The Loma Prieta earthquake of 1989 & Electromagnetic Precursors

On October 17, 1989 at 5:04 P.M., a magnitude 6.9 earthquake struck the San Francisco Bay area and lasted for 15 to 20 seconds. The earthquake happened along the San Andreas Fault, with the epicenter happening near the Loma Prieta peak within the Santa Cruz Mountains. This is the reason for the earthquake’s name. The fault slipping started 18 kilometers deep and the western side moved1.9 meters north and 1.3 meters upward, a result of the fault being right lateral and strike slip. The fault movement did not cause any crust or surface breakage.

Representation of seismic activity

The earthquake caused an estimated $6 billion to $10 billion in structural damage. 63 people were killed, 3757 injured, and an estimated 12,000 people lost their homes. 42 out of the 63 deaths were caused by the interstate 880 upper level collapsing onto its lower level. In the picture below you can see only a small section is still intact, while the rest of the freeway has collapsed onto itself, crushing everything in between. The bridge was not reopened for a month and full re-construction did not finish until a year later. A local World Series baseball game was taking place at the time of the earthquake so many people were at home or at the game. This prevented many deaths because without the game, many more people would have been on the freeway in rush hour traffic.

The double-decker freeway collapsed onto itself.

The western portion of the bay sits on soft mud; this amplified the shaking by 5 to 8 times in this area. Interstate 880 collapsed in the region where it runs through this “soft area.” This portion of the freeway was 90 kilometers away from the epicenter but experienced severe shaking due to the ground it was built over and the vibration frequency of the structure matching the frequency of the ground motion: 2 to 4 lateral cycles per second.

The earthquake also caused ground failure which led to mudslides, fires, and broken water lines. The most spectacular landslides ocurred along the coast, as seen in the picture below. Liquefaction caused the destruction of many buildings. The area with the most reported damage was the Marina District. This was primarily because it was built on water –saturated, sandy, and uncompacted ground. Liquefaction caused many gas lines to break which lead to fires in this area. Meanwhile, firefighters had trouble putting out fires due to broken water lines, but they were able to pump some water from the bay.

A spectacular example of a landslide triggered by the quake

Since the earthquake, building codes have changed with respect to what kind of ground an area resides over. The California Division of Mines and Geology (CDMG) and the U.S. Geological Survey (USGS) have developed maps which depict shake hazard based on information of over 200 active faults, historical earthquake records, and frequencies of different ground areas. USGS and CDMG have proven records to show that areas over soft mud experience much more severe shaking than the other bay areas which are over bedrock and sand and gravel. Building codes have since been revised to promote “extra strength” for buildings constructed on soft ground. Several buildings were destroyed beyond repair, while others, like the ones on the right side of the picture, remain intact. Building codes are set in place to prevent total loss like this building here.

Some structures were completely destroyed

A year prior to the earthquake, USGS had worked with the Working Group on California Probabilities (WGCEP) to estimate that within 30 years following 1988, a magnitude 6.5 or greater would occur near the fault section of the future Loma Prieta earthquake. This estimation was formed because the portion of the fault had not seen any major earthquakes for some time; hence it was thought to be a seismic gap. Research also suggests that electromagnetic precursors may have existed.

Research done by Anthony C. Fraser-Smith and others of Stanford University suggests that weeks before and after the earthquake, there was an increase in ultra-low-frequency (ULF) magnetic noise 52 km from the epicenter, near Corralitos, California. This was discovered by two separate monitoring systems with measurements in the range of .01-32kHz. The time resolution for ULF (range: .01-10 Hz) was 30 minutes and as low as 1 second with extremely low frequency or very low frequency (range: 10 Hz-32 kHz). On September 12th a signal between .05-.02 Hz appeared and continued until October 5th, when there was a significant increase background noise going as low as .01 kHz. A day before the earthquake, an unordinary dip of background noise decreased to .5-5 Hz. On the day of the earthquake, 3 hours prior, an increase of activity between .01-.05 Hz (extremely low). Because there were no significant shocks prior to the earthquake, these measurements show significant evidence for being magnetic precursors. The picture below represents low frequency magnetic signals recorded for 31 days around the 1989 Loma Prieta earthquake.


More recently, scientists are suggesting there is increasing evidence for electromagnetic precursors. A group of scientists sponsored by a satellite company known as QuakeFinder detected what they call a “quake alarm” before the October 30th, 2007 San Jose earthquake. To detect these pulses QuakeFinder engineers had installed 70 sensors spread across California. Theses sensors contain a search-coil magnetometer which detects the “quake alarm” signal. They determined electromagnetic pulses continued for eight minutes solitary to the Alum Rock area nineteen hours before the following 5.6 earthquake. A NASA scientist by the name Friedemann T. Freund also strongly supports the earthquake-alarm theory. He conducted a lab experiment in which he measured the electromagnetic activity in rocks after putting high pressure on them. This modeled pre-earthquake conditions and showed developing electrical currents concurrent to higher pressure. The experiment also showed a decline in electric activity when the pressure was relieved, which corresponds to electromagnetic activity after earthquakes. This next picture shows the USGS recording during the actual quake, real time. Seismographs like these help us monitor ground motions on a precise level.

USGS Recording seismic activity during the 89 quake

Despite the overwhelming evidence for electromagnetic precursors, there are still skeptics. Some believing these signals may not stem from earthquakes at all, but rather from other natural or artificial sources such as solar activity or auto engines. Robert J. Geller and his colleagues make some arguments against the supporting evidence. For instance, despite there being hundreds of papers presenting data for these precursors, the data quality and physical arguments are what should be considered more important than the quantity of research. Another argument is that research has taken too long in this field with no strong supporting statistical data.

Both the critics and the supporting researchers for electromagnetic precursors have valid arguments, but the reality is that there has yet been solid proof from either side. It does seem however that there is more valid evidence coming from the supporters. According to UCLA space physicist Jacob Bortnik, "There are at least a dozen theories that predict these (electromagnetic precursors) should occur (Davidson).” Electromagnetic activity seems to swarm areas prior to and after earthquakes, an example being the Loma Prieta earthquake and the more recent San Jose quake.


Davidson, Keay. “Evidence Mounts for Electromagnetic Earthquake Precursors.” Wired. 14 Dec. 2007. 01 Dec. 2008

Geller, Robert J., Alex I. Braginski, and Wallace H. Campbell. “Earthquake Precursors or Background Noise?.” Apr. 2006 ieee
spectrum. Dec. 2008 <>

Hyndman, Donald and David Hyndman. Natural Hazards and Disasters. 1st ed. Belmont, CA: Thompson Brooks/Cole, 2006.

Page, Robert, et al. “U.S. Geological Survey Fact Sheet 151-99.” 03 Nov. 1999. USGS. 04 Nov. 2008 <>

“Profile of mortality from the 1989 Loma Prieta earthquake using coroner and medical examiner reports.” 18 Jun. 1994. NCBI. 04 Nov. 2008 <>

The Loma Prieta, California, Earthquake of October 17, 1989—Preseismic Observations. Ed. Malcolm J.S. Johnston. 1993. U.S. Geological Survey. 01
Dec 2008 <>

"Historic Earthquakes: Santa Cruz Mountains (Loma Prieta) California, 1989." USGS.GOV Dec 2009

"Anticipating Earthquakes." Science At Nasa.GOV. Dec 2009