases were tracked across the arrays and used to derive slowness vectors. These were compared with slowness vectors computed from a standard layered Earth model to derive tremor locations. Locations were stable within a volume roughly 250 km2 in epicenter and 20 km in depth for hours to days before moving to a new volume. The migration between volumes was not smooth, and the movement of the sources within the volume followed no specific pattern. Overall migration speeds along the strike of the subduction zone were between 5 and 15 km/d; smaller scale migration speeds between volumes reached speeds up to 2 km/min. Uncertainties in the best locations were 5 km in epicenter and 10 km in depth. For this data set and processing methodology, tremor does not locate predominately on the primary subduction interface. Our favored model for the generation of tectonic tremor signals is that the tremor is triggered by stress and fluid pressure changes caused by slow slip and is composed, at least in part, of low-frequency earthquakes broadly distributed in location.

A close examination of seismic data from the Denali earthquake of November 2002, which triggered seismicity across much of western North America, has revealed non-volcanic tremor triggered by the passage of surface waves across the northern Cascadia subduction zone. Previously fluid flow had been thought to be associated with such tremor, but the new work demonstrates that tremor can be triggered by shear stress at the plate interface. Non-impulsive seismic radiation or ‘tremor’ has long been observed at volcanoes1 and more recently around subduction zones2. Although the number of observations of non-volcanic tremor is steadily increasing, the causative mechanism remains unclear. Some have attributed non-volcanic tremor to the movement of fluids2,3,4,5,6, while its coincidence with geodetically observed slow-slip events at regular intervals7,8 has led others to consider slip on the plate interface as its cause7,8,9,10,11,12,13,14. Low-frequency earthquakes in Japan, which are believed to make up at least part of non-volcanic tremor9, have focal mechanisms10 and locations11 that are consistent with tremor being generated by shear slip on the subduction interface. In Cascadia, however, tremor locations appear to be more distributed in depth than in Japan3,4, making them harder to reconcile with a plate interface shear-slip model. Here we identify bursts of tremor that radiated from the Cascadia subduction zone near Vancouver Island, Canada, during the strongest shaking from the moment magnitude Mw = 7.8, 2002 Denali, Alaska, earthquake. Tremor occurs when the Love wave displacements are to the southwest (the direction of plate convergence of the overriding plate), implying that the Love waves trigger the tremor. We show that these displacements correspond to shear stresses of approximately 40 kPa on the plate interface, which suggests that the effective stress on the plate interface is very low. These observations indicate that tremor and possibly slow slip can be instantaneously induced by shear stress increases on the subduction interface—effectively a frictional failure response to the driving stress.

SUMMARY We estimate the source parameters for a selection of microearthquakes that occurred at Mount St Helens in the period 1995–1998. Excluding the activity of 2004 September, this time period includes the most intense episode of earthquake activity since the last dome-building eruption in 1986 October. 200 seismograms were processed to obtain seismic moments, source radii, stress drops and average fault slip. The source parameters were determined from the spectral analysis of P waves, after correction for attenuation and site effects. In particular, P-wave quality (Qp) and site (S) factors have been previously calculated in the frequency ranges 2–7 Hz and 18–30 Hz. Because it was impossible to perform corrections for Qp and S over the whole spectrum we applied a new approach, based on the notion of ‘holed spectrum’, to estimate spectral parameters. The term ‘holed spectrum’ indicates a spectrum lacking corrected spectral amplitude values at certain frequencies. We carried out a statistical study to verify that dealing with the ‘holed spectrum’ does not lead to significant differences in the estimates of spectral parameters. We also investigated the dependence of spectral parameters (low-frequency level, corner frequency and high-frequency decay) on the bandwidth of spectral hole, and defined the threshold values for three different spectral models. Displacement ‘holed spectra’, corrected by attenuation and site response, are then used to determine spectral parameters in order to calculate seismic source parameters. Seismic moments range from 1017 to 1019 dyne-cm, source dimensions from 100 to 350 m, and average fault slip from 0.003 to 0.1 cm. Self-similarity seems to break down in that stress drops are very low (0.1–1 bars). We postulate that seismicity is associated with a brittle shear failure mechanism occurring in a highly heterogeneous material under a relatively low stress regime.

We present a multifractal analysis of Mount St. Helens seismic activity during 1980–2002. The seismic time distribution is studied in relation to the eruptive activity, mainly marked by the 1980 major explosive eruptions and by the 1980–1986 dome building eruptions. The spectrum of the generalized fractal dimensions, i.e. Dq versus q, extracted from the data, allows us to identify two main earthquake time distribution patterns. The first one exhibits a multifractal clustering correlated to the intense seismic swarms of the dome building activity. The second one is characterized by an almost constant value of Dq ≈ 1, as for a random uniform distribution. The time evolution of Dq (for q = 0.2), calculated on a fixed number of events window and at different depths, shows that the brittle mechanical response of the shallow layers to rapid magma intrusions, during the eruptive periods, is revealed by sharp changes, acting at a short time scale (order of days), and by the lowest values of Dq (≈ 0.3). Conversely, for deeper earthquakes, characterized by intense seismic swarms, Dq do not show obvious changes during the whole analyzed period, suggesting that the earthquakes, related to the deep magma supply system, are characterized by a minor degree of clustering, which is independent of the eruptive activity.

It is customary in these parting words of the outgoing President to reflect on issues of current interest to our science and to SSA. To deviate only slightly from this tradition, I want to explore an issue of interest to me. It is not one of science but rather an issue of the “culture” of the seismologist. By “culture” I mean as used in the work of the social or anthropological sciences. While I have no training or experience in social science, I won't let that stop me from speaking authoritatively, using and misusing their terms to examine the relationships between the various different seismological groups, or camps. For, as in society in general, it's often the relationships between groups of people that determine the level of success and maybe even survival of both groups and individuals. Of course there are many different ways that seismologists can be grouped, both formal ones and informal ones. It's primarily the informal groups that I find the most interesting and to which I will devote most of the rest of this presentation. Such groups may have some alignment with formal organizations. Informal groups could be discipline-based, such as those seismologists working on the inner core, as separate from those working on soil amplification effects. They could be historically based, such as all of the professors and their academic offspring from a particular university as opposed to another. Of course, any individual may be a member of several different classes of groups at the same time. Such categorizing of seismologists into different groups may seem pointless until one thinks about the interactions between groups and particularly how those interactions affect success. Just as different tribes in a primitive culture may coexist peacefully with neighboring, even related, tribes, too often jealousies, insults (real or perceived), …

⇓ ![][1] Dr. Anthony “Tony” Qamar, 62, a research associate professor in the Department of Earth and Space Sciences at the University of Washington, and Dr. Daniel Johnson, a geophysicist at the University of Puget Sound, were fatally injured in an auto accident on October 4, 2005, while traveling to the Olympic Peninsula to retrieve a GPS instrument deployed to study episodic tremor and slip. Both deaths leave a large hole in the seismological community as well as in the lives of their many friends. Dr. Qamar was the Washington State Seismologist and a co-principal investigator for the Pacific Northwest Seismograph Network. Born in 1943 in Redding, California, he was raised in the Berkeley area and traveled extensively with his parents, professional dancers who performed in many major venues in the … [1]: /embed/graphic-1.gif

Following 18 years of relative quiescence, Mount St. Helens volcano (MSH) became restless and began erupting again during September–December 2004. On 23 September, the U.S. Geological Survey's (USGS) David A. Johnston Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismograph Network (PNSN) at the University of Washington detected the onset of a shallow earthquake swarm beneath the 1980–1986 lava dome. The Mount St. Helens Emergency Response Plan defines three alert levels that differ from normal background activity: Level 1, Notice of Volcanic Unrest (unusual activity detected); Level 2, Volcano Advisory (eruption likely but not imminent); and Level 3, Volcano Alert (eruption imminent or in progress).

[1] Four hundred forty-seven well-recorded earthquakes at Mount St. Helens during the late 1990s are relocated with high precision using a combination of a cross-correlation technique and then a procedure to obtain a one-dimensional velocity structure solution together with station corrections. The resulting high-resolution pattern of the spatial-temporal evolution of the earthquake activity, along with focal mechanisms of events provide additional evidence for the presence of a magma reservoir in the depth range 5.5–10 km, connected to the surface by a thin vertical conduit. Moreover, the distribution of the events below 5.5 km yielded a much clearer view of the structures that generate the seismicity at that depth. Some of the deeper events occur along a sequence of faults under the influence of a radial stress field; however, a large number of the events occur on a NNE-SSW striking steeply dipping fault with slip consistent with magma being periodically injected into a truncated dike on the northwest side of this fault.

Mount Rainier, the highest (4392 m) volcano in the Cascade Range, towers over a population of more than 2.5 million in the Seattle-Tacoma metropolitan area, and its drainage system via the Columbia River potentially affects another 500,000 residents of southwestern Washington and northwestern Oregon (Figure 1). Mount Rainier is the most hazardous volcano in the Cascades in terms of its potential for magma-water interaction and sector collapse. Major eruptions, or debris flows even without eruption, pose significant dangers and economic threats to the region. Despite such hazard and risk, Mount Rainier has received little study; such important topics as its petrologic and geochemical character, its proximal eruptive history, its susceptibility to major edifice failure, and its development over time have been barely investigated. This situation may soon change because of Mount Rainier's recent designation as a “Decade Volcano.”

Earthquakes following the May 18, 1980, and June 12, 1980, explosive eruptions of Mount St. Helens were concentrated at depths below 6 km. The hypocenters of these posteruption earthquakes define two seismic lobes separated by an aseismic volume of 2 km width that is located beneath the position of the crater. The timing and location of the posteruption seismicity suggests that the earthquakes occurred in the country rock surrounding a magma body located within the aseismic volume. The withdrawal of magma caused a pressure decrease within the reservoir, and the earthquakes occurred as a brittle response to the resulting stress change. Focal mechanism solutions for posteruption earthquakes distributed within the seismic lobes are calculated using polarity and amplitude data. Results show that a perturbation from the regional stress field occurs between depths of 7 and 11 km. This anomalous stress field is successfully modeled by a decrease in pressure within a cylindrical magma body located between the seismic lobes and subject to conditions representing the regional shear stress regime. In the best fitting model the magma body is centered at 46.204°N, 122.187°W, 600 m to the north of the present dome, and is under a regional shear stress of 20 bars. The radius of the chamber is calculated to be between 0.65 and 0.75 km when using pressure drops within the cylinder of 220 and 150 bars respectively. The magma body extends from 7 to 11 km and has a volume of between 5 and 7 km3 Posteruption earthquakes located below 11 km fall on a northeast striking fault that is preferentially aligned with the regional tectonic stress regime. This fault may be a conduit to transport magma to the shallow reservoir from greater depths within the crust.

The landslide and cataclysmic eruption of Mount St. Helens on May 18, 1980 triggered a sequence of explosive eruptions over the following five months. The volume of explosive products from each of these eruptions decreased uniformly over this period, and the character for each eruption progressed from a fairly continuous eruption lasting more than eight hours on May 18 to a series of short bursts, some of which were spaced 12 hours apart, on October 16–18. The transition in the character of these eruption sequences can be explained by a difference between the magma supply rate and the magma discharge rate from a shallow reservoir. The magma supply rate (MSR) is the rate with which magma is supplied to the level where disruption due to vesiculation occurs. It is determined by dividing the dense-rock-equivalent volume of eruptive products by the total duration of each eruption sequence. The magma discharge rate (MDR) is the rate with which the disrupted magma is discharged through the vent. It is determined by dividing the volume of erupted products by the duration of each explosive burst. The relative magnitude of these two quantities controls the temporal evolution of an explosive event. When MDR∼MSR the explosive phase of the eruption lasts for several hours as a single continuous event. When MDR>MSR, an eruption is characterized by a series of short explosive bursts at intervals of several minutes to several days. The MSR of the eruptions of 1980 decreased with time from 5500 m' s −1 on May 18 to 7 m 3 s −2 on October 16–18 and approximately fits an exponential decay. The MDR for the same events remained approximately constant at 2000 m 3 s −1 . Each explosive event has been followed by an aftershock-like series of earthquakes located beneath the volcano at depth mostly between 7 and 14 km. The seismic energy released during each of these series is proportional to the corresponding volume of erupted magma. Deformation data between June and November, 1980 indicate a subsidence of the volcanic structure which can be modeled by a volume collapse of 0.25 km 3 located at 9 km depth. We propose a model in which magma is supplied from depths of 7–14 km through a narrow conduit during each eruption. It erupts to the surface at a uniform rate during each eruption. The deep seismic activity following each eruption is related to a readjustment and volume decrease in the deep feeding system. The decrease of the MSR over time is explained by an increase in the viscosity of a progressively water-depleted magma. The amount of water necessary to explain the observed decrease of the MSR is of the order of 4.6%.

Low frequency seismic events have been recorded on Mt. Saint Helens, an historically active stratovolcano in the Cascade Range of Washington State. While the character of the events is similar to volcanic seismic events recorded on active volcanoes in many parts of the world, the seismic evidence gathered during an experiment on Saint Helens indicates the glaciers on the mountain as the source of the low frequency activity. The experimental results illustrate the current problem of predicting volcanic behavior from seismic data in such geophysically complex areas as glacier-clad volcanoes.

Several earthquake sequences have occurred at Mount St. Helens as clusters of shallow events with nearly identical waveforms (multiplets). We present a technique that allows the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques. A multiplet is studied in pairs of events called doublets. Each doublet is made up of a reference event and one other event from the multiplet. The cross spectrum of a short window beginning just before the P arrival is used to compute differences in arrival times between seismograms from the same station for each doublet. These time differences, computed with a precision of 1–2 ms, are then used to locate each earthquake relative to the reference event. Engineering explosions with well-known locations are used to test the relocation technique. Explosions can be relocated with accuracy better than 20 m for events within 250 m of the reference explosion. The relocation technique is applied to earthquakes preceding the September 10, 1984, and the May-June 1985 dome-building eruptions of Mount St. Helens. Forty events that occurred during a 12-hour period in September 1984 are located in a volume, approximately 30 m in diameter, beneath the lava dome. The earthquakes following the extrusion do not occur in multiplets and appear to be distributed over a much larger volume. In the May-June 1985 eruption we recognize three multiplet sets that took place successively in time. The multiplets are interpreted as repeated failure or slip within a relatively small volume due to high strain rates around the magma supply conduit.

Small-magnitude earthquakes began beneath Mount St. Helens 40 days before the eruption of 20 March 1982. Unlike earlier preeruption seismicity for this volcano, which had been limited to shallow events (less than 3 kilometers), many of these earthquakes were deep (between 5 and 11 kilometers). The location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 March eruption than prior to any of the earlier dome-building eruptions. The depth-time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir.

The SH impulse response of layered medium resulting from the application of a point source shear dislocation is studied by the generalized ray technique. Numerical seismograms of ground displacement in the range 10–100 km are constructed for a number of different crustal models and source descriptions. The results for shallow events show severe wave form modifications at epicentral distances as small as twice the source depth when models contain soft surface layers. A comparison of synthetic seismograms with observations from a central California earthquake of magnitude 4.6 indicates a moment of 2.1×1022 ergs with a faulting duration of 0.5 s. This moment is noticeably smaller than that estimated from the low frequency level of the whole seismogram spectrum assuming the usual homogeneous half-space model. This occurs because the energy arriving with the surface waves and deep crustal reflections boosts the long period portion of the spectrum, giving the impression of an anomalously large moment when calculated in the conventional manner. The close agreement between synthetic results and the actual data both in the time and in the Fourier transform domain suggests that our deterministic approach can be quite useful in understanding the complexity of seismograms recorded in the local field.

Mount St. Helens lies along the western front of the Cascade Range between Mount Rainier, Washington, and Mount Hood, Oregon. This is an area of transition, both geologically and geophysically. North of Mount Rainier, late Cenozoic volcanism is limited to the major stratovolcanoes, whereas south of Mount Hood the volcanic cover from both stratovolcanoes and monogenetic vents is nearly continuous for 300 km. The dominant stratovolcanoes in the southern Washington Cascade Range are physio-graphically similar to those in the North Cascades, but the volcanic cover is similar to the Oregon Cascades. A saddle occurs in the Bouguer gravity data over the Columbia River, interrupting a west-east gravity gradient observed along the rest of the Oregon Cascade Range. Heat flow values are intermediate, being lower than those observed in Oregon but higher than those observed north of Mount Rainier. Seismicity is mostly concentrated along the St. Helens zone (SHZ), and all crustal earthquakes greater than magnitude 5 since 1960 in Washington and northern Oregon have occurred between Mount Rainier and Mount Hood. Little seismicity is known within the Cascade Range south of Mount Hood, and few earthquakes occur within the North Cascades of Washington. Magnetotelluric studies indicate a conductor within the area bounded by Mount Rainier, Mount Adams, and Mount St. Helens, and the conductor is interpreted as an east dipping, compressed marine terrain in the upper plate. Mount St. Helens is located at a dextral offset in the SHZ where the SHZ intersects a set of older, mapped fractures that are preferentially aligned with the contemporary regional principal stress direction. Aeromagnetic and gravity data suggest the presence of an intrusive body beneath the volcano; petrogenesis studies favor multiple intrusions of small batches of magma into the shallow volcanic system. Studies of pyroclastic flows indicate that turbulence within the head of pyroclastic flows may be important for the generation of ash clouds and in forming basal deposits. Observations of the May 18, 1980, eruption and subsequent erosion of pyroclastic flow deposits has allowed a revision of the chronology of the eruption. A numerical study of the slope failure of May 18 suggests that gravity alone was sufficient to trigger the avalanche that initiated the cataclysmic eruption. Hundreds of similar earthquakes recorded during eruptions in 1984 and 1985 suggest that repeated failure or slip within a small volume occurs around the magma supply conduit because of very high strain rates.

Three microearthquake swarms in the Columbia River basin of eastern Washington have been studied by means of a small portable seismic network. Earthquakes in this area typically occur in swarms, concentrated both temporally and spatially. One unusual characteristic of the three swarms studied was the shallow focal depths of all events. Most events located had depths less than 1 km; none were deeper than 2 km. Composite focal mechanism solutions indicate that more than one fault surface is active in any one swarm. All events had some thrust component with the axis of maximum compression oriented roughly in a north-south direction.

A sudden increase in fumarolic activity at Sherman Crater near the summit of Mount Baker, Washington, occurred in March, 1975. Various estimates of the increase in heat flow range from 3 to 15 times the previous level over the roughly half a square kilometer area of the crater. Temperatures of many of the old fumaroles have increased, some by more than 40°C and numerous new fumaroles have formed. Gravity stations were established on the crater rim in May, 1975, as well as control stations at the base of the mountain, and have been resurveyed periodically since then. A maximum decrease in gravity of 0.55 mgal has been observed at the crater stations relative to base stations. Applying corrections to the gravity data to adjust for seasonal ice and snow loads results in a curve which appears to be reaching an equilibrium value about 0.33 mgal lower than the first gravity reading. Several models are proposed to explain this change by hydrothermal effects in the rock below Sherman Crater. An elevation increase of a little more than a meter is necessary to explain the gravity on a geodetic mechanism alone. Some constraints on this model are the almost total lack of seismicity during this process and the apparent absence of tilt on the flanks of the volcano as reported by the U.S. Geological Survey. A model of mass loss due to fumarole discharge resulting in a local density decrease is also proposed.

Seismic monitoring on three Cascade volcanoes in Washington State, U.S.A., has shown that small seismic events are generated by the active glaciers found on each mountain. Detailed seismic experiments have been conducted to investigate the sources for these icequakes. Considerable evidence indicates that the events are the result of a stick–slip type of motion taking place at the bed of the glaciers. The few events we have been able to locate had depths comparable with the glaciers’ thickness. The similarity of wave form from an explosion at the bottom of a glacier and natural icequakes suggests that the complexity of the seismic wave form is due to the path and not the source. The events exhibit an annual trend with more events being recorded in the summer than during the winter. A ten-fold increase in the number of events preceded a large ice avalanche that involved the entire glacier thickness, suggesting that seismic monitoring may be useful in predicting catastrophic ice movements.

Wire strain meters and seismometers spaced longitudinally along the upper part of Variegated Glacier, Alaska, showed quasi-periodic episodes of increased velocity (mini-surges), which lasted about 1 day and recurred at intervals of a few days to 2 weeks during the early part of the melt seasons of 1979, 1980, and 1981. The zone affected by these mini-surges corresponds to the zone of highest velocity and basal stress increase over the previous decade, and the initiation of the most recent surge in 1982. Mini-surges initiate locally; as a single melt season progresses, the later mini-surges start at higher locations and show a distinct down-glacier propagation of a characteristic strain pattern and associated zone of acoustic emissions at speeds of 0.1–0.6 km h−1. During mini-surges, extensile and compressive strain-rates exceed 10 × 10−4d−1and 40 × 10−4d−1, respectively; typical strain-rates between mini-surges were less than 2 × 10−4d−1in magnitude. Seismic activity jumped by two orders of magnitude and was distinctly audible during a mini-surge. Maximum strain-rate during mini-surges decreased from year to year. The high time resolution of the strain allows short time-scale structure of velocity variations to be deduced. As a propagating mini-surge passes, the velocity anomaly at a fixed location is characterized by a rapid initial rise over a few hours to two distinct peaks separated by a few hours, followed by a slower return to normal velocity taking up to a day. The double peak in velocity may arise from a single, very sharp, transient peak in the basal slip velocity associated with the initial opening of cavities at the base in response to a sudden rise in basal water pressure (observed by Kamb and Engelhardt). This supports an important role for basal cavitation in the mini-surge mechanism.

The distribution of microearthquakes in west-central Nevada correlates well with fault-plane solutions for this area and defines a zigzag series of crustal fractures that vary in length from a few to several tens of kilometers. The main Fairview fault strikes northwest, and motion on this fault is right lateral oblique slip. In other parts of the active zone northeast-striking faults have mainly dip-slip motion. Focal mechanisms are generally consistent with an interpretation of simple block faulting, faults of different orientation having the same slip direction. For the Rainbow Mountain and Fairview Peak areas, crustal blocks to the east of the fracture zone move down and southeast with respect to blocks on the west side. Faulting in this region appears to be related to regional extension, acting in the direction N60°W–S60°E.

The University of Nevada's three-component quartz-rod strain meter installation at Round Mountain, Nevada (38°42.1′N, 117°04.6′W) has recorded a number of underground nuclear explosions at the Nevada Test Site, beginning with the megaton-sized JORUM event September 16 1969. Both that explosion and the larger HANDLEY event on March 26 1970 produced static strain offsets of a few parts in 109 at Round Mountain. These offsets did not decay within the first few hours after the explosions. In both cases, the strain offsets were in the sense of ground extension radial to the shot point, which is inconsistent with the assumption of a pure compressive source of strain. The strain-change ellipse for the HANDLEY event was found to have a major strain axis of 11 × 10−9 extensional, oriented N 34°W, and a minor axis of 7.4 × 10−9 compressional. A single-component strain meter at Mina, Nevada, (38°26.3′N, 118°9.3′W) was operated for the HANDLEY event, and recorded a strain offset of 2.6 × 10−9 in the direction N 74°E. Strain offsets at the time of the largest collapse events following HANDLEY were observed at Round Mountain. These offsets had the same sense on each component as those following the explosion itself. This is interpreted as support for the hypothesis that the strain changes are tectonic in origin, and the explosion initiates the strain release. Small offsets were observed for three smaller explosions out of a total of 13 studied. The relationship between body-wave magnitude mb and maximum dynamic strains at Round Mountain may be described empirically by the equation Log S = − 13.4 + 1.10 mb. Because of its high sensitivity and stability, the Round Mountain strain meter is capable of obtaining useful measurements of dynamic and static strain effects of intermediate- to large-sized explosions, at distances ranging from 160 to 200 km.

This paper presents a focal determination routine developed for use in locating microearthquakes recorded on a small tripartite seismic array. The method uses P-wave arrival times at four stations and the S-P time measured at one station. A solution is obtained for each trio of stations. These solutions can be compared to obtain an indication of the precision of the location. Practical problems such as velocity layering and station corrections, and a number of applications are discussed. The advantage of the method is that it leads directly to improved information about local structure.

Six categories of seismic events are recognized on the seismograms from stations in the vicinity of Mount St. Helens. Two types of high-frequency earthquakes occur near the volcano and under the volcano at depths of more than 4 kilometers. Medium- and low-frequency earthquakes occur at shallow depths (less than 3 kilometers) within the volcano and increase in number and size before eruptions. Temporal changes in the energy release of the low-frequency earthquakes have been used in predicting all the eruptions since October 1980. During and after eruptions, two types of low-frequency emergent surface events occur, including rockfalls and steam or gas bursts from the lava dome.

In March 1975 Mount Baker showed a large increase in thermal emission, which has persisted for more than 1 year. Fumarole ejecta accompanied the thermal activity from March to September, but the ejecta had no constituents that suggest a magmatic source. Estimates of that part of the total heat flux that would account for the observed snow and ice loss show that the heat-flow increase was roughly one order of magnitude, from about 2 megawatts at 10 watts per square meter, averaged over Sherman Crater before 1975, to about 30 megawatts at 180 watts per square meter, during 1975. Almost half of the glacier that occupied the basin of Sherman Crater was melted in 1975. In 1975 an interdisciplinary program of seismic, tilt, gravity, gas, hydrologic, petrologic, thermal infrared, and photographic studies by Federal and university scientists was initiated to evaluate the impact of the current thermal activity and to monitor changes that might indicate an impending eruption. By March 1976 only one small earthquake had been identified beneath Mount Baker. Tilt and gravity changes have been observed but cannot be attributed solely to volcanic causes. The data available thus far provide no evidence of an impending eruption, but theymore » cannot be fully interpreted without many additional geophysical and geochemical measurements, as it is not yet possible to clearly distinguish volcanic effects from nonvolcanic background effects. Inasmuch as current activity continues unchanged--without steam explosions, eruptions, or frequent or large earthquakes--the probability of a suitable trigger for large avalanches and mudflows should decrease and should approach that of a more average year. Such an average year would have a hazard probability at least as great as that which existed before 1975, although that level of hazard was not recognized at the time by the public or by administrative agencies.« less

Fault plane solutions of 459 events occurring between 1995 and 1998 at Mount St. Helens (State of Washington, Northwest U.S.A.) were considered in order to infer the state of stress beneath the volcano. These events occurred in two distinct depth zones. The shallower zone is between 2 and 5.5 km, with shocks clustering in a tight cylindrical distribution about 1 km in radius directly beneath the crater. The deeper events are spread over a larger volume from 5.5 to 10 km depth and surround an aseismic zone below and slightly west of the lava dome. Faulting is characterized by a mixture of strike-slip, reverse and normal faults with maximum compression axes which do not cluster around a single direction. In the deep zone, between 5.5 and 10 km, P axes define a wheel-spoke pattern pointing radially away from the center of the aseismic zone. The 459 fault plane solutions were inverted for stress tensor parameters using the algorithm of Gephart and Forsyth. The inversion of the whole data set revealed that faulting was not produced by a uniform stress distribution. The subdivision of the zone into smaller volumes significantly reduced misfit and confidence areas of the solutions, whereas temporal subdivision of the sample did not lead to significant improvements in terms of stress uniformity. We suggest that the inhomogeneous stress field is consistent with a varying pressure source originating from the inferred crustal magma chamber and a thin conduit extending above it.

Tectonic stress in the Pacific Northwest Washington is dominated by a N-S major compressive axis, σ1, and a minor compressive axis, σ3, which varies from E-W to near vertical. Some variations in this pattern occur in different parts of the region. In this study, we used 550 earthquakes in the central Washington Cascade Mountains to study, in detail, the uniformity of the stress tensor in this volcanic arc. Earthquakes from the Pacific Northwest Seismograph Network (PNSN) catalog were divided into several subsets based on epicentral and depth groupings, and stress-tensor inversions using the Gephart and Forsyth technique were computed for each group. As in previous similar studies, the maximum compressive stress axis, σ1, is nearly horizontal and trending N-S to NNE-SSW in all but one subset. Shallower events directly under Mount Rainier have a near-vertical σ1. For other subsets, the minimum compressive stress axis, σ3, deviates from vertical to horizontal for different groups of events. In particular, events in the depth range of 10 to 14 km in the western Rainier seismic zone (WRSZ) have near-vertical σ3 direction, whereas other depth ranges in this area show a near-horizontal, E-W σ3 orientation. We hypothesize that the change in orientation of σ3 in the 10 to 14 km depth range in the WRSZ may be due to the influence of the nearby Mount Rainier magmatic system. Independent evidence for magma at this depth comes in the form of a few deep long-period (LP) events.