Patterns of earthquakes and the effect of solid earth and ocean load tides at Mount St. Helens prior to the May 18, 1980, eruption

Journal Article · Thu May 10 00:00:00 EDT 1984 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JB089iB05p03075· OSTI ID:6753486

McNutt, S R; Beavan, R J

Seismographs near Mount St. Helens Volcano recorded an earthquake swarm lasting nearly 2 months prior to the May 18, 1980, eruption. The earthquakes are divided into four classes based on station CPW (..delta.. = 116 km) seismogram characteristics: (1) events with Sv:P amplitude ratio > 3 and dominant frequency > 3 Hz; (2) events with Sv:P ratio between 1 and 3 and dominant frequency > 2 Hz; (3) events similar to characteristic 2 but with a strong (probably surface wave) phase just after the S phase; and (4) events with frequencies between 1 and 2 Hz lacking a clear S phase. The seismicity pattern for each of the four classes is unique. Solid earth stress and strain tides were calculated at the average hypocentral depth of 4 km. Stress and strain tides induced by ocean loading were also calculated; their amplitudes are typically 20-40% those of the solid earth tides at the location of Mount St. Helens. A weak but significant correlation exists between the latter two classes of events and the tides for a time interval of about 5 days preceding the first onset of volcanic tremor and about 5 days thereafter. The polarity of the correlation is opposite for the two classes of events. In each case, the phase of the correlation changes systematically with time, the changes coinciding with the onset of tremor on March 31 and with a pronounced decrease in earthquake energy release rate on April 3. There are no significant correlations between the tides and the number of events or energy release of these two classes of earthquakes during any other interval between March 20 and May 18, 1980. The first two classes of events show no evidence of significant tidal correlation at any time during the study period. 20 references, 8 figures, 2 tables.

• Research Organization: Columbia Univ., Palisades, NY
• DOE Contract Number: AC02-76ER03134
• OSTI ID: 6753486
• Journal Information: J. Geophys. Res.; (United States), Vol. 89:B5
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
MT ST HELENS
EARTHQUAKES
STRESS ANALYSIS
CLASSIFICATION
CORRELATIONS
EARTH CRUST
PACIFIC OCEAN
SEISMIC P WAVES
SEISMIC S WAVES
SEISMIC SURFACE WAVES
SEISMICITY
STATIC LOADS
STRAINS
STRESSES
TIDE
CASCADE MOUNTAINS
FEDERAL REGION X
MOUNTAINS
NORTH AMERICA
SEAS
SEISMIC EVENTS
SEISMIC WAVES
SURFACE WATERS
USA
WASHINGTON
580201* - Geophysics- Seismology & Tectonics- (1980-1989)