By Matthew Horns
This is the first in a series of articles in which I describe the various geologic hazards that we face
here where the crumbling mountains meet the tempestuous ocean meets active tectonic faults, and so on.
These are the array of geologic hazards that are real concerns for everyone in Malibu, starting locally
and moving globally:
Flooding: Malibu enjoys more than 300 days each year of perfect calm weather. Once in a while, though,
big storms hit here that produce monumental amounts of storm runoff. When that happens, everything within
the flood zone is either washed away, destroyed, or damaged by flood waters. Any and every obstruction
to storm flow backs up the flow and sends it out of the channel, causing additional damage.
Landslides: Landslides are often triggered by floods, earthquakes, grading projects, leaky water pipes,
and other obvious events, and sometimes they happen seemingly randomly with no apparent cause.
Steep slopes throughout Malibu are rife with known and accurately mapped landslide zones.
On many of these slopes (and immediately below) are located residents, businesses, roads,
and other infrastructure. New landslides occur regularly where they were not expected.
Earthquakes: The worst-case earthquake scenario for Malibu is a rupture along a local fault.
A complex web of faults runs east-west along the Malibu coast. When one or more of these faults
cut loose, surface rupture is likely along fault traces that have not yet been accurately mapped
and included among the Alquist-Priolo fault zones where development is prohibited.
Houses will literally be ripped apart.
Activity on these faults has a recurrence interval on the order of thousands to tens of thousands
of years, so such an event is highly unlikely in the foreseeable future.
What is absolutely certain is that large earthquakes will occur on nearby much more active faults
every 20-30 years or so. All or most structures in Malibu are designed to withstand these events
without collapsing, but a lot of objects will fall from walls and ceilings. Injuries from these
quakes can be mitigated with simple actions such as not having any heavy objects on walls or
ceilings near beds.
Storm Wave Damage: Every few decades, monstrous storm waves slam the Malibu Coast and wreak havoc.
It comes with the territory. Deal with it.
Tsunamis: Malibu's coast line lies within what Oceanographers call the “Southern California Bight.”
This is the stretch of coastline between Point Conception and San Diego that is inside a series of
islands and sea mounts that to a large extent block the energy from incoming tsunamis.
Yet, severe tsunami hazards include subduction zone earthquakes from Cascadia Subduction Zone,
that runs offshore from Northern California to Southern British Columbia that ripped in the year 1700,
the subduction zone in South-central Alaska that rocked the Pacific Basin in 1964, and last year's Japan
subduction zone earthquake in which (I was surprised to learn) a piece of the North America Continental
Plate was ravaged by the Pacific Ocean Plate.
Plus, it has been recently discovered that periodically a humongous piece of a Hawaiian Island detaches
from an island and creates a mega-submarine landslide. These mega-events generate mega tsunamis that
pummel the coasts throughout the Pacific Basin.
Other potential geologic/environmantal hazard issues include: Swelling clay soils, tectonic uplift and
subsidence, groundwater hydro-geology, ocean acidification, fisheries management, global climate change,
changes in relative and absolute sea level elevation, and many more.
Ensuing articles will discuss each of these issues in more detail.
411 readers, please let us know of any other natural hazard issues that you are concerned with.