ISNS 4359
EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 8. Historic Earthquakes I: San Francisco 1906, Alaska 1964, Lisbon 1755
Three distinct margins of the North American and Eurasian Plates
1. San Francisco, CA: 18 April 1906 [“Earthquake City”]
Fault & Plate Tectonics San Andreas Fault (NW-trending, 1200 km long, Right-Lateral Strike-Slip Fault Zone, main
boundary system separating N American & Pacific plates). The SAF is ~29 million years old (Ma), has moved ~560 km
since 10 Ma, and is currently moving at ~4 cm/yr. The SAF was discovered by Andrew Lawson in 1893, following the
1857 EQ (M8.3), and is the most studied fault in the world.
Total length of rupture zone ~430 km Depth to Hypocenter ~12 km
Maximum Horizontal Offset 6 m (average 4 m)
Time & Duration At 5:13 AM, a foreshock was followed by the great earthquake ~ 20 to 25 seconds later; 2 violent
shocks with strong shaking for 45-90 seconds; then 10 minutes of gentle rolling.
Felt Area From southern Oregon to south of Los Angeles and inland as far as central Nevada.
Magnitude & Intensity MS = 8.3; MW = 7.8-8.0 (debated); Max Mercalli Intensity = XI
Impact & Other Effects ~3,000 killed (mostly by fire); 225,000 homeless (population 450,000); 28,000 buildings
destroyed; ~$400 million damage (>$50 billion today), including a 3 day fire; ~153 aftershocks recorded in 14 months; No
tsunami (<10 cm ) due to sub-sea strike slip offset (little local change in sea level).
Recurrence Interval ~200 yrs; >7 million presently live in the bay area (70% chance M6.7 in next 25 years).
2. Prince William Sound, AK, (Good Friday) 27 March 1964
Fault & Plate Tectonics Patton Bay Reverse Fault (NE-trending Subduction Zone Megathrust due to NW subduction of
Pacific Plate beneath N American Plate).
Total length of rupture zone ~600-900 km (fault area=500 km x 300 km) Depth to Hypocenter >6 km
Maximum Vertical Offset 6-12 m (raised shorelines) Horizontal Offset ~20 m [average offset-7 m]
Time & Duration At 5:36 PM; gentle rolling motion for < 1 min, followed by hard shaking for 3-4 min.
Felt Area Over ~1,300,000 km2 nearly all of AK & parts of NW Canada.
Magnitude & Intensity MS = 8.4; MW = 9.2 (Largest ever recorded in N America); MMI=X-XI.
Impact & Other Effects Up to 2.3 m coastal subsidence of an area 800 x 150 km (drowned forests); 124,000 km 2
Land and seafloor dropped and 155,000 km2 rose. Pacific tsunami damaged AK (<30 m high wave), W USA & Canada coast,
reached Antarctica (1-2 m), in ~1 day. Sound waves were heard in San Diego. Lower Atmosphere wave propagated at 0.3
km/sec and the ionosphere rippled 90 km above sea level, both due to the Tsunami. ~131 people killed (all but 9 from tsunami),
~$500 million damages: Over 2000 Landslides & Avalanches, Rock Avalanches, Ground Fissures, Consolidation Subsidence,
Cable Breaks, Railroad Tracks & Bridges, Tsunami. Over 70 homes were destroyed in the Turnagain Heights neighborhood.
Over 12,000 aftershocks were detected (19 with M>6) during several subsequent months.
Recurrence Interval ~600-950 years based on previous Aleutian events; ~350 years based on release of strain at current plate
tectonic rates of 5.7 cm/yr
3. Lisbon, Portugal: 1755 Probably the greatest seismic event recorded in western Europe.
Fault & Plate Tectonics On the south flank of the NE-trending Gorringe Ridge, hanging wall block of a 40ºSE dipping
oblique reverse fault, an uplifted section of oceanic lithosphere along the Azores-Gibralter Fault in the Gulf of Cadiz, the main
boundary between the Eurasian & African Plates.
Total length and width of rupture zone ~80 km wide x ~200 km long
Maximum Horizontal Offset ~12 m
Time & Duration ~9:30 AM, All Saint’s Day, 1 Nov 1755; shaking lasted ~10 min [three 2-4 min shocks separated by 1-2
min., possibly indicating multiple events.]
Felt Area Over 800,000 km2 area; much of Europe.
What Magnitude & Intensity Mw = 8.5; Max Mercalli Intensity=X-XI
Impact & Other Effects Terror & panic shocked western civilization more than any event since the 5th Century
fall of the Roman Empire. >70,000 deaths in Lisbon (pop. 250,000) alone; 5 day fire; 3 great waves poured over the
banks of the Tagus River; 15-20 m high tsunami affected the Atlantic Iberian Peninsula & NW Africa (2PM
England, 6PM W Indies); severe damage through liquefaction, landslides and ground shaking, the extent highly
dependent on near-surface geology; all of Lisbon’s 40 churches were damaged and 35 were destroyed; only 10 out
of the 75 convents remained and 33 palaces, the Arsenal, the Royal Library and the Patriarchal Palace were
destroyed. Only 3000 of ~20,000 dwellings housing 38,000 families could be used after the earthquake; 10% loss in
Portugal’s wealth in Lisbon alone.
Recurrence Interval ~600±100 yrs