Earthquake Science in the U.S. Before 1906
The 1906 earthquake marked the dawn of modern scientific study of the San Andreas fault system in California. Before 1906, earthquake research in the U.S. had advanced slowly compared to efforts in Japan and Europe. The first seismographs in the U.S. were installed in 1887, at the Lick Observatory (Mount Hamilton, California) and at the University of California at Berkeley. Around the turn of the century, a small number of geology professors at U.S. universities and geologists working for the U.S. Geological Survey contributed some of the earliest observations of earthquake-related features and compiled the earliest lists of historic earthquakes in the U.S. Relatively little was understood at the time about earthquakes, how and where they occurred, or the hazard they would present to our westward-bound, expanding nation. The theory of plate tectonics was still more than a half-century away.
The 1906 Earthquake Spawns a Flurry of Scientific Investigation
When the 1906 earthquake struck on April 18, nearly all scientists in California began to assemble observations of the earthquake and its effects. Professor Andrew C. Lawson, chairman of the geology department at the University of California, Berkeley, took the first steps that led to Governor George C. Pardee's order, three days later, to appoint a State Earthquake Investigation Commission to unify the work of scientific investigations then under way. Professor Lawson headed the Commission. Scientists (including professors of geology and astronomy) from Stanford University, the Lick Observatory, the Chabot Observatory in Oakland, University of California, Johns Hopkins University and the U.S. Geological Survey served on the Commission. Thus, the first integrated, government-commissioned scientific investigation into earthquakes in the U.S. was launched. No State funds were available, however, to support the effort; funding was provided by the Carnegie Institution of Washington.
The Commission's final report (published in 1908, and now commonly referred to as the Lawson report) was an exhaustive compilation of detailed reports from more than twenty contributing scientists on the earthquake's damage, the movement on the San Andreas fault, the seismograph records of the earthquake from around the world, and the underlying geology in northern California. The Lawson report includes numerous photographs of damaged buildings, detailed maps of their locations, and data from surveys of the earth's movement in the earthquake along the San Andreas fault. To this day, the report remains a document of the highest regard among seismologists, geologists and engineers -- a benchmark for future, integrated investigations into the effects of earthquakes in the U.S.
The Investigations Pay Off... A Coherent Picture Emerges
The 1906 earthquake and the Lawson report demonstrated the importance of accurate, widespread and repeated observations of earthquakes, their effects, and the faults on which they occur. The report provided the basis for much of what is now known about earthquakes in California. The detailed surveys described in the Lawson report show that the damage to buildings in the earthquake was strongly related to both the design and construction of the structure and the local geology -- the type of soil or rock on which it was built. Maps included in the report of apparent shaking intensity in San Francisco clearly show that some of the strongest shaking in 1906 occurred in the soft sedimentary soils of China Basin and in the present Marina district -- two San Francisco neighborhoods that would, some 83 years later, again (and for the same reason) be shaken hard and damaged in the 1989 Loma Prieta earthquake. Other maps of the shaking effects state-wide show that the soft soils bordering San Francisco Bay from San Mateo to San Jose and around the Bay also experienced especially strong shaking. Geological observations carried out after the earthquake by individuals who walked virtually the entire length of the earthquake rupture (often through poison oak-covered hills) showed where and how much the fault had slipped. Maps of triangulation surveys carried out during 1906-1907 also documented the earth movement. This information plays a key role in our current models of how much (and when) the San Andreas is expected to slip in the next earthquake.
The New Knowledge Leads to an Important Theory...
Among the most significant observations made after the 1906 earthquake were triangulation surveys -- optical measurements that detect changes in the angles between lines connecting monuments fixed in the ground. By repeating previous triangulation measurements, it was found after 1906 that displacements in the earthquake were largest at the fault and decreased with distance from it, so that a previously straight line crossing the fault had become curved. These observations provided the basis for the landmark "theory of elastic rebound", introduced by Professor H.F. Reid of Johns Hopkins University, who served on Lawson's Commission. Reid's theory extended earlier insights into the earthquake process by G.K. Gilbert of the U.S. Geological Survey, who also served on the Lawson Commission. This theory, which forms the basis for our modern understanding of earthquakes, describes how the earth's crust gradually and elastically distorts with accumulating plate motion until it is suddenly returned to its undistorted state by rapid slip along a fault, releasing the years of accumulated strain and, in the process, generating seismic waves that produce shaking. Before 1906 and Reid's theory, this basic mechanical concept of earthquakes -- that the faulting causes the earthquake and that the earthquake does not cause the faulting -- was not well recognized as a universal principle applicable to the San Andreas fault.
From 1906 to the Present -- More Advanced Earthquake Monitoring Leads to Greater Understanding...and Reduction of...Earthquake Hazards
Since 1906, a vigorous earthquake research program has developed in the U.S. Today, seismographs measure the shaking intensity at over 1000 sites in California. Geodetic instruments and repeated surveys track the plate motion and the related stressing and distortion of the Earth's crust (which causes earthquakes) throughout California. Data from these instrument networks are analyzed with the aid of computers. From these observations, scientists have formed a detailed picture of the location and activity of the hundreds of faults that make up the San Andreas fault system. They have achieved a growing understanding of which faults are likely to produce strong earthquakes, what their long-term probabilities for occurrence are, and how intense the shaking from them is likely to be. From this understanding, maps have been created that anticipate where shaking is likely to be strong, and these maps guide regional zoning and land use decisions affecting the location of schools, hospitals, homes and nuclear power plants. Also from this understanding, engineers have developed building codes designed to produce buildings and bridges that can withstand the shaking. Earthquake research also guides insurance companies in their formulation of earthquake insurance policies that help businesses and individuals deal affordably with earthquake risk.
Significant Advances... and Challenges for the Future
Of course, there are still major questions about earthquakes for which we really have no answers at this time. Probably the most asked question is Exactly when will the next damaging earthquake happen? At present we cannot answer this more precisely than by stating the long-term probabilities for large earthquakes. But when one considers how much we have learned about the other important questions --
- Where is a strong quake likely to happen?
- How probable is it?
- How intense will the shaking be?
- How well will our infrastructure fare?
- Where should we avoid building critical structures?
- How do we design and build earthquake-resistant structures?
-- when one considers these questions and what has been learned since 1906, it is clear that tremendous progress has been made. In this very real sense, society today is better off because of the great foresight of Governor Pardee and wisdom and scientific insight of Professors Lawson, Reid and Gilbert, who, in the wake of the 1906 earthquake launched California into the modern age of earthquake science 90 years ago.