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Transcript
Seismic Hazard Forecasting Andrew B. Lockman Skylar L. Primm 6 April 2004 General Outline 1. 2. 3. Seismic Hazards in Southern California: Probably Earthquakes, 1994 to 2024 Prospects for Larger or More Frequent Earthquakes in the Los Angeles Metropolitan Region Bigger Jolts Are on the Way for Southern California 1 (State of California, 2003) Probability Modeling Steps: 1. Identify Faults 2. Determine Characteristic Magnitudes and Recurrence Intervals 3. Develop Probabilistic Shake Map Sometimes, this is very straightforward More often, complicated analysis is necessary 2 Identifying Faults (WGCEP, 1995) Identifying Faults (WGCEP, 1995) 3 Identifying Faults (WGCEP, 1995) Type A Fault Zones 4 Characteristic Magnitude M = µ⋅ H ⋅ L⋅ D € M = seismic moment (N*m) µ = crustal rigidity (3*1010 N*m-2) H = brittle crustal thickness (11,000 m) L = fault segment length (m) D = characteristic displacement (m) Then: m = 2 3⋅ log10 M − 6 m = moment magnitude € Recurrence Interval T = D /V € T = time between successive earthquakes (yr) D = displacement in earlier earthquake (m) V = long-term slip rate (m/yr) Note: possibility of cascading ruptures means that rupture rates are not necessarily the same as earthquake rates 5 Type B & C Fault Zones Probabilistic Methods Segmentation, displacement, and paleoseismicity are not known, so the straightforward mathematical methods are not applicable Relevant Data: 1. 2. 3. 4. Historical Earthquakes Regional Strain Accumulation (Geodetics) Geological Slip Estimates Surface/Blind Thrusts 6 Historical Earthquakes Record goes back as far as 1852 Most magnitudes prior to 1932 are uncertain, but may be approximated using the Mercalli Intensity Scale and historical records Future earthquakes are more likely to occur at historical earthquake sites Historical Earthquakes (http://www.tnema.org/EP/EP_EQ.htm) 7 Regional Strain Accumulation Geodetics (GPS) can show areas of strain accumulation along locked faults (known and unknown) Strain accumulation is directly related to seismic moment accumulation, hence areas with greater strain rates have greater earthquake potential Bulk strain is found to be distributed throughout the broader region, rather than only being concentrated on certain discrete faults Regional Strain Accumulation (WGCEP, 1995) 8 Geological Slip Estimates Used to identify surficial faults with slip rates > 3 mm/yr These faults probably exhibit recurrence intervals of hundreds of years Magnitude 7 earthquakes are possible on some of these segments, though this has not occurred in the historical record Thrust Faults Surface Thrusts Slip rates can be estimated using stratigraphy, structures, and geomorphology Blind Thrusts Slip rates are generally unknown, though may be related to fold growth rates Recurrence intervals and characteristic behavior are unknown 9 Thrust Faults (WGCEP, 1995) Regional Earthquake Potential Given that we know where the faults are and have some kind of measure or estimate of their potential magnitudes and recurrence intervals, we can finally turn to hazard potentials 10 Regional Earthquake Potential Characteristic earthquake potential is based directly on recurrence interval For randomly distributed earthquakes, it is based on the Gutenberg-Richter distribution N ( m) = 10 a−b⋅ m In general, the annual number of distributed earthquakes of magnitude ≥ m is related to the characteristic seismicity rate (f) and the maximum magnitude (m (mx, assumed to be the characteristic magnitude), with the number approaching 0 as m € approaches mx Regional Earthquake Potential Type A Zones Characteristic earthquakes are dealt with in the cascading earthquake model Remaining (distributed) seismicity rate is determined from the earthquake catalog, with characteristic earthquakes removed 11 Regional Earthquake Potential Type B Zones Distributed seismicity rate is determined from the complete earthquake catalog Remaining (characteristic) seismicity rate is determined from geological and geodetic information Regional Earthquake Potential Type C Zones There are no characteristic earthquakes, and distributed seismicity rate is the average of the earthquake catalog rate and geodetic rate Maximum seismicity mx is assumed from global seismic catalogs, so it’ it’s not at all wellconstrained 12 Regional Earthquake Potential The upshot is that for each of the 65 seismic source zones, they produce: Predicted rates of distributed (m ≥ 6) and characteristic (m = mx) earthquake ruptures Predicted moment rates for both Regional Earthquake Potential 0.61/yr 0.32/yr 0.067/yr 0.035/yr (WGCEP, 1995) 13 Regional Earthquake Potential According to the data, there is an 81-90% probability of an m ≥ 7 earthquake in Southern California between 1994 and 2024 An alternative model (assuming different cascade behavior and increased maximum magnitudes) matches the observed distribution somewhat better, and predicts an 80-89% probability of an m ≥ 7 earthquake in Southern California between 1994 and 2024 Uncertainty is probably greater than implied here, because of various assumptions in the data Finally, Seismic Hazard Analysis Seismic hazard analysis takes into account all ground-motion effects, including smaller earthquakes It also accounts for attenuation of ground motion with distance from the seismic source The authors use a peak ground acceleration (PGA) of 0.2 g (≈ (≈ 1.96 m/s2) as a baseline The probability of PGA at a given site exceeding 0.2 g is related to the annual seismicity rates, earthquake magnitudes, and earthquake probabilities at each of the 65 seismicity zones, summed using multiple integrals 14 Seismic Hazard Analysis at Los Angeles City Hall (WGCEP, 1995) Seismic Hazard Analysis for Southern California (WGCEP, 1995) 15 Future Work Models could be improved through: A better understanding of site-specific responses to ground motion Increased understanding of earthquake size versus fault geometry Increased study of blind thrusts (segmentation, strain accumulation, relationships with other faults) Wider and higher resolution geodetic measurements Conclusion/Segue Earthquakes with m ≥ 7 have been less frequent than predicted for the last 150 yr, though an m ≥ 7 earthquake by 2024 might make up for the deficit How would this affect the Los Angeles Metropolitan Area? 16 Los Angeles' Seismic Deficit There have been too few earthquakes in the Los Angeles area over the past 200 years to account for the amount of strain accumulation over the same period Major Fault Systems in the Los Angeles Region (Dolan et al., 1995) 17 Earthquake Data Regressions (Dolan et al., 1995) Potential Moderate Earthquake (Mw ~ 6.7) Sources (Dolan et al., 1995) 18 Conclusions Two scenarios: Moderate-Earthquake: predicts one Mw 6.7 earthquake every 11 years Large-Earthquake: predicts one Mw 7.2 to 7.5 earthquake every 140 years In either case, there is a seismic deficit (either a quiet period between earthquake clusters or between major events) Dolan et al. prefer the Large-Earthquake Scenario Further Implications Buildings constructed according to the California Building Code of 1991 may not be able to withstand the predicted earthquakes, particularly along the fault rupture 19