2015 US Geological Survey earthquake forecast for California
The 2015 Uniform California Earthquake Rupture Forecast, Version 3, or UCERF3, is the latest official earthquake rupture forecast (ERF) for the state of California, superseding UCERF2. It provides authoritative estimates of the likelihood and severity of potentially damaging earthquake ruptures in the long- and near-term. Combining this with ground motion models produces estimates of the severity of ground shaking that can be expected during a given period (seismic hazard), and of the threat to the built environment (seismic risk). This information is used to inform engineering design and building codes, planning for disaster, and evaluating whether earthquake insurance premiums are sufficient for the prospective losses.[1] A variety of hazard metrics[2] can be calculated with UCERF3; a typical metric is the likelihood of a magnitude[3] M 6.7 earthquake (the size of the 1994 Northridge earthquake) in the 30 years (typical life of a mortgage) since 2014.
California (outlined in white) and buffer zone showing the 2,606 fault subsections of UCERF 3.1. Colors indicate probability (as a percentage) of experiencing an M ≥ 6.7 earthquake in the next 30 years, accounting for the stress accumulated since the last earthquake. Does not include effects from the Cascadia subduction zone (not shown) in the northwest corner.
Highlights
A major achievement of UCERF3 is use of a new methodology that can model multifault ruptures such as have been observed in recent earthquakes.[5] This allows seismicity to be distributed in a more realistic manner, which has corrected a problem with prior studies that overpredicted earthquakes of moderate size (between magnitude 6.5 and 7.0).[6] The rate of earthquakes of magnitude (M[7]) 6.7 and greater (over the entire state) is now believed to be about one in 6.3 years, instead of one in 4.8 years. On the other hand, earthquakes of magnitude 8 and larger are now expected about every 494 years (down from 617).[8] Otherwise the overall expectations of seismicity are generally in line with earlier results.[9] (See Table A for a summary of the overall rates.)
The fault model database has been revised and expanded to cover over 350 fault sections, up from about 200 for UCERF2, and new attributes added to better characterize the faults.[10] Various technical improvements have also been made.[11]
Table A: Estimated probabilities (minimum, most likely, and maximum) of an earthquake of the given magnitude in the next thirty years for different regions of California1
Location of main faults in following table, with segments color-coded to show slip-rate (up to 40 mm per year).
Of the six main faults evaluated in previous studies the Southern San Andreas fault remains the most likely to experience an M ≥ 6.7 earthquake in the next 30 years. The largest increase in such likelihood is on the Calaveras fault (see main faults map for location), where the mean (most likely) value is now set at 25%. The old value, of 8%, is less than the minimum now expected (10%). The previous under-estimate is believed to be due mostly to not modeling multifault ruptures, which limited the size of many ruptures.[13]
The largest probability decrease is on the San Jacinto fault, which went from 32% to 9%. Again this is due to multifault rupturing, but here the effect is fewer earthquakes, but they are more likely to be bigger (M ≥ 7.7) [14]
Table B
Table B: Aggregate probabilities of an M ≥ 6.7 earthquake within 30 years (and change from UCERF2)1
1. Adapted from Table 6 in Field et al. 2015, p.525. Values are aggregated from the fault sections comprising each fault. Some sections have higher individual probabilities; see Table 4 in Field et al. 2015, p.523. "M" is moment magnitude (p. 512).
2. These are the six faults for which UCERF2 had enough data to do stress-renewal modeling. The Hayward fault zone and Rodgers Creek fault are treated as a single fault; the San Andreas fault is treated as two sections.
3. UCEF3 fault sections, with links to "participation" maps for each section (outlined in black), showing the rate (in color) that section participates in ruptures with other sections. Participation maps for all fault sections available at http://pubs.usgs.gov/of/2013/1165/data/UCERF3_SupplementalFiles/UCERF3.3/Model/FaultParticipation/ Some faults have had sections added or split since UCERF2.
5. Lengths from UCERF-2, Table 4; may vary from QFFDB values.
6. Min. and Max. probabilities correspond to the least and most likely alternatives in the logic tree; the Mean is a weighted average.
7. Slip-rates not included due to variation across sections and deformation models. See figure C21 (below) for an illustration.
Methodology
California earthquakes result from the Pacific Plate, heading approximately northwest, sliding past the North American continent. This requires accommodation of 34 to 48 millimeters (about one and a half inches) of slippage per year,[19] with some of that taken up in portions of the Basin and Range Province to the east of California.[20] This slippage is accommodated by ruptures (earthquakes) and aseismic creep on the various faults, with the frequency of ruptures dependent (in part) on how the slippage is distributed across the various faults.
Modeling
UCERF3's four levels of modeling, and some of the alternatives that form the logic-tree.
Like its predecessor, UCERF3 determines this based on four layers of modeling:[22]
The fault models (FM 3.1 and 3.2) describe the physical geometry of the larger and more active faults.
Deformation models determine the slip rates and related factors for each fault section, how much strain accumulates before a fault ruptures, and how much energy is then released. Four deformation models are used, reflecting different approaches to handling earthquake dynamics.
The earthquake rate model (ERM) fits together all this data to estimate the long-term rate of rupturing.
The probability model estimates how close (ready) each fault segment is to rupturing given how much stress has accumulated since its last rupture.
The first three layers of modeling are used to determine the long-term, or Time Independent, estimates of the magnitude, location, and frequency of potentially damaging earthquakes in California. The Time Dependent model is based on the theory of elastic rebound, that after an earthquake releases tectonic stress there will be some time before sufficient stress accumulates to cause another earthquake. In theory, this should produce some regularity in the earthquakes on a given fault, and knowing the date of the last rupture is a clue to how soon the next one can be expected. In practice this is not so clear, in part because slip rates vary, and also because fault segments influence each other, so a rupture on one segment triggers rupturing on adjacent segments. One of the achievements of UCERF3 is to better handle such multifault ruptures.[23]
The various alternatives (see diagram), taken in different combinations, form a logic tree of 1440 branches for the Time Independent model, and, when the four probability models are factored in, 5760 branches for the Time Dependent model. Each branch was evaluated and weighted according to its relative probability and importance. The UCERF3 results are an average of all these weighted alternatives.[24]
"The Grand Inversion"
In UCERF2 each fault was modeled separately,[25] as if ruptures do not extend to other faults. This assumption of fault segmentation was suspected as the cause of UCERF2 predicting nearly twice as many earthquakes in the M 6.5 to 7.0 range then actually observed, and is contrary to the multifault rupturing seen in many earthquakes.[26]
UCERF3 subdivides each fault section (as modeled by the Fault Models) into subsections (2606 segments for FM 3.1, and 2665 for FM 3.2), then considers ruptures of multiple segments regardless of which parent fault they belong to. After removing those ruptures considered implausible there are 253,706 possibilities to consider for FM 3.1, and 305,709 for FM 3.2. This compares to less than 8,000 ruptures considered in UCERF2, and reflects the high connectivity of California's fault system.[27]
Fig. C21 from Appendix C. Plots of slip rates on two parallel faults (the San Andreas and the San Jacinto) as determined by three deformation models, and a "geologic" model based entirely on observed slip rates, showing variations along each segment. The grand inversion solves for these and many other variables to find values that provide an overall best fit.
A significant achievement of UCERF is development of system-level approach called the "grand inversion".[29] This uses a supercomputer to solve a system of linear equations that simultaneously satisfies multiple constraints such as known slip rates, etc.[30] The result is a model (set of values) that best fits the available data. In balancing these various factors it also provides an estimate of how much seismicity is not accounted for in the fault model, possibly in faults not yet discovered. The amount of slip occurring on unidentified faults has been estimated at between 5 and about 20mm/yr depending on the location (generally higher in the LA area) and deformation model, with one model reaching 30mm/yr just north of LA.[31]
Assessment
While UCERF3 represents a considerable improvement over UCERF2,[32] and the best available science to-date for estimating California's earthquake hazard,[33] the authors caution that it remains an approximation of the natural system.[34] There are a number of assumptions in the Time Independent model,[35] while the final (Time Dependent) model explicitly "assumes elastic rebound dominates other known and suspected processes that are not included in the model."[36] Among the known processes not included is spatiotemporal clustering.[37]
There are a number of sources of uncertainty, such as insufficient knowledge of fault geometry (especially at depth) and slip rates,[38] and there is considerable challenge in how to balance the various elements of the model to achieve the best fit with the available observations. For example, there is difficulty fitting paleoseismic data and slip rates on the southern San Andreas Fault, resulting in estimates of seismicity that run about 25% less than seen in the paleoseismic data. The data does fit if a certain constraint (the regional Magnitude-Frequency Distribution) is relaxed, but this brings back the problem over-predicting moderate events.[39]
An important result is that the generally accepted Gutenberg-Richter (GR) relationship (that the distribution of earthquakes shows a certain relationship between magnitude and frequency) is inconsistent with certain parts of the current UCERF3 model. The model implies that achieving GR consistency would require certain changes in seismological understanding that "fall outside the current bounds of consensus-level acceptability".[40] Whether the Gutenberg-Richter relation is inapplicable at the scale of individual faults, or some basis of the model is incorrect, "will be equally profound scientifically, and quite consequential with respect to hazard."[41]
↑ Field et al. 2013, pp.86–87. Specifically, GR consistency seems to require one or more of the following: "(1) a higher degree of creep both on and off faults; (2) higher long-term rate of earthquakes over the whole region (and significant temporal variability on faults such as the SAF); (3) more fault connectivity throughout the state (for example, ~M8 anywhere); and (or) (4) lower shear rigidity."
Field, Edward H.; Biasi, Glenn P.; Bird, Peter; Dawson, Timothy E.; Felzer, Karen R.; Jackson, David D.; Johnson, Kaj M.; Jordan, Thomas H.; Madden, Christopher; Michael, Andrew J.; Milner, Kevin R.; Page, Morgan T.; Parsons, Tom; Powers, Peter M.; Shaw, Bruce E.; Thatcher, Wayne R.; Weldon, Ray J. II; Zeng, Yuehua (2013), "Uniform California earthquake rupture forecast, version 3 (UCERF3) – The time-independent model", U.S. Geological Survey, Open-File Report 2013–1165. Also California Geological Survey Special Report 228, and Southern California Earthquake Center Publication 1792. Also published in the BSSA as Field et al. 2014.
Field, Edward H.; Arrowsmith, Ramon J.; Biasi, Glenn P.; Bird, Peter; Dawson, Timothy E.; Felzer, Karen R.; Jackson, David D.; Johnson, Kaj M.; Jordan, Thomas H.; Madden, Christopher; Michael, Andrew J.; Milner, Kevin R.; Page, Morgan T.; Parsons, Tom; Powers, Peter M.; Shaw, Bruce E.; Thatcher, Wayne R.; Weldon, Ray J. II; Zeng, Yuehua (June 2014), "Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) – The Time-Independent Model"(PDF), Bulletin of the Seismological Society of America, 104 (3): 1122–1180, doi:10.1785/0120140093 .
Oppenheimer, D. H.; Bakun, W. H.; Parsons, T.; Simpson, R. W.; Boatwright, J. B.; Uhrhammer, R. A. (2010), "The 2007 M5.4 Alum Rock, California, earthquake: Implications for future earthquakes on the central and southern Calaveras Fault", Journal of Geophysical Research, 115 (B8), Bibcode:2010JGRB..115.8305O, doi:10.1029/2009jb006683.
Parsons, Tom; Johnson, Kaj M.; Bird, Peter; Bormann, Jayne; Dawson, Timothy E.; Field, Edward H.; Hammond, William C.; Herring, Thomas A.; McCaffrey, Rob; Shen, Zhen-Kang; Thatcher, Wayne R.; Weldon II, Ray J.; Zeng, Yuehua (2013), "Appendix C – Deformation Models for UCERF3", U.S. Geological Survey, Open-File Report 2013–1165.
In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in the same area of the main shock, caused as the displaced crust adjusts to the effects of the main shock. Large earthquakes can have hundreds to thousands of instrumentally detectable aftershocks, which steadily decrease in magnitude and frequency according to a consistent pattern. In some earthquakes the main rupture happens in two or more steps, resulting in multiple main shocks. These are known as doublet earthquakes, and in general can be distinguished from aftershocks in having similar magnitudes and nearly identical seismic waveforms.
The moment magnitude scale is a measure of an earthquake's magnitude based on its seismic moment. It was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. Similar to the local magnitude/Richter scale (ML ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale; small earthquakes have approximately the same magnitudes on both scales. Despite the difference, news media often says "Richter scale" when referring to the moment magnitude scale.
The New Madrid Seismic Zone, sometimes called the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes in the Southern and Midwestern United States, stretching to the southwest from New Madrid, Missouri.
The Cascadia subduction zone is a 960 km fault at a convergent plate boundary, about 112-160 km off the Pacific Shore, that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is capable of producing 9.0+ magnitude earthquakes and tsunamis that could reach 30m. The Oregon Department of Emergency Management estimates shaking would last 5-7 minutes along the coast, with strength and intensity decreasing further from the epicenter. It is a very long, sloping subduction zone where the Explorer, Juan de Fuca, and Gorda plates move to the east and slide below the much larger mostly continental North American Plate. The zone varies in width and lies offshore beginning near Cape Mendocino, Northern California, passing through Oregon and Washington, and terminating at about Vancouver Island in British Columbia.
Coulomb stress transfer is a seismic-related geological process of stress changes to surrounding material caused by local discrete deformation events. Using mapped displacements of the Earth's surface during earthquakes, the computed Coulomb stress changes suggest that the stress relieved during an earthquake not only dissipates but can also move up and down fault segments, concentrating and promoting subsequent tremors. Importantly, Coulomb stress changes have been applied to earthquake-forecasting models that have been used to assess potential hazards related to earthquake activity.
Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at a given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram. Magnitude scales vary on what aspect of the seismic waves are measured and how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used.
The Queen Charlotte Fault is an active transform fault that marks the boundary of the North American plate and the Pacific plate. It is Canada's right-lateral strike-slip equivalent to the San Andreas Fault to the south in California. The Queen Charlotte Fault forms a triple junction south with the Cascadia subduction zone and the Explorer Ridge. The Queen Charlotte Fault (QCF) forms a transpressional plate boundary, and is as active as other major transform fault systems in terms of slip rates and seismogenic potential. It sustains the highest known deformation rates among continental or continent-ocean transform systems globally, accommodating greater than 50mm/yr dextral offset. The entire approximately 900 km offshore length has ruptured in seven greater than magnitude 7 events during the last century, making the cumulative historical seismic moment release higher than any other modern transform plate boundary system.
Earthquake forecasting is a branch of the science of seismology concerned with the probabilistic assessment of general earthquake seismic hazard, including the frequency and magnitude of damaging earthquakes in a given area over years or decades. While forecasting is usually considered to be a type of prediction, earthquake forecasting is often differentiated from earthquake prediction, whose goal is the specification of the time, location, and magnitude of future earthquakes with sufficient precision that a warning can be issued. Both forecasting and prediction of earthquakes are distinguished from earthquake warning systems, which, upon detection of an earthquake, provide a real-time warning to regions that might be affected.
The San Jacinto Fault Zone (SJFZ) is a major strike-slip fault zone that runs through San Bernardino, Riverside, San Diego, and Imperial Counties in Southern California. The SJFZ is a component of the larger San Andreas transform system and is considered to be the most seismically active fault zone in the area. Together they relieve the majority of the stress between the Pacific and North American tectonic plates.
The 2007 Alum Rock earthquake occurred on October 30 at 8:04 p.m. Pacific Daylight Time in Alum Rock Park in San Jose, in the U.S. state of California. It measured 5.6 on the moment magnitude scale and had a maximum Mercalli intensity of VI (Strong). The event was then the largest in the San Francisco Bay Area since the 1989 Loma Prieta earthquake, which measured 6.9 on the moment magnitude scale, but was later surpassed by the 2014 South Napa earthquake. Ground shaking from the Alum Rock quake reached San Francisco and Oakland and other points further north. Sixty thousand felt reports existed far beyond Santa Rosa, as far north as Eugene, Oregon.
The Puente Hills Fault is an active geological fault that is located in the Los Angeles Basin in California. The thrust fault was discovered in 1999 and runs about 40 km (25 mi) in three discrete sections from the Puente Hills region in the southeast to just south of Griffith Park in the northwest. The fault is known as a blind thrust fault, as the fault plane does not extend to the surface. Large earthquakes on the fault are relatively infrequent but computer modeling has indicated that a major event could have substantial impact in the Los Angeles area. The fault is now thought to be responsible for one moderate earthquake in 1987 and another light event that took place in 2010, with the former causing considerable damage and deaths.
In seismology, doublet earthquakes – and more generally, multiplet earthquakes – were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as single earthquakes having two main shocks of similar magnitude, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude – often within 0.4 magnitude – distinguishes multiplet events from aftershocks, which start at about 1.2 magnitude less than the parent shock and decrease in magnitude and frequency according to known laws.
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An earthquake occurred off the coast of the Alaska Peninsula on July 28, 2021, at 10:15 p.m. local time. The large megathrust earthquake had a moment magnitude of 8.2 according to the United States Geological Survey (USGS). A tsunami warning was issued by the National Oceanic and Atmospheric Administration (NOAA) but later cancelled. The mainshock was followed by a number of aftershocks, including three that were of magnitude 5.9, 6.1 and 6.9 respectively.
The 1933 Sumatra earthquake or Liwa earthquake occurred in West Lampung Regency, Lampung Province, Indonesia on June 25. The earthquake had an estimated surface-wave magnitude (Ms ) of 7.7 occurring at a shallow depth of 20 km. It had an epicenter onshore, devastating the city of Liwa. At least 76 people were reported killed, although the death toll may have been in the thousands. Aftershocks followed, including one which was strong enough to cause additional fatalities. The mainshock also triggered a nearby volcanic eruption two weeks later, killing some people.
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