Instrumentation and Quantification of Tsunamis

With an Emphasis on the Santa Barbara Channel

Quantifying Earthquake & Tsunami Strength

Seismographs & seismometers—calculate magnitude, and epicenter

Tide Gauges

Run-up measurements

Satellite Altimetry

Numerical Model Simulations

ADVs, ADCPs, pressure gauges, etc on ocean floor

Proxies for Tsunami strength

Wave amplitude (Height)

Wave speed

Vertical/horizontal displacement by crustal plates

Seismic energy, wave energy and propagation

Seismology Data

•Broadband Seismometers (top right) measure different frequency motion

•Identify long-period events from far away, and local events

•Detect all motions, including local landslides and distant earthquakes

•Seismograph measure P and S waves. Difference in arrival time used to calcuate epicenter

of earthquake

•Use these data to issue Tsunami Warnings; takes ~5min

•Data are also input into models for simulation of potential Tsunami

Tidal Gauges

•Tide gauges (top) and Buoys (bottom) often provide

first confirmation of tsunami generation.

•Buoys with bottom pressure sensors contribute real-

time data to Tsunami warning system

•Tide gauges measure changes in pressure relative to

mean trends and bottom depth

•Problems: not all tide gauges sample at same frequency

—ever 2 min, 6 min, 30 min, etc. varies by location

Schematic of Tide Gauge (left) and deployed Tide Gauge (right)

Tide gauge results from Sumatra Tsunami 2004

Deep-ocean Assessment and Reporting of Tsunamis (DART)

•~39 Surface buoys with bottom

pressure sensors

•Conversion factor ~10dbar = 10

meters

•DARTS also incorporates data

~400 tide gauges worldwide.

•DARTS transmit data every 15 min;

during Tsunami warning data is time

averaged every minute and

transmitted every 8 minutes

Terrestrial run-up and inundation by tsunamis

Tsunamis behave like shallow-water waves because of long wavelength; used to forecast inundation and amplitude of waves by modelers

By knowing ocean bathymetry, wave speed can be predicted, giving estimate of arrival time on land

Short-term Inundation Forecast for Tsunamis (SIFT)

Developed by NOAA Pacific Marine Environmental Laboratory, or PMEL, to predict tsunami propagation

Model uses seismological data and DARTS data to predict impact on different coastal locations

MOST (method of splitting tsunami) model

MOST (method of splitting tsunami) model

Numerical model that uses forecast techniques and real-time data

Incorporates satellite altimetry data, tide gauge data, and seismographic data

seafloor displacement, horizontal extent of displacement, and its location are most important for determining propagation

Offshore Southern California Tsunamigenic Hazards

Several active submarine thrust fault systems

The walls of the basin forming the channel are susceptible to submarine slope failures

Fault Lines in the Santa Barbara Region

Historical Tsunamis and Earthquakes Offshore Southern California

December 21, 1812Affected over 60km of the coast~7.2 magnitude – one of the largest in California

historyRunup: 4m at El Refugio, ~2m in SB and Ventura,

possibly a 4m in Kona, Hawaii “the sea receded and rose like a high mountain”Residents relocated their settlements further inland

after the tsunami

Historical Tsunamis and Earthquakes Offshore Southern California

November 4, 1927 – Point Arguello-LompocLargest and best observed locally generated tsunamiMagnitude ~7.0Offshore thrust or oblique-reverse fault west of Pt.

ConceptionRunup: 2m at SurfExceptionally high tide elsewhere

Santa Barbara Basin & the Goleta slide

Submerged extension of the Ventura Basin

Goleta slide: 9 miles x 6.5 miles Steep slide from 300 ft

to >1800 ft Slopes as steep as 45

deg 8-10 kilo annum years

ago

Cause of concern because of oil drilling!

Santa Barbara Basin

Triggers

Landslide Major earthquakes are the

most obvious landslide triggers

Subsurface fluid flow

The rate of sediment accumulation in the shelf-edge delta

Destabilizing influence of structural growth

30-40% probability of landslide in next 60,000 years

Landslide to Tsunami

Water depth: too deep, and landslide would propagate southward waves

Landslide volume: all or part of the remaining volume of the Goleta slide could potentially fail

Landslide speed: 20-125 m/s are required to trigger a tsunami

Potential runup: 2-20m of 10km shoreline

Good news: Goleta slide is relatively stable!

Landslides are South-facing