Sistematik aktifasi gempa sepanjang Sistem Sesar Sumatra [bukan preshock]

Udah baca berita kompas tentang kemungkinan preshock ? Wuiih ngeri ya ?

Tapi jangan panik dulu ini ada jawaban lain yang mungkin lebih bisa saya terima. Seperti yang pernah saya singgung tentang segment-segment gempa adanya reaktifasi satu segment dengan segmen lain. Pak Irwan Meilano, seorang ahli gempa dari ITB yang sedang ngangsu kawruh di Nagoya, Jepang, menuliskan dalam tulisan lebih komprehensif tentang reaktivasi ini.

Reaktifasi, adalah rentetan akibat getaran atau gempa di segment yang lain, pre-shock mengarah ke pemikiran gempa di segmen yang sama tetapi kekuatan lebih besar. Ada pelajaran yang penting adalah “Reaktifasi bukanlah pre-shock”

Ini uraian Pak Irwan itu…

Saya coba plot lokasi gempa Maret 2007, ternyata lokasinya persis dengan gempa thn 1926 yang menelan korban lebih dari 222 jiwa [1], dengan asumsi bahwa lokasi dlm katalog gempa [1] adalah benar, maka secara esensi bahwa gempa M6.4 Maret 2007 adalah pengulangan dari gempa 1926. Dan bukan preshock seperti yang ditulis di Kompas. Yang menarik : bahwa menutut katalog [1] gempa thn 1926 tgl 28 juni juga terjadi 2 kali. Dan mungkin ini merupakan karekterik gempa moderated segmen sesar sumatra tsb.

Trus apabila kita pilih solusi tensor dari harvard untuk gempa mekanisme strike-slip sepanjang sesar sumatra dari tgl 26 des 2004 (sesudah gempa aceh) maka paling tidak sampai maret ini telah terdapat gempa dengan >M5 sebanyak 15 (rata-rata 6 gempa/thn) apabila kita bandingkan dengan perioda 1997-25 dec 2004 (sebelum gempa Aceh) terdapat 8 gempa >M5 (rata-rata 1.1 gempa/thn).

Kenaikan aktifitas gempa secara sistematik ini dpt dijelaksan dengan perubahan stres (dCFF) sebagai akibat gempa Aceh dan Nias ang mentriger gempa sepanjang sesar sumatra [2].

Menurut [3] dan [4] besarnya dextral rata2 dari sesar sumatra yaitu 10mm/yr. Bila diasumsikan slip rata2 per gempa besar 1-2m, maka interval etiap gempa besar yaitu 200 tahun. Tetapi efek stres transfer akan mempercepat interval gempa ini. Tapi ini hanya perkiraan kasar saja 🙂
Besarnya pergeseran horizontal akibat gempa Solok yaitu maksimum 5 cm, yang dihitung berdasarkan model tensor dari harvard cmt.

Secara lengkap Irwan menuliskan dibawah ini

The March 6, 2007 M6.4 Sumatra-Solok Earthquake (systematic activation of seismic activity after the 2004 Sumatra Earthquake)
by irwan@seis.nagoya-u.ac.jp

Introduction
A powerful earthquake struck centralwestern of Sumatra on March 6, 2007, killing over 73 people and destroyed much of the city of Solok, Payah Kumbuh, Batusangkar and Simabur . The USGS National Earthquake information center is reported a moment magnitude (Mw) of 6.4.
The location of this earthquake is very close to the 1926, M6.9 which killed more than 200 people around the epicenter[1].

Tectonic setting and historical earthquake
Along the west coast of Sumatra the oblique convergence is partitioned into subduction at trench, which is nearly perpendicular to the arc and arc-parallel motion of for-arc along the Sumatran Fault [2]. The trench-normal convergence rate is about 3 cm/year in the south and slower to the north. The oblique subduction beneath Sumatra results in partitioning of the convergent motion into thrust and strike-slip faulting of SFZ (Sumatran Fault Zone). Most of the focal mechanism solutions along the SFZ, including the 2006 M5.8 earthquake and the 2007 M6.4 show pure right lateral strike-slip faulting,


Figure 1. Plate age (ma), velocity and moment tensor of historical earthquake

Earthquake mechanism and static displacement

figure 2. HIstorical earthquake around west Sumatra. The red beach balls
show the 2006 M5.8 earthquake and the 2007 M6.4 earthquake. Both are strike-slip and located just on the SFZ (Sumatran Fault Zone)

figure 3. Theoretical displacement associated with the 2007 earthquake. This horizontal displacement is calculated by using Okada [3]formula, for static dislocation

Systematic activation of seismicity along SFZ after the 2004 Great Sumatra Earthquake
The location and magnitude of the 2007 M6.4 earthquake is similar with the 1926 M6.8 earthquake which killed more than 200 villager. So essentially the 2007 earthquake is a repetition of the 1926 earthquake.

figure 4. comparison of seismicity along SFZ before and after the 2004 Sumatra earthquake.

The moment tensor solution was obtained from harvard CMT. The left figure shows earthquakes from 1997 until 25 Dec 2004 (7 years),while the right figure shows earthquake from 26 Dec 2004-6 March 2006 (2.3 years). The most obvious is the large increase in number of strike slip earthquake along SFZ. There appears to have been a systematic activation of seismic activity after the 2004 Sumatra Earthquake. The increase in seismic activity following the 2004 Sumatra earthquake is strong evidence that the stress level in the elastic lithosphere was significantly increase, due to stress transfer from the 2004 Sumatra Eq and 2005 Nias earthquake.

Implication to hazard mitigation along SFZ
Bellier et al., [4] mentioned, an average dextral slip rate of SFZ is 10mm/yr. Considering an average slip per major event of D=1-2 m, a minimum recurrence interval of 200-400 years can be deduced. But the stress transfer from the 2004 and 2005 earthquake shorten this recurrence interval. Consequently, hazard potential along SFZ significantly increase.

Reference
[1] Utsu, World Damaging Earthquake, 1990
[2] McCaffrey, 1987
[3] Okada, 1992
[4] Bellier et al., j.geodynamics,1997