Introduction to seismology Mathilde B Srensen and Jens

Introduction to seismology Mathilde B. Sørensen and Jens Havskov

Course content • Introduction: Eartquakes and seismicity • Seismic rays and Earth structure • Instruments and networks • Seismic phases and location of earthquakes • Magnitude • Fault plane solutions • Seismotectonics • Tsunamis • Volcano seismology • Hazard

What is an earthquake?

Why study earthquakes ? Obtain earth structure Shearer, 1999

Why study earthquakes? Understand tectonics

Why study earthquakes? Mitigate effects from earthquake disasters M. Sørensen, 2008

Why study earthquakes: Tsunami warning March 11, 2011

Why study earthquakes: Warn of volcanic eruptions

Earthquake and volcano locations Yellow dots are earthquakes and red triangles active volcanos

Earthquakes in Norway NNSN 1970 – 2010

Earthquakes in the Caribbean s. Seismicity 1974 -2013 from USGS Harbitz et al. 2012

Deep seismicty used to discover subduction

Seismicity of volcanos, Koryaksky

Tectonic plates

The seismic cycle An earthquake occur as a sudden release of strain which has built up at either side of the fault. The earthquake causes a permanent displacement across the fault.

San Francisco, 1906

Three types of plate boundaries

Geometry of faults Fault geometry is described by three parameters: Strike (direction of the fault) Dip (inclination of the fault) Rake/slip (direction of rupture) Foot wall Hanging wall

Fault types

Normal fault

Reverse fault

Strike-slip fault Dekstral eller sinistral?

Fault types Most earthquakes will be a mix of the three main types

Earthquake energy travels through Earth as seismic waves

Types of seismic waves Body waves Surface waves Waves cause destruction Waves used to locate the earthquake and determine magnitude of the earthquake

A simple seismometer Principle behind the inertial seismometer. The damping of the motion can be mechanical, but is usually electro-magnetic.

Seismometers register seismic waves arriving at the surface at seismic stations Nearby earthquake Distant earthquake

Seismic wave velocity and global structure P-waves are faster than S-waves. Surface waves are slower than S-waves

Local crustal structure

Global crustal thickness

Principle of fault plane solution First motion of P observed at different directions relative to the fault plane for a strike-slip fault. The 2 arrows in the fault plane show the relative slip direction in the fault plane. Figure modified from Stein and Wysession, 2003.

Some fault plane solutions from seismology

Earthquake location A seismogram

Earthquake location The time difference between P- and S-waves will increase as the distance to the epicenter increases. We can therefore use the time difference to determine the distance between the earthquake and the station

Earthquake location • 1 station: infinite amount of possible locations • 2 stations: two possible locations • 3 stations: one possible location Station 1 Station 2 Station 3

How strong was the earthquake • There are different measures for the strength of an earthquake • Some describe the rupture, other the effect on the surface of Earth • The most important for the general public is magnitude often referred to as the Richter magnitude

Magnitude determination Magnitude can be determined by measuring the amplitude of the seismic waves on a seismogram We must also take into account that shaking becomes weaker with increasing distance to the epicenter Amplitude

The Richter scale Magnitude must be corrected for distance to the epicenter

General magnitude scale A: Maximum amplitude B: A constant R: Distance C: A constant

Seismic moment M 0 • • • Measure for the strength of an earthquake Determined from the area of the rupture and the displacement during the earthquake: M 0 area Can be used to calculate the most accurate magnitude

Earthquake effects, what controls the damage? • Amplitude of the seismic waves • Magnitude • Distance • Local geology • Frequency content of ground shaking • Building quality • Indirect damages (secondary effects) • Fire • Landslide/rock fall • Tsunami • Liquefaction

Building quality

Frequency content

Seismic hazard in California

Actual shaking during an earthquake

Volcano hazard example from Colombia

Conclusion Seismology is much more then earthquake studies seismology Seismology is useful to study in many fields of geoscince Seismology also essential in many prospection methods