The 1906 San Francisco earthquake as photographed by Arnold Genthe, looking toward the fire on... [+]
The 1906 San Francisco earthquake is widely believed to be the most significant earthquake of all time with regards to destruction, loss of life, and subsequent learnings. On the 110th anniversary of the April 18th, 1906 earthquake, we delve into how our understanding of earthquakes has developed and how this particular earthquake helped further our understanding.
In 1906 San Francisco was a thriving city of 400,000 and an important economic and trading center of the U.S. west coast. On April 18th in the early morning a M7.8 earthquake struck offshore the California coast, violently shaking San Francisco and producing widespread fires throughout the city.
The earthquake, despite its tragic destruction, birthed our modern understanding of earthquakes in the United States. Extensive research in the aftermath of the 1906 earthquake led to the formulation of the elastic-rebound theory related to earthquake source by Reid (1910). With the theory of plate tectonics coming more than 50 years after the earthquake, it's appropriate to say this event helped to motivate and develop a better understanding of how such earthquakes come about.
The 1906 San Francisco Earthquake Triggered Rapid Geologic Advancement
Following the 1906 earthquake, the United States Geological Survey undertook an exhaustive study of the San Andreas Fault system in California. A commission of over 25 geologists, seismologists, and other scientists worked to provide The Report of the State Earthquake Investigation Commission, published in May of 1906, with a subsequent report published by
Fundamental knowledge of how earthquakes behave and effect overlying geology formed our modern understanding of earthquakes. Scientists noted that the intensity of shaking was significantly stronger on soft ground compared to bedrock. Hence, the increased shaking and damage occurring on the soft soils surrounding the San Francisco Bay. In addition, through triangulation surveys, geologists were able to determine that the displacement resulting from the earthquake were largest at the fault and on average decreased with distance from the fault. This was the observation that led to the elastic rebound theory of earthquakes. The earth will stretch similar to a rubber band until it is broken, when the energy will suddenly be released.
Schematic of the elastic rebound theory developed by professor Henry Fielding Reid based on the 1906... [+]
This is the basic mechanism that creates earthquakes from plate movement. Over time plate movement or subduction causes increasing strain on "sticking points" within the crust. Similar to if you rub two pieces of sand paper together, they will initially resist sliding until enough strain is built up. At that point, the "sticking point" is weakened and releases the built up strain. Before this theory was developed, scientists were unsure whether earthquakes caused faulting (plate movement) of faulting caused earthquakes.
Today, over 1000 seismographs measure minute shaking over California, much of which goes unnoticed by human detection. This array of seismographs helped to develop a subsurface model of fault systems and an understanding of which faults potentially hold higher strain. This helps us understand which faults would be more likely to produce strong earthquakes, the relative probability of earthquake occurrence, and expected shaking. Unfortunately, the occurrence of earthquake detection is probabilistic in nature and therefore is limited to long-term predictions. This is similar to the 100-year flood. Scientists determine a 100-year flood based on historical records of flooding, but this doesn't stop two 100-year floods from happening back to back and then none for several hundred years.
Unfortunately, we will never be able to precisely predict the occurrence of a future earthquake. However, with advanced monitoring systems and computing power, we are able to better prepare ourselves for future earthquakes. The loss of life and destruction propagating from the 1906 San Francisco earthquake thankfully led to a united front against understanding earthquakes. An understanding that will continue for generations through the eager minds of geologists.
