Earthquake Report: Southern California

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.