Long Beach and Seal Beach Seismic Hazard
Robert Clayton, Yan Yang, Caltech
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.
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.
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.
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.
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.
The seismic imaging has identified a number of active subsurface faults that have not been previously recognized. These are shown in Figure 5.
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.