ISNS 4359
EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 9. Historic Earthquakes II: Loma Prieta 1989 & Northridge 1994
Two CA EQ near the end of the last century (neither directly on the SAF) both lacking fault rupture at the surface !!
Loma Prieta, Northern CA: 17 Oct 1989 (World Series EQ)
Fault & Plate Tectonics In a seismic gap along the Sargent Fault, a SW-dipping reverse fault near a restraining bend within
4 km of the San Andreas Right-Lateral Strike-Slip Fault (SAF) (segment with low slip rate of 1.5 cm/yr), the main boundary
separating the NAM & PAC plates. The SAF has allowed offset of ~560 km in the last 10 Ma, and is currently moving at ~4
cm/yr. Slip was oblique-nearly equal amounts of Reverse and Right-Lateral strike slip at 10-20 km depth.
Total length of surface rupture zone Not observed, but expected for this magnitude and depth (Earth misbehaves).
Total length of subsurface rupture zone 40 km (southernmost part of 1906 rupture)
Depth to Hypocenter ~17 km (deeper than most San Andreas EQ
Maximum Offset Horizontal ~2 m; Vertical ~1.3 m
Magnitude & Intensity ML = 6.7; MS = 7.1; MW = 6.9; MMIntensity = IX (VI in bedrock regions).
Time & Duration At 5:04 PM Friday (local); Interrupted Game 3 of the World Series (Oakland Athletics-4 vs. San
Francisco Giants-0); observed by millions of TV viewers. CA was in the midst of a drought and many reservoirs were
fortunately low, averting potential dam disasters. The fault ruptured for-6-8 sec, ground shaking lasted ~10-15 sec.
Felt Area Most of central California and part of western Nevada.
Impact & Other Effects >63 killed (43 in vehicles on the Cypress Viaduct); 3,757 injured; >12,000 homeless; >1,300
buildings destroyed and 28,000 damaged; ~$10 billion damage (0.2% of US GDP), most intense damage in Oakland and SF,
~100 km N of slipped fault segment. Liquefaction and motion amplification (6-10x) < 110 km from the epicenter were major
factors in damage to the Marina District in San Francisco; >1000 landslides in Santa Cruz Mtns; 80 of 1500 bridges damaged
(10 closed; major repair: eg. Double-decker Nimitz Expressway Oakland collapsed due to soft sediments; 50 of the 124 spans
of the I-880 Cypress Viaduct collapsed due to resonance); largest EQ on the San Andreas Fault since San Francisco 1906
(wake-up call). Fewer aftershocks occurred than were expected: 87 EQ M>3 within 3 weeks. W Santa Cruz Mountains were
elevated 1 m. Shear waves were reflected and refracted by the Moho and were focused near Bay area, increasing damage in
the Marina district. The EQ was successfully predicted by many seismologist because of the “seismic gap.”
Recurrence Interval ~130±25 yrs; (USGS2003-62% probability of at least one M6.7 EQ in the SF Bay region before 2031).
Northridge, Southern CA: 17 Jan 1994 (unrecognized blind thrust – Urban – Most costly in US)
Fault & Plate Tectonics Previously unrecognized “blind” (no surface rupture) S-dipping Pico Reverse fault (a reactivated
Miocene Normal Fault caused by a 15 Ma caldera to the S) associated with a restraining bend (forming the Transverse
Ranges located on the Pacific Plate) ~40 km SW of the San Andreas Fault (Right-Lateral Strike-Slip, the main boundary
separating NAM & PAC plates). The 1971 San Fernando EQ (M W = 6.7, 67 deaths, $1 billion damage) was caused by the
nearby N-dipping Santa Susana Reverse fault, and had surface offset.
Total length of surface rupture zone Not observed, but predicted for this magnitude.
Total length of subsurface rupture zone ~16 km along strike, 14 km down dip.
Depth to Hypocenter 18 km
Maximum Vertical Offset 2.5 m
Magnitude & Intensity ML = 6.4, MS = 6.8; MW = 6.7; Max Mercalli Intensity = IX
Time & Duration At 4:31 AM Monday, (Martin Luther King Day); ground shaking lasted ~15-30 sec.
Felt Area Over 200,000 km2; most of southern California, N Mexico, and part of western Nevada.
Impact & Other Effects This was the first major urban EQ in USA since 1933 Long Beach event that killed 115. Of the 12
million who felt it in LA, 57 were killed (most fatalities due to Northridge Meadows Apartment collapse); 9,000 injured;
11,000 houses destroyed (many due to fires); 20,000 people homeless and displaced; 9 highway overpasses crumbled; ~$44
billion damages (0.7% US GDP; $30 billion fed & insured losses)- the most costly EQ in US history. Welds in steel beams of
many buildings were severely damaged. Fungus released from Santa Susana Mountains soil caused an outbreak of valley
fever. Surrounding mountains were elevated 30 cm after several thousand aftershocks. Ground motions were the largest
recorded in any US urban setting (up to 1.7 m/sec) and were locally focused: Cedar Hill Nursery (20 m hill, 6 km S of
epicenter) experienced ~2 g acceleration, whereas Encino Reservoir 2 km away saw <0.3 g. If the event had occurred during
normal working hours, an additional 3,000 deaths were estimated based on building damage.
Recurrence Interval ~200-2000 yrs