wiki:ShmDocMenuAmplitudeMl
Last modified 14 years ago Last modified on 02/05/2009 02:14:07 PM

Menu entry ml

Determination of local (Richter) magnitude ml. This operation must needs Wood-Anderson simulated seismograms (see menu entry Filter).

Also needed is a source location, done e.g. by LocSAT or by writing a known location into the analysis parameter box followed by External Loc.

After selecting this ml entry the next two phase picks (clicks with the left mouse button) are used to define a time window. This time window must be marked on a horizontal trace (either N or E component, this doesn't matter). The program then finds the second horizontal component of the selected station and determines the maximum amplitude (zero-peak) independently on both horizontal components within the selected time window. Then it takes the arithmetic mean of both values, a, and uses the formula:

ml = log10(a) + sigma(distance[km])

sigma is a tabulated function of epicentral distance in km (see below). SHM interpolates this function linearly. Since SHM attaches magnitude information to phases, at least one (local) phase must be found on each station record.

   sigma  1.4 1.4 1.5 1.6 1.7 1.9 2.1 2.3 2.4 2.5 2.6 2.7 2.8 2.8 2.8
distance  0   5   10  15  20  25  30  35  40  45  50  55  60  65  70

   sigma  2.8 2.9 2.9 3.0 3.0 3.0 3.1 3.1 3.2 3.2 3.3 3.3 3.4 3.4 3.5
distance  75  80  85  90  95  100 110 120 130 140 150 160 170 180 190

   sigma  3.5 3.6 3.7 3.7 3.7 3.8 3.8 3.9 3.9 4.0 4.0 4.1 4.1 4.2 4.2
distance  200 210 220 230 240 250 260 270 280 290 300 310 320 330 340

   sigma  4.3 4.3 4.3 4.4 4.4 4.5 4.5 4.5 4.6 4.6 4.6 4.6 4.7 4.7 4.7
distance  350 360 370 380 390 400 410 420 430 440 450 460 470 480 490

   sigma  4.7 4.8 4.8 4.8 4.8 4.8 4.9 4.9 4.9 4.9 4.9 5.2 5.4 5.5 5.7
distance  500 510 520 530 540 550 560 570 580 590 600 700 800 900 1000

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