Earthquake Seismology 2017/4/17 Earthquake Seismology Earthquake descriptors Seismic waves Earthquake location

2017/4/17 Snell’s Law

Reflection and refraction of seismic waves 2017/4/17 Reflection and refraction of seismic waves

2017/4/17 Seismic Phases

Near-earthquake phases 2017/4/17 Near-earthquake phases Pg, Sg (or p, s) Waves in the upper crust. Pn, Sn Longitudinal and transverse waves refracted below the Mohorovicic discontinuity (head waves). Pb, Sb (or P*, S*) Waves in the lower crust or along the Conrad discontinuity. PmP, SmS Waves reflected from the Mohorovicic discontinuity.

Phases of distant shallow earthquakes 2017/4/17 Phases of distant shallow earthquakes P, S Direct longitudinal or transverse waves. PKP (or P') Direct longitudinal waves traversing the Earth's core without detailed identification. PKIKP (or P') Core P phase through the inner core. PP, PPP, SS, SSS P or S waves reflected once or twice at the Earth's surface. PcP, ScS P or S waves reflected at the Earth's core boundary. SKS S waves passing through the core as P waves, transformed back into S waves on emergence. PS, SP, PPS, SPP, PSPS, PPSS, SPSP, etc. P and S waves reflected and transformed at the Earth's surface. SKP S wave transformed into P on refraction into the core.

Phases of deep-focus earthquakes 2017/4/17 Phases of deep-focus earthquakes The major branches of the travel-time curves carry the same descriptions as for shallow-focus events. Waves leaving the focus in an upward direction, and reflected at the surface are described by the letters p, s, as follows: pP, pPKP, sP, sPP, etc. P or S waves reflected from the surface as P waves. pS, sS, pSS, etc. P or S waves reflected from the surface as S waves

Surface waves L G LQ LR Lg 2017/4/17 Surface waves L Long waves, unidentified, the beginning of the surface wave group. G A group of long-period Love waves often in the form of a large pulse for transoceanic paths. LQ Love waves. LR Rayleigh waves. Lg Crustal channel wave with characteristics similar to surface waves, it travels only along continental paths; in research papers the subdivision is more detailed (Lg1, Lg2, Li, Rg) (Bath, Oliver).

Locating the source of earthquakes 2017/4/17 Locating the source of earthquakes Earthquake epicenters: plate tectonics Earthquake depths Earthquake foci arbitrarily classified as shallow (surface to 70 kilometers), intermediate (between 70 and 300 kilometers), and deep (over 300 kilometers) Earthquakes originate at depths ranging from 5 to nearly 700 kilometers

Single Station Location 2017/4/17 Single Station Location Estimate Distance From S – P travel time Estimate azimuth from P-wave polarization in 3 dimensions

Hypocenter Location Techniques 2017/4/17 Hypocenter Location Techniques Linearized Least Squares Use of Depth Phases Centroid Moment Tensor (waveform modelling) see http://www.seismology.harvard.edu/CMTsearch.html

Earthquake Seismology -II 2017/4/17 Earthquake Seismology -II Intensity and magnitude Earthquake focal mechanism Velocity of the Earth Seismic tomography

Measuring the size of earthquakes 2017/4/17 Measuring the size of earthquakes Two measurements that describe the size of an earthquake are Intensity – a measure of the degree of earthquake shaking at a given locale based on the amount of damage Magnitude – estimates the amount of energy released at the source of the earthquake

Measuring the size of earthquakes 2017/4/17 Measuring the size of earthquakes Intensity scales Modified Mercalli Intensity Scale was developed using California buildings as its standard The drawback of intensity scales is that destruction may not be a true measure of the earthquakes actual severity

Measuring the size of earthquakes 2017/4/17 Measuring the size of earthquakes Magnitude scales Richter magnitude - concept introduced by Charles Richter in 1935 Richter scale Based on the amplitude of the largest seismic wave recorded Accounts for the decrease in wave amplitude with increased distance

Measuring the size of earthquakes 2017/4/17 Measuring the size of earthquakes Magnitudes scales Other magnitude scales Several “Richter-like” magnitude scales have been developed Moment magnitude was developed because none of the “Richter-like” magnitude scales adequately estimates the size of very large earthquakes Derived from the amount of displacement that occurs along a fault

Earthquake Magnitude Ml - Local (Richter) magnitude 2017/4/17 Earthquake Magnitude Ml - Local (Richter) magnitude Mb - Body wave magnitude Ms - Surface wave magnitude Mw - Seismic moment magnitude

Seismic Moment Moment Magnitude Shear modulus (~3x1010 N/m2) 2017/4/17 Seismic Moment Shear modulus (~3x1010 N/m2) Average amount of slip on the fault plane Area of the fault plane that ruptured Moment Magnitude

Example: 1994 Northridge earthquake (Los Angles) 2017/4/17 Example: 1994 Northridge earthquake (Los Angles) Estimated rupture area: A=430 km2 Average slip = 1.5 m m = 3x1010 N/m2 Mo = 430x106 (m2) x 1.5 (m) x 3x1010 N/m2=1.9x1019 (N.m) Mw=(2/3)log (1.9x1019) -6.0 = 6.8

Magnitude and Energy Log E = 11.8 – 1.5 Ms 2017/4/17 Magnitude and Energy Log E = 11.8 – 1.5 Ms Note: because of the logarithm relation, a M=7 event is 101.5, or ~ 32 times, larger than a M=6 event in terms of energy release, and ~1000 times larger than a M=5 event!

2017/4/17