Geophysical hazard risks

The point **underground** where the rock ruptures and the earthquake begins.

The point on the **surface** directly above the focus.

**Magnitude** = the energy released (moment-magnitude scale); **intensity** = how strongly the shaking is felt at a place, which falls with distance.

A **shallow** focus causes far more violent surface shaking and damage than a deep focus of the same magnitude.

A series of large sea waves caused when an undersea earthquake (or landslide) suddenly **displaces the seabed and water column**.

**Tsunamis, landslides, liquefaction and fires** - knock-on hazards triggered by the shaking.

Saturated, soft ground loses strength and behaves like a **liquid** when shaken hard, so buildings sink, tilt or collapse.

Most (landslides, liquefaction, fires) **weaken** with distance from the epicentre; a **tsunami** is the exception and stays destructive far away.

Mainly the **plate margin**: subduction (destructive) zones give the largest, most frequent quakes + tsunamis; constructive margins give smaller, shallow quakes.

**Human triggers** - filling large reservoirs, or deep fluid extraction/injection - can set off earthquakes away from any margin.

Despite being a smaller quake, Haiti hit a poor, densely built city with weak buildings and no preparedness - **vulnerability**, not geophysics, drove the toll.

Two named events compared, two+ developed geophysical factors with data, a weighing of geophysics vs vulnerability, and a clear judgement.

Any threat from a volcano - **lava flows, pyroclastic flows, lahars, ashfall (tephra)** and toxic gases.

The **downslope movement of rock, soil or debris under gravity** - rockfalls, landslides, debris flows, slumps.

A **fast, super-hot cloud of gas and ash** - the deadliest volcanic hazard, impossible to outrun.

A **volcanic mudflow** of ash and water that races down valleys and buries settlements.

Fragments thrown out by an eruption that **collapse roofs and ruin farmland**.

The event that sets a slope failing - **heavy rain, an earthquake, undercutting, or an eruption**.

It is **fast and super-hot**, so there is no escape; lava is **slow**, destroying property but giving time to leave.

Its **type and speed**, the **warning time**, **population density**, and the people's **vulnerability** (wealth, housing).

A small eruption melted summit ice; **lahars** buried Armero, killing about **23,000** - warnings were not acted on.

A moderate eruption killed nobody, but its **ash cloud grounded European flights** - a huge economic impact.

The earthquake triggered **landslides** on deforested slopes; over **200,000** died, worsened by poverty and weak housing.

**Identify** the modal band, **estimate** by counting events above a threshold, and read coordinates/distance off the **key or scale**.