Earthquakes and Volcanism in Western North America

LAST MODIFIED:

POSTED: 10 June 2020

Seismic and Volcanic Monitoring

• Monitoring Earthquakes

Unlike volcanic eruptions, scientists currently cannot forecast earthquakes and no 'tell-tale signs' have led to evacuations. Scientists, however, have carefully recorded seismic events to better understand EQ processes which perhaps eventually may lead to limited forecasting capabilities.
• traditional monitoring:
  • seismometers: record seismic activity; locate events; locations of aftershocks trace active fault; increase in seismicity can potentially be foreshocks to a larger event
  • trenches: paleoseismologists dig trenches across faults that had EQs in the past; by comparing the relative offset between layers and their ages, sizes of past EQs and recurrence times can be estimated
  • reflection seismic imaging to image lower reaches and geometry of EQ faults: emission of small seismic signals (explosions or vibrator trucks) and recording on seismic sensors; reflections off EQs faults show up as waveforms with increased amplitudes
• additional recent tools
• GPS and laser strainmeters: measure the relative movement between two blocks; this has led to surprising findings that plates can creep aseismically (i.e. move without causing EQs)
• satellite-based interferometry (InSAR); measure altimetry with satellite and see how this changes over time; use is very limited to areas with no changes otherwise, e.g. areas with no vegetation (e.g. Southern California deserts)
• computer simulations of physically consistent EQ fault models
• statistical models: trying to understand the repeatability of EQs by hindcasting past EQ catalogs using statistical theories; if hindcasts are consistent with catalogs, try to forecast next EQ; no physics involved
• tsunami warning
  • traditionally with tide gauges and comparison of suspicious signal with tide calendars
  • may use a variety of new sensors after the Dec 26, 2004 Sumatra earthquake (e.g. hydrophone arrays)
  A fully functional tsunami warning system is in place around the Pacific Ocean but not elsewhere. Most earthquake-caused tsunami are generated in the Pacific Ocean. Danger: last EQ that generated large tsunami that crossed the Pacific Ocean was the Good Friday 1964 EQ in Alaska. People may 'forget' what to do when tsunami sirens go off.

• Monitoring Volcanism

• heat flow and surface temperature
• shape of the volcano by geodetic survey (theodolites, GPS, tiltmeters, laser strainmeters, InSAR)
• uplift of caldera floor (in cases there is a caldera)
• seismic activity; tremors (magmatic) and earthquakes (tectonic); changes in source depth
• chemical analysis of ground water (acidity and chemical content)
• fumarole activity (gas emission); chemistry; occurrence
• above/below ground CO₂ emission (basement of houses/tree kill)
• remote sensing and chemical cloud analysis using satellite systems

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Earthquakes in Western North America

• Current Geometry of the North American-Pacific Plate Boundary System

• North of Mendocino Triple Junction: Subduction of the Juan de Fuca plate
• Mendocino Triple Junction to Gulf of California: Transform Boundary (S.A.F.) with Big Bend north of Los Angeles
• Southern Extension of S.A.F. east of Salton Sea unknown (but could lead into Imperial Fault)
• Gulf of California: oceanic spreading center, leading south into a triple junction with another mid-ocean ridge system and a subduction zone (Rivera Microplate)



• History of the San Andreas Fault (SAF) and the North American-Pacific Plate Boundary System

• More than 30 million years ago: ancient Farallon plate subducts beneath the North American plate
• 30 million years ago: parts of the Farallon ridge reaches the continent and subduction there stops
• subduction to the north (Cascadia) continues
• Strike-slip motion in the south causes San Andreas Fault; San Diego on North American plate
• Western North America experiences extension due to shearing, creating Basin and Range
• 17 million years ago: opening of the Gulf of California; San Diego jumps plates and now is on the Pacific Plate, moving north relative to continent




• Features of the North American/Pacific Plate Boundary System

CASCADIA
• subduction on section north of Mendocino Triple Junction (offshore N. California) -> Cascadia Subduction Zone
• until recently thought to have moderate local seismicity
• recently found evidence for very large mega-thrust earthquake in 1700 and related historical tsunami reports in Japan (previously called the orphan tsunami because no corresponding Japanese EQ could be found in the historical records)
THE SAF
• SAF accompanied by several other faults
• faults mostly parallel to SAF, but some are not
• SAF not only fault on which earthquakes occur; e.g. in San Francisco area, Hayward and Calaveras faults also quite active
• structure and seismicity along entire length of SAF complex and not uniform; some sections are creeping and currently have low seismicity, if at all
• Big Bend in SAF north of L.A. causes SAF to go east before continuing south, east of Salton Sea
• many earthquakes in Southern California occur in Mojave desert, east of SAF; and on faults west of SAF



• Where does San Diego come in?

• compared to other southern CA areas, San Diego county has sparse seismicity
• but active faults do exist, even within the City of San Diego
• seismic records (instruments and historic) poor (no records prior to 1800)
• currently, San Jacinto fault most active fault in San Diego county
• recurrence time for mag. 6 EQ along San Jacinto 8-12 years
• last mag. 6 EQ along San Jacinto: 1987!!!!! (we are overdue)
• Rose Canyon fault (RCF) along I-5 thought to be one of San Diego city's most hazardous
• RCF continues off-shore north of La Jolla for at least 25km
• Right-lateral slip has occurred along the RFC within the last 8000 years at an average rate of about 1-2mm/yr
• if slip occurred in EQs (and not aseismically), then RCF might have been fault of two mag 6 EQ (1800/1862)
• epicenter of two intensity VII EQ within city limits in last 30 years
  (1964 San Diego Bay; 1986 S.E. San Diego)
• Oct. 7, 2003, intensity III EQ in San Ysidro largest within city limits since 1986
• many active faults off-shore
• connectivity of off-shore faults uncertain
  (e.g. Does RCF connect north to Inglewood fault, making a very large EQ more likely)
• Jun. 15, 2004, mag. 5.2 EQ along San Clemente fault (100km off-shore)
  caused intensity IV shaking in San Diego;
  closing down Amtrak and Sea World for damage inspection.



• Particular Seismic Hazards in San Diego

• Mission Bay, Lindberg Field, North Island build on unconsolidated sediments (liquefaction)
• vertical movement on off-shore faults can cause tsunami



• A few EQ web sites

  • The US Geological Survey National Earthquake Information Center
  • Recent EQs in California and Nevada at the Southern California Earthquake Center (SCEC)
  • Recent Bay Area EQs at the Southern California Earthquake Center (SCEC)
  • EQ info at the Berkeley Seismological Laboratory
  • Important public input after an EQ: shake maps did you feel it? questionnaires at the USGS

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  Volcanism in Western North America

• Volcano Case Studies

Lectures 8-11 are accompanied by several case studies. The case studies only give examples and the list is by no means complete. These case studies DO NOT imply that a particular feature (e.g. mud flows/lahars) is observed only at a particular volcano or a particular eruption. In fact, most eruptions exhibit a combination of these.



• Yellowstone Hot Spot/Columbia River LIP

The Yellowstone hot spot is thought to be stationary while the American plate moves to the southwest, creating a hot spot track (oldest calderas found at the southwestern end of the hotspot track).
Activity 17 Mio years ago was that of a very hot plume causing rifting in the crust and producing vast amounts of flood basalts (up to 30m thick lava sheets flowed as far as 550km).
Several lava flows now pile up as a ''large igneous province'' in an up to 500m thick and 220,000km² large plateau. Noteworthy: the Deccan Traps that formed 65Mio years ago and are thought to be one of the mechanisms (the other one is an asteroid impact) responsible for the extinction of the dinosaurs at the K-T boundary was much larger.


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The volcanic activity (that continues today in the Yellowstone National Park in form of geyser activity) started a hot spot track across the Snake River Plain. A humongous eruption took place ca. 630,000 years ago. Pyroclasts were carried as far as the Mississippi River. The last volcanic eruption occurred 70,000 years ago.

Currently, the only "volcanic activity" includes geysers, hot springs, mud pools and fumaroles. Yellowstone National Park currently harbors more than 200 active geysers. Some are quite irregular but Old Faithful (which is located in the more active Upper Basin) is known to erupt every 75-95 min, depending on the weather, tides, rainy season and other factors. The mineral-rich Mammoth Hot springs are located near the northern end of the park.


In the middle sits Norris Basin that exhibited increased heat and steam emission at the end of July 2003. Changes noticed near the Back Basin Trail include:
• new mud pots
• changes in geyser activity (eruption intervals)
• a significant increase in ground temperature (which is now at 95^oC; 200^oF)
• dying vegetation
• a heightened rate of steam discharge

The Basin was closed on July 23, 2003 to tourists and was carefully monitored by the Yellowstone Volcano Observatory with seismometers, GPS (Global Positioning System) and temperature loggers. Scientists DID NOT expect a volcanic eruption but did not exclude violent hydrothermal (phreatic) explosions. Parts of the Basin were reopened in October. A year later, scientists noticed increased seismic activity, called a swarm, which has happened repeatedly in the last 50 years. Seismicity has decreased since then. The caldera floor is currently rising at 5 cm/year, which indicates the "normal" uplift and subsidence cycle of an active caldera and is no sign of an imminent eruption.
Visit the USGS Yellowstone Observatory website.
Visit the University of Utah Yellowstone website.

• The Cascade Mtn. Range

The Cascade volcanoes (sometimes referred to as the Cascades) stretch from Northern California through Washington into Canada. They are the surface expressions of large intruding batholiths, just like the long-gone volcanoes related to the batholiths of the Sierra Nevada (intrusions more than 50 Mio years ago). The volcanism is typical continental volcanic arc volcanism along the ocean-continent convergent plate boundary (featuring an ocean-continent subduction zone). The volcanic rocks are mostly intermediate to silicic (andesitic to rhyolitic). This means that volcanism in the Cascades are typically explosive, and can be very violent.
The majority of volcanoes are considered dormant but can erupt again (i.e. they have been active during the past 2000 years). Three of the most hazardous volcanoes, Mt. Rainier, Mt. Shasta and Lassen Peak, are described in the case studies.
Visit the USGS Cascade Volcano Observatory website


• Mount St. Helens

Mt. St. Helens is located in the northern Cascades in Washington and is the most active of the Cascade volcanoes. For decades, Mt. St. Helens showed no activity. The last eruption was 1857. Nevertheless, the explosive eruption on May 18th 1980 did not come unexpectedly. Mt. St. Helens is the most active of the Cascade volcanoes and usually produces rocks with a high silica content (sometimes up to 67%). Activity restarted with a small earthquake on March 20, 1980. The USGS started monitoring the volcano after a small crater opened at the summit emitting gas and pyroclastic debris and seismic activity increased within a week of the earthquakes. Within the next month however, seismic activity decreased, roadblock were removed and there was some dispute as to what precautions should be taken and how to proceed with an evacuation. Then the northern end started to bulge. The growth of the bulge was monitored and well documented with geodetic surveys (theodolites and tilt meters*) and on May 18th Mt. St. Helens erupted. The eruption was more violent than expected because the hillside started to slide and give way to a huge explosion. The eruption killed 57 people . Four of these people were inside the restricted evacuation area: David Johnson, a USGS scientist who was supposed to observe and document the eruption from the north side, Harry Truman (84), who lived in his lodge at Spirit Lake all his life and had special permission to stay and two amateur volcanologists, Bob Kaseweter and Beverly Wetherald who had permission to take readings at Spirit Lake. Some of the other victims were as far as 13 mi from the mountain that were considered safe.
*NB: At the time of the 1980 eruption, GPS (Global Positioning System) that is now used to monitor small displacements, was not yet available (started in mid 1980ies).
Activity at Mt. St. Helens has significantly increased in late September 2004 that included the growth of a new dome. Some scientists likened many signs of an imminent eruption to those found prior to the 1980 eruption. Check out the newsclips for 2004 updates. In subsequent years, the dome growth slowed and stopped completely in January 2008, and the volcano has so far been quiet. Check it out at USGS.


• Mammoth and the Long Valley Caldera

The Long Valley caldera formed after a cataclysmic eruption about 760,000 years ago that was not quite as large as the eruption at Yellowstone, 160,000 years later. No classic hotspot seems to be associated with its activity. Seismic activity along the Sierra Nevada and White Mountain fault systems and volcanic activity in the Long Valley Caldera area started about 4 Mio years ago. Volcanic activity near the Mono Lake area to the north started only about 400,000 years ago. There were eruptions in the Long Valley 600 years ago and in the Mono Lake area just 150-250 years ago. On May 25-26, 1980, shortly after the Mt. St. Helens eruption, the Long Valley area experienced numerous earthquakes, incl. four magnitude 6 events. The resort town of Mammoth Lakes reported damage to structures. On May 27, 1980 the U.S. Geological Survey issued a "Earthquake Hazard Watch". In October 1980, a survey showed that the caldera floor was resurging by more than 25cm (10in). By early 1982, some magma found its way from 10km depth to only 3km. New fumarole and seismic activity prompted the U.S. Geological Survey on May 25, 1982 to issue a "Notice of Potential Volcanic Hazard" (the lowest level of alert). Real estate prices fell 40% over night and tourism dropped dramatically but there was no serious volcanic activity. This incident was highly publicized and geologists were accused of misleading the public.
In the early 1990ies, trees started to die on Mammoth Mountain, due to increased emission of subsurface CO₂. Seismicity picked up and the ground surface rose. To this date, there has not been an eruption. The 1982 events are considered "false alarm" but volcanologist continue to have a close eye on the area.
In the 1990s, seismicity picked up again near the south rim of the caldera but this time geologists did not issue a warning. Subsequent analyses of the seismic data have revealed that magma migrated upwards and came very close to the surface. Some scientists take this as indication that an eruption might have been imminent, and call their findings 'a close call'. But lessons learnt in the 1982 debacle made them more cautious not to issue warnings prematurely.
Visit the USGS Long Valley Caldera Observatory website

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1. true
2. false

1. measure surface temperature
2. geodetic survey of volcano's shape
3. monitoring radioactive decay
4. remote sensing using satellites
5. monitor seismic activity

1. strike-slip
2. divergent
3. transform
4. convergent

1. North American
2. Pacific
3. Mendocino
4. Juan de Fuca

1. thrust
2. convergent
3. transform
4. divergent