Comments on the Java Earthquake of 26 May 2006 and the Ongoing Eruption of Mount Merapi
Compiled and edited by Willie Lee from many sources (June 14, 2006)
The tectonics of Java are dominated by the subduction of the Australia plate
north-northeastward beneath the Sunda plate with a relative velocity of about 6 cm/year. The
Australia plate dips north-northeastward from the Java trench, attaining depths of 100-200 km
beneath the island of Java, and depths of 600 km north of the island. The earthquake of 26
May 2006 occurred at shallow depth in the overriding Sunda plate well above the dipping
Australia plate as shown in Figure 1. A cross-section (A-A’) of the seismicity from 1964-2006 is
shown in Figure 2 (A), and the earthquake focal mechanism is shown in Figure 2 (B).
From presently available data, we do not know if there is a direct link between the May 26
earthquake and the ongoing eruption of Mount Merapi (a 3-kilometer tall volcano), which is
about 30 kilometers to the north. The occurrence of shallow-focus earthquakes near volcanoes
is not unusual worldwide. Sometimes the association of earthquakes and volcanic eruptions is
so close in space and time that it is clear that the earthquakes are triggered by the magmatic
processes that are causing the eruption. In the cases of many earthquakes that occur in the
general vicinity of volcanoes, however, there are not obvious links to volcanic eruptions. In
these cases, the general spatial association of volcanoes and earthquakes is probably due to
both seismic and magmatic phenomena being localized by a broader process of plate
subduction.
Mount Merapi is one of the most active volcanoes of the world, and has erupted 68 times since
1548. Several of its eruptions have caused fatalities, e.g., thirteen villages were destroyed and
1400 people killed by pyroclastic flows in 1930. It was erupting from 1992 to 2002, and a
particularly large explosion killed 43 people in 1994. It began erupting again in 2006, and
scientists believe a large eruption is imminent. Some recent photos taken on June 6 are shown
in Figures 3 and 4. The Indonesian government has issued a RED alert, the highest level, for
Mount Merapi since May 13, and ordered the immediate evacuation of all residents on the
mountain.
On May 27th (local time), an earthquake of magnitude 6.3 struck roughly 30 miles southwest
of Mount Merapi, killing at least 5,000 and leaving at least 200,000 people homeless in the
Yogyakarta region. At the request of the Central Weather Bureau (CWB), Willie Lee has been
collecting information and relaying daily updates on the eruptions of Mount Merapi to CWB.
These daily updates are being issued by Chris Newhall, one of the world’s foremost volcano
experts, in consultations with the local Indonesian volcanologists. The June 14 update is
summarized as:
Optimism yesterday that Merapi might be slowing down was premature. Merapi is still in high
gear and BPPTK/CVGHM today raised the alert level back up to AWAS. They are
recommending (re-)evacuation of areas near rivers in the south and southwest out to a
distance of 8 km.
Pyroclastic flow activity that picked up late yesterday increased substantially today. Two
unusually long collapse events have occurred thus far today, one beginning around 1145 h
and the other around 1430-1445 h. These are multiple collapse events in which the collapse
front "eats" its way back into the dome. Clouds have obscured the volcano most of the day so
we don't know yet how far back into the dome the collapse front has reached; should it reach
directly over the vent, there is some chance that it could cause the eruption to escalate further.
Figure 1. Seismicity (1964-2006) in the Java, Indonesia region. The 26 May earthquake is
shown as a yellow “star”. Volcanoes are shown as yellow “triangles”. Earthquake epicenters
are shown as “circles”, with red color indicating shallow focal depth, green color, immediate
focal depth, and blue, deep focal depth. Cross section of seismicity along A-A’ is shown in
Figure 2(A). The line pointing to B indicates an insert of the earthquake parameters, which has
been removed and is now shown in Figure 2(B).
Figure 2(A). Cross section of seismicity along A-A’ of Figure 1. Figure 2(B) shows the
earthquake parameters, including the focal mechanism.
Figure 3. Photos of Mount Merapi taken on June 6, 2006, showing eruption activities.
Figure 4. Pyroclastic flows from Mount Merapi (photos taken on June 6, 2006).