Earthquake Notes

Earthquakes

Introduction

• Earthquakes are among Earth's deadliest natural phenomena.
• Ground shaking can topple buildings, liquefy ground, cause landslides, and unleash tsunamis.
• The 2004 Sumatra earthquake (magnitude 9.1) caused a massive tsunami, resulting in over 220,000 deaths or missing persons.
• This earthquake occurred due to movement on a fault where the Indian-Australian plate is subducting beneath the Eurasian plate.
• The fault ruptured for several minutes, uplifting the seafloor and displacing seawater, creating a tsunami that traveled at speeds approaching $800 \text{ km/hr}$ ($500 \text{ mi/hr}$).
• Coastal areas were inundated by as much as $20$ to $30 \text{ m}$ ($65$ to $100 \text{ ft}$) of water in Indonesia and $12 \text{ m}$ ($40 \text{ ft}$) in Sri Lanka.

What Is an Earthquake?

• An earthquake occurs when energy stored in rocks is suddenly released.
• Most earthquakes are produced when stress builds up along a fault, eventually causing the fault to slip.
• Energy is released as seismic waves, which are recorded by seismic stations.
• The hypocenter (or focus) is the place where the earthquake is generated, typically at depths less than $100 \text{ km}$ ($60 \text{ mi}$), but can be as deep as $700 \text{ km}$ ($430 \text{ mi}$) in subduction zones.
• The epicenter is the point on Earth's surface directly above the hypocenter.

Types of Faults

• Normal Faults: The hanging wall moves down with respect to the footwall. Common along divergent plate boundaries and continental rifts.
• Reverse and Thrust Faults: The hanging wall moves up with respect to the footwall. Formed by compressional forces, such as in subduction zones and continental collisions.
• Strike-Slip Faults: The two sides of the fault slip horizontally past each other. Often found at transform plate boundaries, like the San Andreas fault in California.

How Does Faulting Cause Earthquakes?

• Faults slip when the stress applied exceeds the rock's ability to withstand it.
• Rocks either flex and bend or break and slip in response to stress.
• Slippage happens in a sudden jump.
• Rocks return to their original shape after being strained - elastic behavior.
• This is called stick-slip behavior, where the fault sticks (does not move) and then slips.

Earthquake Cycle

• Stress builds up gradually on a fault.
• When the stress equals the fault's strength, the fault slips, and the stress decreases.
• The average time between repeating earthquakes is called the recurrence interval.

Earthquake Ruptures

• A rupture starts on a small section of the fault and expands along the fault plane.
• The edge of the rupture may reach Earth’s surface, causing a break called a fault scarp.
• Rupturing stops when the remaining stress can no longer overcome friction along the fault surface.

Where Do Most Earthquakes Occur?

• Most earthquakes occur along plate boundaries.
• Earthquake activity is more widespread in some regions, reaching far away from plate boundaries and into the middle of continents.
• Approximately 90% of significant earthquakes occur along subduction zones.
• Shallow earthquakes are closer to the trench, with progressively deeper earthquakes farther away in subduction zones.

Earthquake Distribution

• Divergent Plate Boundary: The belt of earthquakes north of Iceland marks a divergent plate boundary along a mid-ocean ridge.
• Continental Rift: A diffuse zone of seismic activity cuts across eastern Africa, following the East African Rift.
• Convergent Boundaries: A seismically active zone stretches along the southern part of Europe and continues eastward into Asia.
• Mid-Ocean Ridges: Only have shallow earthquakes.
• Subduction Zones: Concentrated in the western Pacific (Tonga, Java, the Philippines, and Japan), with shallow, intermediate-depth, and deep earthquakes.
• Western Hemisphere: A belt of strong seismic activity occurs along the southern part of mainland Alaska and the Aleutian Islands.
• Earthquakes follow the west coast of North America.
• Intense seismic activity occurs along the western coasts of Central America and South America.
• A belt of shallow earthquakes follows the Mid-Atlantic Ridge.

What Causes Earthquakes Along Plate Boundaries and Within Plates?

Mid-Ocean Ridges

• Earthquakes are common along mid-ocean ridges, where two oceanic plates spread apart.
• Hypocenters are less than about $20 \text{ km}$ ($12 \text{ mi}$) deep.
• Earthquakes are relatively small and shallow due to the thin lithosphere.

Subduction Zones

• A subduction zone undergoes compression and shearing along the plate boundary.
• Large earthquakes occur along the megathrust (the contact between the subducting plate and the overriding plate).
• The downgoing oceanic plate produces earthquakes from shearing along the boundary and from downward-pulling forces on the sinking slab.
• Subduction zones are typically the only place in the world producing deep earthquakes (as deep as $700 \text{ km}$ ($430 \text{ mi}$)).

Continental Collisions

• During continental collisions, one continental plate underthrusts beneath another.
• Earthquakes are widely distributed along the collision zone.
• Large thrust faults form near the plate boundary in both the overriding and underthrusting plates.

Continental Rifts

• Continental rifts produce normal faults.

Continental Transform Faults

• Strike-slip motion causes earthquakes that are mostly shallower than $20$ to $30 \text{ km}$ ($10$ to $20 \text{ mi}$).

Major North American Earthquakes

San Francisco, 1906

• Magnitude (Mw) 7.8 earthquake that ruptured $470 \text{ km}$ ($290 \text{ mi}$) of the San Andreas fault.
• Ground shaking destroyed most of the brick and mortar buildings.
• More than 3,000 people were killed, and much of the city was devastated by fires.

New Madrid, Missouri, 1811–1812

• Series of large (Mw 7.8–8.1) earthquakes generated over an ancient fault zone.

Charleston, 1886

• Estimated magnitude of Mw 7.3, the largest ever recorded in the southeastern United States.

Hebgen Lake, Yellowstone Area, 1959

• Magnitude (Mw) 7.3 event generated by slip along a normal fault.
• The earthquake set loose the massive Madison Canyon slide.

Alaska, 1964

• Magnitude (Mw) 9.2 earthquake.
• Caused by thrust faults associated with the Aleutian Islands subduction zone.
• Generated a tsunami.

Mexico City, 1985

• Magnitude (Mw) 8.0 earthquake.
• Damage was extensive because Mexico City is built on lake sediments.

Northridge, Los Angeles Area, 1994

• Magnitude (Mw) 6.7 earthquake was generated by a thrust fault.

Earthquakes in the Interiors of Continents

• Stresses can reactivate ancient faults.
• The New Madrid zone is underlain by an ancient rift basin that formed about 750 million years ago.

Can Earthquakes Be Predicted?

• There is no reliable way to predict exactly when one will strike.
• We try to predict which areas will have earthquakes by understanding the frequencies and sizes of historic earthquakes, geologic record of prehistoric earthquakes, and tectonic settings of different regions.

Earthquake Hazard

• Seismic-hazard maps show the intensity of shaking expected on land.
• The risk of earthquakes is greatest in the most tectonically active areas, especially near the plate margin in the western United States.
• The San Andreas fault is responsible for about one magnitude 8 or greater earthquake per century.

What Is the Potential for Earthquakes Along the San Andreas Fault?

• The USGS has forecasted a 99% probability that California will have a magnitude 6.7 or larger earthquake in the next 30 years.
• The San Andreas fault has distinct segments that behave differently.

Segments of the San Andreas Fault

• Northern Segment: Responsible for the famous 1906 earthquake.
• Central Creeping Segment: The two sides of the fault creep past one another somewhat continuously and slowly.
• Parkfield Segment: Produces moderate-sized earthquakes that occur, on the average, every couple of decades.
• Locked Segment: Has the potential to cause a great earthquake, commonly called “the big one.”