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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.

SummaryWe interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr−1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr−1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.

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.

High-precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along-strike variation in Cascadia margin kinematics. These results con- strain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of mag- nitude longer than the decades-long history of seismic moni- toring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station veloci- ties along the coast. After correction for a simple elastic dis- location model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3-7 mm/yr)to northern and central Oregon (-9 mm/yr), con- sistent with oblique convergence and geologic constraints on permanent deformation. The margin-parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, north- westward velocities reflect the influence of Pacific-North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin.

The analysis of 3 years of continuous data from 7 permanent GPS stations along the western Washington section of the Cascadia Subduction Zone indicates that the direction of the observed horizontal velocities (with respect to station DRAO, nominally representing the stable North American continent) is roughly parallel to the relative plate convergence direction of the Juan de Fuca and North America plates and that their magnitude decreases away from the trench. Most of this deformation can be attributed to inter-seismic strain accumulation due to the locking of the thrust interface. When the dislocation model predictions are subtracted from the observed velocities, there is evidence for an additional N-S oriented contraction at a rate of ∼4 mm/yr over a distance of 250 km. This signal presumably represents a more long-term deformation pattern than the periodic accumulation and release of elastic strain connected with subduction earthquakes and is most likely related to the occurrence of shallow earthquakes in western Washington that are characterized by predominantly N-S oriented maximum principal stress.

We use regional broadband seismograms to obtain seismic moment-tensor solutions of the two September 20, 1993, Mw=6, Klamath Falls, Oregon earthquakes, their foreshock and largest aftershocks (My>3.5). Several sub-groups with internally consistent solutions indicate activity on several fault segments and faults. From the estimated moment-tensors and depths of the main shocks and from the aftershock distribution we deduce that both main shocks occurred on an east-dipping normal fault, pos- sibly related to the Lake of the Woods fault system. Rotation of T-axes between the two main shocks is consistent with the two dominant trends of the aftershocks and mapped faults. We pro- pose that a change in fault strike acted as temporary barrier sepa- rating the rupture of the main shocks. Empirical Green's function analysis shows that the first main event had a longer rupture du- ration (half-duration 1.7 s) than the second (1.2 s). In December, vigorous shallow activity commenced near Klamath Lake's west- ern shore, 5-10 km east of the primary aftershock zone. It ap- pears a Mw=5.5 aftershock occurring the day before, though within the primary aftershock zone, triggered the activity.

The Washington Regional Seismograph Network (WRSN) earthquake catalog shows more earthquakes in the day than at night, probably because some local explosions are misidentified as earthquakes. We have implemented a method to help discriminate likely local blasts from local earthquakes. Probable blasts are identified based on time of occurrence (year, month and hour), magnitude, location, and depth. We have defined thirteen areas in Washington and northern Oregon, with enhanced daytime activity probably due to blasts, and have developed specific time, magnitude, and depth criteria for each area to best identify probable blasts. Except in a few regions, most of the smallest, shallowest events occurring during the day in Washington and northern Oregon are probably blasts.

[1] The Mw7.9 Denali, Alaska earthquake of 3 November, 2002, caused minor damage to at least 20 houseboats in Seattle, Washington by initiating water waves in Lake Union. These water waves were likely initiated during the large amplitude seismic surface waves from this earthquake. Maps of spectral amplification recorded during the Denali earthquake on the Pacific Northwest Seismic Network (PNSN) strong-motion instruments show substantially increased shear and surface wave amplitudes coincident with the Seattle basin. Because Lake Union is situated on the Seattle basin, the size of the water waves may have been increased by local amplification of the seismic waves by the basin. Complete hazard assessments require understanding the causes of these water waves during future earthquakes.

Except for the recent rumblings of a few moderate earthquakes and the eruption of Mt. St. Helen's, all has been relatively quiet on the Pacific Northwestern front. The Cascades region in the Pacific Northwest, a sporadically active earthquake and volcanic zone, still has great seismic potential [Atwater, 1987], as comparisons with other subduction zones around the world have shown [Heaton and Kanamori, 1984]. Recent tsunami propagation models [Satake, 1996] and tree ring studies suggest that the last great Cascadia earthquake occurred in the winter of 1700 A.D. and had a magnitude of −8.9. The North Cascades or Wenatchee earthquake followed in 1872. With an estimated magnitude greater than 7, it was the largest earthquake in the written history of Washington and Oregon.

Attenuation in the core is examined by using spectral ratios of seismic core waves. In the outer core a Qα of 5000–10,000 is obtained from the amplitude radio P7KP/P4KP. From the PKIKP/PKP amplitude ratio, Qα in the inner core has a value between 120 and 400. An unusual result is that P7KP is consistently higher in frequency than P4KP, which may indicate frequency-dependent reflection and transmission coefficients at the mantle-core boundary for rays that nearly graze the core.

The intense seismic activity which preceded the 18 May 1980 eruption of Mount St. Helens, Washington, released 2 to 3 × 1018 ergs/day in earthquakes that did not correlate temporally with phreatic eruptions which occurred during the same period. Although the b value and amplitude ratios (long-period/short-period) of the earthquakes vary with time, there are no definitive precursors to the 18 May earthquake and eruption. A Mogi type II frequency-magnitude relation, with critical magnitude Mc = 4.6, constrains the characteristic dimension of the highly stressed region under Mount St. Helens to approximately 3 km, preceding the eruption. A major increase in seismic energy release and a decrease in b value around 1 April 1980 may indicate the first major influx of magma into the upper portion of the volcano.Seismic waves from low-frequency volcanic earthquake have large periods at all epicentral distances. Recordings of volcanic earthquakes from 2 to 4 April 1980 at sites 4 to 9 km from Mount St. Helens show two predominant periods of 0.55 and 1.0 sec. We speculate that seismic signals from the low-frequency volcanic earthquakes have a tectonic origin, but may be modified by pressure oscillations in nearby magma.

Since 1900, more than 290 earthquakes have been reported near Flathead Lake, Montana. Surprisingly, none has exceeded magnitude 5 to 512. Most recent earthquake swarms appear to result from east-west or northwest-southeast extension along short fault segments west and north of the lake. Major normal faults like the Swan and Mission faults east of the lake may pose higher risk, but they appear dormant today. Deformation of sediments in Flathead Lake may be caused by several large earthquakes more than 10,000 years ago but is more probably due to glacial processes accompanying the last retreat of the Cordilleran ice sheet.

Twenty-eight earthquakes in the Baffin Bay region have been relocated using the method of Joint Hypocenter Determination. The revised locations indicate two parallel, linear trends, one along the northeast coast of Baffin Island and the other in the western part of Baffin Bay. The seismicity does not appear to be controlled by glacial rebound but may be a remnant of sea-floor spreading which occurred 40 to 60 m.y. ago. Early P arrivals at near seismograph stations (Δ < 20°) can be explained by a high-velocity (8.5 km/sec) upper mantle in the Baffin region.

Travel times and amplitudes of PKP and PKKP from three earthquakes and four underground nuclear explosions are presented. Observations of reflected core waves at nearly normal angles of incidence provide new constraints on the average velocities in the inner and outer core. Interpretation of these data suggests that several small but significant changes to Bolt's (1962) core velocity model (T2) are necessary. A revised velocity model KOR5 is given together with the derived travel times that are consistent with the 1968 tables for P. Model KOR5 possesses a velocity in the transition zone which is 112 per cent lower than that in model T2. In addition, KOR5 has a velocity jump at the transition zone boundary (r = 1782 km) of 0.013 km/sec and a jump at the inner core boundary (r = 1213 km) of 0.6 km/sec. These values are, respectively, 1/20 and 2/3 of the corresponding model T2 values.

Summary An analysis is given of longitudinal waves which are observed at distances between 105° and 115° some minutes after the arrival of the P waves diffracted by the mantle-core boundary. A number of seismograms which clearly show these wave trains is examined; recordings at WWSS stations from the Chilean shock of 1965 March 28 are specially studied. The observations show (i) that discernible PKiKP waves with wavelengths of order 10 km are reflected from the boundary of the Earth'sinner core back to distances of at least 105° and (ii) that many longitudinal wave onsets (the PdP phase) having travel-times up to 60 s before PKiKP and 90 s before PP arrive near 110° by means of reflection from the lower side of physical discontinuities in the upper mantle of the Earth. The first result is consistent with a relatively sharp increase in P velocity between the transition zone and the inner core at a radius of about 1220 km. The second suggests the existence of a number of discrete shells of different elastic properties in the Earth'supper mantle above 400 km; in particular, one prominent group of PdP waves of order 2 s period is consistent with reflection from a discontinuity near 385 km. This result provides confirmation of the overall high velocity gradient near this depth inferred by L. Johnson; there is an indication, however, of first-order discontinuities in the velocity function assumed to be smooth by Johnson.

THE Earth has an inner core about the size of the Moon. Convincing observations of the seismic waves PKiKP, reflected at steep angles from the inner core boundary (ICB), were announced by Engdahl et al. (annual meeting of the Seismological Society of America), using the Large Aperture Seismic Array (LASA) in Montana (epicentral distances Δ

Before and during the first week of the March-April 1980 eruptions of Mount St. Helens, Washington, infrared thermal surveys were conducted to monitor the thermal activity of the volcano. The purpose was to determine if an increase in thermal activity had taken place since an earlier airborne survey in 1966. Nine months before the eruption there was no evidence of an increase in thermal activity. The survey during the first week of the 1980 eruptions indicated that little or no change in thermal activity had taken place up to 4 April. Temperatures of ejected ash and steam were low and never exceeded 15 degrees C directly above the vent.

A hydraulic mechanism is postulated as the source of certain low-frequency (1-5 Hz) seismic signals that are recorded in the vicinity of temperate glaciers. To illustrate that the mechanism is a plausible seismic source, the equations of motion and continuity for transient flow in a subglacial conduit are developed. The solution to these equations is obtained for the case of the flow of water being terminated in a glacial conduit. Using reasonable values of physical parameters taken from the literature, the displacement of the conduit walls is determined as a function of time and distance along the tunnel. The spectral character of the proposed source and the period of time over which it operates is compared with seismic records of typical type II signals. The agreement in terms of these parameters is good.To further examine the validity of the proposed seismic source a preliminary energy calculation is made which demonstrates that the proposed mechanism is energetically reasonable. Evidence of hydraulically generated seismic waves from a dam site is introduced to support the theory of a hydraulic source.Based on the evidence presented, it is suggested that the flow of water from glaciers is modulated by an internal flow regime that is intermittent in time and space.The substance of this paper is included in a report by the present authors entitled “Hydraulic transients: a seismic source in volcanoes and glaciers”, published in Science, Vol. 203, No. 4381, 1979, p. 654–56.

The 33rd annual meeting of the Pacific Northwest Branch of AGU (PNAGU) was held September 4, 1986, at the University of Washington, Seattle. There were several reports on exploration methods utilizing piezoelectric and magnetometer techniques. Most of the other reports emphasized tectonics, seismology, and paleomagnetics in the Pacific Northwest. New evidence was presented that volcanism in Washington has had a longer history than previously reported. The Washington Department of Natural Resources announced work on four revised geologic maps that cover the state of Washington at a scale of 1:250,000. These maps will be published from 1987 to 1992.

Low-frequency (

Low-frequency icequakes produced by Columbia Glacier, Alaska, are caused by large icebergs which break off the glacier terminus into Prince William Sound. Occasionally, many ice seracs fall in rapid succession, producing complex, low-frequency icequake signals with durations sometimes exceeding 10 min. An empirical relationship V = 4000 T between the volume V of an iceberg in cubic meters and the duration T of the seismic signal in seconds, can be used to estimate the calving rate of Columbia Glacier from seismological data alone. During 1984 and 1985, a period of rapid retreat of the glacier, the calculated number of icequakes per month showed seasonal fluctuations that corresponded to variations in calving rate determined by photogrammetry. It is likely that most previously reported low-frequency icequakes in southern Alaska are produced by calving, tidewater glaciers.

Six AGU members have been elected members of the Regional Committee of the AGU Pacific Northwest Region. Their terms are July 1, 1986, to June 30, 1988. The Regional Committee, which directs the activities of the branch, is composed of a representative of each of the AGU sections taking part in branch activities. Those elected are Robert M. Ellis, Tectonophysics Section; Stephen L. Gillett, Geomagnetism and Paleomagnetism Section; Tark S. Hamilton, Volcanology, Geochemistry, and Petrology Section; Renner B. Hofmann, Seismology Section; Charles W. Slaughter, Hydrology Section; and Richard E. Thomson, Ocean Sciences Section.

The Alvord Valley in southeast Oregon has considerable geothermalpower potential (Groh, 1966; Godwin and others, 1971; White and Williams, 1975; Muffler, 1979). The Mickey Springs system is estimated to have an electrical potential of 160 megawatts for a life of 30 yr, and Alvord Springs and Borax Lake to the south are estimated to have 49 and 91 megawatts, respectively (Muffler, 1979). By comparison, the Geysers in California have a potential of 1,610 megawatts for 30 yr.