Late last night there was a sequence of earthquakes in southern California. The mainshock is a M 4.5 earthquake.
https://earthquake.usgs.gov/earthquakes/eventpage/ci38695658/executive
This temblor was widely felt across the southland (including by my mom, who was warned by earthquake early warning). This sequence happened in the same area as the 1987 Whittier Narrows Earthquake Sequence (which I felt as a child, growing up in Long Beach, CA).
https://earthquake.usgs.gov/earthquakes/eventpage/ci731691/executive
The tectonics of southern CA are dominated by the San Andreas fault (SAF) system. The SAF system is a right-lateral strike-slip plate boundary fault marking the boundary between the Pacific and North America plates.
Basically, the Pacific plate is moving northwest relative to the North America plate. Both plates are moving northwest relative to an Earth reference frame, but the Pacific plate is moving faster.
The SAF system goes through a bend in southern CA, which causes things to get complicated. There are sibling faults to the SAF, also right-lateral strike-slip (e.g. the San Jacinto and Elsinore faults).
Also, because of the fault geometry, there is considerable north-south compression that forms the mountain ranges to the north of the Los Angeles Basin. Some of the faults formed by this compression are the Sierra Madre, Hollywood, Compton, and Puente Hills faults.
A recent earthquake (2014) happened along one of these thrust fault systems. On 28 March 2014 (one day after the 50th anniversary of the Good Friday Earthquake in Alaska) there was an oblique thrust fault earthquake beneath La Habra, CA. My cousins felt that sequence and I remember them mentioning how their children kept waking up after every aftershock, some epicenters were located within a half km from their house.
https://earthquake.usgs.gov/earthquakes/eventpage/ci15481673/executive
This La Habra sequence appears to be related to the Puente Hills Thrust fault system (same for the Whittier Narrows Earthquake). Last night’s M 4.5 also appears to have slipped along a thrust fault on this system. Based on the depth, it looks like the earthquake slipped along the Lower Elysian Park ramp (see poster).
There were a few aftershocks. However, two of them I would rather interpret them as triggered earthquakes. The M 1.6 and M 1.9 earthquakes have strike-slip earthquake mechanisms (focal mechanisms = orange). These also have shallower [hypocentral] depths. There is mapped the Montebello fault, a right-lateral strike-slip fault, just to the east of the M 4.5 epicenter. The Montebello fault is a strand of the Whittier fault system.
So, while this may be incorrect, my initial interpretation is that these two M1+ events happened on the Montebello fault system and were triggered by the M 4.5 event.
There was also an historic earthquake on the Sierra Madre fault system. On 28 June 1991, there was a M 5.8 earthquake beneath the San Gabriel Mountains to the north of the LA Basin. This was also an oblique thrust earthquake.
https://earthquake.usgs.gov/earthquakes/eventpage/ci2021449/executive
Something that all these earthquakes share is that they occurred on blind thrust faults. Why are they called blind? Because they don’t reach the ground surface, so we cannot see them at the surface (thus, we are blind to them).

Below is my interpretive poster for this earthquake

• I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 4.5.
• I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
• A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page.
• Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

• In the upper left corner I include a map that shows the USGS tectonic faults and the USGS seismicity from the past 3 months. I highlight the North America and Pacific plates and their relative motion along the San Andreas fault system.
• In the lower right corner I plot the epicenters related to this sequence. The topographic data here are high resolution LiDAR data from 2016 (publically available).
• In the lower center left is a low-angle oblique block diagram from Daout at al. (2016) that shows the geometry of the major faults in this area (along with estimates of the slip rates for these faults).
• Between the aftershock map and the oblique block diagram are two panels from Rollins et al. (2018). On the left is a map that shows the major fault systems, some historic earthquake mechanisms, and GPS derived plate motion vectors (the direction of relative motion is the orientation of the arrow and the velocity is the length of the arrow). I placed a blue star in the location of last night’s M 4.5. On the right are some cross sections through the subsurface (the location of these cross sections is shown as a dashed gray line on the map). The M 4.5 hypocentral depth is 16.9 km, which clearly plots on the Lower Elysian Park ramp (part of the Puente Hills fault system). Note how the Whittier fault, a strike-slip fault at the surface, soles into the Peunte Hills thrus.
• In the upper right corner is a map where I plot a comparison of the CSIN intensity model results (using the MMI Intensity scale, read more about that here) and the USGS “Did You Feel It?” (dyfi) reports. The intensity map is based on a model of how intensity diminishes with distance from the earthquake. The dyfi results are from real observations from real people. See the plot below the map to check out how these data compare, but in a plot not a map.
• To the left of the intensity comparisons is another map from Rollins et al. (2018) that shows how much these thrust fault systems are accumulating energy over time. Basically, the warmer colors (e.g. red) shows an area of the fault that is storing more energy per year relative to part of the fault that have less warm colors (e.g. yellow). The Sierra Madre fault system is storing the most energy, per year, of all thrust faults that Rollins and his colleagues studied.

• Here is the map with a month’s seismicity plotted.

• Two of the most notable historic earthquakes in southern CA are the 1971 Sylmar and 1994 Northridge earthquakes. Both earthquakes had a significant impact on the growth of knowledge about earthquake hazards in southern CA (and elsewhere), but they also resulted in major changes in how seismic hazards are recognized, codified, and mitigated throughout the state (with impacts nationwide and worldwide). And, both of these earthquakes also happened on blind thrust faults, just like last night’s M 4.5!
• The 1906 San Francisco and 1933 Long Beach earthquakes led to major changes in the state too. 1933 Long Beach particularly led to changes in how schools are built and resulted in the strongest building code (relative to earthquakes) in the country as the time. These changes were eventually adopted statewide, nationwide, and globally (via the universal building code). Check out the Field Act to learn more about this.
• The 1971 Sylmar Earthquake happened on a previously unrecognized fault (because it is blind) and caused lots of damage and many casualties. Perhaps most notably was the veterans hospital which was built across a fault. This fault slipped during the earthquake (triggered by the mainshock). Because the fault slipped beneath the hospital, the hospital was cut in half.
• This was quite educational, to learn that when an earthquake fault slips beneath a building, the building does not (generally) perform well. After this earthquake, state senators Alquist and Priolo wrote and helped to get passed the Alquist-Priolo Act. This act required the state (via the California Geological Survey (CGS), where I work) to identify all active faults in the state. The Board of Mines and Geology (BMG) prepared regulations that help manage development (i.e. construction of buildings) in AP zones. Read more about the AP Act here.
• The 1994 Northridge Earthquake, with a similar magnitude as the 1971 Sylmar quake, caused extensive damage throughout the San Fernando Valley (like, totally dude) and beyond. There are famous photos of the damage to bridges of the 5 and 14 interchange (interstate 5 and state route 14). The 1994 Northridge Earthquake led to the development of the Seismic Hazards Mapping Act. The CGS and the BMG both have mandates related to the SHMA (I work as part of the Seismic Hazards Mapping Program at CGS). Read more about the SHMA here.
• Read more about the 1971 Sylmar Earthquake here.
• Read more about the 1994 Northridge Earthquake here.

Other Report Pages

Some Relevant Discussion and Figures

• Here is a great map from Wallace (1990) that shows the major faults associated with the San Andreas fault system.




Generalized topographic map of southern California, showing major faults with Quaternary activity in the San Andreas firnit system. Faults dotted where concealed by water: hachures on contours indicate area of closed low.

• This is a more updated map from Tucker and Dolan (2001) prepared for their study of the Sierra Madre fault.




Regional neotectonic map for metropolitan southern California showing major active faults. The Sierra Madre fault is a 75-km-long active reverse fault that extends along the northern edge of the metropolitan region. Fault locations are from Ziony and Jones (1989), Vedder et al. (1986), Dolan and Sieh (1992), Sorlien (1994), and Dolan et al. (1997, 2000b). Closed teeth denote reverse fault surface trace; open teeth on dashed lines show upper edge of blind thrust fault ramps. Strike-slip fault surface traces shown by double arrows. Star denotes location of Oak Hill paleoseismologic trench site of Bonilla (1973). CSI, Clamshell-Sawpit fault; ELATB, East Los Angeles blind thrust system; EPT, Elysian park blind thrust fault; Hol Fl, Hollywood fault; PHT, Puente Hills blind thrust fault; RMF, Red Mountain fault; SCII, Santa Cruz Island fault; SSF, Santa Susana fault; SJcF, San Jacinto fault; SJF, San Jose fault; VF, Verdugo fault; A, Altadena study site of Rubin et al. (1998); LA, Los Angeles; LB, Long Beach; LC, La Crescenta; M, Malibu; NB, Newport Beach; Ox, Oxnard; P, Pasadena; PH, Port Hueneme; S, Horsethief Canyon study site in San Dimas; V, Ventura. Dark shading denotes mountains.

• This is a great low angle oblique view of the faults in the southland from Fuis et al. (2001). Note that the SAF geometry creates North-South compression in this area (that causes the thrust faults, some of hem are blind).




Schematic block diagram showing interpreted tectonics in vicinity of LARSE line 1. Active faults are shown in orange, and moderate and large earthquakes are shown with orange stars and attached dates, magnitudes, and names. Gray half-arrows show relative motions on faults. Small white arrows show block motions in vicinities of bright reflective zones A and B (see Fig. 2A). Large white arrows show relative convergence direction of Pacific and North American plates. We interpret a master de´collement ascending from bright reflective zone A at San Andreas fault, above which brittle upper crust is imbricating along thrust and reverse faults and below which lower crust is flowing toward San Andreas fault (brown arrows) and depressing Moho. Fluid injection, indicated by small lenticular blue areas, is envisioned in bright reflective zones A and B.

• This is an updated figure from Daout et al. (2016). The slip rates are included for each fault.




Three-dimensional schematic block model across the SGM [after Fuis et al., 2001b] superimposed to the digital elevation model, the seismicity (yellow dots), the Moho model (red line), and interpreted active faults summarizing the average interseismic strike-slip (back arrows) and dip-slip (red arrows) rates extracted from the Bayesian exploration. Shallow faults (dashed lines) that formed a complex three-dimensional system at the surface [Plesch et al., 2007] are locked during the interseismic period, while the ramp-décollement system (solid lines) decouples the upper crust from the lower crust and partitioned the observed uniform velocity field (blue vector) at the downdip end of the structures.

• Here is a summary of the historic earthquakes in southern CA from Hauksson et al. (1995). They include earthquake mechanisms (B) and the regions impacted (A).




(a) Significant earthquakes of M > 4.8 that have occurred in the greater Los Angeles basin area since 1920. Aftershock zones are shaded with cross hatching, including the 1994 Northridge earthquake. Dotted areas indicate surface rupture, including the rupture of the 1857 earthquake along the San Andreas fault. (b) Lower hemisphere focal mechanisms (shaded quadrants are compressional) for significant earthquakes that have occurred since 1933 in the greater Los Angeles area.

• This is an important figure from Leon et al. (2007) that shows their interpretation of the different faults in the Puente Hills fault system. They highlight the location of the 1987 Whittier Narrows Earthquake, which was to the north of last nights M 4.5.

• This is also an important figure as it shows some additional faults (Shaw et al., 2002). The M 4.5 most likely occurred on the Lower Elysian Park fault.




Structure contour map of the PHT in relation to other major thrust and strike-slip systems in the northern LA basin. Contour interval is 1 km; depths are subsea. Map coordinates are UTM Zone 11, NAD27 datum.

• What follows are a series of figures from Rollins et al. (2018). They studied the strain accumulation (the accumulation of energy in a fault system over time) for the three main thrust fault systems in the LA Basin.
• Here is their first figure that shows the relative plate motions as observed using GPS sites.




(a) Tectonics and shortening in the Los Angeles region. Dark blue arrows are shortening-related GPS velocities relative to the San Gabriel Mountains (Argus et al., 2005). Contours are uniaxial strain rate (rate of change of εxx) in the N ~5° E direction estimated from the GPS using the method of Tape et al. (2009). Background shading is the shear modulus at 100-m depth in the CVM*, a heterogeneous elastic model based on the Community Velocity Model (Süss & Shaw, 2003; Shaw et al., 2015) that we create and use in this study (section 4). Black lines are upper edges of faults, dashed for blind faults. Epicenters of the 1971, 1987, and 1994 earthquakes are from Southern California Earthquake Data Center; focal mechanisms are from Heaton (1982) for 1971 and Global CMT Catalog for 1987 and 1994. Profile A-A0 follows LARSE line 1 (Fuis et al., 2001) onshore and line M-M0 of Sorlien et al. (2013) offshore. SGF = San Gabriel Fault; SSF = Santa Susana Fault. VF = Verdugo Fault. SAF = San Andreas Fault. CuF = Cucamonga Fault. A-DF = Anacapa-Dume Fault. SMoF = Santa Monica Fault. HF = Hollywood Fault. RF = Raymond Fault. UEPF = Upper Elysian Park Fault. ChF = Chino Fault. WF = Whittier Fault. N-IF = Newport-Inglewood Fault. PVF = Palos Verdes Fault. (b) GPS velocities on islands. (c) Tectonic setting. Black lines and pairs of half-arrows, respectively, are major faults and their slip senses. Black arrow is Pacific Plate velocity relative to North American plate from Kreemer et al. (2014). GF = Garlock Fault. SJF = San Jacinto Fault. EF = Elsinore Fault. SB = Santa Barbara. LA = Los Angeles. SD = San Diego.

• Here is their cross section through this part of the LA Basin. The location of this cross section is marked on the above map as a gray dashed line.




(a) Cross sections of faults, structure, north-south contraction, and seismicity along profile A-A0 . Red lines are fault surfaces as meshed here (Figure 3), dashed where uncertain (Shaw & Suppe, 1996; Shaw & Shearer, 1999; Fuis et al., 2012). Geometries of basin, basement, and mantle are from Shaw et al. (2015); geometry of base of Fernando Formation (boundary between beige and tan units of the basin) is interpolated from Sorlien et al. (2013; offshore), Wright (1991; coastline to Whittier Fault), and Yeats (2004; Whittier Fault to Sierra Madre Fault); topography is from Fuis et al. (2012). (b) Projections of Argus et al. (2005) GPS velocities (relative to San Gabriel Mountains) onto the direction N 5° E and 1σ uncertainties. Note that stations on Palos Verdes are plotted left of the coastline as the offshore section of profile A-A0 passes alongside Palos Verdes (Figure 1a). (c) Seismotectonic features. Distribution of shear modulus is from the CVM*, the heterogeneous elastic model used in this study (section 4). Translucent white circles are relocated 1981–2016 M ≥ 2 earthquakes whose epicenters lie within the mesh area of the three thrust faults and decollement (Hauksson et al., 2012 and updated). PVF = Palos Verdes Fault; N-IF = Newport-Inglewood Fault; WF = Whittier Fault.

• I love this map because it shows how these thrust faults dip into the Earth to the north.




geometries of the three main thrust faults beneath the Los Angeles basin (section 4), colored by depth, and 1981–2016 M ≥ 2.5 earthquakes within the mesh area from Hauksson et al. (2012 and updated), scaled by magnitude (white-filled circles). Gray-filled circles are 1981–2016 M ≥ 4.5 earthquakes outside the mesh area. Inferred paleoearthquakes are from Rubin et al. (1998) and Leon et al. (2007, 2009). SAF = San Andreas Fault.

• Finally, we see how they model the amount of plate tectonic motion is accumulated as tectonic strain on these faults. Chris is one of the smartest plate tectonicists I know, so read his paper (several times).




Estimates of moment deficit accumulation rate from combining the four interseismic strain accumulation models. (a) Spatial distribution of moment deficit accumulation rate per area. (Values are on the order of ~108 N m ^-1 yr ^-1 as the moment deficit accumulation rate per patch is on the order of 1015 N m ^-1 yr ^-1 [Figure S11] and the patches are a few kilometers (a few thousand meters) on a side.) (b) Unified PDF of moment deficit accumulation rate (dark blue object) formed by combining the PDFs from the four strain accumulation models. The PDF would follow the red curve if strain accumulation updip of the tips of the Puente Hills and Compton faults (PHF and CF) were counted.

San Andreas plate boundary Earthquake Reports

General Overview

• 1906.04.18 M 7.9 San Francisco

Earthquake Reports

Northern CA

• 2017.12.14 M 4.3 Laytonville
• 2016.11.06 M 4.1 Laytonville, CA
• 2016.11.03 M 3.8 Laytonville, CA
• 2016.08.10 M 5.1 Lake Pillsbury, CA
• 2016.08.04 M 4.5 Honey Lake, CA
• 2015.08.30 M 3.6 Mendocino County, CA
• 2015.07.27 M 3.5 Point Arena, CA

Central CA

• 2018.07.30 M 3.7 San Pablo Bay
• 2018.01.04 M 4.4 Berkeley
• 1989.10.18 M 6.9 Loma Prieta

Southern CA

• 2020.09.19 M 4.5 El Monte
• 2020.06.24 M 5.8 Lone Pine
• 2019.07.04 M 6.4 Ridgecrest
• 2019.07.05 M 6.4 / 7.1 Ridgecrest Update #1
• 2019.07.18 M 6.4 / 7.1 Ridgecrest Update #2
• 2019.07.20 M 6.4 / 7.1 Ridgecrest Update #3
• 2019.06.05 M 4.3 San Clemente Island
• 2018.04.05 M 5.3 Channel Islands
• 2018.04.05 M 5.3 Channel Islands Update #1
• 2016.02.23 M 4.9 Bakersfield
• 2015.12.30 M 4.4 San Bernardino, CA
• 2015.05.03 M 3.8 Los Angeles, CA
• 2015.04.13 M 3.3 Los Angeles, CA
• 2014.04.01 M 5.1 La Habra p-3
• 2014.03.29 M 5.1 La Habra p-2
• 2014.03.28 M 5.1 La Habra p-1
• 1994.11.17 M 6.7 Northridge, CA
• 1971.02.09 M 6.7 Sylmar, CA

Social Media

#EarthquakeReport for M 4.5 #Earthquake near El Monte #California

appears related to Puente Hills thrust system, possibly on Lower Elysian Park fault

triggered Montebello fault strike-slip earthquakes (?)https://t.co/3imJv5OUZ7

report here https://t.co/TnsLc2U1AF pic.twitter.com/LCsrcN4mAY

— Jason "Jay" R. Patton (@patton_cascadia) September 19, 2020

Just before midnight a 4.5 magnitude earthquake occurred in the Los Angeles Area. @MyShakeApp sent an alert to 20,000 phones! Be prepared with the #MyShake mobile app! Designed to send users alerts ASAP so they can drop, cover and hold on. download here: https://t.co/9zF3qAPeTh pic.twitter.com/bmfl8YBQXP

— Cal OES (@Cal_OES) September 19, 2020

M4.5 #earthquake S California: Mystery, late surface-wave arrival on seismogram at USC. Particle motion suggests Love wave arriving from northeast or southwest.https://t.co/CAzDqXbva7 pic.twitter.com/PuSS7Jm8oj

— Anthony Lomax 😷🇪🇺🌍 (@ALomaxNet) September 19, 2020

Tonight's M4.6 quake was VERY close to the epicenter of the 1987 Whittier earthquake (M5.9), but almost twice as deep as '87. Focal mechanism shows it was on a reverse fault about 11 miles deep. pic.twitter.com/gO8mE713jq

— Brian OLSON (@mrbrianolson) September 19, 2020

Over 26,000 Did You Feel It? responses already! Stronger shaking than average (orange curve) for a California M4.5, more like an east coast 4.5 (green curve)https://t.co/f1wXvxZExQ pic.twitter.com/AO0ePXkvvC

— Susan Hough 🦖 (@SeismoSue) September 19, 2020

M4.5 #earthquake S California: Upwards bump in felt reports around 100km shows shaking amplification due to waves bouncing off the base of the crust (Moho). Also note usual, mistaken ("internet effect"?) reports at large distance.https://t.co/sc07C0qCXwhttps://t.co/1ffVEDCxLo pic.twitter.com/PQiiTK9xcl

— Anthony Lomax 😷🇪🇺🌍 (@ALomaxNet) September 19, 2020

The 1987 M5.9 Whittier earthquake is the reason my house, which was not my house at the time, has a modular chimney rather than its original 1920s brick chimney. https://t.co/qHlhNBymfB

— Susan Hough 🦖 (@SeismoSue) September 19, 2020

References:

Basic & General References

• Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
• Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
• Holt, W. E., C. Kreemer, A. J. Haines, L. Estey, C. Meertens, G. Blewitt, and D. Lavallee (2005), Project helps constrain continental dynamics and seismic hazards, Eos Trans. AGU, 86(41), 383–387, , https://doi.org/10.1029/2005EO410002. /li>
• Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
• Kreemer, C., J. Haines, W. Holt, G. Blewitt, and D. Lavallee (2000), On the determination of a global strain rate model, Geophys. J. Int., 52(10), 765–770.
• Kreemer, C., W. E. Holt, and A. J. Haines (2003), An integrated global model of present-day plate motions and plate boundary deformation, Geophys. J. Int., 154(1), 8–34, , https://doi.org/10.1046/j.1365-246X.2003.01917.x.
• Kreemer, C., G. Blewitt, E.C. Klein, 2014. A geodetic plate motion and Global Strain Rate Model in Geochemistry, Geophysics, Geosystems, v. 15, p. 3849-3889, https://doi.org/10.1002/2014GC005407.
• Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. https://doi.org/10.7289/V5H70CVX
• Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
• Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
• Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
• Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198

Specific References

• Daout, S., S. Barbot, G. Peltzer, M.-P. Doin, Z. Liu, and R. Jolivet, 2016. Constraining the kinematics of metropolitan Los Angeles faults with a slip-partitioning model, Geophys. Res. Lett., 43, p. 11,192–11,201 doi:10.1002/2016GL071061.
• Fuis, G.S., Ryberg, T., Godfrey, N.J., Okaya, D.A., and Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1, p. 15-18
• Hauksson, E., Jones, L.M., and Huttn,K., 1995. The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects in JGR, v. 100, no. B7, p. 12,235-12,255
• Leon, L. A., S. A. Christofferson, J. F. Dolan, J. H. Shaw, and T. L. Pratt, 2007. Earthquake-by-earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles, California: Implications for fold kinematics and seismic hazard, J. Geophys. Res., 112, B03S03, doi:10.1029/2006JB004461.
• Rollins, C., Avouac, J.-P., Landry, W., Argus, D. F., & Barbot, S., 2018. Interseismic strain accumulation on faults beneath Los Angeles, California. Journal of Geophysical Research: Solid Earth, 123. https://doi.org/10.1029/2017JB015387
• Shaw, J.H., Plesch, A., Dolan,. J.F., Pratt, T.L., and Fiore, P/, 2002. Puente Hills Blind-Thrust System, Los Angeles, California in BSSA v. 92, no. 8,pp. 2946-2960
• Tucker, A.Z. and Dolan, J.F., 2001. Paleoseismologic Evidence for a
  8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault, Northern Los Angeles Metropolitan Region, California in BSSA v. 91, no. 2, p. 232-249
• Wallace, Robert E., ed., 1990, The San Andreas fault system, California: U.S. Geological Survey Professional Paper 1515, 283 p. [http://pubs.usgs.gov/pp/1988/1434/].

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• Sorted by Day of the Year
• Sorted By Region

This earthquake was the second earthquake in the state of CA to lead to major changes in how people in the state handled earthquake hazards and risk and today is the 47th anniversary of this earthquake. The first important earthquake was the 1933 Long Beach Earthquake, which led to major changes in the building code (first in Long Beach, then later adopted by the entire state). These changes in the building code have continued to evolve and improve, eventually adopted globally. The 1971 M 6.7 Sylmar Earthquake (a little larger than the M 6.4 damaging earthquake sequence recently that happened in Taiwan) caused major damage to buildings and other infrastructure in southern CA (e.g a hospital was destroyed, which caused many casualties). The 1906 San Francisco Earthquake was important too, so I don’t want the SAF to feel left out. Though the 1933 Long Beach and 1971 Sylmar earthquakes seem to have led to more significant changes in how people approach earthquake hazards and risk.
A major positive result from the Sylmar Earthquake was the Alquist Priolo Act. The AP Act created a requirement to characterize all the active faults in the state of CA and to regulate how to consider how structures could be built in relation to these active faults. More about the AP Act can be found here. After several years of no support from the state, the CA Geological Survey has recently supported work in this regard, resulting in an update of their guidelines in how to apply the AP Act in Special Publication 42.
I put together a commemorative #EarthquakeReport interpretive poster to discuss the tectonics of the region. The San Andreas fault (SAF) system is the locus of ~75% of the Pacific-North America plate boundary motion. The SAF is in some places a mature fault with a single strand and in other places, there are multiple strands (e.g. the Elsinore, San Jacinto, and SAF in southern CA or the Maacama, Bartlett Springs, and SAF in northern CA). In southern CA, the SAF makes a bend (called the “Big Bend”) that forms a region of compression. This compression is realized in the form of thrust faults and folds, creating uplift forming the mountain ranges like the Santa Monica Mountains. Some of these thrust faults breach the ground surface and some are blind (they don’t reach the surface).
In 1971 there was a large earthquake (M 6.7) that caused tremendous amounts of damage in southern CA. A hospital was built along one of the faults and this earthquake caused the hospital to collapse killing many people. The positive result of this earthquake is that the Alquist Priolo Act was written and passed in the state legislature. I plot the moment tensor for the 1971 earthquake (Carena and Suppe, 2002).
Then, over 2 decades later, there was the M 6.7 Northridge Earthquake. This earthquake was very damaging. Here is a page that links to some photos of the damage. Here is the USGS website for this 1971 M 6.7 Sylmar Earthquake.

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 4.5.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange) for the M 6.7 earthquake, in addition to some of the significant earthquakes in southern CA.

• I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
• I also include the shaking intensity on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
• I include a legend showing the relative age of most recent activity for faults shown on the map. These faults are from the USGS Active Fault and Fold Database. More can be found about this database here.

• I include some inset figures.

• In the upper left corner is a map of the faults in southern CA (Tucker and Dolan, 2001). Strike-slip faults (like the SAF) have arrows on either side of the fault desginating the relative motion across the fault. Thrust faults have triangle barbs showing the convergence direction (the triangles are on the side of the fault that is dipping into the Earth).
• Below this fault map is a low-angle oblique block diagram showing the configuration of thrust faults in the region of the Big Bend. These thrust faults are forming the topography in southern CA. The 1971 and 1994 earthquakes occurred along thrust faults similar to the ones shown in this block diagram.
• In the upper right corner is a cross section of seismicity associated with the 1971 and 1994 earthquakes (Tsutsumi and Yeats, 1994). 1971 main and aftershocks are in blue and 1994 main and aftershocks are in red. Note how both earthquakes occurred along blind thrust faults. Also note that these faults were dipping in opposite directions (1971 dips to the north (south vergent) and 1994 dips to the south (north vergent).
• In the lower right corner is another figure showing the aftershocks from the 1971 and 1994 earthquakes (Fuis et al., 2003). This shows their seismic velocity model (with fault interpretations). The 1971 and 1994 earthquake focal mechanisms are shown.
• In the lower left corner is an illustration that shows the Likelihood of an earthquake with M ≥ 6.7 for the next 30 years. This is based upon the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3). More about UCERF3 can be found here. I placed a blue star in the general location of the 1971 Sylmar Earthquake.

• Here is the same map, but the MMI is plotted as contours.

Some Relevant Discussion and Figures

• Here is the fault map from Tucker and Dolan (2001).




Regional neotectonic map for metropolitan southern California showing major active faults. The Sierra Madre fault is a 75-km-long active reverse fault that extends along the northern edge of the metropolitan region. Fault locations are from Ziony and Jones (1989), Vedder et al. (1986), Dolan and Sieh (1992), Sorlien (1994), and Dolan et al. (1997, 2000b). Closed teeth denote reverse fault surface trace; open teeth on dashed lines show upper edge of blind thrust fault ramps. Strike-slip fault surface traces shown by double arrows. Star denotes location of Oak Hill paleoseismologic trench site of Bonilla (1973). CSI, Clamshell-Sawpit fault; ELATB, East Los Angeles blind thrust system; EPT, Elysian park blind thrust fault; Hol Fl, Hollywood fault; PHT, Puente Hills blind thrust fault; RMF, Red Mountain fault; SCII, Santa Cruz Island fault; SSF, Santa Susana fault; SJcF, San Jacinto fault; SJF, San Jose fault; VF, Verdugo fault; A, Altadena study site of Rubin et al. (1998); LA, Los Angeles; LB, Long Beach; LC, La Crescenta; M, Malibu; NB, Newport Beach; Ox, Oxnard; P, Pasadena; PH, Port Hueneme; S, Horsethief Canyon study site in San Dimas; V, Ventura. Dark shading denotes mountains.

• This is a figure that is based upon Fuis et al. (2001) as redrawn by UNAVCO that shows the orientation of thrust faults in this region of southern CA. Below the block diagram is a map showing the location of their seismic experiment (LARSE = Line 1; Fuis et al., 2003).




Schematic block diagram showing interpreted tectonics in vicinity of LARSE line 1. Active faults are shown in orange, and moderate and large earthquakes are shown with orange stars and attached dates, magnitudes, and names. Gray half-arrows show relative motions on faults. Small white arrows show block motions in vicinities of bright reflective zones A and B (see Fig. 2A). Large white arrows show relative convergence direction of Pacific and North American plates. We interpret a master decollement ascending from bright reflective zone A at San Andreas fault, above which brittle upper crust is imbricating along thrust and reverse faults and below which lower crust is flowing toward San Andreas fault (brown arrows) and depressing Moho. Fluid injection, indicated by small lenticular blue areas, is envisioned in bright reflective zones A and B.




Shaded relief map of Los Angeles region, southern California, showing Quaternary faults (thin black lines, dotted where buried), shotpoints (gray and orange filled circles), seismographs (gray and orange lines), air-gun bursts (dashed yellow lines), and epicenters of earthquakes .M 5.8 since 1933 (focal mechanisms with attached magnitudes: 6.7a—Northridge [Hauksson et al., 1995], 6.7b—San Fernando [Heaton, 1982], 5.9—Whittier Narrows [Hauksson et al., 1988], 5.8—Sierra Madre [Hauksson, 1994], 6.3—Long Beach [Hauksson, 1987]). Faults are labeled in red; abbreviations: HF—Hollywood fault, MCF—Malibu Coast fault, MHF—Mission Hills fault, NHF—Northridge Hills fault, RF—Raymond fault, SF—San Fernando surface breaks, SSF—Santa Susana fault, SMoF—Santa Monica fault, SMFZ—Sierra Madre fault zone, VF—Verdugo fault. NH is Newhall.

• Here are the figures from Hauksson et al. (1995) showing the regions effected by earthquakes in southern CA.




(A) Significant earthquakes of M >= 4.8 that have occurred in the greater Los Angeles basin area since 1920. Aftershock zones are shaded with cross hatching, including the 1994 Northridge earthquake. Dotted areas indicate surface rupture, including the rupture of the 1857 earthquake along the San Andreas fault. (B) Lower hemisphere focal mechanisms (shaded quadrants are compressional) for significant earthquakes that have occurred since 1933 in the greater Los Angeles area.

• Here is the seismicity cross section plot from Tsutsumi and Yeats (1999).




Cross section down to 20 km depth across the central San Fernando Valley, including the 1971 Sylmar and 1994 Northridge earthquake zones. See Figure 2 for location of the section and Figure 3 for stratigraphic abbreviations. Wells are identified in the Appendix. Aftershock data for the 1971 (blue) and 1994 (red) earthquakes within a 10-km-wide strip including the line of this section are provided by Jim Mori at Kyoto University. Abbreviation for faults: MHF, Mission Hills fault; NHF, Northridge Hills fault; SSF, Santa Susana fault.

• Here is the figure from Fuis et al. (2003) showing their interpretation of seismic data from the region. These data are from a seismic experiment also plotted in the map above. The panel on the left is A and the panel on the right is B. This is their figure 3.




Cross section along part of line 2 with superposition of various data layers. A: Tomographic velocity model plus line drawing extracted from reflection data (see text); heavier black lines represent better-correlated or higher-amplitude phases. B: Velocity model plus relocated aftershocks of 1971 San Fernando and 1994 Northridge earthquakes (brown and blue dots, respectively); main shock focal mechanisms (far hemispheres) are red (San Fernando; Heaton, 1982) and blue (Northridge; Hauksson et al., 1995). Aftershocks are projected onto line 2 from up to 10 km east.

• This is a smaller scale cross section from Fuis et al. (2003) showing a broader view of the faults in this region. This shows the velocity model color legend that also applies to the above figure. This is their figure 4.




Similar to Fig. 3, with expanded depth and distance frame. See caption for Fig. 3 for definition of red, magneta, and blue lines; orange line—interpreted San Andreas fault (SAF); yellow lines—south-dipping reflectors of Mojave Desert and northern Transverse Ranges; “K” —reflection of Cheadle et al. (1986), which is out of plane of this section. SAF is not imaged directly; interpretation is based on approximate northward termination of upper reflections (best constrained) in San Fernando reflective zone (magenta lines). (See similar interpretation for SAF on line 1—Fig. 5.) Wells shown in Mojave Desert are (s) H&K Exploration Co., (t) Meridian Oil Co. (Dibblee, 1967). For well color key, see caption for Fig. 3. Thin, dashed yellow-orange line—estimated base of Cenozoic sedimentary rocks in Mojave Desert based on velocity. Darker, multicolored region (above region of light violet) represents part of velocity model where resolution ≥ 0.4 (see color bar).

• Here is a fascinating figure from Carena and Suppe (2002) showing the 3-dimensional configuration of the faults involved in the 1971 and 1994 earthquakes.




Perspective view, looking from the SE, of the modeled Northridge and San Fernando thrusts. The Northridge thrust stops at a depth of about 6 km, and its upper tip east of the lateral ramp (Fig. 4) terminates almost against the San Fernando thrust, as was suggested by Morti et al. (1993). The San Fernando thrust loser tip is at a depth o 13 km, whereas the Northridge thrust lower tip is at 32 km.

• Here is a map view of the Carena and Suppe (2002) interpretation of these fault planes.




Schematic geological map showing the position of the main faults and folds, as well as the depth contours (contour interval = 1 km) of the Northridge (solid) and San Fernando (dashed) thrusts.

• Here is a structural cross section across this region (Carena and Suppe, 2002).




Cross-section through the San Fernando Valley with projected aftershocks of the 1994 Northridge earthquake and of the 1971 Sylmar earthquake. The Northridge aftershocks are projected from a distance of 1 km or less on each side of the cross-section (main shock projected from 2 km W), whereas those of the Sylmar earthquake are projected from 1.5 km or less (main shock projected from 5 km ESE). The sources that we used for near-surface geology and structure are Dibblee (1991) and a seismic line (Fig. 11). The large N-S changes in Upper Tertiary stratigraphic thicknesses in this region (Dibblee, 1991, 1992a), prevent detailed stratigraphic correlation across fault blocks (this figure and Fig. 12). This face suggests that the shallow faults and possible the deeper San Fernando thrust itself, are reactivating old normal faults of the southern margin of the Ventura Basin (Yeats, et al., 1994; Huftle and Yeats, 1996; Tsutsumi and Yeats, 1999). Location of cross-section is in Fig. 13.

• Here is a comparison of the ground shaking intensity for these two earthquakes (1971 Sylmar vs. 1994 Northridge). These earthquakes had similar magnitudes, but the 1994 earthquake had a higher MMI. The upper panels are the USGS Shakemaps, which are model based estimates of shaking intensity, based on Ground Motion Predicti0on Equations (GMPE; attenuation relations). The lower panels plot two different sets of data. The orange lines are regression lines that represent how shaking intensity diminishes (attenuates) with distance from the earthquake. These are regressions based upon these GMPE relations. More about GMPE relations can be found here. The dots are data from real observations made by people who have reported this on the USGS Did You Feel It? website for each of these earthquakes. More about the DYFI program can be found here.

Some Background Materials

• For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

• Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).

• There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. Many of the earthquakes people are familiar with in the Mendocino triple junction region are either compressional or strike slip. The following three animations are from IRIS.

Strike Slip:

Compressional:

Extensional:

San Andreas fault

General Overview

Earthquake Reports

Northern CA

• 2017.12.14 M 4.3 Laytonville
• 2016.11.06 M 4.1 Laytonville, CA
• 2016.11.03 M 3.8 Laytonville, CA
• 2016.08.10 M 5.1 Lake Pillsbury, CA
• 2015.08.30 M 3.6 Mendocino County, CA
• 2015.07.27 M 3.5 Point Arena, CA

Central CA

• 2018.01.04 M 4.4 Berkeley

Southern CA

• 2016.02.23 M 4.9 Bakersfield
• 2015.12.30 M 4.4 San Bernardino, CA
• 2015.05.03 M 3.8 Los Angeles, CA
• 2015.04.13 M 3.3 Los Angeles, CA
• 2014.04.01 M 5.1 La Habra p-3
• 2014.03.29 M 5.1 La Habra p-2
• 2014.03.28 M 5.1 La Habra p-1

Eastern CA

• 2016.08.04 M 4.5 Honey Lake, CA

Southern CA

Earthquake Reports

• 1994.11.17 M 6.7 Northridge, CA
• 1971.02.09 M 6.7 Sylmar, CA

Documentaries

Social Media

128. The Los Angeles area remained quiet, earthquake-wise, during post-WW2 growth. Lull was broken with M6.6 Sylmar earthquake on morning of 9 Feb. 1971, 47 yrs ago #OTD. 1.25g recorded at Pacoima Dam, highest PGA recorded to-date at that time. #200EQFacts pic.twitter.com/q1UyFzqRUw

— Susan Hough (@SeismoSue) February 9, 2018

#OnThisDay in 1971, M6.7 EQ strikes San Fernando Valley, CA. Two hospitals suffered catastrophic damage (including partial collapse),killing 47. EQ spurred effort to better understand building seismic design & performance https://t.co/ukvwER1NEK pic.twitter.com/PmEJfqhzxp

— USGS (@USGS) February 9, 2018

47 years ago this very minute: The 6.7 #Sylmar Earthquake. Were you there (The Militant may or may not have been)? #SFV https://t.co/LYkULwYGwk

— Militant Angeleno (@militantangleno) February 9, 2018

On this day in 1971, the #SanFernando (aka #Sylmar) #earthquake struck in the San Gabriel Mountains. Although a sparsely populated area, 65 people died and more than 2,000 people were injured. Property damage was estimated at $505 million. Be quake ready: https://t.co/LJAQBv9ysm. pic.twitter.com/pLeLlbQWRC

— CEA (@CalQuake) February 9, 2018

A photo of the Olive View Hospital that was damaged during the Sylmar earthquake, which struck #OTD in 1971. @LAPLPhotos: https://t.co/WgWbLv4gUN pic.twitter.com/l65l9j00YE

— ICW: California & The West (@HUSC_ICW) February 10, 2018

Today is the anniversary of the 1971 Sylmar Earthquake. Our article on how the collapse of 12 freeway overpasses that morning changed freeway construction in Southern California: https://t.co/xz0YTN7u9U

— Metro Library (@MetroLibrary) February 9, 2018

February 9: This Date in Los Angeles Transportation History (1971: The 6.6 Sylmar Earthquake jolts L.A., bringing down freeway overpasses) https://t.co/1ndsnV1gXP

— Metro Library (@MetroLibrary) February 9, 2018

Fifty-two people died in the collapse of several concrete structures, including buildings at the San Fernando Veterans Administration Hospital, above, in the 1971 Sylmar earthquake. Such structures are targets for seismic retrofits in L.A. https://t.co/K3Lx8WtoUq

— Christian Joli (@ChristianJoli) January 4, 2018

February 1971 – 6.6 Sylmar Earthquake Shakes the Southland https://t.co/8RRgSgaRMq pic.twitter.com/pDcVHKFcdf

— KCET-TV SoCal (@KCET) August 26, 2016

In the 1971 San Fernando quake #OTD the Santa Susanna Mountains grew by 2 feet. pic.twitter.com/BN5DPQV4OG

— Dr. Lucy Jones (@DrLucyJones) February 10, 2018

Naturally a lot of Northridge guesses, bc I appear to have successfully come off as an Angeleno. BUT I wasn't one til I moved from Minnesota in '97.
Framed seismogram is the '99 7.1 Hector Mine quake in the Mojave. At age 13, I decided at 2:50 that morning that I'd study quakes. pic.twitter.com/jkVHAm8y2o

— Austin Elliott (@TTremblingEarth) February 10, 2018

References

• Carena, S. and Supper, J., 2002. Three-dimensional imaging of active structures using earthquake aftershocks: the Northridge thrust, California in Journal of Structural Geology, v. 24, p. 887-904.
• Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
• Fuis, G.S>, Ryberg, T., Godfrey, N.J>, Okaya, D.A., and Murphy, J.M., 2001. Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California in Geology, v. 29, no. 1. p. 15-18.
• Fuis, G.S. et al., 2003. Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II in Geology, v. 31, no. 2, p. 171-174.
• Hauksson, E., Jones, L.M., and Hutton, K., 1995. The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects in Journal of Geophysical Research, v., 100, no. B7, p. 12235-12355.
• Tsutsumi, H. and Yeats, R.S., 1999. Tectonic Setting of the 1971 Sylmar and 1994 Northridge Earthquakes in the San Fernando Valley, California in BSSA, v. 89, p. 1232-1249.
• Tucker, A.Z. and Dolan, J.F., 2001. Paleoseismologic Evidence for a ~8 Ka Age of the Most Recent Surface Rupture on the Eastern Sierra Madre Fault, Northern Los Angeles Metropolitan Region, California in BSSA, v. 91, no. 2, p. 232-249.