Lecture 4: Earthquakes -­‐ Case Example: San Andreas fault Ch. 3: p. 37-­‐38 / Ch. 4: p.72-­‐74, 85-­‐89 “There was a deep rumbling, deep and terrible, and then I could see it actually coming up Washington Street. The whole street was undulating. It was as if the waves of the ocean were coming towards me, billowing as they came.” Jesse Cook, San Francisco police sergeant, 1906 The Great San Francisco earthquake: April 18, 1906. San Francisco. 5 am. A M7.8 earthquake shook 430 km of California for 45-­‐60 seconds from Cape Mendocino to San Juan Bautista. Near the epicenter at San Francisco, unreinforced masonry (brick) buildings collapsed into the streets. Many buildings collapsed due to liquefaction. What the earthquake spared, fire claimed: The fire burned for 3 days and destroyed ~500 city blocks. It claimed >28,000 buildings over 4 square miles. 56% of the population lost their homes. 3,000 fatalities. $7.5 billion in damages. The city was left in ruins. Damage at Stanford University campus: http://quake06.stanford.edu/centennial/gallery/structures.html Other consequences and long-­‐term effects of the 1906 earthquake: • City Hall and the County Courthouse were destroyed, along with all records, property deeds, marriage licenses, birth certificates, and business licenses. • Many insurance companies went bankrupt, but the Bank of America got its start. • Stock markets fell, triggering nationwide financial panic that lasted for months. • There was a major charge to remove corrupt politicians and those who influenced them. Mayor Eugene Schmitz was indicted for bribery. • Rebuilding was so rapid that the 1915 World’s Fair was held in San Francisco with most traces of earthquake damage already gone. Unfortunately, the lack of good planning led to development on dangerous reclaimed land, including whole neighborhoods developed on the dumped rubble of the 1906 earthquake. • These same areas suffered the most collapse and fire damage in the 1989 EQ. • Nonetheless, the EQ led to the revamping of building codes in CA and the development of a reinforced concrete water reservoir to aid in fire control. • Los Angeles became the dominant financial center in CA as people avoided the earthquake-­‐
prone and unsafe San Francisco region (little did they know…) • Study of the San Andreas fault led to the development of the elastic rebound theory California owes its existence to the rendings of plate tectonics: We see its effects in the earthquake history of the state. Earthquakes in California/Nevada in the past week: Most earthquake activity clusters along the San Andreas fault. (http://earthquake.usgs.gov/earthquakes/recenteqscanv/) A continental transform fault: 1200 km long. Accommodates the relative motion between the Pacific and North American plates. The amount of average motion is 3.5 cm/yr or 3.5 m/century. What does the amount of motion have to do with earthquakes? A fault that slips by 3.5 m should produce an earthquake with a magnitude of M______. A 1200 km long fault, if rupturing along its entire length, should produce an earthquake with M_________. Fortunately, the entire fault length does not rupture all at once during earthquakes. 1 The rupture length during the 1906 earthquake was about _________ km. This would be expected to produce a M________ earthquake. About 50 km of rupture length is needed to produce a M7.0 EQ with 3.5 m of motion. Multiple earthquakes are needed to relieve the stress along the entire fault length: Unfortunately, there is no clear pattern for these earthquakes in either space or time. For example, the earthquake history of the San Francisco: • 1830s-­‐1906: M6-­‐7 earthquakes every 10-­‐15 years • 1906: M7.8 San Francisco earthquake • 1911-­‐1979: Three earthquakes with M>6 • 1979-­‐1989: Three M>5.8 earthquakes • 1989: M6.9 Loma Prieta earthquake • 1989-­‐2010: No earthquakes with M>5.5 So how can we be predictive about earthquake behavior along the San Andreas fault when it seems to be unpredictable? Or is it? Different parts of the fault seem to slip differently? Some parts are creeping and some parts are locked. Are the creeping parts of the fault safe from earthquakes? Are the locked parts of the fault safe from earthquakes? The San Andreas is actually a system of interacting faults: Is there a pattern to how these faults move and interact? Differences in motion velocities on different sides of a fault imply the need for earthquakes along the fault. Earthquake history of the San Francisco Bay area: The region is at great risk from the San Andreas, Calaveras, Hayward, and Rodgers Creek faults. There has not been an EQ on the Hayward fault since the “Great San Francisco Earthquake” of 1868. Based on measured slip rates along the faults, and the buildup of strain, any 100-­‐km-­‐long fault segment should produce a M6 every 8 years, or a M7 every 60 years, or a M8 every 700 years. It could produce a M7.5 every 120-­‐
170 years. The time since last M7.8: 104 years. Resultant earthquake probability in the Bay Area (2003): There is a 62% probability of an earthquake with M>6.7 between 2003 and 2032. New numbers released by the USGS in April 2008. Earthquake probability in California (2008): There is a >99% probability of an earthquake with M>6.7 between 2008 and 2038. Why do probabilities change through time? The probability of an earthquake with M>6.7 between 2008 and 2038 is 93% in N. California and 97% in S. California. A M7 EQ along the Hayward fault would likely result in 1000s of fatalities and the loss of 57,000 buildings. Modern analog: Kobe, Japan: M6.8 EQ in 1995 killed 6,000. Are we gearing up for a reign of terror? There WAS a pattern of earthquake behavior prior to the 1906 event. A sequence of earthquake events due to a process called STRESS TRIGGERING (an earthquake triggers the next one, like falling dominoes). Was Loma Prieta the start of another cycle? What about southern California? M6.7: largest recorded EQs in 1971 (Sylmar) and 1994 (Northridge). Both of these earthquakes occurred on thrust faults. 2 th
Northridge Earthquake (January 17 , 1994 4:31 am) – M6.7: Total fatalities: 61. Buildings damaged: 10,000. Cost estimate: $15.2 billion. The fault did not reach the surface so it is a blind thrust. In L.A., blind thrusts prevail. This is BAD! The faults are capable of producing M7.2-­‐7.6 EQs (10s of times more energy than a M6.7). So, has a sufficient amount of built up strain been relieved by earthquakes? The mystery (and horror) of not enough earthquakes: Based on measured plate motion rates, the LA Basin should experience a moderate (M~6.7) earthquake about once every 10 years. Since 1857, there should have been 17 such earthquakes. There have been 2. What about strike-­‐slip fault earthquakes in L.A.? Historical events: Fort Tejon EQ (1857) – M7.9 (Largest EQ in CA history. Only 2 deaths. 1-­‐3 mins of shaking. 9.5 m of slip) Long Beach EQ (1933) – M6.4 What’s going on along the parts of the San Andreas fault that historically produced the biggest earthquakes? What is happening in the southernmost portion of the fault? _________________________ More horror: Based on analyses of the walls of trenches dug across the San Andreas fault north of L.A. (a type of analysis called paleoseismology), there is evidence of liquefaction related to 9 large (M~7.5) EQs from ~550 AD – 1857. They occurred at intervals of 55 to 275 years (average: 160 years). It has been 153 years since the last one. 3