Previous Question and Answer Session
The "Earthquake DOC" - May 2005
Thanks for asking these great questions!
Q1: How many faults are there known to be in the Bay Area? Thanks in advance.
A1: The actual number of faults in the Bay Area is a difficult question to answer, but we can discuss some of the more significant faults that affect the region. Because there are too many inactive faults in the region to count, we will focus on the active faults (faults with movement in the past 11,000 years)…the faults most likely to cause earthquakes.
The following link is an interactive map of the active faults in the “greater” Bay Area (http://www.consrv.ca.gov/cgs/rghm/psha/fault_parameters/htm/CA_flt_parameters_sfbay1.htm). The active faults in the area adjacent to the San Francisco Bay include: 1) the San Andreas fault, which lies in the middle of the San Francisco peninsula and extends up through the Marin Headlands, 2) the Hayward fault, which runs along the western base of the East Bay Hills, 3) the Calaveras fault, part of which is south of the Hayward fault and another part forms the east side of the East Bay Hills, and 4) the smaller Monte Vista thrust fault, west of San Jose. Earthquakes on these faults would likely cause the most damage to the cities bordering the bay, including San Francisco, Oakland, and San Jose. During earthquakes, these faults might rupture along a “segment” of these faults or, less likely, along multiple segments of the fault. The longer the fault segment that ruptures, the bigger the earthquake it will produce.
There are a number of other active faults of significance in the area. The Rodgers Creek, West Napa, and Green Valley faults exist within the Napa-Santa Rosa region. The Mount Diablo thrust, Concord, and Greenville faults, along with the Northern Calaveras fault, lie in the area east of the East Bay Hills. The San Gregorio, Monterey Bay-Tularcitos, Rinconda, and Zayante-Vergeles faults parallel the San Andreas Fault in the Santa Cruz-Monterey Bay area. As we learned from the 1989 Loma Prieta Earthquake, which was centered in the Santa Cruz Mountains east of Monterey Bay, any of these faults can cause earthquakes of consequence to the entire Bay Area.
Q2: My zip code is 91351 (Canyon Country in Southern California). How can I find a map with that on it showing recent activity on fault lines?
A2: The following websites identify the significant faults and their relative activity in your area of interest (http://www.data.scec.org:3128/faults/lafault.html; http://www.consrv.ca.gov/CGS/rghm/ap/Map_index/index.htm).
A more precise mapped location of the active surface faults can be seen at CGS’s Los Angeles office (directions: http://www.consrv.ca.gov/CGS/information/publications/offices/la_office.htm) or purchased from CGS at the following website: http://www.consrv.ca.gov/CGS/geologic_hazards/regulatory_hazard_zones/ap_cd_htm.htm.
Q3: I want to know what fault line does Los Angeles Southwest College sit on?
A3: A portion of the Newport-Inglewood fault zone appears to go through the area. The Newport-Inglewood fault was the source for the magnitude 6.4, 1933 Long Beach Earthquake…though no surface rupture was reported. The following website gives more detailed information about the fault (http://www.data.scec.org:3128/fault_index/newping.html). A more detailed map showing the fault can be seen at CGS’s Los Angeles office (directions: http://www.consrv.ca.gov/CGS/information/publications/offices/la_office.htm) or purchased from CGS at the following website: http://www.consrv.ca.gov/CGS/geologic_hazards/regulatory_hazard_zones/ap_cd_htm.htm.
Q4: I was just curious as to earthquake predictions and their probability. Do you have any information on that?
A4: First let’s clarify that there is a difference between earthquake predictions and earthquake probabilities. Earthquake prediction is foretelling that an earthquake of significant size will occur before a certain time (a day, a month, a year, etc.). A wide range of prediction “indicators” have been used…everything from strange animal behavior to patterns in smaller, precursory earthquakes.
An earthquake probability is where a certain size earthquake is given an assigned “odds” or percent-chance of occurring over a period of time; it is similar to weather forecasting: “50% chance of rain” means that there is just as much chance of rain occurring as it not occurring…50-50. An earthquake probability for a fault is calculated in two parts: 1) recognizing known historic and pre-historic earthquakes for the fault and calculating a “recurrence interval” (the average time between major ruptures) on that fault, and 2) using the recurrence interval and the time since the last earthquake to project the chance (percent) of another earthquake occurring over a certain period of time.
See the following web links for more information on earthquake prediction techniques: (http://wwwneic.cr.usgs.gov/neis/predict/predict.html), (http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes_myths.htm) .
One recent and highly publicized “earthquake prediction” was made for a large earthquake to occur somewhere within a large portion of the Southern California desert before September 5, 2004 (more information at: http://earthquake.usgs.gov/eqinthenews/2004/KB_prediction.html). Called the Keilis-Borok California Earthquake Prediction, the prediction model uses recent small earthquake patterns as precursors to predict a larger earthquake. Though these researchers have had some success, predicting the two large earthquakes in 2003 (the magnitude 8.1 earthquake in Hokkaido, Japan, and the magnitude 6.5 San Simeon Earthquake), the prediction model is still incomplete and speculative. Additional work and testing must be done to verify the model and call it a success.
Though short-term earthquake predictions are not yet attainable, 30-year probability analyses have been done for the San Francisco Bay Area and the greater Los Angeles area…both studies wherein CGS geologists have been participants. In the Bay Area, summarized in the following link (http://quake.wr.usgs.gov/research/seismology/wg02/), there is a 62% probability of an earthquake magnitude 6.7 or greater by the year 2032. The faults most “probable” of producing a large earthquake are the San Andreas, Hayward, and Rodger Creek faults, each with a probability over 20%. The 30-year probability analysis for the Los Angeles area, done in 1988, is a bit out of date but still provides some indication of the chance of a large earthquake occurring on significant faults in Southern California. For large earthquakes before 2018, the probabilities for rupture of the southern San Andreas, San Jacinto, and Imperial faults were about 60%, 50%, and 50%, respectively. Though these percentages are no longer correct because of the time that has transpired and the new information that has been gathered about the faults since 1988, it shows that a large earthquake on these or other faults in Southern California is more likely than not in the next couple decades.
Q5: When was the last time the Rose Canyon Fault in San Diego moved?
A5: Since rupture along the Rose Canyon fault has not occurred historically, geologists must rely on information from exploratory soil trenches across the fault to determine the age of the most recent surface-rupturing earthquakes. Soil layers that are crosscut by faults indicate that those layers were deposited prior to that fault rupture (implied earthquake); likewise, undisrupted layers above the fault were deposited after the rupture. Therefore, geologists identify the upper-most soil layers that have been disrupted by these faults, and use Carbon-14/age dating on organic material above and within or below those layers to bracket when the fault ruptures occurred. According to one such study by geologists (Lindvall and Rockwell, 1995), there have been a minimum of three surface fault-rupture events in the last approximately 8,100 years, which suggests a return period for large, surface-rupturing earthquakes of about 4,000 years. Also according to this study, the last large earthquake may have occurred between 225 and 500 years ago. Considering that large earthquakes seem to average 4,000 years between events, and the last event occurred within the last 500 years, it would seem the probability of an earthquake on this segment of the Rose Canyon fault in the near future would be low. However, with such a small number of known earthquakes (three) recognized in the trench, the return rate of 4,000 years and any probability for the next rupture is still very speculative. More work like this needs to be done to better define the recurrence interval for large earthquakes on the Rose Canyon fault.
(Reference used: Lindvall, S.C., and Rockwell, T.K., 1995, Holocene activity of the Rose Canyon fault zone in San Diego, California: Journal of Geophysical Research, v. 100, no. B12, p. 24,121-24,132.)
Q6: How much danger is my home in to be damaged in an earthquake? I am 1 mile west of the 15 freeway and 2 miles north of Mira Mesa Blvd in San Diego. The area is considered Rancho Penasquitos. Am I in an earthquake hazard area? I tried to determine if this area is but is difficult to read the maps.
A6: Without a site-specific analysis of your property, it is difficult to know if you are or are not in an area of a particular type of earthquake hazard. However, as you may have found out, there are a number of resources available to give you a general idea if hazards exist without hiring a geologist or engineer.
The dominant earthquake hazards to consider are: severity of ground shaking, potential for fault rupture, potential for liquefaction, and potential for landslides. As in most of California, you can expect ground shaking to be intense for large earthquakes that occur in your area. How your house will perform during a large earthquake will depend on the type and age of your structure; typically, houses that are smaller, newer (built to more modern building standards), and have wood or metal framing, will perform better than structures that are larger, older, and constructed of unreinforced masonry. Also, large or overhanging items in the house (water heaters, book cases, etc.) should be secured to help keep them stable during earthquake shaking.
The City of San Diego’s Seismic Safety Element (an abridged version online at The City of San Diego’s Seismic Safety Element (an abridged version online at http://www.sandiego.gov/cityofvillages/pdf/generalplan/seismicsafetyelement.pdf) contains information and maps indicating where areas of potential earthquake hazards might exist. From looking at the full version of this report (most cities and counties will let the public see these reports in their public works or planning departments), the area of your residence does not appear to be susceptible to fault rupture; the closest mapped active fault, the Rose Canyon fault (discussed in the previous answer), is approximately 10 miles away. Although liquefaction and landslides also do not appear to be a problem, these maps don’t likely consider local variations that might increase your potential for these hazards.
If you live in a relatively new house or housing development, one other resource is a site-specific geology, soils, or geotechnical report that the City of San Diego, your property tract developer, or the previous homeowner might have. These reports may have been completed for your home or subdivision as a requirement prior to construction. They might contain evaluations for liquefaction and/or landslides for your property. Though these reports might be hard to track down, they will be the most site-specific and informative to help you with your questions about your property.
Q7: I live in Long Beach. When the Offshore Baja temblor hit on June 15, I felt it up here in Long Beach. When I turned on the news, it was reported that a 4.1 earthquake had hit 13 miles east of Lancaster (north Los Angeles County).
However, a private source later said that the temblor I felt was from the Offshore Baja earthquake, and what was thought to be an earthquake in Lancaster was in fact an echo of the Baja quake. I was told that the Lancaster quake was a "non-quake"…
I've never heard of non-quakes before. I have heard of aftershocks which I'm told are actually smaller earthquakes in the same area. Since I haven't heard anything more about the Lancaster event, was the temblor I felt then from the Baja quake - and why did the information on the news say that it was from Lancaster? Can echoes be picked up on seismograph machines?
Thank you for your time with this question.
A7: There really is no such thing as a “non-quake.” The term was possibly used by a news service to indicate that, as you stated, an earthquake that was previously reported, didn’t really happen.
Concerning the magnitude 5.3, offshore earthquake on June 15th, here is the official explanation of what happened from the California Integrated Seismic Network (CISN), the group of collaborative agencies that perform earthquake monitoring in California (http://www.cisn.org/special/evt.04.06.15/): “Because this event occurred offshore, the nearest CISN station was over 76 km away. The location outside of the network created complications for the automatic earthquake monitoring system and an initial, erroneous location near Lancaster was reported.” This error was observed and fixed shortly after.
Q8: Recently I discovered what appears to be a warm springs near my house in the creek bed of a creek running near by. It appears to be bubbling a gas though the water. It is small but an odor of sulfur is present. There is a black patch where the water is flowing. The water is coming out clear and warm. I was wondering if the gas can be dangerous. Also I never noticed it in previous years. Could this be a sign of possible seismic activity? The area is in Boyes Hot Springs so the area has had previous activity.
A8: As implied by the name, hot spring activity is common in the Boyes Hot Springs area (the following link provides a brief history: http://www.iktome.com/svvb/waters.html). The hot springs are a product of the volcanic activity that exists in the region.
As for the potential hazards you might be facing, we don’t think you are in any danger from the hot spring as long as: 1) the sulfur gas (likely sulfur dioxide) bubbling up is being released out into the open air, which dilutes the gas, and 2) the water in the creek is not being used for consumption. If the gas were being released in an enclosed area or the water was part of a drinking supply, it might then become dangerous to humans. Only by sampling the water, gas, and air in the area of the hot spring can you determine how dangerous it really is. If you are concerned about potential hazards from the hot spring, contact your local city or county public works agency for more information. Also, there appears to be no substantive evidence of a new hot springs being used as an earthquake predictor. However, the opposite does occur: earthquakes have been known to cause new hot springs, or increase the activity of existing ones. As a recent example, the magnitude 6.5 San Simeon Earthquake (2003) opened a hot-springs fissure in the parking lot of the City Hall in the City of Paso Robles (http://www.consrv.ca.gov/CGS/geologic_hazards/earthquakes/ground_deformation.ppt). In another case, a study showed that the magnitude 7.9 Denali (Alaska) Earthquake of 2002 caused changes to hot spring and geyser activity 2,000 miles away in Yellowstone National Park (http://www.eurekalert.org/pub_releases/2004-05/uou-qia052704.php).
Q9: People always refer to when "The Big One" is going to strike. What is a "Big One"? A 6.5? A 7.4? A 10.5? Isn't it fair to say that "The Big One" may never come and that California might just keep having a few smaller earthquakes every year with the occasional 6+ every so often?
A9: You are right to be confused by the term “The Big One”…there is really no geologic or seismologic “definition” for it. We think that the people that use the term are referring to the largest event that can happen on the San Andreas fault, which in the past were earthquakes close to magnitude 8 in size (http://pubs.usgs.gov/gip/earthq3/where.html). For Southern California, it happened in 1857 when a magnitude 7.9 earthquake centered near Fort Tejon ruptured a significant length of the San Andreas fault (http://earthquake.usgs.gov/regional/states/events/1857_01_09.php). In northern California, the magnitude 7.8 Great San Francisco Earthquake of 1906 ruptured the northern part of the San Andreas fault (http://earthquake.usgs.gov/regional/states/events/1906_04_18.php). While it is true that relatively smaller earthquakes in the magnitude 6 range have and will be more common, history and scientific evidence has shown us these bigger events will also likely occur again…the question no one can answer is “when?”
With all that said, keep this in mind: ground shaking in close proximity to a magnitude 6 or 7 earthquake can be as bad or worse than a greater distance from a magnitude 8 earthquake. Most seismic hazards and damage to homes is related to earthquake shaking. Therefore, distance from an earthquake might be more important than how big “The Big One” will be.
Q10: The San Andreas fault is determined to be a slipping fault, one plate sliding against the other. Is it possible for this fault ever to become a thrusting fault, and has it ever been a thrusting fault?
A10: The San Andreas fault is a right-lateral, strike-slip fault, with a relative motion of the land west of the fault (the Pacific Plate) moving north compared to the land on east side (the Continental Plate) of the fault. An example of an affects of this motion is: the Los Angeles area is moving north and will someday be a “suburb” of San Francisco…in millions of years though!
The popular theory amongst geologists is that prior to the development of the San Andreas fault, there was a subduction zone [where the oceanic tectonic plate is pushed (subducted) under the continental plate…along the present day West Coast (illustrated in: http://pubs.usgs.gov/publications/text/Farallon.html). Starting about 30 million years ago, the edge of a transform (strike-slip) fault on the oceanic plate came in contact with the continental plate and extended with lateral motion along this boundary over time. So, although the San Andreas fault never was a thrust fault, the same area the San Andreas fault presently occupies was a subduction zone. Over geologic time (millions of years), it is difficult to say what the motion on the San Andreas fault will be…but it will likely remain a strike-slip fault for a long time.
Q11: Are there areas in the San Fernando Valley that are further away from major fault lines than others? Before my wife and I buy a house we'd like to find out what type of ground our home would be on and how far it is from major, active fault lines. We are looking in Calabasas, Agoura, Westlake, and Oak Park. Any suggestions as to how we can get this information and what as should we be looking for? Bedrock?
A11: The following link shows the active surface faults in and around the San Fernando Valley (http://www.data.scec.org:3128/faults/lafault.html). The buried thrust fault, that was responsible for the 1994 Northridge Earthquake, is not shown but it underlies much of the valley and, therefore, must be considered when evaluating sources for earthquakes in the valley.
Though faults and, therefore, earthquake sources exist throughout the valley, you can still reduce your chance of damage to your home or property by choosing land that is less prone to surface fault rupture, liquefaction, or landslides. CGS has identified zones where potential surface fault rupture might occur, shown at the following website: http://www.consrv.ca.gov/cgs/rghm/ap/Map_index/index.htm. CGS has also produced Seismic Hazard Zone Maps for the area in and around the San Fernando Valley; these maps can be viewed at the following website: http://gmw.consrv.ca.gov/shmp/html/pdf_maps_so.html. Though the purpose of these maps are to identify areas where site-specific studies must be performed prior to new construction, they can be used to identify where earthquake hazards are more likely to occur (this doesn’t mean that areas outside the zones won’t have problems, they are just less likely to considering the geologic, topographic, seismic, and ground-water data CGS gathered). In addition, sellers of homes located in either of these types of zones must disclose this information to the prospective buyer.
Other sources of information include the City of Los Angeles Safety Element (http://cityplanning.lacity.org/Cwd/GnlPln/SaftyElt.pdf), other city or county safety elements, and any geotechnical or soils report that was prepared for the property in the past. The site-specific reports are the most accurate for identifying potential geologic or seismic hazards for a property.
Q12: Please send me a copy of the Natural Hazard Disclosure Statement so that I can either download it or print it out.
A12: The following websites give information about the Natural Hazard Disclosure Statement (http://ceres.ca.gov/planning/nhd;
http://www.ceres.ca.gov/planning/nhd/ab1195.html;
http://www.consrv.ca.gov/cgs/rghm/ap/disclose.htm;
http://www.consrv.ca.gov/cgs/shzp/Pages/shmprealdis.aspx). If you perform an Internet search, you will find that there are some real estate companies that have a copy of a form that appears to be endorsed by the California Association of Realtors (we can’t list the websites for you because they are for private companies).
Q13: My husband and I are considering buying a second home in California. We are considering the San Diego area, Laguna Beach, or Malibu. Are any of these areas safer than the others…or are there any areas near the ocean anywhere in California that are safer? Thank you for your help.
A13: You should be able to find a relatively safe house in any of these communities as long as you consider the following (some of these are handled in more detail in other questions on this page):
1) the age of the structure (newer homes are built to better building standards),
2) the type/construction of the house (smaller, wood-frame houses perform better during an earthquake than larger, masonry structures),
3) the geologic conditions underlying the house [this information might be available in CGS’s Seismic Hazard Zone Maps (http://gmw.consrv.ca.gov/shmp/html/pdf_maps_so.html…especially for Laguna Beach and Malibu), the city or county seismic safety elements, or site-specific geotechnical/soils reports],
4) the geologic conditions in proximity to the house and property (nearby active faults, potential landslide debris from surrounding properties, wave-cut erosion adjacent to property, etc.), and,
5) secure heavy objects overhead and high on the walls of your house.
The more you consider the above information, the more comfortable you will become with your home purchase with regard to earthquake hazards.
Q14: Hello, I'm from Minneapolis and will soon be moving to California, the Los Angeles area. I have searched and searched online for a website with information on earthquakes. My problem is that I don't understand the terminology they use. I want to find out which areas earthquakes are going to be something to seriously consider. I was thinking about moving towards the Santa Monica area. Somewhere near the UCLA campus... Is it safe? What sort of earthquake problems do I need to deal with? I know nothing about these things.
Thank you for your time.
A14: Well, as our Director Darryl Young likes to say, “We have to learn to live with our faults.” As “pun-ny” as this statement sounds, it really is true for Californians, new and old. Because there are active faults that will cause earthquakes that can affect all portions of our state, we have to be prepared for them. The following link tells you the kinds of things you can do to be prepared for an earthquake (http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes_what.htm).
The links at the bottom of this page contain the most on-line information about earthquakes in California. Some of these might be more understandable than others depending on your familiarity with the terms (and some of these sites define the terms).
As for your concerns about earthquake hazards in the Santa Monica/UCLA campus area, there are a number of resources available for your use. CGS’s Alquist-Priolo Program, (http://www.consrv.ca.gov/cgs/rghm/ap/Map_index/index.htm), which identifies zones of potential surface rupture from active faults, will be re-evaluating the Santa Monica fault for potential zoning within the next year. This fault is located approximately 6,000-feet south of the UCLA campus. In addition, there are potential liquefaction and landslide hazard zones shown on CGS’s Seismic Hazard Zone Map for the area, at the following link (http://gmw.consrv.ca.gov/shmp/download/pdf/ozn_bevh.pdf). Keep in mind that just because these areas are in hazard zones doesn’t mean that the hazard exists…it just means that it potentially exists (site-specific analysis is needed for confirmation). Earthquake ground shaking is another factor that should be considered (see some of the above questions-and-answers of how to deal with ground shaking issues). Further information about geologic and seismic hazards, as well as other hazards, can be obtained from the City of Los Angeles Safety Element (http://cityplanning.lacity.org/Cwd/GnlPln/SaftyElt.pdf).
Though these potential hazards exist in your area of concern, as we’ve said before, we can all better protect ourselves from earthquakes when they occur by being prepared for them.