1. EARTHQUAKE AND MACROSEIMIC ANALYSYS
On October 8, 1909 an earthquake with the macroseismic intensity VIII °MCS struck the valley of the Kupa river. The epicentre was near Pokupsko, 40 km south-east of Zagreb. Brick and stone masonry buildings were damaged considerably, but there was no damage to wooden (oak) frame houses. The earthquake was also felt in Zagreb, where a number of chimneys toppled.
2. SEISMOGRAM ANALYSYS
The earthquake was registered on many European seismological stations. The nearest seismographs were in Zagreb. Mohorovičić wanted to study the earthquake in more detail, so he asked his colleagues to send him copies of the seismograms or their readings of the phase arrival times. All stations readily sent him the data, so Mohorovičić gathered the data of good quality, especially for the distances up to 800 km.
Phase arrival times for the Pokupsko mainshock and aftershocks as well as for some other earthquakes in the period 1904–1905 provided enough data to allow Mohorovičić to explain how the earthquake waves were propagating through the Earth’s interior.
3. EMPIRICAL SEISMIC TRAVEL-TIME TABLE
The first step was to draw the travel-time curves - graphs of seismic travel time versus epicentral distance for an earthquake at the Earth's surface. Immediately, Mohorovičić realized: “…the beginning of a travel time of the earthquake – undae primae (P) – can not be expressed by only one curve, there are two curves: one beginning in the epicentre reaching the distances up to 700 km, certainly not beyond 800 km. Second, lower curve begins certainly at 400 km, but it is possible that it has already started at 300 km… On the basis of the data collected from our earthquake this curve can be drawn up to 1800 km if necessary …”. He follows to state: “...when I was certain … that there are two types of individual primary waves that both reach all places at distances from 300 to 700 km, and… only the first type of waves reach the distances from epicentre to 300 km, while only the second type reach the distances from 700 km, I wanted to investigate this, up to now, unknown fact...”
4. THE ASSUMPTION: VELOCITY INCREASES CONTINUOSLY WITH DEPTH – TEORETHICAL TIME-TABLE OF THE INDIVIDUAL PH
The location of the Pokupsko earthquake epicentre was known from macroseismic investigations. The attempt to calculate the depth of the earthquake focus failed until Mohorovičić gave up the assumption of rectilinear propagation of the waves (with constant velocity). Instead, he proposed a simple law - that the wave velocity (c) increases with depth according to the exponential function:
c = c0 (ρ0 /ρ)k (see Figure). This expression is nowadays known as the Mohorovičić law. The rays of elastic waves in such a medium will be smooth concave curves. With this hypothesis Mohorovičić succeeded in answering all the questions he posed to himself regarding the propagation of seismic waves. He derived the parametric equation of travel-time curves (graphically presented in Figure) – the system of analytical equations (with the parameter e0) which express for the duration of wave travel from the focus to the point on the Earth’s surface at the epicentral distance j. If the theoretical travel-time table was to be applicable to practical calculation it was necessary to determine wave velocity at the focus (co), the depth of hypocentre (h = 6371–r0) and the exponent k. Mohorovičić used those values for which theoretical travel-time table best described measured (empirical) values: co = 5.60 km/s, h = 25 km, k = 3.0. Theoretical travel-times derived with those constants matched the measured data very well, having thus confirmed his assumption regarding the exponential increase of velocity with depth.
5. THE NECESSITY OF DISCONTINUITY EXISTANCE, NORMAL PHASES
The question still remaining to be answered was why the individual phases do not reach stations at distances greater than 700 km (Strassbourg - at the epicentral distance of 720 km - was the furthest station that recorded this wave) and why some stations were reached by two longitudinal (and two transversal) waves. Mohorovičić knew that “… it is entirely impossible that two different kinds of longitudinal waves with different velocities leave the earthquake focus”, so that is why “… both kinds of waves…are of the same type, that differs one from another only because they reach the surface of the Earth by different paths…”
This observation led directly to the conclusion that the Earth is not homogenous, i.e. that at a specific depth there has to be a boundary surface which separates two media with different elastic properties, and through which the waves, therefore, must propagate with different velocities.
The smooth increase of velocity with depth is assumed valid in both media, but on the boundary surface seismic wave velocities suddenly increase. Mohorovičić distinguished “individual” waves ( P, S, nowadays called Pg, Sg), whose rays lie only in the crust, and “normal” waves (P, S - nowadays known as Pn, Sn) whose rays enter the mantle and are then refracted back towards the surface of the Earth.
It was now easy to calculate the depth of the boundary surface – it corresponded to the depth of the vertex T of the ray that reache the furthest epicentral distance (at about 720 km).
Mohorovičić found by the numerical experimentation that the observed data match the theoretical equations best if the assumed value for the thickness of the upper medium was 54 km*.
He could now calculate the velocity of the longitudinal wave just above the discontinuity surface: c = 5.68 km/s. By analogously applying empirical values for the Pn phase and the fundamental laws of optics he also succeeded in determining the velocity just below the discontinuity surface and obtained: c = 7.75 km/s.
* We know today that Mohorovičić overestimated the thickness of the Earth's crust in that area due to rather poor quality of available data.
Confirming the existence of a discontinuity surface, Mohorovičić concluded that reflections and conversions of seismic waves should exist, and he calculated theoretical travel-time tables for seven fundamental reflections that matched well with the observed data from, by that time, not identified phases.
contour map of the
The discovery of the Mohorovičić discontinuity in the Earths’s interior came as the solution of one of the first inverse problems in geophysics – on the basis of the data observed on the Earth’s surface Mohorovičić concluded about the properties of the media in the Earth’s interior through which the seismic waves have travelled. In this way the seismological procedures were established for determination of other properties of the deep Earth’s interior which is inaccessible to direct measurements.
Mohorovičić discontinuity exists everywhere on the Earth. It is the largest named natural object on our planet. On the average it lies at the depth of 33 km under. Under the oceans it is the thinnest (5-10 km), while under the mountains it may be up to 70 km deep. In Croatia it is the deepest under Velebit and Dinara Mts. (about 42 km), and the shallowest under the Southern Adriatic Sea and eastern Slavonia (25 km).
The structure of the Earth's interior
A. Mohorovičić (1910): Godišnje izvješće zagrebačkog meteorološkog opservatorija za godinu 1909. Godina IX, dio IV. - polovina 1. Potres od 8. X. 1909.
D. Skoko, J. Mokrović (1998): Andrija Mohorovičić, DHMZ i Školska knjiga, Zagreb.