Few such detailed and precise monitoring efforts had been attempted on active stratovolcanoes or composite volcanoes before 1980 (a notable exception is the study of Usu volcano, Japan [Yokoyama and others, 1981]), when the reawakening of Mount St. Helens provided an ideal opportunity to test the utility of such measurements. In late April 1980, distance and angle measurements were initiated from instrument sites on the apron of the volcano to targets on its flanks. This monitoring documented a remarkable bulging of the north flank as magma intruded the volcano before the large landslide and explosion of May 18, 1980 (Lipman and others, 1981). The need to monitor all sides of a volcano was indicated by the localized development of the bulge, which covered 3-4 square kilometers, extended nearly 2 kilometers downslope from the summit area, and was mostly confined to a 60 degree radial sector of the cone. Points on the bulge moved tens of meters northward, whereas points just off the bulge and elsewhere on the one were nearly immobile. After the May 18 events, geodetic monitoring of the volcano's flanks suggested slight horizontal expansion before other explosions in 1980 and slight contraction afterward (Swanson and others, 1981). Since 1980, distance measurements inside the crater of Mount St. Helens have been used to predict dome-building extrusions of dacite a few days to 3 weeks in advance (Swanson and others, 1983; Chadwick and others, 1983). The unprecedented success of horizontal strain monitoring at Mount St. Helens suggests that this technique can be used for surveillance of other composite volcanoes, such as those of the Cascade Range.
Slope distances were measured with an EDM (Hewlett-Packard 3808A) and vertical angles with a theodolite (Wild T-2, both old and new styles) from benchmarks generally at low elevations around each volcano to reflectors at benchmarks high on the cones. Vertical angles were measured primarily to establish station elevations and to make mark-to-mark distance reductions; they were not measured reciprocally (table 2). Temperature, pressure, and humidity corrections were applied to EDM data. Temperature was measured with a thermistor or thermometer generally about 2.5 m above ground at each benchmark. Pressure was measured with a pressure transducer at each reflector site and a transducer backed up by a precision aneroid barometer at the instrument site. In 1983 and 1984, humidity was read at the instrument site with a sling psychrometer; no end-point humidity measurements were made in 1981 and 1982, when a nominal correction of -0.5 ppm was assumed.
Measured distances several kilometers long commonly cross 1 km or more of elevation on the steep volcanoes; consequently atmospheric properties change significantly along the line path. Determining the atmospheric refractivity using end-point measurements of temperature, pressure, and humidity provides some correction for the EDM measurements, but temperature and humidity do not necessarily change linearly along such steep lines with large elevation differences.
We attempted to improve the precision of the slope-distance measurements by using a helicopter to take semi-continuous temperature and.humidity readings along most line paths while distance measurements were being made. (This procedure was not followed at Crater Lake, where the lines are nearly horizontal and the noise of a helicopter would be particularly disturbing to tourists.) Temperatures and humidities were determined with a thermistor and hygrister mounted on the front of the left helicopter skid and recorded on a data logger inside the aircraft. The helicopter generally flew downslope at a nearly constant air speed and was guided by radio from the reflector end in order to stay within a few meters of the line path. Most lines were flown at an air speed of about 45 mph, but some were flown as slowly as 25 mph and others as rapidlv as 60 mph.
Programs for CVO's VAX 11/750 computer (Endo and others, 1985), adapted from similar programs used in the U. S. Geological Survey's Tectonophysics Branch in Menlo Park, California, use the flight-line data to calculate an average refractive index for the entire line and correct the measured slope distances accordingly. Savage and Prescott (1973) and Bornford (1980) describe in detail the measurement techniques and related procedures. Slope distances for 1982-1984 were calculated using flightline atmospheric data (table 4). Most distance measurements in 1981 were accompanied by flightline readings, but equipment malfunctions led to so many obviously incorrect temperature measurements that we decided to discard all of the 1981 flightline data.
The manufacturer's stated precision for the EDM is +(5 mm+l ppm) in the temperature range of interest. If two measurements of the same line differ bv twice this value or less, the difference cannot be considered significant. This is an instrumental precision only, however, and does not take into account inaccuracies in measuring the atmospheric index of refraction. An overall precision of +3 ppm, including errors resulting from end-point temperature, pressure, and humidity measurements, was found for relatively flat lines measured with the same instrument model at Long Valley, California (R. P. Denlinger, personal commun., 1984). By assuming no strain in our networks, we calculate an overall precision of +3.9 ppm from the differences in 142 end-point line lengths given in tabl" 3 (exclusive of the long lines at Crater Lake) (fig. 2). This value may be larger than that at Long Valley owing to the differences in terrain, to some strain or benchmark instability in our networks, or to problems in our method of temperature measurement. In addition, the figure was calculated using some data from Mount Shasta and Lassen Peak in 1984 that we believe are of poorer quality than normal owing to windy conditions. Lacking objective evidence of this, however, we use the figure of +3.9 ppm as a guide for evaluating apparent changes.
A significant improvement in precision was apparently not obtained by flying the lines. We had expected a precision of perhaps about +2 ppm (a't Long Valley it is +1.5 ppm), but instead we find a precision of about + 3.4 ppm for the 72 measurements in table 4 under the assumption of no train (fig. 3). This result is addressed in the discussion section.
Surveys at Mount Baker (table 3A) show no evidence of deformation between 1981 and 1983 (fig. 4). All repeat measurements agree within twice the assumed error of one end-point measurement (3.9 ppm), consistent with the lack of seismicity at the volcano during the same time. Measurement conditions were excellent during both surveys: light winds, clear air, and moderate day and night temperatures. Such conditions probably contribute much toward the relatively high quality of the surveys. In 1983, distances calculated using flightline data are longer (mean=+3.1 ppm, s.d.=l.l) than those using end-point data (tables 3A, 4A, and 5A). This difference could be explained by an average end-point temperature about 3oc lower than that obtained by the aircraft. Pressure and humidity have relatively little effect on the calculations, and all other variables--instrument height, uncorrected slope distance readings, station elevations, etc.--are the same for both sets of calculations.
At Mount Rainier, most repeat measurements agree within twice the expected error of a single measurement between 1982 and 1983 (tables 3B and 4B). Some lines could not be measured in 1983 owing to poor weather. As at Mount Baker, distances calculated from flightline data are generally longer than those calculated from end-point data, in 1982 by a mean of 2.3 ppm (s.d.=l.5) and in 1983 by a mean of 2.0 ppm (s.d.--l.8) (table 5B). These comparisons suggest that the average end-point temperatures were about 2oC lower than the flightline temperatures. Conditions were poor during measurements of several lines. Particularly strong, gusty winds 30 knots) badly vibrated both the EDM and reflector during measurement of line 2 in both years. Past experience at Mount St. Helens with a Rangemaster 3 has shown that such windy and gusty conditions shake the instrument and reflector out of plumb an often accompany air instability, both factors adversely affecting measurements. The windy conditions may account for the large apparent change in length of line 2, which is much above the expected error and was excluded from the precision analysis for that reason (this is the only measurement excluded from any statistical treatment in this report). Other shots involving the end points (McClure Tilt and Camp Hazard) of line 2 are within expected error, so that both benchmarks are probably stable. In addition, the relative elevation of McClure Tilt mark was determined in both years by precise levelling and showed no undue change (Daniel Dzurisin, unpublished data). The apparent length change on line 29 (Iron Mountain to St. Andrews Rock) is beyond that of expected error for the end-point calculation and barely within" expected error for the flightline calculation. Local site stability of St. Andrews Rock cannot be checked because it is not sighted from another station. The apparent change in flightline distance for line 8 is above expected error, but the end-point calculation is acceptable; the reason for this discrepancy is unknown. A longer history of measurements will be necessary before such changes can be evaluated adequately. The inconsistent changes on adjacent lines (fig. 5) argues against but cannot exclude the possibility of deformation of the cone.
At Mount Hood, several apparent changes between 1980 and 1983 are relatively large (table 3C; fig. 6), but little stock can be placed in them owing to the lack of adequate temperature and pressure equipment and. the use of a different EDM (Rangemaster 3) in 1980, when a limited number of PK masonry, nails were installed (the nails were left in place when benchmarks were emplaced in 1983). The apparent changes are small by comparison with those expected should the volcano begin to swell in rsponse to magma intrusion at depth.
Apparent changes in line length at Mount Hood from 1983 to 1984 are within expected error for flightline calculations and, except for line 20, also for end-point calculations (tables 3D and 4C; fig. 7). Flightline data yield longer distances than end-point data; in 1983, the mean difference is 3.6 ppm (s.d.=l.5), and in 1984, 2.8 ppm (s.d.=0.9) (table 5C). This is consistent with end-point temperatures being about 3.5oc and 3o C lower respectively than flightline temperatures. Three of the four distances measured from Cathedral in 1984 (lines 17, 18, and 20) are longer by 4.5, 6.6, and 10.7 ppm than in 1983, but the fourth (line 19) is nearly the same length. Flightline data for these lines are not available owing to equipment malfunction. Perhaps the Cathedral benchmark is unstable or the setup was not properly centered, although neither of these possibilities by itself can account for the small change on line 19. Random error is possibly the best explanation for the apparent changes.
At Crater Lake, repeat measurements are well within expected precision limits (table 3E). The presence of the lake beneath virtually the entire length of each line may help stabilize air density. Moreover, the lines are nearly flat (table 2D) and are high enough above lake surface to be unaffected by scintillation and enhanced humidity. End-point measurements should suffice across the caldera under most conditions.
Most line lengths at Mount Shasta were similar in 1981 and 1982 (table 3F; fig. 8)). Most of the apparent changes were small extensions, generally within expected error. However, lines 12 and 13 show apparent changes greater than expected; no reason is evident. The apparent 1981-1982 extensions were cancelled or changed to contractions by measurements in 1984 (fig. 9), and the net apparent change on most lines between 1981 and 1984 is contraction (tables 3F and 4D). The small 1981-1982 extensions and somewhat larger 1982-1984 contractions probably reflect slightly different atmospheric conditions. In 1982, the comparison between calculated line lengths using end-point and flightline data (table 5D) is closer flightline data slightly longer, mean=0.9 ppm, s.d.=l.5) than in 1984 at Mount Shasta mean=3.1 ppm, s.d.=l.3) and at other volcanoes in 1982-1984 (Mount Baker, Mount Rainier, Mount Hood, and Lassen Peak). This suggests that end-point temperatures were closer to average air temperatures in 1982 than normal. In 1984, conditions at Mount Shasta were very windy, and the overall quality of the survey was probably less than in other years. This may account for the large apparent changes on lines 9, 12, and 20 end-point calculation only), although downslope movement of Shastina (line 9) and Wishbone (line 12) is also possible. On balance, the data probably should be considered as reflecting measurement errors rather than real strains, although we cannot eliminate the possibility of slight areal contraction around the mountain between 1981 and 1984.
The pattern of apparent changes at Lassen Peak is very similar to that at Mount Shasta. Generally small extensions within expected error were recorded between 1981 and 1982 (fig. 10) and larger contractions, shown both by end-point and flightline data, accrued between 1982 and 1984 (fig. II), with a net overall contraction between 1981 and 1984 (tables 3G and 4E). Calculated distances based on flightline data (table 4E) are slightly longer than those based on end-point data in 1982 (mean=l.3 ppm, s.d.=0.9) and significantly longer in 1984 (mean=3.2, s.d.=2.1) (table 5E), a pattern similar to that at Mount Shasta. The 1984 survey was made during strong winds, just as at Mount Shasta, and the quality of the data is probably less than in previous years and likely accounts for large ap- parent changes on many of the lines beyond those expected from our estimates of overall precision. The 1982 weather at Lassen was favorable except for one day of strong winds ending with a thunderstorm. Mornings were cooler than during other surveys at Lassen, however, and this apparently caused the great disagreement 7 ppm) between flightline and end-point calculations for line I0, which was measured early in the morning when the ground was cold and the average end-point temperature was 9.4oC cooler than in 1982. We conclude that there is no strong evidence that deformation is occurring at Lassen Peak and that the spread in the line lengths probably reflects measurement error. We cannot, however, rule out the possibility of slight areal contraction between 1982 and 1984.
We have no good reason but can suggest several possibilities. Our temperature-measuring setup on the slid of the helicopter may need improvement, and we are investigating this now. To date, however, we have found nothing that leads us to believe that the thermistor is inferior. Perhaps the air speed of the helicopter is too slow to enable proper corrections for frictional effects of airspeed to be made to the raw thermister readings. Perhaps rotor wash affects the readings in a way unaccounted for. Possibly inherent but unrecognized errors exist when using a helicopter on short, generally steep lines that are absent when using an airplane on long, relatively flat lines. We will investigate and test means to improve the quality of the flightline data, but if they cannot be improved, we will probably revert to making only end-point measurements of temperature and humidity.
The end-point results differ from flightline results in a predictable manner, as described above for each volcano. Combining all 1980 differences between flightline and end-point results yields a mean of 2.4 ppm (s.d=1.7), with flightline calculations consistently longer than end-point calculations (fig. 12). This suggests that end-point temperatures are systematically about 2.4 degrees C +/- 1.7 cooler than the effective flightline temperatures, regardless of time of day. This was surprising to us, for we had thought that the end-point temperatures would generally be warmer than the air temperatures or at least cooler in only early morning hours. If the flightline temperatures were incorrect, this conclusion is of course invalid.
The distance measurements, although of less than desired quality, indicate no significant deformation of any of the monitored volcanoes. We cannot rule out the possibility of slight areal contraction at Mount Shasta and Lassen Peak between 1982 and 1984 but prefer an alternative interpretation that the data from 1984 are adversely affected by the windy conditions during the survey.
The data presented in this report should define adequate baselines for detecting changes of a few centimeters but no less. We will attempt to improve the quality of these baselines, but already they are far superior to those existing at Mount St. Helens before 1980. We now have a way to recognize and interpret the early stage of deformation at the monitored volcanoes that may be precursory to future eruptions, although we must realize that during a typical winter and spring snow cover will make reoccupation of most of each network impractical.
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