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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] 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.

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.

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.