Introduction to Earthquakes مقدمة علم الزلازل

Topics What are earthquakes ? Why do earthquakes occur ? How do we describe earthquakes ? Where do earthquakes occur ? How often do earthquakes occur ? How do we record earthquakes ? Can we predict earthquakes ? What to do in an earthquake ?

What is an Earthquake? An earthquake is a sudden movement of the ground that releases elastic energy stored in the rocks and generates seismic waves. After the initial ground movement along the fault, seismic waves propagate outward and vibrate the ground.

What is an Earthquake? Other names for earthquakes include: event (or seismic event) teleseism - an earthquake from far away seismicity - the general pattern of earthquakes in a region mainshock, foreshock, aftershock - earthquakes that form part of a sequence over time tremblor - mainly used by media

Why Do Earthquakes Occur? Short answer: Most earthquakes occur naturally as a consequence of the cooling of the Earth. Earth was very hot when it formed 4.5 billion years ago. Long answer: involves plate tectonics and mantle convection, which we will thoroughly discuss later in the class.

Why Do Earthquakes Occur? Some earthquakes are actually induced by human activity. These earthquakes are very rare and quite small. The most common type of induced seismicity has to do with the injection of high pressure waste fluid into the Earth

How are Earthquakes Described? By Location: We use two numbers to describe where an earthquake has occurred: latitude and longitude Latitude varies from 90˚ S to 90˚ N, or -90 to +90 Longitude varies from 180˚ W to 180˚ E, or -180˚ to + 180˚ These two numbers together are known as the earthquake’s epicenter

How are Earthquakes Described? By Location: Distances from earthquake epicenters to seismic stations are usually given in terms of degrees 1 degree is about 111 km. How do you figure this out ? (Hint: Earth’s radius is 6371 km) Distance is given along great circle paths (the same paths that airplanes take) Why was Steve Fossett’s trip bogus ?

How are Earthquakes Described?

How are Earthquakes Described? By Location: It turns out that earthquakes often occur beneath the earth’s surface at various depths When we combine the earthquake’s epicenter with its depth we call it the hypocenter Another word for hypocenter is focus The world’s deepest earthquakes occur at a depth of about 700 km

How are Earthquakes Described? By Time: The time when an earthquake starts is known as the origin time Origin times are reported in terms of Greenwich Mean Time (GMT) Sometimes the day and month are combined into the julian day Example: 2002 073 20:23:14.2 , means March 14, 2002 at 3:23:14.2 pm (St. Louis time)

How are Earthquakes Described? By size: The oldest method for describing the size of an earthquake is its maximum intensity (amount of damage caused) These methods were used extensively in the past before seismometers had been invented (seismometers have been around for only about 100 years) The most well known intensity scale is known as the modified Mercalli intensity scale. It varies from I for very small damage to XII for very extensive damage

How are Earthquakes Described? By size: Definition of modified Mercalli intensity V: Felt by nearly everyone, many awakened. Some dishes, windows, and so on broken; cracked plaster in a few places; unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop.

How are Earthquakes Described? By size: The more modern method of describing earthquake size is by magnitude Magnitudes are related to the size of the waves that are recorded by seismometers Magnitudes are much more accurate than maximum intensities in describing earthquake size

How are Earthquakes Described? By size: Magnitudes are defined with a logarithmic scale That means that 1 unit is really a factor of ten For instance, a magnitude 5.0 earthquake has 10 times larger seismic waves than a magnitude 4.0 earthquake In terms of energy (different than amplitude) a 5.0 is 30 times bigger than a 4.0

How are Earthquakes Described? By size: There are four main magnitude scales used by seismologists. The Richter scale is never used anymore. It is strictly valid only for earthquakes in southern California. It is impossible to convince journalists of this fact.

How are Earthquakes Described? By size: The four main magnitude scales are: Ml (local magnitude) Mb (body wave magnitude) Ms (surface wave magnitude) Mw (moment magnitude) Each one has pluses and minuses. We will discuss them in detail later in this class.

Where Do Earthquakes Occur ? Earthquakes occur in along geological structure known as faults. A fault is a fracture in a rock a that has been offset. Faults generally occur in groups known as fault zones.

Where do Earthquakes Occur ? Faults may be 100’s to 1000’s of km long, 10’s of km deep, but are generally relatively narrow.

How Often Do Earthquakes Occur? Earthquakes are quite common: Earthquakes occur in the outer 700 km of Earth and most are located in well-defined belts.

How Often Do Earthquakes Occur? Earthquakes seldom occur alone, but are generally part of a sequence of events consisting of a mainshock and aftershocks (occasionally foreshocks also occur). Generally aftershocks are usually much smaller than the mainshock - but can still be damaging in large sequences.

How Often Do Earthquakes Occur? Larger mainshocks tend to have larger aftershocks and longer aftershock sequences The decay rate of aftershocks is given by Omori’s Law: earthquake frequency ~ 1/time

How Often Do Earthquakes Occur? Some earthquakes repeat on the same fault at periodic intervals. These earthquakes are called characteristic earthquakes and the time between such events is called the recurrence interval. Typical recurrence intervals are hundreds to thousands of years.

How Do We Record Earthquakes? Devising systems that can accurately measure and record ground motions is a difficult task. Large amounts of time and effort are spent on developing and improving seismic recording systems because of the enormous value of seismic data.

How Do We Record Earthquakes? seismoscope – an instrument that documents the occurrence of ground motion (but does not record it over time) seismometer – an instrument that senses ground motion and converts the motion into some form of signal accelerometer – a seismometer that records acceleration, also known as strong ground motion geophone – another name for a seismometer, commonly used in active source seismology

How Do We Record Earthquakes? seismograph – a system of instruments that detects and records ground motion as a function of time seismogram – the actual record of ground motion produce by a seismograph seismometry – the design and development of seismic recording systems

How Do We Record Earthquakes? First seismoscope! Constructed by Chang Heng (a Chinese philosopher) around 130 AD. Ball would roll out of dragon’s mouth into frog’s mouth in the direction of ground motion.

How Do We Record Earthquakes? 132 – first seismoscope (Heng, China) 1751 – seismoscope which etched in sand (Bina, Italy) 1784 – first attempt to record ground motion as a function of time using a series of seismoscopes (Cavalli, Italy) 1875 – first true seismograph (Cecchi, Italy)

How Do We Record Earthquakes? 1889 – first known seismogram from a distant earthquake is generated (Rebeur-Paschwitz, Germany) 1914 – first seismometer to use electromagnetic transducer to sense ground motion (Galitzin, Russia) 1969 – first digital seismograph (data recorded in discrete samples on a magnetic tape) (U.S. researchers) 1990s – broadcast of real time seismic data via internet

How Do We Record Earthquakes? The first SLU seismometer was installed in 1909 (in basement of DuBourg Hall) It was part of the first North American seismological network – the Jesuit Seismological Network The first geophysics department in the Western hemisphere was established at SLU in 1925, by J.B. Macelwane, S.J. At SLU in 1948 Florence Robinson became the first woman geophysicist to obtain a PhD in North America

Can We Predict Earthquakes ? No!! There is no proven, reliable method for predicting earthquakes. A method must provide statistically significant results.

Can We Predict Earthquakes ? Example: Every day I could predict a magnitude 5.5 earthquake in Southern California and eventually I would be right and I will have “predicted” the earthquake. But this would not be statistically significant.

Can We Predict Earthquakes ? Because earthquakes cannot be predicted most of the work is directed toward mitigating the hazard from earthquakes. However, there are legitimate scientists who work on methods of predicting earthquakes.

Can We Predict Earthquakes ? Some of the things people study for possible use in predicting earthquakes: Seismicity patterns Electromagnetic precursors Variations in depth of the water table Variations in radon concentration in well water

Why Study Earthquakes? Large earthquakes can be catastrophic to cultures and societies: Can this damage be mitigated in the future? (Yes) Can earthquakes be predicted? (Probably not) Can earthquakes be prevented? (VERY unlikely)

Why Study Earthquakes? Earthquakes are the dominant source of seismic waves Analysis of seismic waves gives us the best view into the interior of the Earth: Seismologists use seismic waves to image the Earth in the same way that doctors use ultrasound to image unborn babies

Why Study Earthquakes? Techniques used in studying earthquakes can be applied to studying nuclear weapons tests. This is called verification seismology and aims to answer questions like: Did nation X test a nuclear device? How big was the weapon nation Y tested?

Who Studies Earthquakes ? Earthquake science is broad and interdisciplinary and is studied by those in fields such as: Seismology, geophysics, geochemistry, geology, and engineering Economics, history, and sociology Students in EAS-A193 at SLU

What to do in an earthquake … If you are in bed: Remain in bed until the shaking stops Hold on Protect your head with pillow

What to do in an earthquake … If you are driving: Slow down Drive to a clear area and stop Remain in the car until the shaking stops

What to do in an earthquake … If you are outdoors: Find a clear spot away from tree, power lines, and buildings Drop to the ground If you are near a tall building: duck into a doorway to avoid debris from above