The global seismic energy to moment ratio: a tool for basic research and real-time identification of “Tsunami

Earthquakes”

Jaime Andres ConversDr. Andrew Newman

Earth & Atmospheric Sciences

Motivation

• Recent increased activity of large earthquakes• Methods for rapid assessment of earthquake

size have not been adequate for Tsunami potential– Rapid assessment is important for hazard mitigation

from strong shaking and Tsunami generation– Identify slow tsunami earthquakes

• Obtain a discriminant for tsunami earthquakes for use in real time

Tsunami Earthquakes (TsE)

Tsunamigenic Earthquakes:• Shallow events in

subduction zones• Generate observable

Tsunami

Tsunami Earthquakes:• Shallow events in

subduction zones• Generate observable

Tsunami, larger than expected for its size

• Slow rupture• Deficient in high

frequency energy

pubs.usgs.gov/

Energy to moment ratio

• Measure of how much seismic energy is radiated related to the size of its corresponding earthquake.

• Theoretically θ assumed constant (constant stress drop).

• TsE are deficient in energy at high frequencies

• Useful for identifying energy deficient TsE

010 /log ME

Global Catalog

• Mw>6.6 from 1997+

• Radiated seismic energy in vertical component

• Between 30º and 80º.

• Above 30º we avoid crustal triplications

• At distances less than 80º we prevent effects from core diffraction

Mantle

Core

Global Catalog

• 10 years (1997 - 2007) Mw=6.7+

• 250+ earthquakes

•11,000+ recordings•avg. 45 rec/ event•E determined in f-domain

Global catalog results

• Global average for

θ= -4.5• Slow Tsunami

earthquakes show a value of <-5.7

Central America

• Location of a big TsE (Nicaragua 1992)• 50 events from 1981 to 1997 (Okal &

Newman, 2001)

• 38 events of Mw > 5.7 from 1997• depth less than 70 km.• θ= -5.1 • no along-strike or down-dip trends are

apparent.• events may rupture more slowly, or

more generally, with a lower stress drop than the global average.

Rupture durations (Tr)• We can obtain estimations

for Tr from radiated energy at high frequencies (0.5-2Hz)

Sumatra 26/12/2004 Mw=9.0

Kuril Islands 13/01/2007 Mw=8.2

TsE: Java 17/07/2006 Mw=7.7

Rupture durations and Energy

Helps to differentiate Tsunami earthquakes, characterized for having long ruptures, Tr, and reduced energy, E

Strong Earthquakes

Tsunami earthquakes

Summary and Conclusions• We have a robust dataset of 10yrs of earthquakes with

estimated released energy, E/M0

• Global average of θ is -4.5

• TsE identifiable for being energy deficient <-5.7

• Central America regional studies:– Show E/M0 averages lower than the global average:

could mean overall slower ruptures or local lower stress drop

• Its possible to obtain rupture times from HF calculations in near real time due to speed of calculations

• Further discrimination of tsunami earthquake