2016-2017 SCEC Technical Report

Introduction:

Model predictions of motion along the Pitas Point Thrust-Ventura Avenue Anticline (PPT-VAA)

call for tsunamis with peak amplitudes of 6-9 m along the Santa Barbara and Ventura coasts (Ryan et al.,

2015; Kie Thio et al., 2014; and Lotto and Dunham, 2014); amplitudes similar to the tsunami generated

by the Tohoku-earthquake. However, the fault motion and geometry used as a source for these model

predictions has been called into question (Nicholson et al., 2015). To date, evidence for such large

tsunamis is lacking. However, the only viable archive along this coastline that has been examined in

detail is Carpinteria Slough, which appears to be undergoing large environmental shifts at the same time

as the purported large earthquakes, potentially masking any potential tsunami deposits within the slough

(Reynolds et al., 2015). Most of the coastline of the Santa Barbara Channel is marked by cliffs or sandy

beaches, with very few marshes like Carpinteria Slough to preserve evidence for tsunami inundation.

However recent work along other coastlines known to have experienced large tsunamis (>6 m) like those

predicted to have struck the Santa Barbara Channel coast have shown that sandy beach ridges also

provide a record of past tsunami inundation and erosion (Meyers et al., 1996; Gouramanis et al., 2015;

Simms et al., 2015; in review). The purpose of this proposal was to collect and examine Ground

Penetrating Radar (GPR) along the coastal beach plain of Ventura and Oxnard, California in search of

evidence for beach erosion from large tsunamis created during earthquakes events along the PPT-VAA.

Activities/Methods:

We collected approximately 12 km of ground-penetrating radar using an EkkoPulse Pro (Fig. 1).

Data were collected with 100, 200, and 500 MHz antennae each with different resolutions. GPR profiles

were processed using DeWow (a Sensors and Software proprietary processing algorithm), automatic gain-

control, and corrected for topography. A common-midpoint-survey was used to determine radar

velocities within the sandy beach deposits. After processing, GPR lines were loading into Kingdom Suite

for viewing and mapping.

Preliminary Findings:

General GPR Characteristics

The GPR profiles from the Ventura-Oxnard Plain contain two major facies assemblages (Fig.

2A). The first is a seaward dipping set of parallel GPR reflections. This set routinely displays sections of

minor truncation. Based on its location and similarity to other published facies models (e.g. Hein et al.,

2013; Rodriguez and Meyer, 2006), this facies assemblage is interpreted to represent shoreface/foreshore

deposits of the prograding beach. The frequency of these minor truncations suggest they are related to

decadal-scale events – potentially strong El Nino seasons (e.g. 1982-1983, 1997-1998). These seaward

dipping reflections are capped and occasionally erosionally overlain by a second GPR facies assemblage

characterized by wavy flat-lying, occasionally landward dipping reflections (Fig. 2A). Based on its

location and similarity to other published facies models, this assemblage is interpreted as eolian dunes

overlying the underlying shoreface/forshore deposits. In a couple locations (e.g. San Buenaventura

Beach), these deposits are underlain by a reflection-free unit likely representing clay-rich deposits,

potentially of lagoonal origin. In addition, sediments from the modern Ventura River Delta were also

Figure 1. Location of the GPR profiles collected as part of this grant.

reflection-free. Both reflection-free packages likely represent poor-penetration of the GPR signal due

either to high clay or salt contents.

Potential Tsunami Surfaces

We have identified only two potential tsunami erosional surfaces, both in the same location at

Mandalay Beach (Fig. 2B and C). They likely represent the same event but imaged in different directions

– one shore-normal and the second shore-parallel. The first, identified in shore-normal profiles, is a

shallow (<1.5 m) seaward-dipping erosional surface onlapped by landward dipping reflections (Fig. 2B).

In this shore-normal perspective the surface appears similar to eolian-erosion but in shore-parallel profiles

it is clearly from different origin as it reaches greater depths (>2 m) and is localized along channel-like

incisions (Fig. 2C). The surface could represent ocean return flow following marine inundation (from a

tsunami or storm) or a small stream. The number of channels that appear un-connected support the

Figure 2. GPR profiles from Ventura-Oxnard illustrating A.) the two major facies assemblages

within the Ventura-Oxnard Plain, B.) the potential tsunami erosional surface in a shore-normal

profile as well as the other minor erosional surfaces, and C.) the potential tsunami erosional

surface in a shore-parallel profile resembling return-flow channels.

marine return flow rather than fluvial origin. Only ages from the channels will determine if they provide

support for tsunami inundation.

If it turns out that these candidate surfaces did not form at the same time as the earthquakes along

the PPT-VAA suggested by Rockwell et al. (2016) and our GPR profiles image deposits representing the

middle to late Holocene (last 6 ka), our survey confirms the work within Carpinteria Slough suggesting

the earthquakes of Rockwell et al. (2016) did no produce tsunamis.

Other GPR results

One of our originally hypothesized tsunami-units after further investigation likely represents bar

welding rather than tsunami erosion (Fig. 3). This interpretation is based on its similarity to other GPR

facies collected on a modern delta, which formed only after the removal of a dam in Washington State.

From the GPR data collected at the mouth of the Santa Clara River, we can provide a model of how

offshore bars weld to comprise the stratigraphy characteristic of beach ridges within wave-dominated

deltas.

Figure 3. Model for bar

welding based on GPR

profiles collected on the

Santa Clara River Delta and

the Elwha River Delta of

Washington.

References:

Borrero, J.C., Dolan, J.F., Synolakis, C.E., 2001. Tsunamis within the eastern Santa Barbara Channel. Geophysical Research Letters 28, 643-646.

Gouramanis, C., Switzer, A.D., Polivka, P.M., Bristow, C.S., Jankaew, K., Dat, P.T., Pile, J., Rubin, C.M., Yingsin, L., Ildefonso, S.R., Jol, H.M., 2015. Ground penetrating radar examination of thin tsunami beds - A case study from Phra Thong Island, Thailand. Sedimentary Geology 329, 149-165.

Hein, C.J., Fitzgerald, D.M., Cleary, W.J., Albernaz, M.B., de Menezes, J.T., Klein, A.D.A.F., 2013. Evidence for a transgressive barrier within a regressive strandplain system: Implications for complex coastal resposne to environmental change. Sedimentology 60, 469-502.

Hubbard, J., Shaw, J.H., Dolan, J., Pratt, T.L., McAuliffe, L., Rockwell, T., 2014. Structure and seismic hazard of the Ventura Avenue Anticline and Ventura Fault, California: Prospect for large, multisegment ruptures in the Western Transverse Ranges. Bulletin of the Seismological Society of America 104, 1070-1087.

Lotto, G. C., and Dunham, E. M., 2014, Modeling tsunami generation and propogation from an earthquake on the Pitas Point fault, Southern California Earthquake Center: Palm Springs, CA.

Nicholson, C., Sorlien, C.C., Hopps, E., and Sylvester, A.G., 2015, Anomalous uplift at Pitas Point, California: Whose fault is it anyway?, Southern California Earthquake Center: Palm Springs, CA.

Reynolds, L.C., Simms, A.R., Rockwell, T.K., and Peters, B., 2015, Holocene evolution of Carpinteria Marsh, Southern California: Storms and Subsidence, Southern California Earthquake Center: Palm Springs, CA.

Rockwell, T.K., Clark, K., Gamble, L., Oskin, M.E., Haaker, E.C., Kennedy, G.L., 2016. Large Transverse Range earthquakes cause coastal upheaval near Ventura, Southern California. Bulletin of the Seismological Society of America 106, 2706-2720.

Rodriguez, A.B., Meyer, C.T., 2006. Sea-level variation during the Holocene deduced from the morphological and stratigraphic evolution of Morgan Peninsula, Alabama, USA. Journal of Sedimentary Research 76, 257-269.

Ryan, K.J., Geist, E.L., Barall, M., Oglesby, R.J., 2015. Dynamic models of an earthquake and tsunami offshore Ventura, California. Geophysical Research Letters 42.

Simms, A.R., 2015, Recording past tsunamis in prograding coastal plains: Examples from Tolowa Dunes, northern California, Southern California Earthquake Center, Palm Springs, CA.

Thio, H. K., Li, W., Shaw, J., Hubbard, J., Plesch, A., and Wilson, R., 2014, Tsunami hazard from the Ventura-Pitas fault and fold system, Southern California Earthquake Center: Palm Springs, CA.