2004 Wave Amplitudes
& Magnitudes
of Ohio Earthquakes

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Technical Presentation for the 2004 OhioSeis Workshop,
August 11, 2004 at Alum Creek, Ohio (ACSO)
Written and presented by Dr. Larry Ruff

(These pages update the original 2002 web presentation.)

Introduction

OhioSeis Network must determine basic earthquake parameters:

• Hypocenter parameters: Epicenter, depth, & origin time (Depends on regional velocity structure).
• Magnitude: requires reliable wave amplitudes & correction factors

Focus on Magnitude

Basic definition of magnitude

In the central & eastern North America, Lg becomes the dominant phase

What is Lg?

Lg is a complex surface waves that grows from the Sg phase

What does Lg look like?

A distinct high-amplitude phase with U=3.5 km/sec, easily seen at distances of hundreds of km from EQs with M>5

What do Ohio Earthquakes look like at regional distances?

We see an unusual regionally coherent "local Lg" wave with U<2.0 km/sec that is a distinct large wave at even small distances

So what should we do to calculate the Ohio EQs magnitude?

We must find a consistent wave arrival & procedure (see below).
Meanwhile, we are using the "LLg" wave & Hermann & Kijko (1983) formula.
 

Amplitude Study of recent Ohio & Michigan earthquakes

Goal: To find the best combination of a large amplitude wave that is also regionally consistent so that a reliable magnitude can be assigned to local earthquakes -- large & small.

Issues involved in practical aspects of wave amplitude & magnitude

• Amplitudes in frequency domain
• Anti-aliasing filter
• Use wave amplitudes at Nyquist freq?

Manipulation of the MN formula suitable for broad-band velocity instruments
The 2001.01.26 Ashtabula OH earthquake
The 2000.08.07 Alliance OH earthquake
The 2001.10.23 Calhoun Co. MI earthquake

2004 Update & extension:

Results from the 2003.06.30 & 2004.06.30 NE Ohio earthquakes
Conclusions & Recommendations

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