April 2025 Meeting of the Chapter

The San Andreas fault is one of the longest and probably the most famous and

well-studied transform fault in the world, but geologists knew surprisingly little

about it before the 1906 San Francisco earthquake. The seismic importance of the

San Andreas fault was only recognized in the aftermath of the devastating 1906

earthquake. The first part of this paper describes the severe damage caused by

the 1906 earthquake and, as is now the case with most major earthquakes,

lessons learned in its aftermath. Immediately after the earthquake, a commission

of prominent geologists was appointed to survey the damage and study the fault

that caused it. Not surprisingly, a detailed survey of the city revealed that buildings

on unconsolidated fill were the most damaged—both from shaking and

subsequent fires. Geologists were amazed when they found that the San Andreas

fault (SAF), which had been mapped only recently as a minor splay, had slip and

ground rupture that extended hundreds of kilometers to the north and south of

San Francisco. Another important, but much less obvious, observation was that

the location of points as measured by geodetic surveys some thirty years before

the earthquake differed systematically with their location measured shortly after

the earthquake. The Earth’s crust out to a distance of about 10 km from the SAF

had been gradually bent during the intervening 30 years. One of the commission’s

members, the geophysicist H. F . Reid, realized that a huge amount of elastic

strain energy must have been stored in the bend. He proposed that the cause of

the earthquake was the sudden release of this energy when the bent rocks on

both sides of the fault suddenly rebounded during the fault slip. “Elastic

rebound” is still considered the energy source of most earthquakes.

The second part of this history traces the decades-long, heated debate about

whether the SAF, or any strike slip fault, could have more than a few kilometers

of offset. What happens at the fault’s ends? At each end, mountains should be

thrust up on one side of the fault and a gigantic hole should be left on the other!

About eighty years after the 1906 San Francisco earthquake, the fortuitous

collaboration between land-based geophysicists who were measuring the

geomagnetic reversal time scale and oceanographers who were measuring

strange magnetic anomaly patterns over the oceans led to the discovery that

earth’s outer 100 km “shell” was broken into plates that can move relative to one

another. In the late nineteen sixties, the Mid-Atlantic Ridge and East Pacific Rise

were easily proven to be seafloor spreading centers. The deep sea trenches

were also quickly shown to be subduction zones. But the oceanographers also

found that crustal slabs slide past one another along a new kind of fault, called a

“transform” fault. AND, if the SAF is a transform fault, there is no geometric

problem with it having hundreds of kilometers of strike-slip displacement! In fact,

Tanya Atwater used the marine magnetic anomalies off the west coast of North

America to show when and how the San Andreas fault was born as a transform

plate boundary between the North American and Pacific plates. Marine magnetic

anomalies at the mouth of the Gulf of California enabled oceanographers to both

date when Baja California rifted away from mainland Mexico (about 5 Ma), and

the rate at which Baja and Alta California have moved north (about 50 mm/yr),

creating the “modern” or present-day San Andreas fault zone (SAFZ).

Because of the important role these marine magnetic anomalies had in

discovering plate tectonic and how the SAFZ formed, we will take a few

minutes to discuss their origin.

 

Finally, I would like to show you how two very different parts of modern

technology, satellites and directional drilling, can help us better understand

aspects of the SAFZ that Reid and his colleagues would have thought

impossible in 1906. Global Positioning System (GPS) instruments can measure

to within a millimeter per year the rate at which the slivers of crust between the

multiple strands of the southern SAFZ are moving. Secondly, directional drilling

enabled geologists to obtain cores from the seismogenic zone of the very

unusual section of the SAF that lies between the south end of the rupture of the

1906 San Francisco and the north end of the rupture of the great 1857 Ft. Tejon

earthquakes. Called the “creeping zone”, the plate movement in this 180

kilometer-long section consists only of aseismic slip that is accompanied by

many small, and occasional moderate earthquakes. The fault gauge was found

to consist of a special clay having a friction coefficient as low as 0.1!

I’m a fourth generation San Diegan and was born in Mercy Hospital. After high

school, I attended Villanova University, in the suburbs of Philadelphia, where I

majored in philosophy and minored in astronomy. Upon returning to San Diego,

I enrolled at San Diego State as an astronomy major, but was lured into

geology by a charismatic geology professor, the friendly students, and drinking

beer around a campfire in the desert! I received my second bachelor’s degree

in geology and geophysics in 1966. I went to Stanford for my PhD in geology

and geophysics. My thesis was measuring the magnetic properties of dredged

seafloor basalt, and showed that they are the main source of the linear marine

magnetic anomalies. I am probably one of the few people still alive who were

sitting in the large auditorium at the 1969 annual AGU convention in San

Francisco, and watched how oceanographers used the linear marine magnetic

anomalies to prove conclusively that seafloor spreading/plate tectonics was

real! We all looked at one another and knew that we had just experienced a

revolution in earth science! :>) After graduating in 1971, I continued my

paleomagnetic research at the USGS paleomag lab in Menlo Park. But, I really

wanted to teach and so, after taking a much needed break (six, adventure-filled

months of biking around western Europe), I joined the SDSU geology department in 1975.

My main courses were geophysics, structural and petroleum geology, and

paleomagnetism and plate tectonics. Along with my students, we conducted

paleomagnetic studies in southern California and gravity studies of the faults in

metropolitan San Diego. I had three sabbatical years—doing research and

teaching at universities in France, Russia, and the Czech Republic. I retired in

2005. My academic life has continued by giving geology talks, teaching the S.D.

Natural History Museum’s new docents and hiking guides, and writing articles

for SDAG’s field trip guidebooks. Tonight’s talk will be the one I prepared for

SDAG’s field trip last Fall to see the San Andreas fault as it passed over the

San Gabriel’s at Wrightwood—until our camp was almost turned to ashes!