Imaging the Subducted Gorda Plate with Converted Phases and Trapped Waves

Session: Amphibious Seismic Studies of Plate Boundary Structure and Processes [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The largest slip in megathrust earthquakes often occurs in the 10-30 km depth range, updip of the zone of Episodic Tremor and Slip, yet seismic imaging in this region has proven difficult owing to its offshore location and significant depth. The Cascadia Initiative amphibious seismic array near the Mendocino Triple Junction (MTJ) provides a good opportunity to characterize the structure of the plate boundary zone between the subducted Gorda plate and the overriding North American plate across the shoreline. Here we use Ps and Sp converted phases and trapped waves recorded by the array to image the geometry and material properties of the subducted Gorda crust.

The Sp and Ps converted phases are generated within a few km of the plate interface at 10-20 km depth from local earthquakes originating within the mantle of the subducted slab. We use their arrival times and amplitudes to infer the location of the top and bottom of the subducted Gorda-plate crust and the velocity contrasts across these discontinuities. Our results indicate that near the MTJ, the Slab 1.0 model typically locates the interplate boundary within the subducted crust, about 1-2 km too deep in the 2-20 km depth range, except between 40.25°to 40.4°N where the Slab 1.0 model is too deep by 6-8 km. Comparing the amplitudes of the converted phases to synthetics for simplified velocity models, the converted phases require models that contain a ~5-km-thick low velocity zone (LVZ) with a 10-20% reduction in S wave velocity. Thus, the plate boundary is likely contained within this LVZ, which potentially indicates a significant porosity and fluid content within the seismogenic zone.

Low velocity zones act as channels that trap seismic energy. We show evidence of guided waves from earthquakes that occur within the LVZ close to the thrust interface. These earthquakes produce longer-duration S waves compared to events located just 5-8 km deeper. We conducted 2D wave propagation simulations to investigate the behavior of the thrust interface guided waves and constrain the physical properties of the plate boundary zone.

Presenting Author: Jianhua Gong

Authors