EAS 116 Earthquakes and Volcanoes

Transcription

1 EAS 116 Earthquakes and Volcanoes J. Haase Forecasting Volcanic Eruptions Assessment of Volcanic Hazard Is that volcano active? Mount Lassen: BP and 1915 Santorini, IT: 180,000 BP, 70,000 BP, BP, 3600 BP, 197BC, 1620 AD, 1950 AD Yellowstone: 2 mya, 1.3 mya, 600,000 BC Can t count on historical records 1

2 Geological investigation Long term forecasting Redoubt Volcano Determine ages of previous eruption deposits Date using radioactive decay of minerals in rock C14 dating of charcoal preserved in ash Date using paleomagnetic sequence Determine long term recurrence interval Determine spatial extent of ash fall and pyroclastic flows Monitoring for short term forecasts Seismic recordings Ground deformation monitoring Small phreatic eruptions Gas chemistry monitoring Webcams visual monitoring Less common: Infrared-temperature observations Magnetic field measurements 2

3 Volcanic precursors - Seismic Earthquake locations Magma forces its way through the rock and cracks it This cracking results in an earthquake Locations of the earthquakes tell where the magma is Earthquake seismograms from Mt. St. Helens March 21-23, 1980 Note individual eq signals starts with small (dark) P-wave and then (lighter colored) larger amplitude S-wave 3

4 Mt St. Helens, Mar 24-25, 1982 There are so many earthquakes, one can t tell where one begins and the other ends Volcanic precursors - Seismic When magma moves it increases stress causing rocks to fracture on faults Earthquake locations Show magma moving from mantle to crust Show magma moving into the magma chamber Show magma moving from magma chamber to eruption 4

5 Earthquakes occur in and around the magma conduit system leading from the magma chamber to the lava dome. Volcanic precursors - Seismic Earthquake locations Deepest earthquakes (as much as 60 km deep in Hawaii) are associated with magma moving from the mantle up into the crust Intermediate depth earthquakes (6-10 km) are associated with magma moving into the magma chamber Shallow earthquakes (less than 3 km to the surface) indicate rock fracturing as the magma forces its way to the surface to erupt 5

6 Interior view of Kilauea Tremor indicates magma moving close to surface as it erupts Earthquakes indicate magma moving into magma chamber Number of earthquakes Moderate earthquakes Aftershocks increase in number after a volcanic event In contrast, typically after a tectonic earthquake, the number of aftershocks decreases in number after the event. (Mt St Helens, first M=4.1 earthquake on March 20, 1980) USGS 6

7 Mount St. Helens Volcanic Precursors Number of earthquakes increases right before eruption Number of earthquakes decreases over time Magnitudes of earthquakes are larger => maintain high energy release Seismic waves detect magma chambers P wave velocities are slower in region with melt (remember low velocity zone below lithosphere, and liquid core) S wave velocities cannot travel through liquid Seismic waves that travel through magma chamber arrive later than expected at the station. 7

8 Volcanic Precursors - Seismic Volcanic tremor Volcanic tremor or Harmonic tremor: A rhythmic continuous ground motion recorded by seismographs during or preceding an eruption Associated with the underground movement of magma In contrast with sudden generation of energy during an earthquake followed by rapid decay Volcanic precursors - Seismic Volcanic tremor from Mt. St. Helens 8

9 Monitoring for short term forecasts Seismic recordings Earthquake locations, number of events, volcanic tremor, seismic velocity anomalies Ground deformation monitoring Small phreatic eruptions Gas chemistry monitoring Webcams visual monitoring Less common: Infrared-temperature observations Magnetic field measurements 9

10 Mount St. Helens a few weeks before the 1980 eruption 1 microradian tilt = 1 mile long board raised by placing a nickel under one end Gradual inflation is due to filling of magma chamber Abrupt deflation is due to magma moving out of chamber to vents for eruption 10

11 Short period earthquakes occur just prior to eruption as magma cracks the crust (gray) Long period earthquakes and tremor occur during the eruption as magma moves through crust during the eruption (pink) Deformation (tilt) gives long term forecast Seismicity and deformation give short term forecast Monitoring for short term forecasts Seismic recordings Earthquake locations, number of events, volcanic tremor, seismic velocity anomalies Ground deformation monitoring Small phreatic eruptions Gas chemistry monitoring Webcams visual monitoring Less common: Infrared-temperature observations Magnetic field measurements 11

12 Steam eruptions Phreatic eruptions Magma makes contact with ground or surface water Near instantaneous evaporation to steam Produces an explosion of stream and rock (no magma) Magma must be within a few km of the surface Mt St Helens phreatic eruptions (USGS) from Mar 27-May 18 Monitoring for short term forecasts Seismic recordings Earthquake locations, number of events, volcanic tremor, seismic velocity anomalies Ground deformation monitoring Small phreatic eruptions Gas chemistry monitoring Less common: Infrared-temperature observations Magnetic field measurements 12

13 Volcanic gases Correlation Spectrometer (COSPEC) Increased release of SO 2 sulfur dioxide and HCl There is a fresh magma source It s a high temperature magma It s not just steam from warming ground water Small eruption on Mount St. Helens March 19, 1982 Monitoring of Earthquakes Deformation of dome Tilt of ground surface SO 2 emissions Led to successful prediction 13

14 Debris landslide Initial Eruption of Mount St. Helens Hyndman & Hyndman, Fig

15 First minute of Mt. St. Helens eruption 8:27:00 8:32:53 8:32:47 8:33:03 Effect of Mount St. Helen lateral blast Lateral blasts are pyroclastic flows (high-density mixtures of hot, dry rock fragments and hot gases) that move away from the vent at very high speeds due to the explosive force of the eruption. (Distinguished from normal pyroclastic flows that move downslope due to gravitational force.) Hyndman & Hyndman, Fig

16 Mount St. Helens Eruption May 18, 1980 Eruption cloud Ash cloud Ash Fall Distribution Hyndman & Hyndman Figure

17 Ash Fall Distribution 80 km 500 km 2000 km 1600 km 1500 km Hyndman & Hyndman Figure 6-9 Mount St. Helens, Washington 1980 Eruption Before Eruption Hyndman & Hyndman Figures 6-6, 6-7 After Eruption 17

18 Products of Volcanic Eruptions Eruption volume It is very difficult to predict the size of the volcanic eruption Estimates are based on historic eruptions Estimates are based on size of magma chamber from seismic velocities 18

19 Yellowstone National Park Old Faithful Geiser 19

20 Yellowstone Hotspot Hot spot track leaves trail of calderas where lava flows and low elevations from collapse distinguish it in the topography 20

21 Eruption volume It is very difficult to predict the size of the volcanic eruption Estimates are based on historic eruptions Estimates are based on size of magma chamber from seismic velocities 21

22 Kilauea Caldera 1km A Caldera goes through inflation and deflation cycles: Eruption Collapse after magma is erupted Resurgent doming of surface Lava dome extrusion 22

23 Yellowstone Caldera 60 km 60 km wide Nearly flat summit Gently sloping sides Kilauea Caldera Caldera walls 23

24 Lowenstein et al., 2006 Seismicity indicates where tectonic stresses are high from magma intrusion Seismic velocities indicate size/ location of magma chamber 24

25 Seismic velocity anomalies Gas emissions from Yellowstone Lowenstern et al

26 Currently large emissions of CO 2 are found in the caldera Measurements of ground deformation Uplift Subsidence Dzursin et al,

27 Resurgent domes 2 areas of uplift within caldera GPS measures vertical ground deformation at points INSAR from satellite measures vertical deformation everywhere Since 2004 there has been VERY fast uplift 27

28 4/26/12 A study of the data indicates The giant Yellowstone "supervolcano" is rising upward faster than ever observed. The red arrows pointing up represent uplift of the Yellowstone caldera, or volcanic crater, while the downward red arrows show sinking of the land near Norris Geyser Basin. The black arrows indicate lateral or horizontal ground movement. University of Utah Yellowstone Yellowstone is a caldera It erupts every ~700,000 years The ash from the eruptions covers an area almost as big as the US There is currently seismicity and a very large magma chamber Recent deformation since 2004 is occuring at a rate of 7 cm/year, three times any previously observed rate. 28

29 Review of Volcanic Precursors For a review of precursory activity leading to an eruption, see Mount St. Helens: teachers-corner/library/pre-eruption-0329.shtml Hawaii: Yellowstone (potential eruption): Hyndman & Hyndman p