GIRGIZAR ƘASA A ƊAKIN GWAJI: RUWAN DA KE JUYA AL'AMARI

Girgizar ƙasa tana faruwa ne idan matsin da ke kan tsagewar ƙasa (fault) ya zarce ƙarfinta: tubalan duwatsu biyu suna zamewa farat ɗaya. A cikin tsagewar da ta jiƙu da ruwa, matsin ruwan da ke cikin ƙananan ramukan dutse yana taka muhimmiyar rawa. Idan ya yi yawa, tubalan ba sa matse juna sosai — kuma tsagewar tana zamewa cikin sauƙi.

Hanyoyi biyu suna gogayya da juna lokacin zamewa:

1. Ƙaruwar matsi saboda zafi (thermal pressurisation). Gogayya tana ɗumama tsagewar. Ruwa yana faɗaɗa fiye da dutse nesa ba kusa ba, don haka matsinsa yana tashi, yana “ɗaga” tsagewar ya raunana ta. Ana yawan ɗauka cewa shi ne ke tuƙa manyan girgizar ƙasa — amma ba kasafai aka gan shi kai tsaye a ɗakin gwaji ba.
2. Ƙarfafawa ta hanyar faɗaɗa (dilatant strengthening). Yayin da dutse ke yankewa yana tsagewa, yana samar da sarari maras komai. Matsin ruwa yana faɗuwa, tubalan suna matse juna da ƙarfi, kuma tsagewar tana ƙara ƙarfi — wataƙila ta birki karyewar.

A fili, matakan ruwa a rijiyoyin da ke kusa da tsagewar sun sauka cikin sauri bayan girgizar ƙasar Hualien ta 2018 a Taiwan (ƙarfi 6.3) da girgizar ƙasar Kumamoto ta 2016 a Japan (ƙarfi 7.0). To wanne tasiri ne ke yin nasara, kuma yaushe?

Ƙananan girgizar ƙasa

Ƙungiyar Caiyuan Fan, Harsha Bhat da Alexandre Schubnel a Ɗakin Gwajin Ilimin Ƙasa na École normale supérieure da ke Paris ta yi amfani da silinda na granite na Westerly, tsawon mm 85 da faɗin mm 40. An ɗumama dutsen zuwa 450 °C don ya tsage, sannan aka yanka shi da zarto a kusurwar 30° don samar da tsagewa, aka goge fuskokinta da manna na lu’u-lu’u.

• Na’urorin auna matsin ruwa guda shida: huɗu daidai a gefen tsagewar (cikin rabin milimita), biyu kuma can nesa a cikin dutsen.
• Wata matsewa mai ƙarfin matsi ta matse samfuran a matsin kewaye mai tasiri na megapascal 30, 45 da 60, da ruwa a MPa 25 ko 45.
• An ɗora nauyi a hankali sosai (micrometre 0.3 a daƙiƙa). Tsagewar tana tara matsi, sannan ta zame lokaci ɗaya — girgizar ƙasa ta ɗakin gwaji. Gwaji ɗaya ya haifar da fiye da 100 daga cikinsu.

Abin da suka gani

Da farko, ƙaruwar matsi saboda zafi — an gan ta kai tsaye.

• A girgizar ƙasa na farko, matsin ruwa ya tashi lokacin zamewa, a daidai lokacin da matsin tsagewar ya faɗi.
• A lamari na farko: faɗuwar matsi ta kusan MPa 2.2, ƙaruwar matsin ruwa ta kusan MPa 7, zamewa ta kusan mm 0.05, da saurin ƙaruwar matsi har zuwa MPa 180 a daƙiƙa.
• Tsarin ka’idar ƙaruwar matsi saboda zafi yana sake samar da waɗannan ƙaruwar da kyau.
• Alamar gabanin aukuwa: matsin ruwa ya fara tashi fiye da daƙiƙa 50 kafin karyewa, yayin da matsin tsagewar kusan bai sauya ba.

Sannan, juyawa zuwa faɗaɗa.

• Girgizar ƙasa bayan girgizar ƙasa, matsin ya fara faɗuwa maimakon tashi — har zuwa kusan MPa 15, da sauri har zuwa MPa 380 a daƙiƙa. A gwaji ɗaya, ya kusa kai sifili.
• A nan ma akwai alamar gabanin aukuwa: matsin ya fara faɗuwa daƙiƙa 10 kafin karyewa.
• An ga wannan juyi a dukkan gwaje-gwaje huɗu.

Da sauri, sannu, da sauri. Tsakanin matakan biyu akwai girgizar ƙasa masu sannu a hankali, sun fi masu sauri jinkiri da mataki ɗaya zuwa uku na girma (orders of magnitude). Ba a gan su ba a cikin samfurin da ya fi barin ruwa ya ratsa.

Dalilin da ya sa tsagewar ta sauya

Yankin yankewar ya yi kauri kusan sau goma, daga kusan mm 0.05 zuwa kusan mm 0.4. Zafin ya bazu a kan dutse mai yawa, kuma ƙiyasin ɗumamar ya faɗi daga kusan digiri 10 zuwa digiri 0.2. A ƙarƙashin na’urar hangen nesa ta electron, tsagewar ta nuna wurare na dutsen da aka niƙa, zaruruwan dutsen da gogayya ta narkar, da yanki mai lalacewa mai zurfin kusan micrometre 60 a matsakaici.

Abin mamaki: a mataki na ƙarshe, faɗaɗar ta fi ƙarfi, amma duk da haka karyewar ta fi sauri. Faɗaɗa ita kaɗai ba ta tabbatar da tsagewar; akwai wasu hanyoyin raunana da ke aiki.

Iyakoki

Marubutan sun yi kira da a yi taka-tsantsan wajen kwatantawa da tsagewar ƙasa ta halitta: waɗannan gwaje-gwaje ba su haɗa da warkewar tsagewa tsakanin girgizar ƙasa ba. Sun kuma ɗauki matsakaicin na’urori huɗun, waɗanda ke nuna filin matsi marar daidaito.

Taga zuwa ga yanayin tsagewa

A cewar marubutan, wannan shi ne nuni na farko a gwaji na martanin matsin ruwa biyu masu saɓani, da jerin girgizar ƙasa masu sauri da masu sannu, a kan tsagewa ɗaya mai sauyawa. Yana ƙalubalantar ra’ayin cewa ƙaruwar matsi saboda zafi ce kullum ke rinjaye a manyan girgizar ƙasa masu sauri. Kuma alamun matsin ruwa za su iya zama taga zuwa ga yanayin tsagewa.