Long Beach and Seal Beach Seismic Hazard


Robert Clayton, Yan Yang, Caltech
3DSeismicSolutions
Eric Campbell, LASeismic; Dan Hollis, Sisprobe
Dec 9, 2020

The seismic hazard in Long Beach and Seal Beach, California is primarily based on the 1933 Long Beach earthquake, which caused wide spread damage in Seal Beach, Long Beach and surrounding areas. It was a magnitude 6.4 event that initiated near Huntington Beach and ruptured NW through Long Beach. The earthquake occurred on the Newport-Inglewood Fault (NIF), although the rupture did not break the surface.

The NIF in this area is defined to be an Alquist-Priolo Zone, which restricts the type of buildings that can be constructed within it. The zone is 300m wide swath that follows the mapped trace of the NIF as shown in Figure 1.

Figure 1. The Alquist-Priolo Zone in Seal Beach and Long Beach. This zone follows the mapped trace of the Newport-Inglewood Fault with a width of 300m.

The NIF is also the location of a number of the oil fields in the Los Angeles Basin, and as a result has been investigated with dense seismic arrays. These typically involve thousands of seismic sensors (nodes) with a station spacing of approximately 100m. They also have an active source (vibrators) component, which allows for a detailed imaging of the subsurface using reflected waves. The array in Seal Beach is shown in Figure 2, and has 5200 sensors with 300 of them deployed offshore.

Figure 2. Seal Beach Array. The left panel shows the 5200 node deployment, and the right panel shows the waves from a small earthquake (magnitude 1.4) crossing the array.

In this paper, we present the combination of two studies. The first is using passive monitoring with the array to determine the seismicity in the region and the second is imaging the subsurface using the active source part of the survey.

Seismicity

The continuous data from the sensor array is used to detect and locate micro seismicity in the Seal Beach area. During the time span that the array was active, no earthquakes were located in the Seal Beach area by the network that monitors earthquake activity in southern California (Southern California Seismic Network). The dense array was however able to detect and locate 3000 micro-earthquakes. Although none of the earthquakes were felt by humans, they are useful for identifying active subsurface structures. The seismicity is shown in Figure 3.

Figure 3. Seismicity in Long Beach and Seal Beach. The earthquakes do not appear to follow the mapped trace of the NIF. There are 3000 earthquakes detected over the three week duration of the survey. The swarm of earthquakes in the lower part of the map is discussed below.

Subsurface Imaging

The Seal Beach array along with the active source component allows for the creation of high-resolution images of the subsurface. This analysis is capable for showing the location of underground faulting and other geologic structures. An example of this imaging is shown in Fig 4.

Figure 4. Subsurface structure and seismicity. Two cross-sections are shown, with their location indicated on the map to the left. The A-A' X-section shown that the Newport-Inglewood Fault (NIF) is not a simple line with depth, but rather a complex splay fault that is spread over 5 km. The seismicity is distributed across this whole zone, which means it is all active. To the right in the A-A image are two linear faults, one of which is the Los Alamitos Fault (LAF) and the other is an unnamed fault that is quite active.

The seismic imaging has identified a number of active subsurface faults that have not been previously recognized. These are shown in Figure 5.

Figure 5. Active Faults in Seal Beach. The red areas are active deformation zones and the blue are lines are active faults. The red lines are the mapped Newport-Inglewood Fault.

As a partial confirmation of the connection between seismicity and the new faults, a comparison of the seismic swarm shown in Figure 1, is shown with the faults in Figure 6.

Figure 6. Seismic Swarm and Imaged Fault. The swarm is occurring along a particular segment of the subsurface faults (denoted by the black arrow) and appears to end where that fault intersects another fault. The small green star dot in both images is the location of the Seal Beach Pier in both plots.